This Article Abstract Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Permissions Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Liu, D.-W. Articles by Antzelevitch, C. Search for Related Content PubMed PubMed Citation Articles by Liu, D.-W. Articles by Antzelevitch, C.

(Circulation Research. 1995;76:351-365.)
© 1995 American Heart Association, Inc.

Articles

Characteristics of the Delayed Rectifier Current (I_Kr and I_Ks) in Canine Ventricular Epicardial, Midmyocardial, and Endocardial Myocytes

A Weaker I_Ks Contributes to the Longer Action Potential of the M Cell

Da-Wei Liu, Charles Antzelevitch

From the Masonic Medical Research Laboratory, Utica, NY.

Correspondence to Dr Charles Antzelevitch, Masonic Medical Research Laboratory, 2150 Bleecker St, Utica, NY 13504.

	Abstract

Top Abstract Introduction Materials and Methods Results Discussion References

 
Abstract Recent studies have described regional differences in the electrophysiology and pharmacology of ventricular myocardium in canine, feline, rat, guinea pig, and human hearts. In this study, we use standard microelectrode and whole-cell patch-clamp techniques to examine the characteristics of the action potential and the delayed rectifier K⁺ current (I_K) in epicardial, M region (deep subepicardial to midmyocardial), and endocardial cells isolated from the canine left ventricle. Cells from the M region displayed much longer action potential durations (APDs) at slow rates. At a basic cycle length of 4 s, APD measured at 90% repolarization was 358±16 (mean±SEM), 262±12, and 287±11 ms in cells from the M region, epicardium, and endocardium, respectively. Steady state APD-rate relations were steeper in cells from the M region. In complete Tyrode's solution, I_K was smaller in myocytes from the M region when compared with those isolated from the epicardium or endocardium. Further characterization of I_K was conducted in a Na⁺-, K⁺-, and Ca²⁺-free bath solution to isolate the slowly activating component of the delayed rectifier (I_Ks) from the rapidly activating component (I_Kr). I_Ks was significantly smaller in M cells than in epicardial and endocardial cells. With repolarization to -20 mV, I_Ks tail current density was 1.99±0.30 pA/pF (mean±SEM) in epicardial cells, 1.83±0.18 pA/pF in endocardial cells, and 0.92±0.14 pA/pF in M cells. Voltage dependence and time course of activation and deactivation of I_Ks were similar in the three cell types. The relative contribution of I_Kr and I_Ks among the three cell types was examined by using 6 mmol/L [K⁺]_o Tyrode's solution with and without E-4031, a highly selective blocker of I_Kr. An E-4031–sensitive current was observed in the presence but not in the absence of extracellular K⁺. This rapidly activating component showed characteristics similar to those of I_Kr as described in rabbit and cat ventricular cells. Deactivation of I_Kr was significantly slower than that of I_Ks. I_Kr (E-4031–sensitive component) tail current density was similar in the three cell types, whereas I_Ks (E-4031–insensitive component) tail current density was significantly smaller in the M cells. Our results suggest that the distinctive phase-3 repolarization features of M cells are due in part to a lesser contribution of I_Ks and that this distinction may also explain why M cells are the main targets for agents that prolong APD in ventricular myocardium. These findings may advance our understanding of the ionic basis for the electrocardiographic T wave, U wave, and long QT intervals as well as our understanding of factors contributing to the development of cardiac arrhythmias.

Key Words: ventricular myocardium • electrophysiology • heterogeneity • M cells • delayed rectifier K⁺ currents

	Introduction

Top Abstract Introduction Materials and Methods Results Discussion References

 
A growing body of evidence suggests that prominent electrophysiological and pharmacological heterogeneity exists within the mammalian ventricle (for reviews see References 1 through 3^{1 2 3} ). Studies from our laboratory have delineated several electrophysiological distinctions between epicardial and endocardial tissues and myocytes isolated from the canine ventricles.^{1 4 5 6 7} Chief among these is the presence of a prominent spike-and-dome morphology of the action potential in epicardial but not endocardial tissues. This distinction has also been described in the canine heart in vivo^{8 9} and in rabbit,¹⁰ feline,¹¹ and human^{12 13} ventricular myocytes studied in vitro. The presence of a more prominent 4-aminopyridine–sensitive transient outward current (I_to1) in canine ventricular epicardial myocytes was shown to be responsible for the more prominent spike-and-dome morphology of the epicardial action potential.^{4 7 10 11}

More recent studies have described a unique population of cells (M cells) in the deep subepicardial to midmyocardial layers (M region) of the canine ventricle. M cells were found to display electrophysiological features intermediate between those of myocardial and conducting cells^{7 14 15} and pharmacological responsiveness different from that of either epicardium or endocardium.^{16 17} The hallmark of the M cell is the ability of its action potential to prolong dramatically with slowing of the stimulation rate. The rate dependence of action potential duration (APD) in the M region is much more accentuated than that of epicardium and endocardium but more akin to that of Purkinje fibers. Pharmacological studies have also shown that early and delayed afterdepolarizations (EADs and DADs, respectively) and triggered activity develop much more readily in tissues from the M region than in epicardial or endocardial tissues.^{16 17}

Although the ionic basis for the prolonged APD of M cells and the unique sensitivity of these cells to agents with class III actions is not known, a diminution in the intensity of net repolarizing current is clearly involved. The delayed rectifier current (I_K) along with transient outward current (I_to) and inward rectifier current (I_K1) are thought to play an important role in the regulation of APD in a variety of tissues from different species.¹⁸ In the canine ventricle, the magnitude of I_K1 is not significantly different among cells isolated from epicardium, midmyocardium, and endocardium.⁷ I_to has been shown to be most prominent in cells from the epicardial region and to diminish gradually across the canine ventricular wall. Because of its fast activation and inactivation kinetics, I_to is believed to play an important role in phase 1 repolarization but to have negligible effects on phase 3 repolarization in the canine ventricle.^{7 19} M cells exhibiting steep APD-rate relations have been shown to possess levels of I_to not significantly different from those found in epicardial cells displaying little rate dependence of APD. Thus, differences in I_K1 and I_to do not appear to account for the marked heterogeneity of repolarization characteristics observed among cells spanning the canine ventricular wall.

Although differences in the contribution of I_K to endocardial and epicardial activity have been described in the feline ventricle,²⁰ the relative contribution of I_K to midmyocardial cell activity has not previously been quantified. The present study was designed to characterize the properties of I_K in myocytes isolated from discrete regions of the canine left ventricle and to test the hypothesis that the unique action potential features of the M cell are due at least in part to the presence of a weaker I_K in these cells.

	Materials and Methods

Top Abstract Introduction Materials and Methods Results Discussion References

 
Isolation of Cardiac Myocytes
Myocytes were isolated by enzymatic dissociation as previously described.⁷ Briefly, adult mongrel dogs of either sex were anesthetized with sodium pentobarbital (30 mg/kg IV), and their hearts were quickly removed and placed in normal Tyrode's solution. A wedge consisting of that part of the left ventricular free wall supplied by the left anterior descending coronary artery (LAD) was excised. The LAD was cannulated and flushed with Ca²⁺-free Krebs' buffer supplemented with 0.1% bovine serum albumin (BSA, fraction V, Sigma Chemical Co) and gassed with 95% O₂/5% CO₂ for 5 minutes at a rate of 12 mL/min. Perfusion was then switched to 75 mL Ca²⁺-free Krebs' buffer containing 75 mg BSA and 37.5 mg collagenase (CLS 2, 171 U/mg, Worthington) for 15 to 20 minutes at 37°C (95% O₂/5% CO₂, with recirculation). After perfusion, thin slices of tissues were dissected from epicardium (<1.5 mm from epicardial surface), M region (2 to 7 mm from epicardial surface), and endocardium (<2 mm from the endocardial surface) by use of a dermatome (Davol Simon Dermatome power handle No. 3293 with cutting head No. 3295). Shavings were made parallel to the surface of the left ventricular free wall midway along the apicobasal axis. Tissues from each region were placed in separate beakers, minced, incubated in fresh Krebs' buffer containing 0.5 mg/mL collagenase, 3% BSA, and 0.3 mmol/L CaCl₂, and agitated with 95% O₂/5% CO₂. Incubation was repeated three to five times at 15-minute intervals with fresh enzyme solution. The supernatant from each digestion was filtered (220-µm mesh) and centrifuged (200 to 300 rpm for 2 minutes). Cells were then stored in a HEPES-buffered Tyrode's solution (see below) supplemented with 0.5 mmol/L Ca²⁺ at room temperature for later use.

Solutions and Drugs
Krebs' buffer used in the cell dissociation procedure contained (mmol/L) NaCl 118.5, KCl 2.8, NaHCO₃ 14.5, KH₂PO₄ 1.2, MgSO₄ 1.2, and glucose 11.1. The composition of the HEPES-buffered Tyrode's solution was (mmol/L) NaCl 132, KCl 4 or 6 (as indicated), CaCl₂ 2, MgSO₄ 1.2, HEPES 20, and glucose 11.1 (pH was adjusted with NaOH to 7.35). The Na⁺-, K⁺-, and Ca²⁺-free external solution contained (mmol/L) choline chloride 140, MgCl₂ 2.0, HEPES (free-acid) 20, and glucose, 11.1 (pH was adjusted to 7.35 with LiOH). In some of the experiments, EGTA (0.5 mmol/L) was included in addition to the above constituents. The pipette solution contained (mmol/L) potassium aspartate 125, KCl 20, MgCl₂ 1, ATP (Mg²⁺ salt) 5, HEPES 5, and EGTA 10. The pH of the pipette solution was adjusted to 7.3 with KOH. Nisoldipine (Sigma) and E-4031 (Eisai Co, Ltd) were prepared fresh before each use.

Recording Techniques
Myocytes were superfused with a HEPES-buffered Tyrode's solution (aerated with 100% O₂) at a flow rate of 2 to 3 mL/min. Only relaxed quiescent cells displaying clear cross striations were used. All experiments were performed at 35°C to 37°C, and temperature was maintained constant within 0.5°C during any given experiment.

To minimize alterations of intracellular milieu, action potential studies were performed with 2.7 mol/L KCl–filled microelectrodes (20- to 50-M resistance) and an Axoclamp-2A amplifier with an HS-2L–gain X0.1 headstage (Axon Instruments) in bridge mode. Cells were stimulated by injection of current pulses of 1- to 2-ms duration at basic cycle lengths (BCLs) ranging from 300 to 8000 ms.

I_K was measured by using standard whole-cell patch-clamp techniques. For the present study, an Axopatch-1D amplifier with a CV-4 1/100 headstage (Axon Instruments) was used. Suction pipettes made of borosilicate glass (outer diameter, 1.5 mm; inner diameter, 1.1 mm; Becton, Dickinson and Co) were pulled on a Flaming-Brown–type pipette puller (Sutter Instrument Co) and heat-polished before use. Pipette tip resistances measured in Tyrode's solution were 2 M when filled with pipette solution. The junction potential between the pipette solution and Tyrode's solution was 10 mV (pipette negative). All voltages in the patch-clamp experiments were corrected for this offset. In one set of experiments, action potential was recorded by using a pipette under whole-cell current-clamp mode; I_K was then measured by switching to voltage-clamp configuration. Na⁺ current was inactivated by holding at -40 mV; Ca²⁺ current was inhibited by the addition of 2 µmol/L nisoldipine to the superfusate. In a second series of experiments, I_K was recorded in Na⁺-, K⁺-, and Ca²⁺-free external solution, in which Na⁺, Ca²⁺, and inward rectifier K⁺ currents, electrogenic Na⁺-K⁺ pump, and Na⁺-Ca²⁺ exchange currents were eliminated. Nisoldipine (1 µmol/L) was also added to the external solution. Because 0 mmol/L [K⁺]_o has been shown to increase the slowly activating component of I_K (I_Ks) and greatly diminish the rapidly activating component of I_K (I_Kr),^{21 22 23} these conditions were used to characterize I_Ks. In a third series of experiments designed to determine the relative contribution of I_Kr and I_Ks, we exposed the cells to Tyrode's solution containing 6 mmol/L K⁺ and 2 µmol/L nisoldipine, with and without E-4031, a specific I_Kr blocker.^{22 24} I_to was not blocked, but it had little influence on our measurement of I_K because of its fast inactivation kinetics (80% inactivated within 40 ms after the onset of depolarization⁷ ).

Cell capacitance was calculated by integrating the area under the uncompensated capacitive transient produced by a 5-mV hyperpolarization step from 0 mV and dividing this area by the voltage step. The average capacitance values for myocytes from epicardium, M region, and endocardium were 156.3±28 (mean±SD, n=32), 157.8±20 (n=35), and 148.2±11 (n=28) pF, respectively. Whole-cell current was not electronically compensated for series resistance and capacitance. The average access resistance (the sum of the pipette resistance and the residual resistance of the ruptured patch) was 5.1±0.91 M (mean±SD, n=21), estimated by dividing time constant of the decay of the capacitance transient by the calculated cell membrane capacitance (measured as indicated above).²⁵ The membrane currents recorded in the present study were <500 to 600 pA in most cases. Thus, the maximum voltage error caused by the series resistance would be expected to be on the order of 2 to 3 mV.

Data Acquisition and Analysis
To exclude the possible effects of I_K rundown²⁶ on our current measurement, the time course of changes of I_K after rupture of the patch membrane was monitored in a set of experiments by repeatedly measuring peak tail current amplitude at -20 mV. No apparent rundown was observed for at least 15 minutes after membrane rupture in the three cell types. Accordingly, all measurements of I_K reported below were obtained between 4 and 11 minutes after rupture of the plasma membrane.

A personal computer equipped with 12-bit AD/DA converters (model 1401, Cambridge Electronic Design) was used for data acquisition and generation of pulse template and command potentials for both current- and voltage-clamp modes (VCLAMP software module). Currents were filtered with a four-pole bessel filter at 0.5 to 1 kHz and digitized at 1 kHz.

Curves were fit by using nonlinear least-squares regression techniques (voltage-clamp analysis module, Cambridge Electronic Design). The goodness of fit could be assessed by examining the minimum variance between the experimental data and the fitted curve. Data are expressed as mean±SEM unless indicated otherwise. Statistical analysis of the data was performed by one way ANOVA coupled with Scheffe's or Tukey's procedure (SIGMASTAT software package, Jandel Scientific).

	Results

Top Abstract Introduction Materials and Methods Results Discussion References

 
Action Potential Characteristics
Action potential characteristics of myocytes isolated from the epicardial, M, and endocardial regions of the canine left ventricle are illustrated in Figs 1 and 2. Transmembrane activity was initially recorded by using conventional microelectrodes to minimize changes in the intracellular milieu. At slow stimulation rates, action potentials recorded from epicardial and M cells displayed a prominent phase 1, giving rise to a spike-and-dome configuration of the action potential. M cells also displayed longer action potentials. These features of the three cell types are illustrated in Fig 1A. APD measured at 90% repolarization (APD₉₀) was 400 ms in the myocyte from the M region and 224 and 223 ms in the epicardial and endocardial cells, respectively. Fig 1B graphically illustrates composite data obtained from 30 animals, showing APD₉₀ values of 69 individual cells. Data are grouped on the basis of the site of origin of the myocyte as defined in "Materials and Methods." APD₉₀ values averaged 358±16 (n=34), 262±12 (n=18), and 287±11 (n=17) ms in M, epicardial, and endocardial cells, respectively.

View larger version (15K): [in this window] [in a new window]   Figure 1. A, Transmembrane action potentials recorded from myocytes isolated from the epicardial region (Epi), midmyocardial (M) region (deep subepicardium to midmyocardium), and endocardial region (Endo) of canine left ventricular free wall. Action potentials were obtained by using standard microelectrodes at a stimulation basic cycle length of 4000 ms. B, Graph showing cumulative data of the action potential duration (APD) of myocytes isolated from Epi, M, and Endo layers. APD90 indicates APD measured at 90% repolarization. Data points represent the results of individual myocytes. [K+]o was 6 mmol/L. Solid lines among data points denote mean values for each group.

View larger version (20K): [in this window] [in a new window]   Figure 2. A, Rate dependence of action potential characteristics of myocytes isolated from the epicardial region, midmyocardial (M) region, and endocardial region of the canine left ventricular free wall. Each section shows superimposed action potentials recorded at basic cycle lengths (BCLs) of 300, 500, 1000, 2000, 4000, and 8000 ms. All action potentials were recorded by using standard microelectrode techniques. [K+]o was 6 mmol/L. B through D, Action potential duration (APD)-rate relations for myocytes isolated from epicardial, M, and endocardial regions of the canine left ventricular free wall. M cell behavior (steep APD-rate relation) is observed almost exclusively in cells isolated from the M region, whereas relatively flat relations are observed predominantly in cells from epicardium and endocardium. Transitional behavior is apparent in all three fractions. APD90 indicates APD measured at 90% repolarization. [K+]o was 6 mmol/L. Results are from 69 cells isolated from 30 different hearts.

The rate dependence of action potential characteristics is illustrated in Fig 2. Each section in Fig 2A comprises six superimposed tracings representing action potentials recorded at progressively longer BCLs (steady state). A prominent rate-dependent spike and dome is apparent in the myocytes of the epicardial and M regions (M cells and transitional cells) but not in those from endocardium. A gradual shift from a BCL of 300 to 8000 ms leads to a progressive accentuation of the spike-and-dome configuration of the action potential in epicardial cells. Phase 1 becomes more prominent, and the peak plateau is achieved later, usually reaching a more positive potential. Accentuation of the notch is seen to contribute to the overall prolongation of APD₉₀ in epicardial cells. Deceleration-induced accentuation of the spike-and-dome morphology of the action potential is also observed in the M cells. The overall contribution of the changes in phase 1 to rate dependence of APD, however, is less important than in epicardial cells. In cells from the M region, deceleration was attended by a remarkable prolongation of the action potential that was principally due to progressive delays in the onset of final repolarization. Many cells isolated from the M region exhibited transitional behavior (second action potential from top), displaying intermediate prolongation of APD. Endocardial cells displayed little rate-dependent change in APD and no change in the early phases of the action potential.

The APD-rate relations for 17 epicardial cells, 34 M cells, and 18 endocardial cells are graphically illustrated in Fig 2B through 2D. At a BCL of 300 ms, all three cell types display relatively brief action potentials of similar duration. With progressive slowing of the stimulation rate, the APD of the M cells is prolonged more than the APD of the epicardial and endocardial cells. Thus, the APD-rate relations recorded in cells from the M region are generally steeper than those observed in cells from the epicardial and endocardial regions of the left ventricular free wall. APD-rate relations with transitional characteristics (intermediate slope) are found in all three panels, whereas steep relations are observed predominantly in cells from the M region, and relatively flat relations are observed exclusively in cells from the epicardial and endocardial regions. These results indicate that transitional cells are present in our epicardial and endocardial as well as M region fractions. These results are consistent with those of recent studies designed to assess the distribution of M cells across the canine ventricular wall by mapping transmural tissue slices. M cells were found to be widely distributed within the ventricular wall, with transitional behavior apparent throughout, particularly between midmyocardium and endocardium, as well as between midmyocardium and epicardium.^{2 15} The similarities in behavior between the tissues and myocytes suggest that regional differences in electrophysiological characteristics are likely due to intrinsic differences in ionic currents underlying the action potential.

Action Potentials and Membrane Currents Recorded Using Whole-Cell Patch-Clamp Technique
The action potential data thus far presented indicate prominent differences in APD among cells from different regions of the ventricle. To obtain a better understanding of the ionic basis for these differences, we characterized I_K in the three cell types by using whole-cell patch-clamp techniques. Fig 3 illustrates the results of a representative experiment. The action potentials in panel A, recorded while using a patch pipette under whole-cell current-clamp mode ([K⁺]_o, 4 mmol/L; BCL, 2000 ms), illustrate that the salient features of transmembrane activity of the three distinct cell types are maintained under these recording conditions. Thus, in this experiment and several others in which ionic currents were evaluated, we were able to distinguish between M cells, epicardial cells, and endocardial cells not only on the basis of their anatomic source (level of the ventricular wall from which they were isolated) but also on the basis of their action potential characteristics. After recording of the action potential, we switched to voltage-clamp mode for the measurement of ionic currents from these same cells. Nisoldipine (2 µmol/L) was then added to the bath solution to block the Ca²⁺ current, and the Na⁺ current was inactivated by holding the membrane potential at -40 mV. Cells were depolarized to +60 mV for 5 s and then repolarized to -20 mV (Fig 3B inset). These pulse parameters were chosen so as to optimize the measurement of I_Ks availability in the three cell types. A time-dependent current is seen to develop during the depolarization, and a decaying I_K tail current is observed on the return to -20 mV (Fig 3B). These tracings point to a smaller I_K in M cells than in epicardial or endocardial cells.

View larger version (16K): [in this window] [in a new window]   Figure 3. Delayed rectifier K+ current (IK) recorded from myocytes from the epicardial region (Epi), midmyocardial (M) region, and endocardial region (Endo) by using whole-cell patch-clamp technique. A, Action potentials recorded in current-clamp mode (basic cycle length [BCL], 2000 ms) when using normal Tyrode's solution. [K+]o was 4 mmol/L. B, Membrane currents recorded from the same cells using the voltage-clamp protocol shown in the inset. Nisoldipine (1 µmol/L) was added just before recording of current. The developing current during depolarization and the tail current observed on repolarization to -20 mV were smallest in the M cell (cell 122905; capacitance, 169 pF), less prominent in the Endo cell (cell 122922; capacitance, 142 pF), and most prominent in the Epi cell (cell 122913; capacitance, 123 pF). C, Graph showing relation between the action potential duration (APD) and IK tail current density. APD90 indicates APD measured at 90% repolarization. BCL was 2000 ms, and [K+]o was 4 mmol/L. Tail current amplitude was measured at -20 mV after a 5-s depolarization step to +60 mV from a holding potential of -40 mV. Conditions were the same as in Fig 3A and 3B.

Fig 3C summarizes data obtained from 17 cells in which I_K and APD₉₀ were recorded in tandem by use of the same protocol. Plotted is the relation between the APD₉₀ values and I_K tail current density, expressed as I_K tail current amplitude divided by the cell capacitance. Data are expressed as mean±SEM. The data indicate that cells from the M region display significantly longer APDs as well as smaller I_K density when compared with cells from the epicardium and endocardium. There was no significant difference in either I_K density or APD₉₀ between epicardial and endocardial myocytes.

Properties of I_Ks
In guinea pig ventricular myocytes, I_K has been shown to comprise two components: I_Kr, which is rapid to activate, and I_Ks, which activates more slowly.^{22 24} These two components also differ in their rectification characteristics as well as their sensitivity to drugs: I_Kr is more selectively blocked by methanesulfonamides such as E-4031^{22 27} and dofetilide.²⁸ Another distinction is that in the absence of extracellular K, I_Kr is greatly diminished, whereas the intensity of I_Ks is augmented.²¹ To assess the contribution of I_Ks in the three cell types, we conducted another series of experiments using a Na⁺-, K⁺-, and Ca²⁺-free external solution. Under these conditions, I_Kr is practically eliminated, and the contribution of Na⁺, Ca²⁺, and inward rectifier K⁺ currents, as well as Na⁺-K⁺ pump current and Na⁺-Ca²⁺ exchange current, are minimized. I_to was partially inactivated at the holding potential of -40 mV. The I_K tail current recorded under these conditions is unaffected by 5 µmol/L E-4031 (Fig 4, tracings 2 and 3); therefore, we refer to it as I_Ks.

View larger version (20K): [in this window] [in a new window]   Figure 4. Effect of E-4031 on the delayed rectifier current in the absence (0 mmol/L) and presence (6 mmol/L) of extracellular K+. A midmyocardial region myocyte was voltage-clamped by using the protocol shown in the top inset. Superimposed current tracings were recorded with the following bath solutions: Tyrode's containing 6 mmol/L [K+]o (tracing 1), 0 mmol/L [K+]o (tracing 2), 0 mmol/L [K+]o+5 µmol/L E-4031 (tracing 3), and 5 µmol/L E-4031+6 mmol/L [K+]o (tracing 4). E-4031 had no effect on the delayed rectifier current recorded in 0 mmol/L [K+]o (tracings 2 vs 3) but decreased both developing current and tail current recorded in 6 mmol/L [K+]o (tracings 1 vs 4). These results indicate that the rapidly activating component (E-4031–sensitive current) of the delayed rectifier is suppressed in 0 mmol/L [K+]o and that the slowly activating component is the predominant component under these conditions. Nisoldipine (2 µmol/L) was present throughout. A diminution of the inward rectifier K+ current is responsible for the shift in background current observed when [K+]o is lowered from 6 to 0 mmol/L.

Fig 5A illustrates representative current tracings recorded in cells bathed in Na⁺-, K⁺-, and Ca²⁺-free external solution. From a holding potential of -40 mV, the cells were depolarized to various voltages for 5 s and then repolarized to -20 mV. The outward current at the end of the 5-s depolarizing pulse shows a gradual outward creep, especially at the more positive potentials, a common finding that has been reported in other studies.^{20 29 30 31} The developing currents observed during the depolarization steps and the tail currents observed on the return to -20 mV are once again smaller in the M cell than in the epicardial or endocardial cell. Fig 5B and 5C plots the cumulative data for I_Ks tail currents measured in the three cell types. Shown are current amplitudes and current densities measured at -20 mV after a 5-s pulse to +60 mV (from a holding potential of -40 mV). Each point represents results from an individual cell. The dotted lines indicate the mean values for each group. The amplitude and current density of I_Ks tails were significantly greater in epicardial and endocardial cells than in M cells (also see Table 1). The average tail current density in cells from the M region was approximately half of that recorded from the epicardial and endocardial cells. Many cells isolated from the M region displayed very low levels of I_Ks; 49% registered I_Ks densities <0.5 pA/pF. No cells from the epicardial or endocardial regions of the left ventricle displayed such low I_Ks densities. The majority of cells isolated from epicardium and endocardium displayed I_Ks densities >1.75 pA/pF. Only two cells (6%) isolated from the M region yielded I_Ks densities of this magnitude (Fig 5C).

View larger version (23K): [in this window] [in a new window]   Figure 5. A, Voltage-dependent activation of the slowly activating component of the delayed rectifier K+ current (IKs). Shown are representative tracings recorded from cells from the epicardium (Epi), midmyocardial (M) region, and endocardium (Endo). Currents were elicited by the voltage pulse protocol shown in the inset. All experiments were performed by using a Na+-, K+-, and Ca2+-free solution. Capacitance (Cm) was as follows for cells with the following numbers: 010501 (Epi, Cm=182 pF), 121014 (M, Cm=235 pF), and 122205 (Endo, Cm=124 pF). B and C, Plots showing IKs tail current amplitude (left) and density (right) in the three cell types measured at -20 mV after a 5-s depolarizing pulse to +60 mV from a holding potential of -40 mV. Each point represents the results from an individual cell. The dotted lines indicate the mean values for each data group. The amplitude and density of IKs tail current measured in myocytes isolated from the M region were significantly smaller than those recorded from cells isolated from epicardium and endocardium. The bath solution was Na+, K+, and Ca2+ free.

View this table: [in this window] [in a new window]   Table 1. Slowly Activating Delayed Rectifier Tail Current Amplitude and Current Density in the Three Cell Types

To assess the contribution of I_Ks under more physiologically relevant conditions, we measured the tail current amplitude and current density elicited by a 400-ms depolarizing pulse to +40 mV. The tail current amplitude averaged 117±29 (n=7), 50±9 (n=13), and 84±14 (n=9) pA in epicardial, M, and endocardial cells, respectively. A significant difference was apparent between epicardial and M cells but not between epicardial and endocardial cells.

Voltage Dependence of Activation of I_Ks
One possible explanation for the regional differences in I_Ks is that the voltage dependence of activation of I_Ks is different in the three cell types. We studied the voltage dependence of steady state activation of I_Ks by using the protocol illustrated in Fig 5A. The relation between the normalized tail current and the voltage of the preceding 5-s pulse was taken as approximate measurement of voltage dependence of steady state activation. Fig 6 shows the relations recorded from the three cell types fitted to the following Boltzmann function:

where V is the voltage at which half activation was achieved, V_m is the membrane potential, and k is the slope factor. No significant difference was detected in the voltage for V or k in the three cell types. The V and k values were 24.6±8 mV and 12.1±0.4 (n=18) in epicardial cells, 24.7±6.0 mV and 13.6±0.8 (n=21) in M cells, and 25.4±6.0 mV and 12.1±0.5 (n=17) in endocardial cells.

View larger version (16K): [in this window] [in a new window]   Figure 6. Steady state activation of the slowly activating component of the delayed rectifier K+ current (IKs) in myocytes from the epicardium (Epi), midmyocardial (M) region, and endocardium (Endo). The voltage dependence of activation of IKs was obtained by measurement of tail currents recorded by using the protocol shown in the inset. Continuous lines are least-squares fits to a Boltzmann function by using the mean values from each data group. Values represent mean±SEM. All experiments were performed by using a Na+-, K+-, and Ca2+-free solution.

Activation and Deactivation Kinetics of I_Ks
Fig 7 illustrates the results of envelope-of-tails tests performed in the three cell types. From a holding potential of -40 mV, membrane potential was depolarized to +40 mV for various durations and then returned to -40 mV once every 20 s. Fig 7A shows superimposed tracings of the developing current activated during progressively longer depolarizations and the tail currents observed immediately after repolarization. The relative magnitude of the developing current recorded during the test pulse and the tail current recorded on repolarization to -40 mV were observed to change in parallel (nearly constant ratio), suggesting that I_Ks represents the activation of a single outward current. Fig 7B plots the amplitude of the tail currents as a function of pulse duration. The data points were fit by least-squares regression to the sum of two exponentials, shown as solid lines for each cell type. Time constants of the fast (_f) and slow (_s) components were not statistically different in the three cell types: _f, 628±57 ms (n=6), 387±45 ms (n=9), and 484±110 ms (n=6); _s, 6.1±1.5 ms (n=6), 4.4±0.9 ms (n=9), and 8.2±4.8 ms (n=6) in epicardial, M, and endocardial cells, respectively. The amplitude of the fast component was 41±7%, 35±6%, and 32±10% of the total amplitude in epicardial, M, and endocardial cells, respectively. Fig 7C depicts normalized plots of the tail current amplitude as a function of pulse duration, illustrating the similar time course for activation of I_Ks in the three cell types.

View larger version (13K): [in this window] [in a new window]   Figure 7. Time course of activation of the slowly activating component of the delayed rectifier (IKs) tail current (envelope-of-tails test) in the three cell types. A, Superimposed current tracings obtained from cells from the epicardium (Epi), midmyocardial (M) region, and endocardium (Endo). Currents were elicited by using voltage-clamp pulses from -40 to +40 mV for various durations, applied once every 20 seconds. All experiments were performed by using a Na+-, K+-, and Ca2+-free bath solution. B, Time course of activation of peak tail current amplitude obtained from the corresponding recordings in panel A. Biexponential fits are indicated by the solid line. Time constants of fast and slow components of activation of the delayed rectifier K+ current were, respectively, as follows: Epi, 553 ms and 1.85 s; M, 350 ms and 2.95 s; and Endo, 602 ms and 2.22 s. Capacitance (Cm) was as follows for cells with the following numbers: 121003 (Epi, Cm=165 pF), 111206 (M, Cm=138 pF), and 122209 (Endo, Cm=155 pF). C, Composite data comparing the time course of activation of IKs in 30 Epi, M, and Endo cells. Normalized tail current amplitudes are plotted as a function of the duration of the voltage-clamp pulse (same protocol as in panel A). Continuous lines represent least-squares fits of double-exponential functions to the mean values of each data group.

The time course of I_Ks deactivation was examined by least-squares fits of current tails recorded on repolarization to -20 mV after a 5-s pulse to various test voltages. A biexponential time course of decay of I_Ks tail current was observed in all three cell types. _f and _s, as well as their relative contributions, are presented in Table 2. No significant difference was observed in the time course of decay of I_Ks among the three cell types.

View this table: [in this window] [in a new window]   Table 2. Kinetic Parameters of the Decay of the Slowly Activating Delayed Rectifier Tail Current in Epicardial, Midmyocardial, and Endocardial Cells

E-4031–Sensitive and –Insensitive Components of I_K: I_Kr and I_Ks
In another series of experiments, we used the I_Kr blocker, E-4031, to dissect out the two components of I_K. In this part of the study, I_K was measured in Tyrode's solution containing 6 mmol/L [K⁺]_o and 2 µmol/L nisoldipine. Fig 8 shows typical currents measured in the absence and presence of E-4031. From a holding potential of -40 mV, the cell was depolarized to various test potentials for 5 s, followed by repolarization to -40 mV. E-4031 blocked both the developing current and the tail current at most voltages tested (Fig 8A). Fig 8B shows the E-4031–sensitive current, revealing the presence of a rapidly activating current with characteristics similar to those of I_Kr as described in the guinea pig. I_Kr in the dog appears to deactivate more slowly than in the guinea pig. Another distinction is that the developing current does not always rectify strongly at positive potentials, as in the guinea pig; we encountered large cell-to-cell variability in the degree of rectification. Fig 9 shows the envelope-of-tails test in the presence and absence of E-4031 from a representative experiment. In the absence of E-4031, the ratio of the tail and developing currents is fairly constant (0.45) for pulses >1000 ms but is seen to increase progressively with shorter pulses. This deviation is eliminated after block of I_Kr with 5 µmol/L E-4031. Thus, the envelope-of-tails test is satisfied once the rapidly activating component, I_Kr, is eliminated. Data from this and two similar experiments indicate that I_Ks in epicardial cells is greater than I_Kr after long pulses. With pulses on the order of 400 ms, the two components are approximately equal.

View larger version (19K): [in this window] [in a new window]   Figure 8. Effects of E-4031 on the delayed rectifier current in a canine ventricular cell from the midmyocardial region. A, Outward current recorded before and after exposure to 5 µmol/L E-4031. Currents were recorded after a 5-s depolarization to various potentials from a holding potential of -40 mV. Normal Tyrode's solution contained 6 mmol/L [K+]o. Nisoldipine (2 µmol/L) was used to block Ca2+ current. E-4031 (5 µmol/L, tracings marked with an asterisk) blocked both developing and tail currents at all test voltages. B, E-4031–sensitive current obtained by digital subtraction of currents shown in panel A.

View larger version (22K): [in this window] [in a new window]   Figure 9. Envelope test of tail currents. A, Superimposed currents recorded in an epicardial myocyte before (control) and after exposure to 5 µmol/L E-4031 during the voltage protocol shown at the top. Pulse duration was 200 to 5000 ms. B, The ratio of delayed rectifier (IK) tail current to IK developing current plotted as a function of pulse duration. Voltage protocol was the same as in panel A. The current shown in the inset was recorded in the presence of E-4031. Values and bars indicate mean±SEM (n=5). Data were obtained from five epicardial myocytes. Normal Tyrode's solution contained 6 mmol/L [K+]o.

Fig 10 illustrates the results of experiments designed to assess the reversal potentials of I_Kr (defined as E-4301–sensitive current) and I_Ks (defined as E-4301–insensitive current): -60.0±1.2 and -79.3±2.1 mV (n=8, P<.001), respectively. The difference between the mean reversal potentials for I_Ks and I_Kr is similar to that reported in guinea pig ventricular myocytes.²² The figure also shows inward rectification of the fully activated I_Kr tail current at plateau potentials. This characteristic has previously been reported in guinea pig ventricular myocytes as well.²²

View larger version (17K): [in this window] [in a new window]   Figure 10. Current-voltage relation for fully activated rapidly and slowly activating components of the delayed rectifier K+ current (IKr and IKs, respectively). A, Representative current tracings recorded in an epicardial cell in the absence and presence of E-4031 (5 µmol/L) during the voltage protocol shown in the top inset. Normal Tyrode's solution contained 6 mmol/L [K+]o with 2 µmol/L nisoldipine. B, Normalized tail currents for both IKs and IKr plotted as a function of voltage. Values and bars indicate mean±SEM (n=8). IKs tail current was measured as the time-dependent E-4031–insensitive component recorded during the repolarizing pulse (difference in current amplitude between the beginning and end of the pulse); at potentials 0 mV, IKs was fully deactivated by the end of the pulse). IKr tail currents were measured as the E-4031–sensitive current recorded during the pulse. Data were pooled from three epicardial, three midmyocardial, and two endocardial myocytes. For each individual cell, IKr and IKs tail currents were normalized to the maximum tail current.

Fig 11 illustrates the tail current-voltage relation for the total current, as well as the E-4031–sensitive and–insensitive components. The activation curves for I_Kr (E-4301–sensitive current) and I_Ks (E-4301–insensitive current) are similar to those described by Sanguinetti and Jurkiewicz²² in guinea pig ventricular cells. I_Kr is seen to activate at more negative potentials (between -20 and -10 mV) than I_Ks and to plateau at potentials positive to 0 mV. I_Ks activates at more positive potentials (0 mV) and does not reach a plateau even at a test potential of +60 mV. Similar results were obtained when sotalol (100 to 500 µmol/L) or WAY 123,398 (10 µmol/L) was used to block I_Kr.³² In all cases, a small rapidly activating and slowly deactivating drug-sensitive component was observed. It is noteworthy that the tail current density of I_Kr was not significantly different among the three cell types, whereas the level of I_Ks (E-4031–insensitive component) was significantly smaller in the M cells than in the epicardial or endocardial cells, consistent with the results obtained in the previous series of experiments conducted under Na⁺-, K⁺-, and Ca²⁺-free conditions. Finally the total tail current, I_K (I_Ks +I_Kr), was also significantly greater in epicardial and endocardial myocytes than in cells from the M region.

View larger version (16K): [in this window] [in a new window]   Figure 11. Graphs showing that the slowly activating component of the delayed rectifier K+ current (IKs), but not the rapidly activating component (IKr), is smaller in cells from the midmyocardial (M) region. A, Tail current density-voltage relation of total IK (IKr+IKs) measured before exposure to E-4031. B, IKs component (current remaining after exposure to E-4031). C, IKr component (E-4031–sensitive current). Voltage protocol and solution were the same as in Fig 8. Epi indicates cells from the epicardium; Endo, cells from the endocardium. Values represent mean±SEM. *P<.05 compared with Epi or Endo.

Figs 12 and 13 examine the kinetics of activation and deactivation of I_Kr (E-4031–sensitive current) and I_Ks (E-4031–insensitive current) in more detail. Panels A and B of Fig 12 illustrate representative tracings showing the voltage dependence of the kinetics of activation (time course of the developing current) and deactivation (decay of the tail current) of I_Kr and I_Ks. Activation of I_Kr was well fitted by a single exponential, whereas activation of I_Ks was better fitted by a double exponential. The time course of deactivation of I_Ks tails was also well fitted by a biexponential function, except at potentials negative to -20 mV, where the best fit was with a single exponential. I_Kr deactivation was better fitted with a biexponential at most potentials. Composite data displaying the voltage dependence of _s and _f for activation and deactivation are summarized in Fig 13. Because no significant differences could be discerned in the time constants recorded from the three cell types, the data for all cell types were pooled. We report deactivation kinetics to potentials as negative as -70 mV but are unable to provide reliable data at more negative potentials because of the proximity to the reversal potential. The results indicate that the kinetics of activation of I_Kr are a very sensitive function of voltage, particularly near the threshold for activation of this current (0 mV). At this potential, I_Kr is very slow to activate, and I_Ks has not yet activated. At +20 mV, both components activate, but I_Ks activates more slowly than I_Kr. At more positive potentials, I_Ks activates more rapidly, although its activation time course is always slower than that of I_Kr. In contrast, deactivation of I_Ks is always faster than that of I_Kr, even at potentials approaching the normal resting membrane potential. It is noteworthy that the time constants recorded for deactivation of I_Ks are faster when the current is isolated as the E-4031–insensitive current ([K⁺]_o, 6 mmol/L) than when I_Ks is isolated by removing K⁺_o (compare Table 2 and Fig 13). These differences are also observed in experiments in which the two methods are used in the same cell, such as the experiment illustrated in Fig 4.

View larger version (21K): [in this window] [in a new window]   Figure 12. Voltage dependence of kinetics of activation (A) and deactivation (B) of the rapidly and slowly activating components of the delayed rectifier K+ current (IKr and IKs, respectively). Activation kinetics were obtained by analyzing developing currents activated during a 5-s depolarizing pulse to various voltages. Deactivation kinetics were obtained by analyzing tail currents at various repolarizing voltages after a 3-s pulse to +40 mV. IKs was measured as the E-4031–insensitive current, and IKr was measured as the E-4031–sensitive current. All current tracings were fitted to either single- or double-exponential functions (continuous lines). Vm indicates membrane voltage. See text for further details.

View larger version (22K): [in this window] [in a new window]   Figure 13. Time constants of activation () and deactivation () of the rapidly and slowly activating components of the delayed rectifier K+ current (IKr and IKs, respectively). Analysis procedure was as described in Fig 12. Values and bars indicate mean±SEM (n=5). f and s indicate time constants of the fast and slow components, respectively. Data were obtained from two cells from the epicardium, two cells from the midmyocardial region, and one cell from the endocardium.

In the potential range (+10 to +30 mV) at which both activating and deactivating currents could be measured for I_Ks, the time course of deactivation was approximately twice as fast as for activation (Fig 13). This result is consistent with Hodgkin-Huxley behavior of the channel assuming two activation gates. In such a case, the increase in conductance is described by [1-exp(-t/)]², whereas the fall is given by exp(-2t/).³³ Similar results have been described in the guinea pig.³⁴ I_Kr, on the other hand, appears to be more consistent with a single-gate model, showing similar time constants for activation and deactivation at potentials where they overlap (Fig 13).^{22 24}

	Discussion

Top Abstract Introduction Materials and Methods Results Discussion References

 
Action Potential Characteristics
Our findings further delineate the distinctions among cells spanning the wall of the canine left ventricle. Initial characterization of the myocytes was performed by using standard microelectrode techniques so as to minimize any alteration of the intracellular milieu. Action potential characteristics and APD-rate relations of myocytes isolated from the epicardial, endocardial, and M regions (deep subepicardium to midmyocardium) were similar to those previously described in syncytial preparations.^{1 2 3 4 5 14} Myocytes isolated from the epicardial surface display action potentials with a prominent spike-and-dome morphology but a fairly flat APD-rate relation. Cells from the deep subepicardium exhibit action potentials with a spike-and-dome morphology as well as a steep APD-rate relation. Finally, myocytes isolated from the endocardial surface show action potentials exhibiting neither a spike-and-dome nor a steep APD-rate relation. Transitional behavior (intermediate APD-rate relations) is observed as well (Figs 1 and 2).

In another set of experiments, using whole-cell patch-clamp recording techniques, we evaluated action potential characteristics in tandem with ionic currents (Fig 3). Action potential characteristics recorded with the patch pipette appeared qualitatively similar to those observed when using standard microelectrode techniques (compare Fig 3 versus Figs 1 and 2). However, quantitative comparison reveals longer APDs in all three cell types recorded when using the patch pipette (Fig 3). The longer APDs appear to be due to equilibration of the intracellular milieu of the cell with the pipette solution, causing Ca²⁺ buffering (EGTA) among other effects. The removal of Ca²⁺-activated outward currents, especially Ca²⁺-dependent I_K,³⁵ likely contributes to this phenomenon.

Our data provide further characterization of the M cell isolated from the deep subepicardial to midmyocardial layers of the canine ventricle. The hallmark of the M cell is its steep APD-rate relation. Previous studies have reported cells with prolonged action potentials in deep myocardial layers of the ventricles of several species, including dog,^{6 36 37} rat,³⁸ and guinea pig.³⁹ Recently, using a transmural monophasic action potential recording technique, Wang et al⁴⁰ provided in vivo evidence of the existence of M cells in intact dogs. Evidence for the existence of M cells in the human heart was recently provided by Drouin et al,⁴¹ who used transmural tissue slices from explanted human hearts.

Studies designed to assess the distribution of M cells across the canine ventricular wall by mapping transmural slices have shown that M cells are widely distributed within the ventricular wall and that transitional behavior is apparent throughout the wall, particularly between midmyocardium and endocardium,^{2 7} as well as in endocardial structures.² In line with these observations, we found a wide range of transitional behaviors in myocytes isolated from the M region (Figs 1 and 2). Intermediate APD-rate relations were also observed in some cells isolated from the epicardial and endocardial layers. Thus, transitional cells were present in all three (epicardial, endocardial, and M) fractions.

Ionic Currents
Several currents contribute to repolarization of the cardiac action potential. In the ventricle, three major outward K⁺ currents are thought to be involved: the transient outward current, I_to; the delayed rectifier current, I_K; and the inward rectifier current, I_K1.

In the canine ventricle, regional differences in APD do not appear to be due to differences in I_to or I_K1. I_to contributes to a more accentuated early repolarization phase (phase 1) in epicardial and M cells,^{7 10 11} but because of its rapid inactivation kinetics, I_to contributes little to phase 3 repolarization.^{3 5} I_K1 differences are unlikely to underlie APD differences among epicardial, M, and endocardial cells, because similar levels of this current have been measured in the three cell types.⁷ The present study suggests that differences in the intensity of I_K may contribute significantly to the regional differences in repolarization characteristics.

I_K was first characterized in sheep Purkinje fibers by Noble and Tsien.⁴² It has been described in calf,⁴³ rabbit,²³ and canine⁴⁴ Purkinje fibers; rabbit^{45 46 47 48} and guinea pig⁴⁹ nodal cells; cat,⁵⁰ guinea pig,^{22 30 51} rabbit,^{52 53} and human⁵⁴ ventricular cells; and guinea pig²⁴ and human⁵⁵ atrial cells. Although early studies suggested that I_K was very small or nonexistent in canine ventricular myocardium,⁵⁶ more recent studies have shown this current to be a relatively important contributor to repolarization in canine ventricular myocytes.²⁹ Our data support this claim and provide a possible explanation for the disparate results obtained in previous studies. Our results suggest that studies using isolation procedures that select out M cells would yield I_K measurements considerably smaller than isolation procedures that include epicardial and endocardial cells.

Two Components of I_K
It has become evident that the delayed rectifier can be composed of more than one component^{22 24} and that striking species as well as regional differences exist in the type and amplitude of the current.^{20 57 58 59 60}

A rapidly activating component of I_K (I_Kr) is well characterized in guinea pig atrial and ventricular myocytes.^{22 24 61} I_Kr can be distinguished from I_Ks on the basis of its more rapid activation kinetics, more negative threshold potential, and sensitivity to methanesulfonamide class III agents, such as E-4031.²² In the guinea pig, this current activates with a time constant of 15 ms at +20 mV and deactivates with time constant of 50 ms at -70 mV. I_Kr amplitude was found to be 10% that of the fully activated I_Ks at more positive potentials. In rabbit preparations (nodal,^{46 47} Purkinje,²³ and ventricular⁵³ cells), only an I_Kr component has been observed. In cat studies, Follmer and Colatsky⁶² found that E-4031 (5 µmol/L) nearly completely blocked the tail currents observed on repolarization to -40 mV, suggesting the lack of an I_Ks component. In contrast, Furukawa et al²⁰ reported an I_Ks-like delayed rectifier current in cat ventricular cells on the basis of the slow activation time course and the experimental condition used (the Na⁺- and K⁺-free external solution used is known to inhibit I_Kr).

The present study demonstrates the presence of both I_Kr and I_Ks in canine ventricular myocytes. Moreover, our results indicate that the regional variation in I_Ks does not apply to I_Kr. Our direct demonstration of an I_Kr component in ventricular myocardium is consistent with reports of the effect of E-4031 (30 to 300 µg/kg IV) to prolong ventricular refractoriness and prevent the induction of ventricular arrhythmias in anesthetized dogs.^{63 64} We have also observed dramatic prolongation of APD and development of EADs after exposure of canine ventricular tissues and myocytes to E-4031 (1 µmol/L) and sotalol (0.5 to 1.0x10^-4 mol/L), another I_Kr blocker.^{2 3 32} Both agents are much more effective in tissues isolated from the M region.

I_Ks in dog activates in a time- and voltage-dependent manner, showing a sigmoidal voltage dependence and a relatively slow time course. These characteristics of I_Ks activation are similar to those previously reported in canine,²⁹ guinea pig,³¹ and cat²⁰ ventricular myocytes. Our data indicate that the average tail current density in cells from the M region is approximately half of that recorded from the epicardial and endocardial cells.

Differences in I_Ks among the three cell types cannot be explained on the basis of differences in the voltage dependence of either activation (Fig 6 and Table 1) or deactivation (Table 2) of the current. The mean half-activation voltage (V) was similar for the three cell types, averaging +25 mV. This value is similar to that (+23.7 mV) reported by Fan and Hiraoka³¹ in guinea pig ventricular myocytes studied under similar experimental conditions (a Na⁺-, K⁺-, and Ca²⁺-free bath solution) but more positive than that reported by Tseng et al²⁹ in canine ventricular myocytes (V, +10 mV; Tyrode's solution) or in feline epicardial and endocardial cells (V, 0; Na⁺- and K⁺-free solution).²⁰ A number of factors, in addition to species differences, can contribute to variability in the quantitation of V, including (1) divalent cations (Co²⁺ and Cd²⁺), commonly used as Ca²⁺ channel blockers in above studies, which have recently been shown to shift the voltage dependence of I_K activation curve to more positive potentials in guinea pig³¹ and feline⁶⁵ ventricular myocytes, and (2) differences in the bath solution used (Tyrode's versus Na⁺-, Ca²⁺-, and K⁺-free solutions). K⁺-free solution is known to greatly diminish I_Kr but to increase I_Ks, and removal of extracellular Ca²⁺ has been shown to shift the voltage dependence of activation of I_Kr and I_Ks in guinea pig ventricular myocytes such that I_Kr activates at more negative membrane potentials and I_Ks activates at more positive potentials.^{21 23}

Differences in activation kinetics, likewise, cannot explain the difference in amplitude of I_Ks among the different cell types (Fig 7). With depolarization to +40 mV, activation of I_Ks (growth of tail current) was found to follow a biexponential time course, as previously reported in canine,²⁹ guinea pig,³⁰ and cat^{20 50} ventricular cells. No significant regional differences could be discerned.

Although the mechanism(s) responsible for the regional differences in I_Ks require further study, our data point to differences in channel density and/or unitary conductance as the basis for the smaller I_Ks in cells isolated from the M region. It is noteworthy that in the cat, Furukawa et al²⁰ reported I_K unitary current to be similar in feline ventricular endocardial and epicardial cells. The authors concluded that the epicardial versus endocardial difference in the amplitude of I_K was due to a difference in channel density.

Relative Contribution of I_Kr and I_Ks to Repolarization
Although I_Ks is slow to activate, protocols involving short pulses in the physiological range indicate that I_Ks can provide an important contribution to action potential repolarization, particularly in epicardial and endocardial cells. Fig 9 shows that I_Ks and I_Kr are approximately equal after a 400-ms pulse to +40 mV (conditions approaching the action potential recorded from similar cells under patch-clamp conditions [see Fig 3A]; epicardial cell). Because the Ca²⁺-dependent component of I_Ks is largely eliminated under our recording conditions (10 mmol/L EGTA),³⁵ the contribution of I_Ks may be underestimated. Moreover, when developing currents measured at plateau potentials are considered, I_Ks is considerably greater than I_Kr because of rectification of the latter (see also Fig 10). Thus, I_Ks could provide an important contribution to repolarization.

The smaller I_Ks contribution in cells from the M region may help explain the much steeper APD-rate relations observed in these cells as well as their greater proclivity to develop EADs and to display pronounced action potential prolongation in response to agents with class III antiarrhythmic actions. These hypotheses remain to be tested.

In the guinea pig, I_Ks has been reported to deactivate more slowly than I_Kr. Jurkiewicz and Sanguinetti²⁸ suggested that rate-dependent APD changes in guinea pig ventricular myocytes are due in part to rate-dependent changes in the degree of residual activation (accumulation) of I_Ks. Because these relations are reversed in the dog, it is tempting to speculate that accumulation of the more slowly decaying I_Kr component in the dog contributes to abbreviation of APD at fast rates. Of relevance to this issue is the demonstration by Carmeliet⁶⁶ that I_Kr tail currents recorded from rabbit ventricular myocytes increase with successive depolarizations when a train of 200-ms pulses is applied at frequency of 1.33 Hz (at -50 mV). I_Kr in the rabbit, as in the dog, is very slow to decay. Follmer and Colatsky⁶² as well as Spinelli et al⁶⁷ demonstrated that I_Kr in cat ventricular myocytes is also slow to deactivate. Of note is the fact that in the feline myocytes the amplitude of tail currents remained stable over a wide range of frequencies, suggesting a lack of residual activation between pulses.⁶⁷

It should be emphasized that the mechanism responsible for the rate dependence of APD is quite complex and most likely involves the interaction of a number of currents other than I_Kr and I_Ks. These include but are not limited to the inward rectifier (I_K1), Ca²⁺ inward current, Na⁺-K⁺ pump current, Na⁺-Ca²⁺ exchanger current, and slowly inactivating Na⁺ current or Na⁺ "window" current.

Regional Heterogeneity of I_K in Other Species
Studies conducted using feline myocytes indicate a much greater I_K in epicardial compared with endocardial cells²⁰ ; this is in contrast to the present study, which demonstrates slightly, but not significantly, greater I_K in epicardial versus endocardial cells isolated from the canine ventricle. This disparity may reflect species differences or alternatively may be due to the inclusion of M cells in the endocardial preparation isolated from the cat ventricle.

Physiological and Clinical Implications
Our results provide further support for the existence of marked electrophysiological heterogeneities among cells spanning the ventricular wall of the canine heart. The identification of cells with diverse action potential morphologies and electrophysiological characteristics at different levels of the ventricular wall may contribute to our understanding of a number of basic electrophysiological and electrocardiographic phenomena.

The presence of M cells displaying accentuated APD-rate relations in the deep subepicardial to midmyocardial regions of the ventricular wall has several implications. The development of a progressively more prominent dispersion of repolarization and refractoriness within the ventricular wall as stimulation rate is slowed is one consequence. M cells are also thought to contribute to registration of the electrocardiographic U wave as well as the development of long QTU intervals.^{2 68}

The demonstration of a weaker contribution of I_K to the M-cell action potential may aid in our understanding of the steeper APD-rate relations as well as the unique pharmacological responsiveness of the M cells.² This ionic distinction may contribute to making the M cells the primary targets for drugs that display class III antiarrhythmic actions as well as the primary targets for other agents that prolong APD in ventricular myocardium.^{2 3 16 17 69 70 71} By the same token, this characteristic may help make the M cells the primary culprits responsible for the development of long QTU, EADs, triggered activity, and atypical ventricular tachyarrhythmias such as torsade de pointes.¹⁶

	Acknowledgments

 
This study was supported by grants HL-37396 and HL-47678 from the National Institutes of Health and grants from the Charles L. Keith and Clara Miller Foundation, Dolgeveille Lodge No. 796, and Governeur Lodge No. 217. We are grateful to Judy Hefferon and Robert Goodrow for their expert technical assistance. E-4031 was kindly donated by EISAI Co, Ltd, of Tokyo, Japan.

	Footnotes

 
Preliminary results have been previously presented in abstract form (PACE Pacing Clin Electrophysiol. 1993;16[suppl II]:870 and PACE Pacing Clin Electrophysiol. 1994;17[suppl II]:755).

Received March 25, 1994; accepted November 23, 1994.

	References

Top Abstract Introduction Materials and Methods Results Discussion References

 
1. Antzelevitch C, Sicouri S, Litovsky SH, Lukas A, Krishnan SC, Di Diego JM, Gintant GA, Liu DW. Heterogeneity within the ventricular wall: electrophysiology and pharmacology of epicardial, endocardial, and M cells. Circ Res. 1991;69:1427-1449. [Free Full Text]

2. Antzelevitch C, Sicouri S. Clinical relevance of cardiac arrhythmias generated by afterdepolarizations: the role of M cells in the generation of U waves, triggered activity and torsade de pointes. J Am Coll Cardiol. 1994;23:259-277. [Abstract]

3. Antzelevitch C, Sicouri S, Lukas A, Nesterenko VV, Liu DW, Di Diego JM. Regional differences in the electrophysiology of ventricular cells: physiological and clinical implications. In: Zipes DP, Jalife J, eds. Cardiac Electrophysiology: From Cell to Bedside. 2nd ed. Philadelphia, Pa: WB Saunders Co; 1994:228-245.

4. Litovsky SH, Antzelevitch C. Differences in the electrophysiological response of canine ventricular subendocardium and subepicardium to acetylcholine and isoproterenol: a direct effect of acetylcholine in ventricular myocardium. Circ Res. 1990;67:615-627. [Abstract/Free Full Text]

5. Litovsky SH, Antzelevitch C. Rate dependence of action potential duration and refractoriness in canine ventricular endocardium differs from that of epicardium: the role of the transient outward current. J Am Coll Cardiol. 1989;14:1053-1066. [Abstract]

6. Litovsky SH, Antzelevitch C. Transient outward current prominent in canine ventricular epicardium but not endocardium. Circ Res. 1988;62:116-126. [Abstract/Free Full Text]

7. Liu D-W, Gintant GA, Antzelevitch C. Ionic bases for electrophysiological distinctions among epicardial, midmyocardial, and endocardial myocytes from the free wall of the canine left ventricle. Circ Res. 1993;72:671-687. [Abstract/Free Full Text]

8. Levine JH, Spear JF, Guarnieri T, Weisfeldt ML, DeLangen CDJ, Becker LC, Moore N. Cesium chloride–induced long QT syndrome: demonstration of afterdepolarizations and triggered activity in vivo. Circulation. 1985;72:1092-1103. [Abstract/Free Full Text]

9. Tande PM, Mortensen E. Rate-dependent differences in dog epi- and endocardial monophasic action potential configuration in vivo. Am J Physiol. 1991;261:H1387-H1391. [Abstract/Free Full Text]

10. Fedida D, Giles WR. Regional variations in action potentials and transient outward current in myocytes isolated from rabbit left ventricle. J Physiol (Lond). 1991;442:191-209. [Abstract/Free Full Text]

11. Furukawa T, Myerburg RJ, Furukawa N, Bassett AL, Kimura S. Differences in transient outward currents of feline endocardial and epicardial myocytes. Circ Res. 1990;67:1287-1291. [Abstract/Free Full Text]

12. Wettwer E, Amos GJ, Posival H, Ravens U. Transient outward current in human ventricular myocytes of subepicardial and subendocardial origin. Circ Res. 1994;75:473-482. [Abstract/Free Full Text]

13. Nabauer M, Beuckelmann DJ. Regional differences in current density and properties of the the transient outward current in human ventricular myocytes. Circulation. 1993;88(suppl I):I-89. Abstract.

14. Sicouri S, Antzelevitch C. A subpopulation of cells with unique electrophysiological properties in the deep subepicardium of the canine ventricle: the M cell. Circ Res. 1991;68:1729-1741. [Abstract/Free Full Text]

15. Sicouri S, Antzelevitch C. Distribution of M cells in the canine ventricle. J Cardiovasc Electrophysiol.. 1994;5:824-837. [Medline] [Order article via Infotrieve]

16. Sicouri S, Antzelevitch C. Afterdepolarizations and triggered activity develop in a select population of cells (M cells) in canine ventricular myocardium: the effects of acetylstrophantidin and Bay K 8644. PACE Pacing Clin Electrophysiol. 1991;14:1714-1720. [Medline] [Order article via Infotrieve]

17. Sicouri S, Antzelevitch C. Drug-induced afterdepolarizations and triggered activity occur in a discrete subpopulation of ventricular muscle cell (M cells) in the canine heart: quinidine and digitalis. J Cardiovasc Electrophysiol. 1993;4:48-58. [Medline] [Order article via Infotrieve]

18. Gintant GA, Cohen IS, Datyner NB, Kline RP. Time dependent outward currents in the heart. In: Fozzard HA, Jennings RB, Haber E, Katz AM, Morgan HE, eds. The Heart and Cardiovascular System. 2nd ed. New York, NY: Raven Press Publishers; 1991: 1121-1169.

19. Tseng G-N, Hoffman BF. Two components of transient outward current in canine ventricular myocytes. Circ Res. 1989;64:633-647. [Abstract/Free Full Text]

20. Furukawa T, Kimura S, Furukawa N, Bassett AL, Myerburg RJ. Potassium rectifier currents differ in myocytes of endocardial and epicardial origin. Circ Res. 1992;70:91-103. [Abstract/Free Full Text]

21. Sanguinetti MC, Jurkiewicz NK. Role of external Ca²⁺ and K⁺ in gating of cardiac delayed rectifier K⁺ currents. Pflugers Arch. 1992;420:180-186. [Medline] [Order article via Infotrieve]

22. Sanguinetti MC, Jurkiewicz NK. Two components of cardiac delayed rectifier K⁺ current. J Gen Physiol. 1990;96:195-215. [Abstract/Free Full Text]

23. Scamps F, Carmeliet E. Delayed K⁺ current and external K⁺ in single cardiac Purkinje cells. Am J Physiol. 1989;257:C1086-C1092. [Abstract/Free Full Text]

24. Sanguinetti MC, Jurkiewicz NK. Delayed rectifier outward K⁺ current is composed of two currents in guinea pig atrial cells. Am J Physiol. 1991;260:H393-H399. [Abstract/Free Full Text]

25. Sigworth FJ. Electronic design of the patch clamp. In: Sakmann B, Neher E, eds. Single-Channel Recording. New York, NY: Plenum Publishing Corp; 1983:3-35.

26. Harvery RD, Hume JR. Autonomic regulation of delayed rectifier K⁺ current in mammalian heart involves G proteins. Am J Physiol. 1991;257:H818-H823.

27. Sanguinetti MC, Jurkiewicz NK, Scott AL, Siegl PKS. Isoproterenol antagonizes prolongation of refractory period by the class III antiarrhythmic agent E-4031 in guinea pig myocytes: mechanism of action. Circ Res. 1991;68:77-84. [Abstract/Free Full Text]

28. Jurkiewicz NK, Sanguinetti MC. Rate-dependent prolongation of cardiac action potentials by a methanesulfonanilide class III antiarrhythmic agent: specific block of rapidly activating delayed rectifier K⁺ current by dofetilide. Circ Res. 1993;72:75-83. [Abstract/Free Full Text]

29. Tseng GN, Robinson RB, Hoffman BF. Passive properties and membrane currents of canine ventricular myocytes. J Gen Physiol. 1987;90:671-701. [Abstract/Free Full Text]

30. Matsuura H, Ehara T, Imoto Y. An analysis of the delayed outward current in single ventricular cells of the guinea-pig. Pflugers Arch. 1987;410:596-603. [Medline] [Order article via Infotrieve]

31. Fan Z, Hiraoka M. Depression of delayed outward K⁺ current by Co²⁺ in guinea pig ventricular myocytes. Am J Physiol. 1991; 261:C23-C31.

32. Liu DW, Antzelevitch C. Evidence for a rapidly activating but slowly deactivating E-4031-sensitive component of the delayed rectifier K⁺ current (Ikr) in canine ventricular myocytes. PACE Pacing Clin Electrophysiol. 1994;17(suppl II):755. Abstract.

33. Leroyer R, Jarreau F, Pays M, Varoquaux O, Advenier C. Reversed phase liquid chromatography and pharmacokinetic study of two analogs of quinidine in dogs. J Pharmacol Sci. 1994;73:844-846.

34. Abildskov JA. Adrenergic effects on the QT interval of the electrocardiogram. Am Heart J. 1976;92:210-216. [Medline] [Order article via Infotrieve]

35. Nitta J, Furukawa T, Marumo F, Sawanobori T, Hiraoka M. Subcellular mechanism for Ca²⁺-dependent enhancement of delayed rectifier K⁺ current in isolated membrane patches of guinea pig ventricular myocytes. Circ Res. 1994;74:96-104. [Abstract/Free Full Text]

36. Solberg LE, Singer DH, Ten Eick RE, Duffin EG Jr. Glass microelectrode studies on intramural papillary muscle cells. Circ Res. 1974;34:783-797. [Abstract/Free Full Text]

37. Hewett KW, Legato MJ, Danilo P, Robinson RB. Isolated myocytes from adult canine left ventricle: Ca²⁺ tolerance, electrophysiology, and ultrastructure. Am J Physiol. 1983;245:H830-H839.

38. Watanabe T, Dellbridge LM, Bustamante JO, McDonald TF. Heterogeneity of the action potential in isolated rat ventricular myocytes and tissue. Circ Res. 1983;52:280-290. [Abstract/Free Full Text]

39. Watanabe T, Rautaharju P, McDonald TF. Ventricular action potential, ventricular extracellular potentials, and the ECG of guinea pig. Circ Res. 1985;57:362-373. [Abstract/Free Full Text]

40. Wang Y, Hariman RJ, Gintant GA, Louie EK, Hwang MH, Loeb HS. A method for recording of intramyocardial monophasic action potential in intact dogs: in vivo evidence of M cells. PACE Pacing Clin Electrophysiol. 1992;15(suppl II):559. Abstract.

41. Drouin E, Charpentier F, Gauthier C, Chevallier JC, Laurent K, Michaud JL, Le Marec H. Evidence for the presence of M cells in the human ventricle. PACE Pacing Clin Electrophysiol. 1993;16(suppl II):876. Abstract.

42. Noble D, Tsien RW. Outward membrane currents activated in the plateau range of potentials in cardiac Purkinje fibers. J Physiol (Lond). 1969;200:205-231. [Abstract/Free Full Text]

43. Bennett PB, McKinney LC, Kass RS, Begenisich T. Delayed rectification in the calf cardiac Purkinje fiber: evidence for multiple state kinetics. Biophys J. 1985;48:553-567. [Medline] [Order article via Infotrieve]

44. Gintant GA, Datyner NB, Cohen IS. Gating of delayed rectification in acutely isolated canine cardiac Purkinje myocytes: evidence for a single voltage-gated conductance. Biophys J. 1985;48:1059-1064. [Medline] [Order article via Infotrieve]

45. Nakayama T, Kurachi Y, Noma A, Irisawa H. Action potential and membrane currents of single pacemaker cells of the rabbit heart. Pflugers Arch. 1984;402:248-257. [Medline] [Order article via Infotrieve]

46. Furukawa T, Tsujimura Y, Kitamura K, Tanaka H, Habuchi Y. Time- and voltage-dependent block of the delayed K⁺ current by quinidine in rabbit sinoatrial and atrioventricular nodes. J Pharmacol Exp Ther. 1989;251:756-763. [Abstract/Free Full Text]

47. Shibasaki T. Conductance and kinetics delayed rectifier potassium channels in nodal cells of the rabbit heart. J Physiol (Lond). 1987;387:227-250. [Abstract/Free Full Text]

48. DiFrancesco D, Noma A, Trautwein W. Kinetics and magnitude of the time-dependent potassium current in the rabbit sinoatrial node. Pflugers Arch. 1979;381:271-279. [Medline] [Order article via Infotrieve]

49. Anumonwo JMB, Freeman LC, Kwok WM, Kass RS. Delayed rectification in single cells isolated from guinea pig sinoatrial node. Am J Physiol. 1992;262:H921-H925. [Abstract/Free Full Text]

50. McDonald TF, Trautwein W. The potassium current underlying delayed rectification in cat ventricular muscle. J Physiol (Lond). 1978;274:217-246. [Abstract/Free Full Text]

51. Roden DM, Bennett PB, Snyders DJ, Balser JR, Hondeghem LM. Quinidine delays I_K activation in guinea pig ventricular myocytes. Circ Res. 1988;62:1055-1058. [Abstract/Free Full Text]

52. Veldkamp MW, van Ginneken ACG, Bouman LN. Single delayed rectifier channels in the membrane of rabbit ventricular myocytes. Circ Res. 1993;72:865-878. [Abstract/Free Full Text]

53. Carmeliet E. Voltage- and time-dependent block of the delayed K⁺ current in cardiac myocytes by dofetilide. J Pharmacol Exp Ther. 1991;262:809-817. [Abstract/Free Full Text]

54. Beuckelmann DJ, Nabauer M, Erdmann E. Alterations of K⁺ currents in isolated human ventricular myocytes from patients with terminal heart failure. Circ Res. 1993;73:379-385. [Abstract/Free Full Text]

55. Wang Z, Fermini B, Nattel S. Delayed rectifier outward current and repolarization in human atrial myocytes. Circ Res. 1993;73:276-285. [Abstract/Free Full Text]

56. Noble D. The surprising heart: a review of recent progress in cardiac electrophysiology. J Physiol (Lond). 1984;353:1-50. [Free Full Text]

57. Giles WR, Imaizumi Y. Comparison of potassium currents in rabbit atrial and ventricular cells. J Physiol (Lond). 1988;405:123-145. [Abstract/Free Full Text]

58. Hume JR, Uehara A. Ionic basis of the different action potential configurations of single guinea-pig atrial and ventricular myocytes. J Physiol (Lond). 1985;368:525-544. [Abstract/Free Full Text]

59. Hiraoka M, Kawano S. Mechanism of increased amplitude and duration of the plateau with sudden shortening of diastolic intervals in rabbit ventricular cells. Circ Res. 1987;60:14-26. [Abstract/Free Full Text]

60. Josephson JR, Sanchez Chapula J, Brown AM. Early outward current in rat single ventricular cells. Circ Res. 1984;54:157-162. [Abstract/Free Full Text]

61. Balser JR, Bennett PB, Roden DM. Time-dependent outward current in guinea-pig ventricular myocytes. J Gen Physiol. 1990; 96:835-863.

62. Follmer CH, Colatsky TJ. Block of delayed rectifier current, I_K, by flecainide and E-4031 in cat ventricular myocytes. Circulation. 1990;82:289-293. [Abstract/Free Full Text]

63. Lynch J, Heaney L, Wallace A, Gehret JR, Selnick HG, Stein R. Suppression of lethal ischemic ventricular arrhythmias by the class III agent E-4031 in a canine model of previous myocardial infarction. J Cardiovasc Pharmacol. 1990;15:764-775. [Medline] [Order article via Infotrieve]

64. Chi L, Mu DX, Lucchesi BR. Electrophysiology and antiarrhythmic actions of E-4031 in the experimental animal model of sudden coronary death. J Cardiovasc Pharmacol. 1991;17:285-295. [Medline] [Order article via Infotrieve]

65. Follmer CH, Lodge NJ, Cullinan CA, Colatsky TJ. Modulation of the delayed rectifier, Ik, by cadmium in cat ventricular myocytes. Am J Physiol. 1992;262:C75-C83. [Abstract/Free Full Text]

66. Carmeliet E. Use-dependent block of the delayed K⁺ current in rabbit ventricular myocytes. Cardiovasc Drugs Ther. 1993;7(suppl 3):599-604.

67. Spinelli W, Moubarak IF, Parsons RW, Colatsky TJ. Cellular electrophysiology of WAY-123, 398, a new class III antiarrhythmic agent: specificity of I_K block and lack of reverse use-dependence in cat ventricular myocytes. Cardiovasc Res. 1993;27:1580-1591. [Abstract/Free Full Text]

68. Nesterenko VV, Antzelevitch C. Simulation of the electrocardiographic U wave in heterogeneous myocardium: effect of the local junctional resistance. In: Proceedings of Computers in Cardiology. Los Alamitos, Calif: IEEE Computer Society Press; 1992;43-46.

69. Antzelevitch C, Di Diego JM, Sicouri S, Lukas A. Selective pharmacological modification of repolarizing currents: antiarrhythmic and proarrhythmic actions of agents that influence repolarization in the heart. In: Breithardt J, ed. Antiarrhythmic Drugs: Mechanisms of Antiarrhythmic and Proarrhythmic Actions. Berlin, Germany: Springer-Verlag; 1994.

70. Zhang ZQ, Antzelevitch C. Erythromycin produces prominent action potential prolongation and early afterdepolarization (EAD)-induced triggered activity in M but not epicardial or endocardial regions of the canine ventricle. Circulation. 1993;88(suppl I):I-327. Abstract.

71. Hariman RJ, Wang Y, Sharif MA, Louie EK, Hwang MH, Loeb HS, Scanlon PJ. Early afterdepolarizations arising from M cells in intact dogs. Circulation. 1992;86(suppl I):I-301. Abstract.

This article has been cited by other articles:

				K. F. Decker, J. Heijman, J. R. Silva, T. J. Hund, and Y. Rudy Properties and ionic mechanisms of action potential adaptation, restitution, and accommodation in canine epicardium Am J Physiol Heart Circ Physiol, April 1, 2009; 296(4): H1017 - H1026. [Abstract] [Full Text] [PDF]

				B. J.D. Boukens, V. M. Christoffels, R. Coronel, and A. F.M. Moorman Developmental Basis for Electrophysiological Heterogeneity in the Ventricular and Outflow Tract Myocardium As a Substrate for Life-Threatening Ventricular Arrhythmias Circ. Res., January 2, 2009; 104(1): 19 - 31. [Abstract] [Full Text] [PDF]

				H. V. Hume-Smith, S. Sanatani, J. Lim, A. Chau, and S. D. Whyte The Effect of Propofol Concentration on Dispersion of Myocardial Repolarization in Children Anesth. Analg., September 1, 2008; 107(3): 806 - 810. [Abstract] [Full Text] [PDF]

				H. Furushima, M. Chinushi, A. Sanada, and Y. Aizawa Ventricular repolarization gradients in a patient with takotsubo cardiomyopathy Europace, September 1, 2008; 10(9): 1112 - 1115. [Abstract] [Full Text] [PDF]

				B. Yang, Y. Lu, and Z. Wang Control of cardiac excitability by microRNAs Cardiovasc Res, September 1, 2008; 79(4): 571 - 580. [Abstract] [Full Text] [PDF]

				A. Burashnikov, W. Shimizu, and C. Antzelevitch Fever Accentuates Transmural Dispersion of Repolarization and Facilitates Development of Early Afterdepolarizations and Torsade de Pointes Under Long-QT Conditions Circ Arrhythm Electrophysiol, August 1, 2008; 1(3): 202 - 208. [Abstract] [Full Text] [PDF]

				C. S. Nicolas, K.-H. Park, A. El Harchi, J. Camonis, R. S. Kass, D. Escande, J. Merot, G. Loussouarn, F. Le Bouffant, and I. Baro IKs response to protein kinase A-dependent KCNQ1 phosphorylation requires direct interaction with microtubules Cardiovasc Res, August 1, 2008; 79(3): 427 - 435. [Abstract] [Full Text] [PDF]

				M. Chinushi, D. Izumi, K. Iijima, S. Ahara, S. Komura, H. Furushima, Y. Hosaka, and Y. Aizawa Antiarrhythmic vs. pro-arrhythmic effects depending on the intensity of adrenergic stimulation in a canine anthopleurin-A model of type-3 long QT syndrome Europace, February 1, 2008; 10(2): 249 - 255. [Abstract] [Full Text] [PDF]

				M. Stoll, M. Quentin, A. Molojavyi, V. Thamer, and U. K.M. Decking Spatial heterogeneity of myocardial perfusion predicts local potassium channel expression and action potential duration Cardiovasc Res, February 1, 2008; 77(3): 489 - 496. [Abstract] [Full Text] [PDF]

				S. Moro, M. Ferreiro, D. Celestino, E. Medei, M. V. Elizari, and S. Sicouri In Vitro Effects of Acute Amiodarone and Dronedarone on Epicardial, Endocardial, and M Cells of the Canine Ventricle Journal of Cardiovascular Pharmacology and Therapeutics, December 1, 2007; 12(4): 314 - 321. [Abstract] [PDF]

				D. L. Weiss, D. U.J. Keller, G. Seemann, and O. Dossel The influence of fibre orientation, extracted from different segments of the human left ventricle, on the activation and repolarization sequence: a simulation study Europace, November 1, 2007; 9(suppl_6): vi96 - vi104. [Abstract] [Full Text] [PDF]

				C. Antzelevitch Role of spatial dispersion of repolarization in inherited and acquired sudden cardiac death syndromes Am J Physiol Heart Circ Physiol, October 1, 2007; 293(4): H2024 - H2038. [Abstract] [Full Text] [PDF]

				C. Antzelevitch Ionic, molecular, and cellular bases of QT-interval prolongation and torsade de pointes Europace, September 1, 2007; 9(suppl_4): iv4 - iv15. [Abstract] [Full Text] [PDF]

				V. Munoz, K. R. Grzeda, T. Desplantez, S. V. Pandit, S. Mironov, S. M. Taffet, S. Rohr, A. G. Kleber, and J. Jalife Adenoviral Expression of IKs Contributes to Wavebreak and Fibrillatory Conduction in Neonatal Rat Ventricular Cardiomyocyte Monolayers Circ. Res., August 31, 2007; 101(5): 475 - 483. [Abstract] [Full Text] [PDF]

				K. H.W.J. Ten Tusscher, R. Hren, and A. V. Panfilov Organization of Ventricular Fibrillation in the Human Heart Circ. Res., June 22, 2007; 100(12): e87 - e101. [Abstract] [Full Text] [PDF]

				R. S. Hansen, S.-P. Olesen, and M. Grunnet Pharmacological Activation of Rapid Delayed Rectifier Potassium Current Suppresses Bradycardia-Induced Triggered Activity in the Isolated Guinea Pig Heart J. Pharmacol. Exp. Ther., June 1, 2007; 321(3): 996 - 1002. [Abstract] [Full Text] [PDF]

				S. Poelzing and R. Veeraraghavan Heterogeneous ventricular chamber response to hypokalemia and inward rectifier potassium channel blockade underlies bifurcated T wave in guinea pig Am J Physiol Heart Circ Physiol, June 1, 2007; 292(6): H3043 - H3051. [Abstract] [Full Text] [PDF]

				S. Vecchietti, E. Grandi, S. Severi, I. Rivolta, C. Napolitano, S. G. Priori, and S. Cavalcanti In silico assessment of Y1795C and Y1795H SCN5A mutations: implication for inherited arrhythmogenic syndromes Am J Physiol Heart Circ Physiol, January 1, 2007; 292(1): H56 - H65. [Abstract] [Full Text] [PDF]

				E. M. Cherry and F. H. Fenton A tale of two dogs: analyzing two models of canine ventricular electrophysiology Am J Physiol Heart Circ Physiol, January 1, 2007; 292(1): H43 - H55. [Abstract] [Full Text] [PDF]

				S. N. Flaim, W. R. Giles, and A. D. McCulloch Contributions of sustained INa and IKv43 to transmural heterogeneity of early repolarization and arrhythmogenesis in canine left ventricular myocytes Am J Physiol Heart Circ Physiol, December 1, 2006; 291(6): H2617 - H2629. [Abstract] [Full Text] [PDF]

				L. Xiao, L. Zhang, W. Han, Z. Wang, and S. Nattel Sex-based transmural differences in cardiac repolarization and ionic-current properties in canine left ventricles Am J Physiol Heart Circ Physiol, August 1, 2006; 291(2): H570 - H580. [Abstract] [Full Text] [PDF]

				M. Dong, X. Sun, A. A. Prinz, and H.-S. Wang Effect of simulated Ito on guinea pig and canine ventricular action potential morphology Am J Physiol Heart Circ Physiol, August 1, 2006; 291(2): H631 - H637. [Abstract] [Full Text] [PDF]

				M. N. Obreztchikova, K. W. Patberg, A. N. Plotnikov, N. Ozgen, I. N. Shlapakova, A. V. Rybin, E. A. Sosunov, P. Danilo Jr., E. P. Anyukhovsky, R. B. Robinson, et al. IKr contributes to the altered ventricular repolarization that determines long-term cardiac memory Cardiovasc Res, July 1, 2006; 71(1): 88 - 96. [Abstract] [Full Text] [PDF]

				S. Rajamani, C. L. Anderson, C. R. Valdivia, L. L. Eckhardt, J. D. Foell, G. A. Robertson, T. J. Kamp, J. C. Makielski, B. D. Anson, and C. T. January Specific serine proteases selectively damage KCNH2 (hERG1) potassium channels and IKr Am J Physiol Heart Circ Physiol, March 1, 2006; 290(3): H1278 - H1288. [Abstract] [Full Text] [PDF]

				Y. L. Protsenko, S. M. Routkevitch, V. Y. Gur'ev, L. B. Katsnelson, O. Solovyova, O. N. Lookin, A. A. Balakin, P. Kohl, and V. S. Markhasin Hybrid duplex: a novel method to study the contractile function of heterogeneous myocardium Am J Physiol Heart Circ Physiol, December 1, 2005; 289(6): H2733 - H2746. [Abstract] [Full Text] [PDF]

				J. M. Nerbonne and R. S. Kass Molecular Physiology of Cardiac Repolarization Physiol Rev, October 1, 2005; 85(4): 1205 - 1253. [Abstract] [Full Text] [PDF]

				J.M. Cordeiro, R. Brugada, Y.S. Wu, K. Hong, and R. Dumaine Modulation of IKr inactivation by mutation N588K in KCNH2: A link to arrhythmogenesis in short QT syndrome Cardiovasc Res, August 15, 2005; 67(3): 498 - 509. [Abstract] [Full Text] [PDF]

				X. Sun and H.-S. Wang Role of the transient outward current (Ito) in shaping canine ventricular action potential - a dynamic clamp study J. Physiol., April 15, 2005; 564(2): 411 - 419. [Abstract] [Full Text] [PDF]

				C. Antzelevitch Cardiac repolarization. The long and short of it Europace, January 1, 2005; 7(s2): S3 - S9. [Abstract] [Full Text] [PDF]

				D. L. Weiss, G. Seemann, F. B. Sachse, and O. Dössel Modelling of short QT syndrome in a heterogeneous model of the human ventricular wall Europace, January 1, 2005; 7(s2): S105 - S117. [Abstract] [Full Text] [PDF]

				S. D. Whyte, P. D. Booker, and D. G. Buckley The Effects of Propofol and Sevoflurane on the QT Interval and Transmural Dispersion of Repolarization in Children Anesth. Analg., January 1, 2005; 100(1): 71 - 77. [Abstract] [Full Text] [PDF]

				F. Extramiana and C. Antzelevitch Amplified Transmural Dispersion of Repolarization as the Basis for Arrhythmogenesis in a Canine Ventricular-Wedge Model of Short-QT Syndrome Circulation, December 14, 2004; 110(24): 3661 - 3666. [Abstract] [Full Text] [PDF]

				W.-G. Ding, F. Toyoda, and H. Matsuura Regulation of Cardiac IKs Potassium Current by Membrane Phosphatidylinositol 4,5-Bisphosphate J. Biol. Chem., December 3, 2004; 279(49): 50726 - 50734. [Abstract] [Full Text] [PDF]

				N. Ueda, D. P. Zipes, and J. Wu Functional and transmural modulation of M cell behavior in canine ventricular wall Am J Physiol Heart Circ Physiol, December 1, 2004; 287(6): H2569 - H2575. [Abstract] [Full Text] [PDF]

				T. J. Hund and Y. Rudy Rate Dependence and Regulation of Action Potential and Calcium Transient in a Canine Cardiac Ventricular Cell Model Circulation, November 16, 2004; 110(20): 3168 - 3174. [Abstract] [Full Text] [PDF]

				P. Zhabyeyev, T. Asai, S. Missan, and T. F. McDonald Transient outward current carried by inwardly rectifying K+ channels in guinea pig ventricular myocytes dialyzed with low-K+ solution Am J Physiol Cell Physiol, November 1, 2004; 287(5): C1396 - C1403. [Abstract] [Full Text] [PDF]

				C. Bellocq, R. Wilders, J.-J. Schott, B. Louerat-Oriou, P. Boisseau, H. Le Marec, D. Escande, and I. Baro A Common Antitussive Drug, Clobutinol, Precipitates the Long QT Syndrome 2 Mol. Pharmacol., November 1, 2004; 66(5): 1093 - 1102. [Abstract] [Full Text] [PDF]

				V. E. Bondarenko, G. P. Szigeti, G. C. L. Bett, S.-J. Kim, and R. L. Rasmusson Computer model of action potential of mouse ventricular myocytes Am J Physiol Heart Circ Physiol, September 1, 2004; 287(3): H1378 - H1403. [Abstract] [Full Text] [PDF]

				C. Antzelevitch, L. Belardinelli, A. C. Zygmunt, A. Burashnikov, J. M. Di Diego, J. M. Fish, J. M. Cordeiro, and G. Thomas Electrophysiological Effects of Ranolazine, a Novel Antianginal Agent With Antiarrhythmic Properties Circulation, August 24, 2004; 110(8): 904 - 910. [Abstract] [Full Text] [PDF]

				S. Poelzing, F. G. Akar, E. Baron, and D. S. Rosenbaum Heterogeneous connexin43 expression produces electrophysiological heterogeneities across ventricular wall Am J Physiol Heart Circ Physiol, May 1, 2004; 286(5): H2001 - H2009. [Abstract] [Full Text] [PDF]

				C. E Conrath, R. Wilders, R. Coronel, J. M.T de Bakker, P. Taggart, J. R de Groot, and T. Opthof Intercellular coupling through gap junctions masks M cells in the human heart Cardiovasc Res, May 1, 2004; 62(2): 407 - 414. [Abstract] [Full Text] [PDF]

				B. Rosati and D. McKinnon Regulation of Ion Channel Expression Circ. Res., April 16, 2004; 94(7): 874 - 883. [Abstract] [Full Text] [PDF]

				G. X. Liu, J. Zhou, S. Nattel, and G. Koren Single-channel recordings of a rapid delayed rectifier current in adult mouse ventricular myocytes: basic properties and effects of divalent cations J. Physiol., April 15, 2004; 556(2): 401 - 413. [Abstract] [Full Text] [PDF]

				A. G. KLEBER and Y. RUDY Basic Mechanisms of Cardiac Impulse Propagation and Associated Arrhythmias Physiol Rev, April 1, 2004; 84(2): 431 - 488. [Abstract] [Full Text] [PDF]

				H. B. Rasmussen, M. Moller, H.-G. Knaus, B. S. Jensen, S.-P. Olesen, and N. K. Jorgensen Subcellular localization of the delayed rectifier K+ channels KCNQ1 and ERG1 in the rat heart Am J Physiol Heart Circ Physiol, April 1, 2004; 286(4): H1300 - H1309. [Abstract] [Full Text] [PDF]

				J. M. Cordeiro, L. Greene, C. Heilmann, D. Antzelevitch, and C. Antzelevitch Transmural heterogeneity of calcium activity and mechanical function in the canine left ventricle Am J Physiol Heart Circ Physiol, April 1, 2004; 286(4): H1471 - H1479. [Abstract] [Full Text] [PDF]

				J. Tamargo, R. Caballero, R. Gomez, C. Valenzuela, and E. Delpon Pharmacology of cardiac potassium channels Cardiovasc Res, April 1, 2004; 62(1): 9 - 33. [Abstract] [Full Text] [PDF]

				Y. Fukuda, S. Miyoshi, K. Tanimoto, K. Oota, K. Fujikura, M. Iwata, A. Baba, Y. Hagiwara, T. Yoshikawa, H. Mitamura, et al. Autoimmunity against the second extracellular loop of beta1-adrenergic receptors induces early afterdepolarization and decreases in K-channel density in rabbits J. Am. Coll. Cardiol., March 17, 2004; 43(6): 1090 - 1100. [Abstract] [Full Text] [PDF]

				C. Antzelevitch, L. Belardinelli, L. Wu, H. Fraser, A. C. Zygmunt, A. Burashnikov, J. M. Di Diego, J. M. Fish, J. M. Cordeiro, R. J. Goodrow Jr, et al. Electrophysiologic Properties and Antiarrhythmic Actions of a Novel Antianginal Agent Journal of Cardiovascular Pharmacology and Therapeutics, March 1, 2004; 9(1_suppl): S65 - S83. [Abstract] [PDF]

				G. Liu, J. B. Iden, K. Kovithavongs, R. Gulamhusein, H. J. Duff, and K. M. Kavanagh In vivo temporal and spatial distribution of depolarization and repolarization and the illusive murine T wave J. Physiol., February 15, 2004; 555(1): 267 - 279. [Abstract] [Full Text] [PDF]

				P. Smetana, V. N. Batchvarov, K. Hnatkova, A. J. Camm, and M. Malik Ventricular gradient and nondipolar repolarization components increase at higher heart rate Am J Physiol Heart Circ Physiol, January 1, 2004; 286(1): H131 - H136. [Abstract] [Full Text] [PDF]

				D. M. Roden A Surprising New Arrhythmia Mechanism in Heart Failure Circ. Res., October 3, 2003; 93(7): 589 - 591. [Full Text] [PDF]

				J. M. Di Diego, L. Belardinelli, and C. Antzelevitch Cisapride-Induced Transmural Dispersion of Repolarization and Torsade de Pointes in the Canine Left Ventricular Wedge Preparation During Epicardial Stimulation Circulation, August 26, 2003; 108(8): 1027 - 1033. [Abstract] [Full Text] [PDF]

				A. N. Mazzadi, X. Andre-Fouet, J. Duisit, V. Gebuhrer, N. Costes, P. Chevalier, C. Rodriguez, J.-J. Schott, H. Le Marec, P. Guicheney, et al. Cardiac retention of [11C]HED in genotyped long QT patients: a potential amplifier role for severity of the disease Am J Physiol Heart Circ Physiol, August 7, 2003; 285(3): H1286 - H1293. [Abstract] [Full Text] [PDF]

				P. G.A. Volders, M. Stengl, J. M. van Opstal, U. Gerlach, R. L.H.M.G. Spatjens, J. D.M. Beekman, K. R. Sipido, and M. A. Vos Probing the Contribution of IKs to Canine Ventricular Repolarization: Key Role for {beta}-Adrenergic Receptor Stimulation Circulation, June 3, 2003; 107(21): 2753 - 2760. [Abstract] [Full Text] [PDF]

				T. Banyasz, L. Fulop, J. Magyar, N. Szentandrassy, A. Varro, and P. P. Nanasi Endocardial versus epicardial differences in L-type calcium current in canine ventricular myocytes studied by action potential voltage clamp Cardiovasc Res, April 1, 2003; 58(1): 66 - 75. [Abstract] [Full Text] [PDF]

				N. lost, L. Virag, A. Varro, and J. Gy. Papp Comparison of the Effect of Class IA Antiarrhythmic Drugs on Transmembrane Potassium Currents in Rabbit Ventricular Myocytes Journal of Cardiovascular Pharmacology and Therapeutics, March 1, 2003; 8(1): 31 - 41. [Abstract] [PDF]

				K. Takenaka, T. Ai, W. Shimizu, A. Kobori, T. Ninomiya, H. Otani, T. Kubota, H. Takaki, S. Kamakura, and M. Horie Exercise Stress Test Amplifies Genotype-Phenotype Correlation in the LQT1 and LQT2 Forms of the Long-QT Syndrome Circulation, February 18, 2003; 107(6): 838 - 844. [Abstract] [Full Text] [PDF]

				K. H. W. J. Ten Tusscher and A. V. Panfilov Reentry in heterogeneous cardiac tissue described by the Luo-Rudy ventricular action potential model Am J Physiol Heart Circ Physiol, February 1, 2003; 284(2): H542 - H548. [Abstract] [Full Text] [PDF]

				C Ramakers, M.A Vos, P.A Doevendans, M Schoenmakers, Y.S Wu, S Scicchitano, A Iodice, G.P Thomas, C Antzelevitch, and R Dumaine Coordinated down-regulation of KCNQ1 and KCNE1 expression contributes to reduction of IKs in canine hypertrophied hearts Cardiovasc Res, February 1, 2003; 57(2): 486 - 496. [Abstract] [Full Text] [PDF]

				J. Wang, K. Della Penna, H. Wang, J. Karczewski, T. M. Connolly, K. S. Koblan, P. B. Bennett, and J. J. Salata Functional and pharmacological properties of canine ERG potassium channels Am J Physiol Heart Circ Physiol, January 1, 2003; 284(1): H256 - H267. [Abstract] [Full Text] [PDF]

				C. Cabo and P. A. Boyden Electrical remodeling of the epicardial border zone in the canine infarcted heart: a computational analysis Am J Physiol Heart Circ Physiol, January 1, 2003; 284(1): H372 - H384. [Abstract] [Full Text] [PDF]

				G. Schram, P. Melnyk, M. Pourrier, Z. Wang, and S. Nattel Kir2.4 and Kir2.1 K+ channel subunits co-assemble: a potential new contributor to inward rectifier current heterogeneity J. Physiol., October 15, 2002; 544(2): 337 - 349. [Abstract] [Full Text] [PDF]

				J. M. Di Diego, J. M. Cordeiro, R. J. Goodrow, J. M. Fish, A. C. Zygmunt, G. J. Perez, F. S. Scornik, and C. Antzelevitch Ionic and Cellular Basis for the Predominance of the Brugada Syndrome Phenotype in Males Circulation, October 8, 2002; 106(15): 2004 - 2011. [Abstract] [Full Text] [PDF]

				G.-R. Li, C.-P. Lau, A. Ducharme, J.-C. Tardif, and S. Nattel Transmural action potential and ionic current remodeling in ventricles of failing canine hearts Am J Physiol Heart Circ Physiol, September 1, 2002; 283(3): H1031 - H1041. [Abstract] [Full Text] [PDF]

				M. Chinushi, H. Kasai, M. Tagawa, T. Washizuka, Y. Hosaka, Y. Chinushi, and Y. Aizawa Triggers of ventricular tachyarrhythmias and therapeutic effects of nicorandil in canine models of LQT2 and LQT3 syndromes J. Am. Coll. Cardiol., August 7, 2002; 40(3): 555 - 562. [Abstract] [Full Text] [PDF]

				J. J. Lynch Jr., J. J. Salata, A. A. Wallace, G. L. Stump, D. B. Gilberto, H. Jahansouz, N. J. Liverton, H. G. Selnick, and D. A. Claremon Antiarrhythmic Efficacy of Combined IKs and beta -Adrenergic Receptor Blockade J. Pharmacol. Exp. Ther., July 1, 2002; 302(1): 283 - 289. [Abstract] [Full Text] [PDF]

				G. Schram, M. Pourrier, P. Melnyk, and S. Nattel Differential Distribution of Cardiac Ion Channel Expression as a Basis for Regional Specialization in Electrical Function Circ. Res., May 17, 2002; 90(9): 939 - 950. [Abstract] [Full Text] [PDF]

				H. Matsuura, T. Ehara, W.-G. Ding, M. Omatsu-Kanbe, and T. Isono Rapidly and slowly activating components of delayed rectifier K+ current in guinea-pig sino-atrial node pacemaker cells J. Physiol., May 1, 2002; 540(3): 815 - 830. [Abstract] [Full Text] [PDF]

				D. Lacroix, P. Gluais, C. Marquie, C. D'Hoinne, M. Adamantidis, and M. Bastide Repolarization abnormalities and their arrhythmogenic consequences in porcine tachycardia-induced cardiomyopathy Cardiovasc Res, April 1, 2002; 54(1): 42 - 50. [Abstract] [Full Text] [PDF]

				T. V. Pham and M. R. Rosen Sex, hormones, and repolarization Cardiovasc Res, February 15, 2002; 53(3): 740 - 751. [Abstract] [Full Text] [PDF]

				C. E Conrath, A. A.M Wilde, R. J.E Jongbloed, M. Alders, I. M van Langen, J Peter van Tintelen, P. A Doevendans, and T. Opthof Gender differences in the long QT syndrome: effects of {beta}-adrenoceptor blockade Cardiovasc Res, February 15, 2002; 53(3): 770 - 776. [Abstract] [Full Text] [PDF]

				M Lei, H Honjo, I Kodama, and M R Boyett Heterogeneous expression of the delayed-rectifier K+ currents iK,r and iK,s in rabbit sinoatrial node cells J. Physiol., September 15, 2001; 535(3): 703 - 714. [Abstract] [Full Text] [PDF]

				C. Antzelevitch Heterogeneity of cellular repolarization in LQTS: the role of M cells Eur. Heart J. Suppl., September 1, 2001; 3(suppl_K): K2 - K16. [Abstract] [PDF]

				Z. Lu, K. Kamiya, T. Opthof, K. Yasui, and I. Kodama Density and Kinetics of IKr and IKs in Guinea Pig and Rabbit Ventricular Myocytes Explain Different Efficacy of IKs Blockade at High Heart Rate in Guinea Pig and Rabbit: Implications for Arrhythmogenesis in Humans Circulation, August 21, 2001; 104(8): 951 - 956. [Abstract] [Full Text] [PDF]

				A. C. Zygmunt, G. T. Eddlestone, G. P. Thomas, V. V. Nesterenko, and C. Antzelevitch Larger late sodium conductance in M cells contributes to electrical heterogeneity in canine ventricle Am J Physiol Heart Circ Physiol, August 1, 2001; 281(2): H689 - H697. [Abstract] [Full Text] [PDF]

				B. B. Lerman, E. D. Engelstein, and D. Burkhoff Mechanoelectrical Feedback: Role of {beta}-Adrenergic Receptor Activation in Mediating Load-Dependent Shortening of Ventricular Action Potential and Refractoriness Circulation, July 24, 2001; 104(4): 486 - 490. [Abstract] [Full Text] [PDF]

				C. Antzelevitch Transmural dispersion of repolarization and the T wave Cardiovasc Res, June 1, 2001; 50(3): 426 - 431. [Full Text] [PDF]

				P. Taggart, P. M.I Sutton, T. Opthof, R. Coronel, R. Trimlett, W. Pugsley, and P. Kallis Transmural repolarisation in the left ventricle in humans during normoxia and ischaemia Cardiovasc Res, June 1, 2001; 50(3): 454 - 462. [Abstract] [Full Text] [PDF]

				T. V. Pham, E. A. Sosunov, R. Z. Gainullin, P. Danilo Jr, and M. R. Rosen Impact of Sex and Gonadal Steroids on Prolongation of Ventricular Repolarization and Arrhythmias Induced by IK-Blocking Drugs Circulation, May 1, 2001; 103(17): 2207 - 2212. [Abstract] [Full Text] [PDF]

				U. C. Hoppe, E. Marban, and D. C. Johns Distinct gene-specific mechanisms of arrhythmia revealed by cardiac gene transfer of two long QT disease genes, HERG and KCNE1 PNAS, April 24, 2001; 98(9): 5335 - 5340. [Abstract] [Full Text] [PDF]

				F. L Burton and S. M Cobbe Dispersion of ventricular repolarization and refractory period Cardiovasc Res, April 1, 2001; 50(1): 10 - 23. [Full Text] [PDF]

				W. Han, Z. Wang, and S. Nattel Slow delayed rectifier current and repolarization in canine cardiac Purkinje cells Am J Physiol Heart Circ Physiol, March 1, 2001; 280(3): H1075 - H1080. [Abstract] [Full Text] [PDF]

				L. Virag, N. Iost, M. Opincariu, J. Szolnoky, J. Szecsi, G. Bogats, P. Szenohradszky, A. Varro, and J. Gy. Papp The slow component of the delayed rectifier potassium current in undiseased human ventricular myocytes Cardiovasc Res, March 1, 2001; 49(4): 790 - 797. [Abstract] [Full Text] [PDF]

				T. Volk, T. H.-D. Nguyen, J.-H. Schultz, J. Faulhaber, and H. Ehmke Regional alterations of repolarizing K+ currents among the left ventricular free wall of rats with ascending aortic stenosis J. Physiol., February 1, 2001; 530(3): 443 - 455. [Abstract] [Full Text] [PDF]

				C. Antzelevitch Electrical Heterogeneity, Cardiac Arrhythmias, and the Sodium Channel Circ. Res., November 24, 2000; 87(11): 964 - 965. [Full Text] [PDF]

				S. Nattel Acquired delayed rectifier channelopathies: how heart disease and antiarrhythmic drugs mimic potentially-lethal congenital cardiac disorders Cardiovasc Res, November 1, 2000; 48(2): 188 - 190. [Full Text] [PDF]

				Y. Tsuji, T. Opthof, K. Kamiya, K. Yasui, W. Liu, Z. Lu, and I. Kodama Pacing-induced heart failure causes a reduction of delayed rectifier potassium currents along with decreases in calcium and transient outward currents in rabbit ventricle Cardiovasc Res, November 1, 2000; 48(2): 300 - 309. [Abstract] [Full Text] [PDF]

				M. Jiang, C. Cabo, J.-A. Yao, P. A Boyden, and G.-N. Tseng Delayed rectifier K currents have reduced amplitudes and altered kinetics in myocytes from infarcted canine ventricle Cardiovasc Res, October 1, 2000; 48(1): 34 - 43. [Abstract] [Full Text] [PDF]

				J. Merot, V. Probst, M. Debailleul, U. Gerlach, N. S. Moise, H. Le Marec, and F. Charpentier Electropharmacological characterization of cardiac repolarization in German shepherd dogs with an inherited syndrome of sudden death: abnormal response to potassium channel blockers J. Am. Coll. Cardiol., September 1, 2000; 36(3): 939 - 947. [Abstract] [Full Text] [PDF]

				W Haverkamp, G Breithardt, A.J Camm, M.J Janse, M.R Rosen, C Antzelevitch, D Escande, M Franz, M Malik, A Moss, et al. The potential for QT prolongation and proarrhythmia by non-antiarrhythmic drugs: clinical and regulatory implications. Report on a Policy Conference of the European Society of Cardiology Eur. Heart J., August 1, 2000; 21(15): 1216 - 1231. [PDF]

				A.J. Camm, M.J. Janse, D.M. Roden, M.R. Rosen, J. Cinca, and S.M. Cobbe Congenital and acquired long QT syndrome Eur. Heart J., August 1, 2000; 21(15): 1232 - 1237. [PDF]

				W. Haverkamp, G. Breithardt, A.J. Camm, M. J Janse, M. R Rosen, C. Antzelevitch, D. Escande, M. Franz, M. Malik, A. Moss, et al. The potential for QT prolongation and pro-arrhythmia by non-anti-arrhythmic drugs: Clinical and regulatory implications: Report on a Policy Conference of the European Society of Cardiology Cardiovasc Res, August 1, 2000; 47(2): 219 - 233. [Full Text] [PDF]

				W. Guo, H. Li, B. London, and J. M. Nerbonne Functional Consequences of Elimination of Ito, f and Ito, s : Early Afterdepolarizations, Atrioventricular Block, and Ventricular Arrhythmias in Mice Lacking Kv1.4 and Expressing a Dominant-Negative Kv4 {alpha} Subunit Circ. Res., July 7, 2000; 87(1): 73 - 79. [Abstract] [Full Text] [PDF]

				J. F. Spear and E. N. Moore Modulation of arrhythmias by isoproterenol in a rabbit heart model of d-sotalol-induced long Q-T intervals Am J Physiol Heart Circ Physiol, July 1, 2000; 279(1): H15 - H25. [Abstract] [Full Text] [PDF]

				J. M Nerbonne Molecular basis of functional voltage-gated K+ channel diversity in the mammalian myocardium J. Physiol., June 1, 2000; 525(2): 285 - 298. [Abstract] [Full Text] [PDF]

This Article Abstract Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Permissions Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Liu, D.-W. Articles by Antzelevitch, C. Search for Related Content PubMed PubMed Citation Articles by Liu, D.-W. Articles by Antzelevitch, C.