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(Circulation Research. 2001;89:807.)
© 2001 American Heart Association, Inc.

Integrative Physiology

Electrical Properties and Conduction in Reperfused Papillary Muscle

Wayne E. Cascio, Hua Yang, Timothy A. Johnson, Barbara J. Muller-Borer, John J. Lemasters

From the Departments of Medicine (W.E.C., H.Y., T.A.J., B.J.M.-B.) and Cell and Developmental Biology (J.J.L.), The University of North Carolina at Chapel Hill, Chapel Hill, NC.

Correspondence to Wayne E. Cascio, MD, University of North Carolina School of Medicine, Division of Cardiology, CB No. 7075, 349 Burnett-Womack Bldg, Chapel Hill, NC 27599-7075. E-mail wcascio{at}med.unc.edu

	Abstract

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The reversibility of ischemia-induced changes of extracellular K⁺ concentration ([K⁺]_o), resting membrane potential (E_M), and passive cable-like properties, ie, extracellular resistance and cell-to-cell electrical coupling, and their relationship to recovery of conduction and contraction is described in 25 reperfused rabbit papillary muscles. No-flow ischemia caused extracellular K⁺ accumulation, depolarization of E_M, an increase in whole-tissue (r_t), external (r_o), and internal (r_i) longitudinal resistances, and failure of conduction and contraction. Muscles were reperfused 10 minutes after the onset of ischemia related cell-to-cell electrical uncoupling, ie, 26±1 minutes after arrest of perfusion. In 11 muscles, incomplete reflow occurred with only partial recovery of [K⁺]_o and r_t. In the remaining 14 muscles, reperfusion caused a rapid and parallel decrease in [K⁺]_o, r_t, and r_o. When complete tissue reperfusion occurred, cell-to-cell electrical uncoupling was largely reversible. Thus, cell-to-cell electrical uncoupling did not indicate irreversible injury. Reperfusion induced a depolarizing current widening the difference between the K⁺ equilibrium potential and the E_M. This difference decreased after longer periods of reperfusion. Conduction was restored and conduction velocity approached preischemic values as cell-to-cell electrical interaction was reestablished and E_M recovered. The recovery of r_o preceded r_i, decreasing the ratio of the extracellular to intracellular resistance early in reperfusion, an effect predicted to influence the amplitude of the extracellular voltage field and electrocardiographic ST segments during reperfusion.

Key Words: ischemia • cable-like properties • cell-to-cell electrical coupling • extracellular potassium • conduction

	Introduction

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Increased extracellular and intracellular resistances contribute to slowing and failure of impulse propagation in ischemic myocardium.^1,2 The onset of cell-to-cell electrical uncoupling, as measured by a sudden increase of tissue resistance after 10 to 20 minutes of ischemia, is associated with the onset of ventricular fibrillation.^3–5 Yet, little is known about the temporal changes of cell-to-cell electrical coupling and extracellular resistance in postischemic reperfused heart and their relationship to the recovery of resting membrane potential (E_M) and impulse propagation. In excised ischemic canine heart "reperfused" by superfusion with an oxygenated solution, the space constant and conduction velocity were decreased relative to the preischemic values.⁶ Such changes in the space constant might be due to a persistently increased extracellular resistance, cell-to-cell electrical uncoupling, or decreased membrane resistance. Yet, only one other study¹ sought to define the time course of changes in passive cable-like properties in reperfused ischemic heart using a method capable of distinguishing changes in extracellular and intracellular resistance. However, cellular injury in the reperfused papillary muscle precluded the measurement of passive cable-like properties. These observations suggested that the onset of cellular uncoupling, which is temporally linked to ischemia-induced ionic and metabolic changes, herald the onset of irreversible injury. Thus, the purpose of this study was to investigate the time course of extracellular K⁺ washout, the recovery of E_M, the amplitude of the transmembrane action potential and extracellular voltage field, and passive cable-like properties, namely extracellular (r_o) and intracellular (r_i) longitudinal resistances in reperfused myocardium after cellular uncoupling was established during 20 to 30 minutes of myocardial ischemia. Subsequently, cell-to-cell electrical interaction, r_o, E_M, and action potential characteristics were then related to recovery of impulse propagation.

	Materials and Methods

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Papillary Muscle Preparation
New Zealand White rabbits (Robinson Inc, Clemmons, NC) of either sex weighing 2 to 3 kg were heparinized (200 U/kg, IV) and anesthetized with Na thiamylal (30 mg/kg, IV) or thiopental (40 to 50 mg/kg, IV) in compliance with guidelines established by the National Institutes of Health (Guide for the Care and Use of Laboratory Animals, NIH publication No. 85-23, revised 1996). A right ventricular papillary muscle of the rabbit was isolated and perfused via the septal artery,⁷ with an erythrocyte-free perfusate composed of Tyrode’s solution (in mmol/L: Na⁺ 149, K⁺ 4.5, Mg²⁺ 0.49, Ca²⁺ 1.8, Cl^- 133, HCO₃^- 25, HPO₄²⁺ 0.4, and glucose 20) plus insulin (1 U/L), heparin (400 U/L), albumin (2 g/L), and dextran (M_r 70 000; 40 g/L). The time elapsed between cross-clamping the aorta and perfusion was <5 minutes. Once perfused, the septum was secured in a Lexan chamber⁷ and perfused with the previous perfusate supplemented with bovine erythrocytes delivered by a peristaltic pump. The hematocrit level was 0.40. A papillary muscle was attached by its tendon to a piezoresistive element (SensoNor) (Figure 1). The preparation was enclosed in the chamber, surrounded by a humidified atmosphere, and maintained between 36.5°C and 37.5°C. A membrane gas exchanger controlled the PO₂ and PCO₂ of the perfusate⁷ and maintained the pH at 7.4. The septal artery pressure measured with a Millar transducer averaged 48±4 mm Hg at flow rates ranging from 0.9 to 1.8 mL/min/g tissue.

View larger version (43K): [in this window] [in a new window]   Figure 1. Experimental preparation. Left, Photomicrograph of a perfused papillary muscle suspended in an atmosphere. Subthreshold and threshold current pulses were applied through a platinum wire attached to the distal end of the muscle. Two silk-wick extracellular recording electrodes were positioned along the long axis of the muscle, designated Eo and Eo,ref. A K+-sensitive electrode was located near the reference electrode, Eo,ref. A floating glass microelectrode was referenced to Eo,ref. Bar=1 mm. Right, Representative data recorded during perfusion: a, subthreshold and excitatory current pulses; b, bipolar extracellular voltage recorded during the subthreshold current pulse, Vo, and during a propagated impulse, Vo; c, differentiated extracellular voltage signal from trace b used for measurement of the conduction times; d, transmembrane voltage recording, Vm; and e, isometric twitch tension. Note the difference in the time scales for panels a through e.

The PCO₂ and PO₂ in the chamber were measured using a gas analyzer. The PCO₂ surrounding the preparation was adjusted to match the PCO₂ of the perfusate. Displacing O₂ from the chamber and arresting flow produced myocardial ischemia. Forty-five seconds before the arrest of flow, the atmosphere in the chamber was changed from a mixture of CO₂ (34±5 mm Hg)/N₂ and O₂ (119±9 mm Hg) to a mixture of CO₂ (41±7 mm Hg) and N₂. During ischemia the PO₂ in the chamber was 6 mm Hg. Reperfusion was accomplished by restoring O₂ to the chamber followed by restoring arterial flow 45 seconds later.

Passive Cable-Like Properties and Conduction
Longitudinal whole-tissue resistance (r_t), intracellular (r_i), and extracellular (r_o) resistances were calculated using the voltage-ratio method.⁷ The distance between the extracellular electrodes averaged 1.8±0.1 mm (n=25). The ratio of the extracellular and intracellular longitudinal resistances, r_o:r_i=q, is the ratio of the absolute amplitudes of the extracellular bipolar electrogram, |V_o|, and the intracellular potential, |V_i|, as shown in Equation 1, where |V_m|-|V_o| equals |V_i|.

The extracellular and intracellular longitudinal resistances (r_o and r_i, in k/cm) were derived from Equations 2 and 3:

During reperfusion, the restoration of cell-to-cell electrical coupling was measured directly as a decrease in r_i. Longitudinal conduction velocity () was calculated by dividing the interelectrode distance by the conduction time, ie, the time between the maximum negative and positive slopes of the differentiated extracellular electrogram, V_o.

[K⁺]_o and Mechanical Properties
[K⁺]_o was measured at the muscle surface with a K⁺-selective electrode² positioned within 500 µm of the reference electrode (see Figure 1). The response of the K⁺-selective electrodes (n=25) was 56.6±0.5 mV per 10-fold change in [K⁺]_o. For the calculation of E_K, extracellular K⁺ activity was calculated by multiplying [K⁺]_o by 0.746.⁸ Isometric twitch and rest tension were measured with a piezoresistive force transducer.^2,7

Study Protocol
Reperfusion was initiated 10 minutes after the onset of cell-to-cell electrical uncoupling rather than after a fixed duration of ischemia. The second increase in r_t during ischemia arising from a plateau of variable duration was defined as the onset of cell-to-cell electrical uncoupling, thus assuring nearly complete cell-to-cell electrical uncoupling before reperfusion was initiated.

Statistics
Data are expressed as mean±SEM. All analyses used StatView (SAS Institute). Testing for differences among the two groups during normal arterial perfusion and reperfusion was conducted with an ANOVA with repeated measures. When the F statistic indicated significance, the Tukey-Kramer HSD method was used to test pairwise mean comparisons for statistical significance among the groups. P<0.05 was considered significant.

	Results

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Longitudinal Whole-Tissue Resistance, [K⁺]_o, and Vascular Resistance
During ischemia, r_t and [K⁺]_o increased.^1,2 When arterial flow was restored, 14 of 25 papillary muscles reperfused immediately as evidenced by an increase in the arterial pressure, a homogeneous expansion of the muscle, and a decrease in [K⁺]_o and r_t (Figure 2). Reperfusion did not occur in 8 muscles and appeared heterogeneous in 3 muscles, ie, perfusion had a patchy distribution. As shown in the Table, muscle diameter, rate of stimulation, E_M, tissue resistivity, transmembrane voltage, amplitude of the extracellular bipolar electrogram, isometric twitch tension, and did not differ between these muscles and the 14 that reperfused homogeneously.

View larger version (19K): [in this window] [in a new window]   Figure 2. A through C, [K+]o, rt, and pressure-to-flow ratio (P/F) for the septal artery. , Muscles reperfusing homogeneously (n=14); , muscles reperfusing poorly or not at all (n=11). *Significant differences between the two groups.

View this table: [in this window] [in a new window]   Table 1. Preischemic Characteristics of Papillary Muscles Subsequently Showing Homogeneous Reflow and No or Partial Reflow After Ischemia

Reperfusion of ischemic tissue expands the intravascular and interstitial compartments, diluting accumulated extracellular ions and metabolites. In this study, the time course of reperfusion was inferred from the washout of extracellular K⁺, ie, the fall in [K⁺]_o. Because r_o is sensitive to changes in the conductivity of the extracellular compartment,⁹ r_t is expected to be sensitive to the expansion of the intravascular and interstitial space. Consequently, the decline in [K⁺]_o and r_t during early reperfusion are expected to be linked. This relationship was assessed by measuring [K⁺]_o and r_t during the first 9 minutes of reperfusion in the 14 papillary muscles reperfusing homogeneously. The linear regression between r_t and [K⁺]_o fit the following equation: r_t (% of the preischemic value)=21.1 · [K⁺]_o+43.1, with a correlation coefficient of 0.889. The strength of this correlation was weakened by persistent cell-to-cell electrical uncoupling during the early minutes of reperfusion in some muscles (see subsequent discussion). These findings confirmed that the decrease in the r_t, like [K⁺]_o, is a sensitive indicator of reperfusion.

Reactive hyperemia occurred in the septal preparation after reperfusion (Figure 2). Among those muscles reperfusing rapidly and homogeneously, the ratio of the intra-arterial pressure to flow, an index of arterial resistance, was 70% of its preischemic value during the first 2 minutes of reperfusion. The intravascular resistance then increased throughout reperfusion and returned to normal after 15 minutes. A hyperemic response was not observed in those septal preparations reperfusing poorly. Restoration of normal intravascular resistance followed the same time course as recovery of V_o and r_o (see subsequent discussion).

Extracellular and Intracellular Resistances
Homogeneous macroscopic conduction returned in 14 perfused muscles, allowing the calculation of r_o and r_i. As shown in Figure 3, the ischemia-induced increase in r_o¹ reversed rapidly during reperfusion. In 12 of 14 muscles, r_o decreased to values lower than those measured before ischemia. The lowest values of r_o for each muscle averaged 78±8% (n=14) of the value present before ischemia and occurred 15±2 minutes after reperfusion. In some muscles, r_o increased steadily after this minimum value was attained resulting in a gradual increase of the average r_o present after 20 minutes of reperfusion.

View larger version (32K): [in this window] [in a new window]   Figure 3. Relative changes of ri () and ro () were measured in homogeneously reperfused muscles. *Significant difference for ri compared with values before ischemia.

During ischemia, r_i increased^1,2 and cell-to-cell electrical uncoupling was nearly complete, as indicated by a plateau in the r_t values in individual experiments. As shown in Figure 3, r_i, decreased during reperfusion indicating the restoration of cell-to-cell electrical coupling. Cell-to-cell electrical uncoupling caused by ischemia was largely reversible in all 14 reperfused muscles. In 11 of the 14 muscles, r_i recovered to 114±6% relative to the value before ischemia after 28 minutes of reperfusion. In 7 of these muscles, r_i decreased to 107±2% during the first 16±4 minutes of reperfusion and remained within 10% of preischemic values, ie, 101±3% throughout the remainder of reperfusion. In 4 muscles, r_i initially returned to 99±8% of the preischemic values after 10±3 minutes of reperfusion, but then slowly increased to 137±5% after 28 minutes. The remaining 3 muscles reperfused more slowly, and r_i recovered slowly from 322±66% at 6±3 minutes after the onset of reperfusion to 162±22% after 28 minutes, but never reached values measured before ischemia. In general, when compared with r_o, the recovery of r_i was delayed.

Transmembrane, Extracellular, and Intracellular Voltages
V_m, V_o, and V_i decreased during ischemia.¹ As shown in Figure 4, both V_m and V_o increased during early reperfusion relative to the final values recorded during ischemia and recovered to 86% and 72% of their preischemic values during the first 5 minutes of reperfusion. Neither V_m nor V_o recovered fully. After 28 minutes of reperfusion, V_m was 84±3 mV (n=11) or 88% of the preischemic value, and V_o was 41±3 mV or 80% of the preischemic value.

View larger version (23K): [in this window] [in a new window]   Figure 4. A, Plot assessing the mean change of the transmembrane voltage, Vm (), and the amplitude of the extracellular voltage field, Vo (), in 14 ischemic muscles after the onset of reperfusion, and in panel B, the corresponding changes, in percent, relative to the value during normal perfusion. *Significant difference for Vm and Vo compared with values before ischemia.

By contrast, V_i increased to 51±8 mV (n=6) after 3 minutes of reperfusion (Figure 5). This value represents 112% of the preischemic value. As reperfusion continued, V_i slowly returned to the value measured before ischemia as cell-to-cell electrical coupling was reestablished.

View larger version (20K): [in this window] [in a new window]   Figure 5. A, Plot shows the intracellular voltage, Vi, in papillary muscles during ischemia and reperfusion. B, Plot shows the corresponding values of q, the ratio of Vo and Vi. *Significant differences of q during reperfusion compared with preischemic values.

During ischemia, the ratio of V_o to V_i, q, decreased from 1.25±0.14 to 1.09±0.31. The partial recovery of V_o and the rapid recovery of V_i during the first 3 minutes of reperfusion decreased q to 0.79±0.16. Subsequently, q gradually recovered to values near the preischemic level after 28 minutes of reperfusion.

Potassium Equilibrium and Resting Membrane Potentials
During ischemia, E_K and E_M depolarized in parallel as extracellular K⁺ increased.¹⁰ After reperfusion, rapid washout of extracellular K⁺ was associated with the polarization of E_K, (Figure 6). In individual muscles, when a transient overshoot of the preischemic [K⁺]_o occurred, E_K hyperpolarized more than E_M as reported previously.^11,12 This hyperpolarization was not apparent in the averaged data because of the temporal variability of the response. E_M recovered only partially after reperfusion. Thus, the difference between E_K and E_M increased to values greater than those present before the onset of ischemia. The widening of the difference between E_M and E_K was greatest in papillary muscles in which [K⁺]_o fell rapidly.

View larger version (19K): [in this window] [in a new window]   Figure 6. A through C, Calculated EK, EM, and EK-EM, respectively, in papillary muscles during ischemia and reperfusion.

Cable-Like Properties, Resting Membrane Potential, and Conduction
Immediately after reperfusion, nonuniform conduction characterized by fragmented extracellular waveforms was observed in several preparations. Nonuniform conduction occurred at a time when r_t and [K⁺]_o were rapidly falling but still elevated. As shown in Figure 7, uniform planar impulse propagation returned rapidly and was present within 5 minutes in 12 of the 14 muscles. After 2 minutes of reperfusion, was 29±2 cm/s, or 64% of the preischemic value. As shown by the stippled line in Figure 7, the decrease in exceeded that predicted by the decrease of the series resistance, r_o+r_i. Thus, during early reperfusion, slowing of conduction was likely the consequence of persistent membrane depolarization, altered membrane excitability, and persistent electrical uncoupling. After 12 minutes of reperfusion, recovered to 37±3 cm/s or 82% of preischemic values. By this time, r_o had returned to the value present before ischemia, while r_i remained elevated slightly. After 12 minutes, the predominant factors decreasing were persistent membrane depolarization and electrical uncoupling. After 28 minutes, the remained slightly decreased at 42±3 cm/s or 91% of the preischemic value.

View larger version (23K): [in this window] [in a new window]   Figure 7. A, Plot shows the time course of changes in longitudinal in reperfused muscles. The stippled line represents the conduction slowing expected because of changes of ro and ri based on linear cable theory, where is inversely related to the square root of the series resistance (ro+ri).36 B, Active isometric twitch tension () and the change of rest tension () are shown for papillary muscles during ischemia and reperfusion. *Significant difference for twitch tension compared with the value present before ischemia.

Recovery of Mechanical Properties
During ischemia, contractile force decreased and resting tension increased (Figure 7). Immediately after reperfusion, contraction was restored and contractile force increased to 85% of its preischemic value within 5 minutes. Subsequently, after 28 minutes of reperfusion, the contractile force decreased to 38% of the value present before ischemia. While resting tension decreased after reperfusion, it did not fully recover to the values present before ischemia.

	Discussion

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This study shows that cell-to-cell electrical uncoupling induced by ischemia is reversible after reperfusion. Moreover, an abrupt and rapid decrease of r_o and r_t during reperfusion is temporally associated with the expansion of the vascular space and washout of accumulated extracellular K⁺. During reperfusion, r_o decreases beyond values measured before ischemia, and probably relates to interstitial edema caused by an ischemia/reperfusion-induced microvascular injury or endothelial dysfunction, analogous to that observed with hypoxia.¹³ In contrast to rapid changes in r_o, r_i recovers more slowly. The difference in the rates of recovery of r_o and r_i in reperfused tissue leads to a change in the ratio, r_o/r_i, that is lower than that measured in the muscle before ischemia. This ratio is an important determinant of extracellular and intracellular current flow in the heart, and the amplitude of the corresponding extracellular and intracellular voltages, ie, V_o and V_i. As such, an altered ratio is predicted to have implications regarding the registration of the ST segment on the surface ECG in reperfused heart. Early after reperfusion the difference between E_K and E_M increased as E_K rapidly repolarized with the washout of accumulated extracellular K⁺ without a commensurate repolarization of the resting membrane potential.

Importantly, cell-to-cell electrical uncoupling during ischemia was not an absolute marker of irreversible cellular injury. On the contrary, cell-to-cell electrical coupling was restored in 14 of 25 reperfused muscles. Despite complete recovery of E_K during early reperfusion, E_M repolarized more slowly, a finding consistent with the activation of a depolarizing current. Impulse propagation recovered as extracellular K⁺ was washed away and E_M approached values present before ischemia. Contractile strength peaked immediately after reperfusion and nearly reached preischemic values before decreasing to 40% of the values present before ischemia. This transient increase in contraction occurred when E_M, r_i, , and intra-arterial pressure were not fully recovered, indicating that this transient recovery of contraction before onset of myocardial stunning was at least partially independent of the recovery of these parameters. This finding is consistent with an increase of Ca²⁺_i during this period.¹⁴

Expansion of the Extracellular Space and Extracellular Resistance
The accumulation of K⁺ in the extracellular compartment is a hallmark of no-flow ischemia, whereas the rapid washout of accumulated extracellular K⁺ is an unequivocal indicator of reperfusion. Similarly, the early increase of r_t in ischemic myocardium is caused by a rise in r_o attendant to the collapse of the vasculature and interstitial space after the arrest of coronary flow.^1,7 Early during reperfusion, expansion of the intravascular and interstitial space decreases r_o and r_t. As shown in this study, the strong positive correlation between the parallel decrease of [K⁺]_o and r_t during the first few minutes of reperfusion supports the use of a decrease of r_t as a reliable indicator of tissue reperfusion.

During reperfusion, r_o falls progressively, reaching values 22% less than values measured before ischemia. Because the magnitude of r_o is known to be sensitive to an increase in interstitial water,⁹ the reduction of r_o is probably related to increased permeability of the vascular endothelium promoting accumulation of extracellular fluid^15,16 and interstitial edema. The gradual rise in r_o during later reperfusion may indicate partial resolution of interstitial edema or an increase in intracellular volume with compression of the extracellular compartment.¹⁶

The consequences of the reduced r_o during early reperfusion of an ischemic heart cannot be addressed by our data. However, a lower r_o provides a reduced resistance to extracellular excitatory current flow, thereby diminishing the extracellular voltage field and possibly facilitating conduction. Alternatively, the low resistance might shunt excitatory current into a larger extracellular compartment, thereby decreasing excitatory current density and contributing to conduction block. Because current flow is scaled by r_o/r_i, injury current and defibrillation current are expected to partition into the extracellular and intracellular compartments differently in normal myocardium, when compared with ischemic and reperfused tissue. These changes in voltage distribution may contribute to spatial and temporal heterogeneity of membrane potentials and increase the likelihood of arrhythmia.

Potassium Equilibrium and Resting Membrane Potentials
As shown previously, during normal arterial perfusion, E_K was more negative than E_M and the difference between E_K and E_M narrowed, as the membrane became more permeable to K⁺ during ischemia.¹⁰ After the onset of reperfusion, E_K and E_M diverged, confirming an observation made previously in reoxygenated Purkinje fibers.¹¹ Hyperpolarization of E_K in association with the transient undershoot of [K⁺]_o was described previously in whole hearts.¹² The undershoot of [K⁺]_o and E_K hyperpolarization were greatest in papillary muscles that reperfused rapidly. Slower reperfusion attenuated the divergence of E_K and E_M. Simultaneous shortening of the action potential (data not shown) and the failure of E_M to polarize with E_K suggests an inward current. Possible sources for such a current may be related to a Ca²⁺-activated current,¹⁷ a volume-regulated Cl^- current,^18,19 or nonspecific cationic current²⁰ related to reperfusion-induced cell swelling. Other possibilities include a depolarizing Na⁺-Ca²⁺ exchange current²¹ or a free radical–activated nonselective cationic current.²² Regardless of the mechanism, the failure of E_M to recover rapidly had significant effects on as discussed next.

Recovery of Cell-to-Cell Electrical Coupling
Cell-to-cell electrical uncoupling was reversed in the majority of reperfused papillary muscles. Thus, cellular uncoupling during ischemia was not an absolute marker for irreversible injury. Nevertheless, 11 of 25 muscles reperfused incompletely or not at all. Thus, the ionic and energetic changes temporally related to the onset of cellular uncoupling might be closely coupled to mechanisms initiating microvascular injury, increased permeability of the vascular, and the no-reflow phenomenon.

During reperfusion, the restitution of cellular electrical coupling, albeit rapid, lagged behind the decrease in r_o. Within 5 minutes, r_i decreased from 299% to 156% of the value before ischemia and remained relatively unchanged between 5 and 12 minutes, thereafter slowly approaching values present before ischemia between 20 and 30 minutes after reperfusion. The rapid recovery of cell-to-cell electrical coupling during reperfusion is difficult to explain given our present understanding of the factors modulating gap junctional conductance, namely Ca²⁺, H⁺, and lipid metabolites. The time course of recovery from ischemia-induced Ca²⁺_i loading during reperfusion in the papillary muscle preparation is not known with certainty, but Ca²⁺_i is likely to increase during early reperfusion.²³ Indeed, Ca²⁺_i loading attendant to reperfusion may account for the increased contractile force measured during early reperfusion. Although ischemia-induced intracellular acidosis recovers from pH 6.4 to 7.0 during the first 10 minutes of reperfusion in the isolated papillary muscles (unpublished data, 1997, courtesy of Dr B. Muller-Borer), and long-chain acyl carnitines are washed from reperfused tissue quickly,²⁴ the time course of pH recovery and loss of acyl carnitine is slow compared with the recovery of cell-to-cell coupling. Therefore, explaining the rapid recovery of cell-to-cell electrical coupling is difficult if the mechanism is related solely to the decrease of cytosolic Ca²⁺ and H⁺ and the loss of lipid metabolites. Recently, Beardslee et al²⁵ showed that the timing of cell-to-cell electrical uncoupling correlated to the appearance of dephosphorylated connexin43 and translocation of connexin43 to intracellular pools. Rapid phosphorylation of connexin43 and translocation of connexin43 from intracellular pools to the gap junctions during early reperfusion might restore cellular coupling. Further work is needed to test this hypothesis as well as evaluate the effects other important metabolites and signaling molecules on gap junctional conductance during reperfusion.

Amplitude of the Transmembrane Action Potential and the Extracellular Voltage Field
Despite resting membrane depolarization, V_m and V_o were unexpectedly high during early reperfusion, an observation most likely reflecting persistent cell-to-cell electrical uncoupling and higher excitatory current density. As recognized by Kléber et al,¹ changes in the ratio of the intracellular to extracellular resistance (r_i/r_o) are predicted to scale the amplitude of the surface ECG. The gradual decline in r_o after reperfusion and the attendant decrease of V_o is anticipated to cause a decrease of voltage on the surface ECG. This observation may account for the loss of R-wave amplitude in ECG electrodes overlying the reperfused myocardium. Such changes have implications regarding the interpretation of the recovery of ST elevation during reperfusion. Loss of ST-segment elevation is generally assumed to represent recovery from ischemia and injury. Our data are in agreement with this concept, but additionally our data suggest that ST elevation might decrease because of a fall in tissue resistance without complete resolution of the ischemia-induced sarcolemmal depolarization.

Relation of [K⁺]_o, Resting Membrane Potential, Conduction, and Cellular Coupling to Recovery of Contractile Function
Linear cable theory accurately describes impulse propagation in the normal and ischemic papillary muscles. Likewise, changes of r_o, r_i, and E_M measured in the reperfused muscle produce predictable changes in .¹ Thus, predictions of cable theory remain valid during the early phase of reperfusion showing that recovery of membrane polarization, restoration of cell-to-cell electrical coupling, and the fall of r_o are sufficient to account for the recovery of .

The initial recovery of isometric twitch tension during reperfusion was temporally related to the recovery of [K⁺]_o, E_M, cell-to-cell electrical coupling, and conduction. The force of contraction increased from 0% to 85% of its preischemic value after 5 minutes of reperfusion. The coincident increase in resting tension implicates a mechanism related to Ca²⁺_i loading. Subsequently, twitch tension decreased, despite further recovery of cell-to-cell electrical coupling, r_o, intra-arterial perfusion pressure, and E_M consistent with the development of myocardial stunning.

Relevance to Ventricular Arrhythmias
Although reperfusion is essential to prevent irreversible cellular injury and preserve ventricular function, reperfusion and the attendant recovery from ischemia-induced metabolic, ionic, and electrical changes cause ventricular arrhythmias,²⁶ cellular injury,²⁷ and sudden death.²⁸ In animal models, reperfusion arrhythmias occur after brief episodes of ischemia, ie, <10 minutes,^12,29 but are most common when the preceding ischemia lasts for 20 to 30 minutes.^30,31 The increased incidence of reperfusion arrhythmias after 20 to 30 minutes of ischemia may relate to the rapid reexpansion of the extracellular compartment, washout of the extracellular K⁺ and H⁺, and the attendant recovery of excitability in the presence of persistent cellular uncoupling.^1,2,24,32 The persistent cellular uncoupling and concomitant recovery of excitability are anticipated to enhance electrical discontinuities^33–35 and contribute to heterogeneity of electrical properties, slowing of impulse conduction, the propagation of delayed afterdepolarizations, and the initiation of arrhythmia.

In conclusion, our data support the hypothesis that cell-to-cell electrical interaction can be restored during reperfusion, yet the slow time course for regaining cellular coupling contributes to slowing of impulse propagation. Persistent membrane depolarization, despite the recovery of E_K, also contributes to impairment of conduction. The more rapid recovery of r_o relative to r_i leads to a reduction in r_o/r_i and has implications regarding the scaling of the extracellular voltage field, the registration of the ECG, and the flow of injury current in reperfused myocardium.

	Acknowledgments

 
The work was supported by P01 HL27430, RO1 HL48769, and T32 HL07793 from the National Heart, Lung, and Blood Institute, Bethesda, Md, and N00014-96-0283 from the Office of Naval Research, Bethesda, Md.

Received April 23, 2001; revision received September 4, 2001; accepted September 4, 2001.

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