Evidence for Two Components of Delayed Rectifier K+ Current in Human Ventricular Myocytes
Abstract Previous voltage-clamp studies have suggested that the delayed rectifier current (IK) is small or absent in the human ventricle and, when present, consists only of the rapid component (IKr); however, molecular studies suggest the presence of functionally important IK in the human heart, specific IKr blockers are known to delay ventricular repolarization and cause the long QT syndrome in humans, and we have shown that the expression of IK is strongly influenced by cell isolation techniques. The present experiments were designed to assess the expression of IK in myocytes obtained by arterial perfusion of right ventricular tissue from explanted human hearts. Of 35 cells from three hearts, 33 (94%) showed time-dependent currents typical of IK. The envelope-of-tails test was not satisfied under control conditions but became satisfied in the presence of the benzenesulfonamide E-4031 (5 μmol/L). E-4031 suppressed a portion of IK in 32 of 33 cells, with properties of the drug-sensitive and -resistant components consistent with previous descriptions of IKr and the slow component (IKs), respectively. Action potential duration to 95% repolarization at 1 Hz was prolonged by E-4031 from 336±16 (mean±SEM) to 421±19 ms (n=5, P<.01), indicating a functional role for IK. Indapamide, a diuretic agent previously shown to inhibit IKs selectively, suppressed E-4031–resistant current. The presence of a third type of delayed rectifier, the ultrarapid delayed rectifier current (IKur), was evaluated with the use of depolarizing prepulses and low concentrations (50 μmol/L) of 4-aminopyridine. Although these techniques revealed clear IKur in five of five human atrial cells, no corresponding component was observed in any of five human ventricular myocytes. We conclude that a functionally significant IK, with components corresponding to IKr and IKs, is present in human ventricular cells, whereas IKur appears to be absent. These findings are important for understanding the molecular, physiological, and pharmacological determinants of human ventricular repolarization and arrhythmias.
The delayed rectifier K+ current (IK) was first described in sheep cardiac Purkinje fibers by Noble and Tsien1 and has since been identified in a wide variety of species and tissue types.2 3 4 5 6 7 8 9 10 11 12 13 Noble and Tsien defined two components of IK that differed in kinetic properties, voltage dependence, and rectification,1 and similar components were subsequently observed in cardiac cells from the guinea pig,4 5 dog,9 10 11 and chick13 heart. Sanguinetti and Jurkiewicz4 5 demonstrated that certain class III antiarrhythmic drugs, notably E-4031 and sotalol, selectively block IKr and that the drug-sensitive (IKr) and drug-resistant (IKs) components of IK differ in terms of voltage dependence, kinetics, rectification properties, and pharmacological sensitivity. These differing properties may have important implications for understanding the physiological function of IK and the effects of IK blockers and agonists in specific tissues.14
A potential molecular equivalent of IKs was cloned several years ago,15 16 17 and electrophysiological and immunolocalization data suggesting that the corresponding protein (called minK or IsK) underlies IKs in the guinea pig heart have been presented.18 Recently, a gene (human ether-a-go-go–related gene, or HERG) coding for channels carrying currents resembling IKr has been identified in the human heart.19 20 Despite the demonstrated presence of genes for both IsK16 17 and HERG20 in the human heart and the presence of high concentrations of HERG mRNA in human cardiac tissue,20 the presence of significant currents corresponding to IKr and IKs has been difficult to demonstrate in human ventricular tissue. Beuckelmann et al21 found small delayed rectifier currents in 41% of cells from failing human left ventricles and in none of six cells from normal hearts. Although a detailed electrophysiological characterization of IK was not performed, the authors concluded that the only component present was IKr, that minimal or no IK was present in most human ventricular cells, and that the major current responsible for repolarization in human ventricle appeared to be Ito. We have recently shown that the expression of IK in cells isolated from the canine right atrium is very dependent on isolation techniques.9 We adapted the techniques we previously used to isolate canine atrial cells with robust IK9 in order to obtain cells from right ventricles of three patients receiving heart transplants for severe left ventricular failure without significant right ventricular pathology. The purpose of the present study was to determine the prevalence of IK in human ventricular cells isolated with these methods and to establish whether components corresponding to IKr and IKs are present.
Materials and Methods
Explanted hearts were obtained at the time of heart transplantation from three patients, aged 31, 53, and 54 years. The underlying heart disease was congestive cardiomyopathy in two patients and heart failure due to aortic valve disease in one patient. Subsequent examination of the right ventricle by a cardiac pathologist revealed it to be macroscopically normal in all patients. Microscopic examination of the right ventricular myocardium was normal in all hearts, with minor extramyocardial abnormalities consisting of fatty infiltration in one heart and subendocardial fibrosis in another.
All hearts were initially placed in cold (4°C) oxygenated Krebs’ solution and then transferred to cardioplegic solution for dissection and coronary artery cannulation. A portion of the free wall of the right ventricle (≈2×4 to 2×5 cm) was removed along with the coronary artery branch irrigating it, with dissection and arterial cannulation completed within 20 minutes of excision of the heart. The free wall was perfused with oxygenated nominally Ca2+-free Tyrode’s solution for 20 to 30 minutes, and the solution was then changed to one containing 200 to 300 U/mL collagenase (CLS II, Worthington Biochemical) for 60 to 100 minutes. The digested tissue was cut into small (≈1.5- to 2-mm3) pieces, placed in a high-K+ storage solution (see below), and gently triturated with a Pasteur pipette. Isolated myocytes were kept in the medium at least 1 hour before use. Atrial cells were isolated from right atrial samples of three additional patients without atrial disease by the use of techniques previously described in detail.22 23
A small aliquot of the solution containing the isolated cells was placed in an open perfusion chamber (1 mL) mounted on the stage of an inverted microscope. Myocytes were allowed to adhere to the bottom of the chamber for 5 to 10 minutes and were then superfused at 2 to 3 mL/min with Tyrode’s solution. Experiments studying classic IK were conducted at 36°C, with the temperature controlled by a Peltier-effect device. In studies of IKur, cells were evaluated at room temperature in order to resolve the very rapid kinetics of the current.23 Only quiescent rod-shaped cells showing clear cross striations were used.
The Tyrode’s solution contained (mmol/L) NaCl 126, KCl 5.4, MgCl2 1.0, CaCl2 1.0, NaH2PO4 0.33, glucose 10, and HEPES 10 (pH adjusted to 7.4 with NaOH). For voltage-clamp studies of IK, external Na+ was replaced by equimolar (126 mmol/L) choline to suppress INa, 4-AP (5 mmol/L, Sigma Chemical Co) was used to block Ito, 0.5 mmol/L BaCl2 (Sigma) was used to inhibit IK1, and 0.2 mmol/L CdCl2 was used to suppress ICa. IKur was studied with the same external solution, except that 4-AP was omitted under control conditions. The high-K+ storage medium contained (mmol/L) KCl 20, KH2PO4 10, glucose 10, potassium glutamate 70, β-hydroxybutyric acid 10, taurine 10, EGTA 5.0, and mannitol 10, along with 0.1% albumin (pH adjusted to 7.3 with KOH). The pipette solution contained (mmol/L) KCl 20, potassium aspartate 110, MgCl2 1.0, HEPES 10, EGTA 5.0, GTP 0.1, Na2 phosphocreatine 5.0, and Mg2ATP 5.0 (pH adjusted to 7.2 with KOH). E-4031 was provided as a kind gift by Eisai Ltd (Ibaraki, Japan) and prepared as a 5 mmol/L stock solution in distilled water.
Data Acquisition and Analysis
The tight-seal whole-cell patch-clamp technique was used. Borosilicate glass electrodes (outer diameter, 1.0 mm) were pulled with a Brown-Flaming puller (model P-87) and had tip resistances of 2 to 4 MΩ when filled with pipette solution. Data were acquired with the use of an Axopatch 200A or 1-D amplifier (Axon Instruments). Command pulses were generated by a 12-bit digital-to-analog convertor controlled by pClamp software (Axon Instruments). Recordings were low-pass–filtered at 2 kHz, and data were acquired by analog-to-digital conversion at a maximum rate of 50 kHz (model TM 125, Scientific Solutions) and stored on the hard disk of an IBM-compatible computer. Junction potentials (2 to 10 mV) were compensated before the pipette touched the cell. A tight seal was obtained, and seals with a resistance of <10 GΩ were rejected. The cell membrane was ruptured by gentle suction to establish the whole-cell configuration.
Rs was electrically compensated to minimize the duration of the capacitive transient. Rs was estimated by dividing τcap by the total membrane capacitance obtained during 5-mV hyperpolarizing steps from a holding potential of −60 mV. Before compensation, τcap in ventricular cells averaged 1043±112 μs, and Rs averaged 5.8±0.7 MΩ (cell capacitance, 179±15 pF). After compensation, τcap decreased to 449±21 μs, and Rs decreased to 3.2±0.5 MΩ. In atrial cells, the initial τcap averaged 509±49 μs (cell capacitance, 71±9 pF), and Rs averaged 6.9±0.9 MΩ. Corresponding values after τcap and Rs compensation were 259±30 μs and 4.1±0.8 MΩ, respectively.
Curve fitting was performed with a Marcquardt algorithm and TableCurve software (Jandel Scientific). Results are presented as the mean±SEM. Statistical comparisons between two group means were by t test, and a two-tailed value of P<.05 was taken to indicate statistical significance. Each series of experiments was performed with roughly equal numbers of cells from all hearts, in order to ensure that the results of each analysis were representative of all the hearts studied.
Prevalence and General Properties of IK
Currents were studied during 3-s depolarizing steps to voltages between −40 and +60 mV from a holding potential of −60 mV, with IKtail observed during subsequent repolarization to −30 mV. Thirty-three (94%) of 35 cells showed IKtail of at least 50 pA after steps to +60 mV. Fig 1A⇓ shows IKstep and IKtail in a representative cell. The addition of 5 μmol/L E-4031 reduced the amplitude of both IKstep and IKtail (Fig 1B⇓), and the E-4031–sensitive current obtained by digital subtraction showed rapid activation and relatively large IKtails (Fig 1C⇓). Action potentials were recorded at 1 Hz with current-clamp techniques during superfusion with normal Tyrode’s solution, and typical results from one cell before and after the addition of E-4031 (5 μmol/L) are shown in Fig 1D⇓. In five cells exposed to 5 μmol/L E-4031, resting potential averaged −83±3 mV, and APD to 95% repolarization at 1 Hz averaged 336±16 ms before and 424±19 ms after (P<.01) exposure to the drug. Of the 33 cells showing IK, an E-4031–sensitive component was seen in all but one, and an E-4031–resistant component was present in all.
Components of IK
The envelope of tails was studied with the use of the protocol illustrated in Fig 2A⇓. Under control conditions, the envelope test was not satisfied, as indicated by the lack of superposition of scaled IKtail on IKstep. After the addition of E-4031, the envelope test was satisfied (Fig 2B⇓). Mean ratios of IKtail to IKstep from eight cells are shown in Fig 2C⇓. Under control conditions (open circles), the ratio averaged 2.8±0.3 after a 200-ms activating pulse, and this value gradually decreased to 1.3±0.1, reaching steady state values at a pulse duration of 1000 ms. In the presence of E-4031 (filled circles), time-dependent changes in the ratio of IKtail to IKstep were eliminated. These data suggest that IK consists of more than one component under control conditions and that in the presence of E-4031 only one component remains.
Fig 3A⇓ shows recordings obtained before and after exposure to 5 μmol/L E-4031 in a different cell from that illustrated in Fig 1⇑. The E-4031–sensitive current (Fig 3B⇓) activated rapidly and had IKtails that were relatively large compared with IKsteps. At the more positive voltages, E-4031–sensitive IKstep density showed a slow time-dependent decay, resulting in strong inward rectification of the end-pulse current as previously shown in human atrial cells.22 Fig 3C⇓ shows mean time-dependent IKstep density (defined as the current level at the end of the pulse relative to the initial current level, normalized to cell capacitance) as a function of test potential in six cells. The current-voltage relation of total current has a flat portion between +10 and +30 mV, whereas the current-voltage relation for E-4031–resistant current is relatively smooth. E-4031–sensitive current shows strong inward rectification.
An analysis of the voltage-dependent activation of E-4031–sensitive and –resistant components is shown in Fig 4⇓. Fig 4A⇓ shows mean IKtail densities in 11 cells at −30 mV after 3-second depolarizing pulses to the test pulse voltages indicated. At voltages below +10 mV, E-4031 inhibited the majority of the IKtail. At more positive potentials, the E-4031–sensitive portion remained constant, whereas the drug-resistant portion continued to increase. Activation voltage dependence (Fig 4B⇓) was determined by normalizing IKtail at each test potential in Fig 4A⇓ to the current at the most positive test potential. Under control conditions, V0.5 averaged 0.9±0.3 mV, and the slope factor was 11.2±3.1 mV. In the presence of E-4031, V0.5 increased to 9.4±2.5 mV (P<.01 versus control), and the slope factor averaged 11.8±2.9 mV. For the E-4031–sensitive component, V0.5 averaged −14±4 mV (P<.01 versus control), and the slope factor was 7.7±2.7 mV.
An analysis of the kinetics of IK activation at +50 mV based on IKtail densities upon subsequent repolarization to −30 mV (same protocol as in Fig 2⇑) is shown (mean±SEM for 10 cells) in Fig 4C⇑. Under control conditions, IK was well fitted by a biexponential function, with a τ1 of 161±32 ms and a τ2 of 533±86 ms. The E-4031 resistant component was also biexponential, with a τ1 of 360±87 ms and a τ2 of 8.5±0.3 s. The E-4031–sensitive current had an activation time constant of 192±53 ms (not significantly different from the fast phase time constant under control conditions) and an inactivation time constant of 10.6±0.6 s.
The reversal potential of IK was closely related to [K+]o. In six cells exposed to [K+]o of 5.4, 10.8, and 21.6 mmol/L, the potential for reversal of IKtail was linearly related to log([K+]o), with a slope of 54.8 mV per decade and a correlation coefficient of .999. Fig 5A⇓ illustrates the effect of E-4031 on the reversal potential in a representative cell. The amplitude of IKtail was determined at various potentials after a 3-second pulse to +40 mV. Under control conditions, the reversal potential was between −70 and −80 mV. After exposure to E-4031, IKtail became smaller, and the reversal potential became less negative (between −60 and −70 mV). Fig 5B⇓ shows mean IKtail densities at various test potentials as measured under control conditions in four cells and in the presence of E-4031 in four cells. The average reversal potential was −75 mV under control conditions and −69 mV in the presence of E-4031. Mean E-4031–sensitive tail current densities, measured in two cells studied under stable conditions before and after E-4031, are shown by the open inverted triangles in Fig 5B⇓. The reversal potential of mean E-4031–sensitive current was −86 mV, very close to the estimated K+ equilibrium potential.
Unlike IKr, for which a variety of highly selective blockers are available, there is no similarly recognized tool for the study of IKs. Recently, however, the diuretic agent indapamide has been reported to be a selective IKs blocker.24 To study further the pharmacological properties of E-4031–resistant current in human ventricle, we exposed cells to 1 mmol/L indapamide (a concentration reported to fully block IKs in guinea pig ventricle24 ). Fig 6A⇓ shows an envelope of tails recorded from one myocyte in the presence of 5 μmol/L E-4031. Exposure to indapamide fully suppressed both IKstep and IKtail (Fig 6B⇓). Partial reversal of current suppression was observed after 15 minutes of indapamide washout (Fig 6C⇓). Similar results were obtained in a total of four cells.
In the final series of experiments, we sought to establish whether the ultrarapid delayed rectifier reported in human atrium is also present in human ventricular myocytes. Fig 7A⇓ (top) shows typical recordings of IKur, obtained in a human atrial myocyte with the use of a 100-ms prepulse to +40 mV (to inactivate Ito) delivered 10 ms before a 150-ms depolarizing test pulse. As described previously,23 50 μmol/L 4-AP substantially inhibited IKur (middle). Similar results were obtained in all of five atrial myocytes studied over the course of these experiments. Typical recordings from a ventricular myocyte obtained with the same protocol are shown in Fig 7B⇓ (top). The small outwardly rectifying current recorded under these conditions shows no time dependence, and 50 μmol/L 4-AP does not cause any obvious inhibition (middle). In contrast to the clear rapidly activating 50 μmol/L 4-AP–sensitive current observed in atrial cells (Fig 7A⇓, bottom), no corresponding component was noted in this (Fig 7B⇓, bottom) or four other ventricular cells studied in the same fashion. Furthermore, exposure to 4-AP concentrations as high as 10 mmol/L failed to reveal any drug-sensitive component comparable to IKur.
In the present study, we have demonstrated that significant quantities of IK are present in a large proportion of human ventricular cells and that the current can be dissociated into components corresponding to IKr and IKs. Before the present study, whole-cell voltage-clamp data in the literature suggested that IK is small in amplitude and is expressed in a minority of human ventricular cells and that the IKs component is absent. We found no evidence for the presence in ventricular cells of an ultrarapid delayed rectifier comparable to IKur previously reported in human atrial cells.
Comparison With Previous Studies of IK
The E-4031–sensitive and –resistant components that we observed share a variety of properties with IKr and IKs previously described in other systems. As in guinea pig ventricle4 and atrium,5 canine ventricle,10 and human atrium,22 the E-4031–sensitive component (IKr) activates more rapidly, at a more negative voltage, and with a more steep slope factor than does the E-4031–resistant component (IKs) and shows inward rectification. The reversal potential of IKr is more negative than that of IKs, which is compatible with previous observations in other species,1 4 10 and suggests a greater K+ selectivity for IKr. Like IKs in guinea pig ventricle,24 E-4031–resistant current in human ventricle is effectively inhibited by the diuretic agent indapamide. The rapid-phase time constant of IK activation in human ventricle is similar to the activation time constant of E-4031–sensitive current, suggesting that it is due to the activation of IKr. E-4031–resistant current activated in a biexponential fashion, with time constants differing by approximately an order of magnitude. This finding differs from results previously described by Sanguinetti and Jurkiewicz5 in guinea pig ventricle and Wang et al22 in human atrium but closely resembles results recently reported in canine ventricular cells.10 The discrepancies may be due to species- and/or tissue-related differences in IKs behavior or to methodological differences among studies. We observed a slow decline of E-4031–sensitive current during sustained depolarization in the present study, similar to previous findings in human atrium.22 This slow decline may be due to IKr inactivation, as suggested by previous reports of experiments in rabbit nodal cells6 and in AT-1 cells.25 26
Beuckelmann et al21 observed small IKr-like currents in a minority of left ventricular cells from explanted failing human hearts and in none of six cells from normal hearts. They concluded that IKs is absent in the human ventricle and that IK is unlikely to be important in human ventricular repolarization. APD in the failing heart cells studied by Beuckelmann et al was >1 s, compared with an average of 336 ms in the cells in the present study. The latter value is in the same range as the mean APD recorded from normal multicellular human ventricular preparations (300 to 360 ms)27 28 and the mean in vivo monophasic APD of ≈300 ms obtained by Bargheer et al29 during clinical electrophysiological studies in 10 patients. The differences in APD between the cells used by Beuckelmann et al and those used in the present study are consistent with the much smaller amounts of repolarizing IK recorded in the former study. Veldkamp et al30 reported the presence of single IKr channels in human ventricular myocytes in a preliminary communication, but the prevalence and kinetics of these channels were not described. In a recently published study, Konarzewska et al31 analyzed the properties of Ito and IK1 in myocytes obtained from biopsies of normal human ventricle but did not detect IK. They noted the discrepancy between indirect evidence pointing toward a role for IK in human ventricular repolarization and the lack of direct recordings of IK in their study and in the previous literature. The present study, which shows that both components of IK are demonstrable in the vast majority of normal human right ventricular myocytes, stands to resolve this discrepancy.
We found no evidence in human ventricular myocytes for the presence of currents resembling IKur in human atrial cells. Konarzewska et al31 were similarly unable to demonstrate a highly 4-AP–sensitive current in human ventricular cells. The molecular component believed to underlie IKur, the Kv1.5 channel,23 32 33 has recently been detected by immunohistochemical techniques in both human atrium and ventricle.34 The apparent discrepancy between the immunohistochemical and electrophysiological evidence requires explanation. One possibility is that IKur is not carried by Kv1.5 channels, but there is substantial evidence pointing to the contrary.23 33 A second possibility is that the isolation procedure damages IKur in the ventricle and renders it nonfunctional; however, we have observed that IKur is more resistant than other K+ currents (like IK and Ito) to damage during isolation of human atrial myocytes. A third potential explanation relates to differences in the pattern of immunohistochemical expression of Kv1.5 channels in human atria and ventricles. Although Kv1.5 protein is found at intercalated disks in both atrium and ventricle, longitudinal staining of the cell membrane is found only in the atria.34 It is possible that only channels in the longitudinal cell membrane carry transmembrane current and that although channels in the intercalated disk are involved in intercellular communication, they do not contribute to ion flux between the intracellular and extracellular spaces. Finally, we examined a limited number of cells from the free wall of the right ventricle, and it is possible that we failed to record Kv1.5-like currents in ventricular cells because of limited sampling and/or regional distribution of the channel. Further work is clearly necessary to determine the mechanisms and functional importance of the cellular localization of cardiac ion channel proteins in general and Kv1.5 in particular.
Our results shed potentially important light on the K+ currents governing cardiac repolarization in humans. These findings are particularly relevant in the context of recent molecular studies, which point to the presence of molecular substrates for IKr and IKs in the human ventricle.15 16 17 18 19 20 The physiological expression of these channels has significant implications, especially in view of the evidence that one form of the congenital long QT syndrome is due to mutations that interfere with the expression of a gene that appears to code for IKr.19 20 IKr blockers are known to be particularly likely to cause the acquired long QT syndrome.14 35 Our results demonstrate the electrophysiological substrate, in terms of IKr expression, for these important clinical problems.
Because of the risks of proarrhythmia attending currently available class III antiarrhythmic drug therapy,35 there has been interest in defining novel ionic targets for new drug development. IKs may contribute to rate-dependent action potential abbreviation,36 and it has been suggested that selective IKs blockers may have a more desirable profile of rate-dependent action and safety than currently available compounds. Recent modeling work indicates an important role for IKs in repolarizing guinea pig ventricular myocytes.37 Our observation of IKs in human ventricular cells is therefore potentially significant for both the understanding of mechanisms of human ventricular repolarization and the development of new antiarrhythmic drugs. The absence of IKur in the human ventricle is also of potential importance for new drug development. Since IKur plays an important role in human atrial repolarization23 and is absent in human ventricle, it is a potentially promising target for the development of drugs that prevent reentrant atrial arrhythmias without a risk of ventricular proarrhythmia.
We studied right ventricular cells from patients with severe left ventricular failure. We cannot exclude the possibility that our results were influenced by the presence of heart disease. However, expert pathological examination at both macroscopic and microscopic levels did not reveal abnormalities in the right ventricular myocardium of our patients, and action potential characteristics of our cells are similar to previous results for normal human ventricle.27 28 There may also be regional differences in the quantity and properties of IKr and IKs. Since all of our studies were based on tissue from a similar zone in the free wall of the right ventricle, our results are not affected by this possibility and certainly cannot exclude it.
Because of the limited availability of normal human ventricular tissue and the exacting requirements of our isolation technique, we have been able to evaluate the presence of IK systematically in only the three hearts presented in this article. In fact, initial observations of a similar type were made in cells from another heart and led to the present studies; however, since protocols described in the present article were not applied systematically in that heart, we have based the present article on only the results from the subsequent three hearts that were studied in the same systematic fashion.
Divalent cations are known to have effects on IK,38 39 and since we used Cd2+ to block ICa, our results must be interpreted in this light. It is important to inhibit ICa in studying the components of IK.40 Although divalent cations can modify IK, organic Ca2+ channel blockers are associated with well-known voltage- and use-dependent effects that can also complicate analysis. Finally, the human ventricular cells that we studied do not readily tolerate repeated prolonged (>5-s) depolarizing pulses. This limits precise kinetic calculations for processes occurring during depolarization to those with time constants of <1.5 s. Thus, the time constants for IKr decay during a depolarizing pulse and for the slow phase of IKs activation should be considered only approximations.
Results presented in this article indicate that IK is detectable under appropriate conditions in most ventricular myocytes obtained from relatively normal human right ventricle and that rapid (IKr) and slow (IKs) components with properties similar to those described in other species can be detected. IKur, present in human atrium, appears to be absent from human ventricle. These results have important implications for our understanding of the physiological, pharmacological, and molecular control of repolarization in the human heart.
Selected Abbreviations and Acronyms
|τcap||=||capacitive time constant|
|τ1, τ2||=||fast and slow time constants|
|APD||=||action potential duration|
|IK||=||delayed rectifier K+ current|
|IK1||=||inward rectifier current|
|IKr||=||rapid component of IK|
|IKs||=||slow component of IK|
|IKstep||=||Ik step current|
|IKtail||=||Ik tail current|
|IKur||=||ultrarapid delayed rectifier current|
|Ito||=||transient outward current|
|V0.5||=||50% activation voltage|
This study was supported by grants from the Medical Research Council of Canada, the Quebec Heart Foundation, and the Fonds de Recherche de l’Institut de Cardiologie de Montréal. Dr Li is a research scholar of the Fonds de la Recherche en Santé du Québec. The authors thank Johanne Doucet for technical support and Carolyn Gillis for secretarial help with the manuscript.
- Received July 14, 1995.
- Accepted December 14, 1995.
- © 1996 American Heart Association, Inc.
Hume JR, Giles W, Robinson K, Shibata EF. A time and voltage-dependent K+ current in single cardiac cells from bull frog atrium. J Gen Physiol. 1986;88:777-798.
Sanguinetti MC, Jurkiewicz NK. Two components of cardiac delayed rectifier K+ current. J Gen Physiol. 1990;96:195-215.
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.
Duan D, Fermini B, Nattel S. Potassium channel blocking properties of propafenone in rabbit atrial myocytes. J Pharmacol Exp Ther. 1993;264:1113-1123.
Carmeliet E. Electrophysiologic and voltage clamp analysis of the effects of sotalol on isolated cardiac muscle and Purkinje fibers. J Pharmacol Exp Ther. 1985;232:817-825.
Yue LX, Feng J, Li GR, Nattel S. Transient outward and delayed rectifier currents in canine atrium: properties and role of isolation methods. Am J Physiol (Heart Circ Physiol). In press.
Liu D-W, Antzelevitch C. Characteristics of the delayed rectifier current (IKr and IKs) in canine ventricular epicardial, midmyocardial, and endocardial myocytes: a weaker IKs contributes to the longer action potential of the M Cell. Circ Res. 1995;76:351-365.
Colatsky TJ, Follmer CH, Starmer CF. Channel specificity in antiarrhythmic drug action: mechanism of potassium channel block and its role in suppressing and aggravating cardiac arrhythmias. Circulation. 1990;82:2235-2242.
Takumi T, Ohkubo H, Nakanishi S. Cloning of a membrane protein that induces a slow voltage-gated potassium current. Science. 1988;242:1042-1045.
Swanson R, Folander K, Bennett C, Antanavage J, Stein RB, Smith JS. Total synthesis, expression and functional assay of a gene encoding a human delayed rectifier potassium channel. Biophys J. 1990;57:211a. Abstract.
Freeman LS, Kass RS. Expression of a minimal K+ channel protein in mammalian cells and immunolocalization in guinea pig heart. Circ Res. 1993;73:968-973.
Beuckelmann DJ, Näbauer M, Erdmann E. Alterations of K+ currents in isolated human ventricular myocytes from patients with terminal heart failure. Circ Res. 1993;73:379-385.
Wang Z, Fermini B, Nattel S. Sustained depolarization-induced outward current in human atrial myocytes: evidence for a novel delayed rectifier K+ current similar to Kv1.5 cloned channel currents. Circ Res. 1993;73:1061-1076.
Turgeon J, Daleau P, Bennett PB, Wiggins SS. Selby L, Roden DM. Block of IKs, the slow component of the delayed rectifier K+ current, by the diuretic agent indapamide in guinea pig myocytes. Circ Res. 1994;75:879-886.
Yang T, Wathen MS, Felipe A, Tamkun MM, Snyders DJ, Roden DM. K+ currents and K+ channel mRNA in cultured atrial cardiac myocytes (AT-1 cells). Circ Res. 1994;75:870-878.
Spear JF, Horowitz LN, Hodess AB, MacVaugh H, Moore EN. Cellular electrophysiology of human myocardial infarction, 1: abnormalities of cellular activation. Circulation. 1979;59:247-256.
Dangman KH, Danilo P Jr, Hordof AJ, Mary-Rabine L, Reder RF, Rosen MR. Electrophysiologic characteristics of human ventricular and Purkinje fibers. Circulation. 1982;65:362-368.
Bargheer K, Bode F, Klein HU, Trappe HJ, Franz MR, Lichtlen PR. Prolongation of monophasic action potential duration and the refractory period in the human heart by tedisamil, a new potassium-blocking agent. Eur Heart J. 1994;15:1409-1414.
Veldkamp MW, van Ginneken ACG, Opthof T, Bouman LN. Single channel recordings of the delayed rectifier current in human ventricular myocytes. Circulation. 1994;90(suppl I, pt 2):I-582. Abstract.
Konarzewska H, Peeters GA, Sanguinetti MC. Repolarizing K+ currents in nonfailing human hearts: similarities between right septal subendocardial and left subepicardial ventricular myocytes. Circulation. 1995;92:1179-1187.
Fedida D, Wible B, Wang Z, Fermini B, Faust F, Nattel S, Brown AM. Identity of a novel delayed rectifier current from human heart with a cloned K+ channel current. Circ Res. 1993;73:210-216.
Tamkun MM, Bennett PB, Snyders DJ. Cloning and expression of human cardiac K+ channels. In: Zipes DP, Jalife J, eds. Cardiac Electrophysiology: From Cell to Bedside. 2nd ed. Philadelphia, Pa: WB Saunders Co; 1995:21-30.
Mays DJ, Foose JM, Philipson LH, Tamkun MM. Localization of the Kv1.5 K+ channel protein in explanted cardiac tissue. J Clin Invest. 1995;96:282-292.
Hondeghem LM, Snyders DJ. Class III antiarrhythmic agents have a lot of potential but a long way to go: reduced effectiveness and dangers of reverse use dependence. Circulation. 1990;81:686-690.
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.
Zeng J, Laurita KR, Rosenbaum DS, Rudy Y. Two components of the delayed rectifier K+ current in ventricular myocytes of the guinea pig type: theoretical formulation and their role in repolarization. Circ Res. 1995;77:140-152.
Fan Z, Hiraoka M. Depression of delayed outward K+ current by Co2+ in guinea pig ventricular myocytes. Am J Physiol. 1991;261:C23-C31.
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.
Jaeger JM, Gibbons WR. Slow inward current may produce many results attributed to IX1 in cardiac Purkinje fibers. Am J Physiol. 1985;249:H122-H132.