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(Circulation. 1999;100:2455.)
© 1999 American Heart Association, Inc.

Basic Science Reports

Downregulation of Delayed Rectifier K⁺ Currents in Dogs With Chronic Complete Atrioventricular Block and Acquired Torsades de Pointes

Paul G. A. Volders, MD, PhD; Karin R. Sipido, MD, PhD; Marc A. Vos, PhD; Roel L. H. M. G. Spätjens, BS; Jet D. M. Leunissen; Edward Carmeliet, MD, PhD; Hein J. J. Wellens, MD, PhD

From the Department of Cardiology (P.G.A.V., M.A.V., R.L.H.M.G.S., J.D.M.L., H.J.J.W.), Cardiovascular Research Institute Maastricht (E.C.), Maastricht University, the Netherlands, and the Laboratory of Experimental Cardiology, University of Leuven, Belgium (K.R.S.).

Correspondence to Paul G.A. Volders, MD, PhD, Department of Cardiology, Cardiovascular Research Institute Maastricht, Academic Hospital Maastricht, PO Box 5800, 6202 AZ, Maastricht, Netherlands. E-mail p.volders{at}cardio.azm.nl

	Abstract

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Background—Acquired QT prolongation enhances the susceptibility to torsades de pointes (TdP). Clinical and experimental studies indicate ventricular action potential prolongation, increased regional dispersion of repolarization, and early afterdepolarizations as underlying factors. We examined whether K⁺-current alterations contribute to these proarrhythmic responses in an animal model of TdP: the dog with chronic complete atrioventricular block (AVB) and biventricular hypertrophy.

Methods and Results—The whole-cell K⁺ currents I_TO1, I_K1, I_Kr, and I_Ks were recorded in left (LV) and right (RV) ventricular midmyocardial cells from dogs with 9±1 weeks of AVB and controls with sinus rhythm. I_TO1 density and kinetics and I_K1 outward current were not different between chronic AVB and control cells. I_Kr had a similar voltage dependence of activation and time course of deactivation in chronic AVB and control. I_Kr density was similar in LV myocytes but smaller in RV myocytes (-45%) of chronic AVB versus control. For I_Ks, voltage-dependence of activation and time course of deactivation were similar in chronic AVB and control. However, I_Ks densities of LV (-50%) and RV (-55%) cells were significantly lower in chronic AVB than control.

Conclusions—Significant downregulation of delayed rectifier K⁺ current occurs in both ventricles of the dog with chronic AVB. Acquired TdP in this animal model with biventricular hypertrophy is thus related to intrinsic repolarization defects.

Key Words: electrophysiology • ventricles • torsades de pointes • myocytes • ions

	Introduction

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Acquired QT prolongation is a common ECG finding in heart disease, but it can also be present as a primary derangement in the absence of structural abnormalities or drugs.¹ It is regarded as 1 of the risk factors of ventricular arrhythmias and sudden cardiac death.¹ Clinical studies on the acquired long-QT syndromes indicate an enhanced susceptibility to polymorphic ventricular tachycardias, notably torsades de pointes (TdP).² Unlike for the congenital long-QT syndromes, in which distinctive defects of ion-channel function in myocardial cells cause delayed repolarization,³ the ionic basis of acquired QT prolongation not related to drugs is poorly understood.

In recent years, we have used the dog with chronic complete atrioventricular block (AVB) as a model for the study of TdP.⁴ QT intervals and ventricular endocardial monophasic action potential durations (APDs) are much longer than expected on the basis of the bradycardia alone and point toward a disturbed ventricular repolarization,⁵ corresponding to clinical findings on acquired AVB.⁶ Anesthetized dogs show an enhanced susceptibility to acquired TdP after several weeks of AVB duration, which is associated with the development of increased interventricular dispersion of repolarization and early afterdepolarizations.⁵ Electrical remodeling is accompanied by the development of biventricular hypertrophy.^{5 7} Approximately 15% of the chronic-AVB dog population dies suddenly, often during circumstances of excitement (eg, during feeding or ambulation). Transmembrane action potential recordings in isolated myocytes indicate that the prolonged ventricular repolarization of chronic AVB is an intrinsic abnormality, which is amplified by class III antiarrhythmic drugs.⁷ In addition, action potential prolongation is more pronounced in left (LV) than right (RV) ventricular myocytes, supporting the in vivo finding of increased regional dispersion of repolarization.

In our search for the ionic mechanisms of electrical remodeling and proarrhythmia in dogs with chronic AVB, we investigated the possible contribution of K⁺-current alterations to ventricular action potential prolongation and increased regional dispersion of repolarization. To this end, we measured whole-cell K⁺ currents in midmyocardial cells of animals with documented TdP and directly compared LV and RV myocytes in the same hearts.

	Methods

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Animal handling was in accordance with the Dutch Law on Animal Experimentation and the European Directive for the Protection of Vertebrate Animals Used for Experimental and Other Scientific Purposes (86/609/EU). The experiments were approved by the Committee for Experiments on Animals of our university.

In Vivo Studies
Twenty-four adult mongrel dogs of either sex and weighing between 22 and 37 kg were used for the experiments. For a complete description of the creation of AVB (n_dogs=15), the perioperative care, and the definition of TdP, we refer to a previous publication.⁵ Anesthesia was induced by sodium pentobarbital 20 mg/kg IV; subsequently, the dogs were ventilated with a mixture of oxygen, nitrous oxide, and halothane (vapor concentration 0.5% to 1.0%). To test the induction of TdP, the class III agent almokalant (0.12 mg · kg^-1 · 10 min^-1 IV) was used.⁸ These experiments were performed during anesthesia 1 week before the dogs were euthanized for cell isolation.

Cellular Experiments
Anesthetized chronic-AVB (9±1 weeks) and control dogs (with sinus rhythm; n_dogs=9) received intravenous heparin on thoracotomy. The hearts were quickly excised and washed in cold cardioplegic solution. Heart weights of chronic-AVB dogs were significantly greater than those of control dogs: 306±12 versus 220±8 g, respectively (P<0.05). When corrected for body weight, this difference remained significant: 11.6±0.3 versus 8.6±0.3 g/kg (P<0.05).

To isolate single LV and RV midmyocardial cells simultaneously, the left anterior descending and right coronary arteries were cannulated. The isolation procedure was the same as recently described.⁹ Whole-cell currents were measured with patch pipettes (borosilicate glass) with resistances of 1.0 to 3.0 M when filled with internal solution. Experiments were performed at 37±0.5°C. Cell capacitance was measured by hyperpolarizing steps from a holding potential of -60 mV. In the LV myocytes, average values were 216±9 pF (n_{Chronic AVB}=35) versus 227±11 pF (n_Control=27; P=NS). In RV myocytes, capacitances were 221±12 pF (n_{Chronic AVB}=28) versus 228±11 pF (n_Control=29; P=NS). Length times width of these myocytes was, on average, LV_{Chronic AVB}=186x36 µm versus LV_Control=193x33 µm (P=NS); RV_{Chronic AVB}=204x35 µm versus RV_Control=192x36 µm (P=NS). Compared with a large population study indicating hypertrophy of the individual myocytes in chronic AVB,⁷ the cells used in this study represent the larger bin size.

For the measurements of I_TO1, we used a holding potential of -70 mV. Na⁺ current was inactivated by a 10-ms prepulse to -40 mV. L-type Ca²⁺ current was inhibited with nifedipine 5 µmol/L. I_TO1 amplitudes were measured as peak amplitudes minus steady-state values at the end of depolarizations. For the measurements of I_K1, I_Kr, and I_Ks, the holding potential was set at -50 mV. I_Kr and I_Ks were measured as the peak tail currents on repolarization. For I_Kr, we used the tail current sensitive to 2 µmol/L almokalant (specific I_Kr blocker) on repolarization to the holding potential of -50 mV. For I_Ks, almokalant-resistant tail currents were used.

The standard buffer solution used for the experiments contained (in mmol/L) NaCl 145, KCl 4.0, CaCl₂ 1.8, MgCl₂ 1.0, NaH₂PO₄ 1.0, glucose 11, and HEPES 10; pH was adjusted to 7.4 with NaOH at 37°C. The patch-pipette solution contained (in mmol/L) potassium aspartate 125, KCl 20, MgCl₂ 1.0, MgATP 5, HEPES 5, and EGTA 10; pH was adjusted to 7.2 with KOH.

The data are expressed as mean±SEM. Intergroup comparisons were made with ANOVA (for multiple comparisons) and with Student’s t test for unpaired and paired data groups, after testing for the normality of distribution. Differences were considered significant if P<0.05.

	Results

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Acquired QT Prolongation and Susceptibility to TdP in the Study Group
In vivo electrophysiological recordings revealed significant QT prolongations in all chronic-AVB dogs. On average, the QT time (measured in ECG lead II) increased from 241±9 ms at sinus rhythm to 316±14 ms on creation of AVB (P<0.05) and to 374±15 ms in the chronic phase at 9±1 weeks thereafter (P<0.05 versus acute AVB). Corresponding RR intervals measured 488±24, 1479±142 (P<0.05), and 1381±80 ms, respectively (P=NS versus acute AVB). An example of these 3 states in the same animal is shown in Figure 1. From acute to chronic AVB, QRS widths did not change significantly (see Figure 1). However, broad-based T waves developed early after the QRS complex. In 13 of the 15 animals with chronic AVB, LV and RV endocardial monophasic action potentials were recorded at the idioventricular cycle length. In all 13, the LV APD was longer than the RV APD, on average 419±25 versus 354±19 ms (P<0.05), yielding an interventricular difference of 65±12 ms (in line with previous results).^{5 8} In 6 of 7 animals tested with almokalant, TdP occurred.

View larger version (60K): [in this window] [in a new window]   Figure 1. Acquired QT prolongation in a dog with AVB. Shown are ECG leads II and aVF and simultaneous monophasic action potential recordings from LV (LV MAP) and RV endocardium (RV MAP). During sinus rhythm (1 Control), cycle length is 585 ms and QT time 280 ms. LV and RV MAP durations measure 230 and 220 ms, respectively. During acute AVB (2 AAVB), idioventricular cycle length is 1570 ms. There is a minimal QT prolongation to 285 ms, and LV and RV MAP durations are 310 and 290 ms. Right (3 CAVB, where C indicates chronic), effect of 6 weeks of AVB on electrophysiological parameters is obvious: at a similarly slow rate (cycle length 1460 ms), QT time has increased significantly to 580 ms with MAP prolongation to 540 and 430 ms in LV and RV, leading to an interventricular difference of 110 ms. Humps in repolarization of LV MAP in CAVB indicate presence of small early afterdepolarizations. TdP occurred spontaneously in this animal during administration of almokalant.

In the sinus rhythm control group used for comparisons in the cellular investigations, the RR interval was 421±22 ms, with a QT time of 231±6 ms (both P=NS versus preoperative sinus rhythm in the chronic-AVB group).

I_TO1 Is Not Altered in Chronic AVB
The activation of I_TO1 was tested during steps of -40 to +70 mV. In all 4 cell types, the current activated at a test voltage (V_test) -20 mV (Figure 2). There was no difference in I_TO1 density between chronic AVB and control in either ventricle, and the normal interventricular difference⁹ was maintained. I_TO1 was nearly completely inhibited by 5 mmol/L 4-aminopyridine in all 4 cell types. I_TO1 inactivation during the 300-ms V_test was best fitted with a single exponential function yielding similar time constants (range, 7 to 14 ms) in chronic AVB and control. As illustrated in Figures 3A and 3B, voltage dependence of steady-state inactivation and time-dependent recovery from inactivation were not different between the cell types.

View larger version (15K): [in this window] [in a new window]   Figure 2. ITO1 activation in chronic AVB. Shown are families of current traces during Vtest of -20 to +70 mV (300 ms; interval 10 seconds) in 2 RV myocytes (capacitances, 264 and 247 pF in chronic AVB and control, respectively). Horizontal bars on left indicate 0-pA level. Bottom, Current density-voltage relations for LV (nChronic AVB=17, nControl=10) and RV (nChronic AVB=16, nControl=12).

View larger version (15K): [in this window] [in a new window]   Figure 3. A, Voltage-dependence of steady-state inactivation of ITO1. Left, Family of traces in a chronic-AVB LV cell. Right, Fractions of ITO1 relative to amplitude at Vtest of +60 mV for a Vcond of -100 mV in LV (nChronic AVB=8, nControl=9) and RV (nChronic AVB=4, nControl=6). Curves are Boltzmann fits to data. Half points and slope factors are similar in 4 groups. B, Time-dependent recovery from inactivation. Left, Original traces with interpulse delays (t) of 5 to 50 ms (5-ms increments), 60 to 180 ms (20-ms increments), and 2000 ms in a chronic-AVB LV myocyte. Middle, Voltage-clamp protocol indicates prepulse to +60 mV followed by Vtest to +60 mV at increasing t. Right, Fractions of ITO1 relative to amplitude in prepulse as a function of t in LV (nChronic AVB=6, nControl=8) and RV (nChronic AVB=3, nControl=7). Horizontal bars on left in original traces indicate 0-pA levels.

Properties of I_K1
Figure 4 shows typical recordings of I_K1 in chronic-AVB and control myocytes, as well as current density-voltage relations at the end of V_test of -140 to -20 mV in both LV and RV. The current was fully inhibited in K⁺-free superfusate (0 [K⁺]_o; not shown). I_K1 outward currents at V_test positive to the K⁺ reversal potential were similar in chronic AVB and control. Only at very hyperpolarizing pulses in RV cells were current densities less negative in chronic AVB. There were no interventricular differences between chronic AVB and control.

View larger version (15K): [in this window] [in a new window]   Figure 4. IK1 in chronic AVB. Examples are from LV myocytes and show currents during Vtest of -70 to -140 mV (200 ms; interval 3 seconds). Bottom, Current density-voltage relations for IK1 steady-state activation in LV (nChronic AVB=8; nControl=5) and RV (nChronic AVB=10; nControl=9). In RV, * indicates significant difference (P<0.05).

Downregulation of I_Ks and I_Kr
To dissect the 2 components of the delayed rectifier K⁺ current at baseline, we applied a protocol in which, after a depolarization to +30 mV, repolarization was divided into 2 steps: to 0 mV for 4.5 seconds and then back to -50 mV (Figure 5).¹⁰ Tail currents at the first repolarization step (to 0 mV) were insensitive to the I_Kr blocker almokalant (Figure 5, left arrow in both panels), indicating that they were largely composed of deactivating I_Ks and largely devoid of I_Kr. Tail currents at the second repolarization step (to -50 mV) decreased significantly on almokalant administration, indicating the opposite: namely, that they were composed largely of deactivating I_Kr with a much smaller contribution of I_Ks than at 0 mV (Figure 5, right arrow in both panels). From these data, it appeared that I_Ks was smaller in chronic-AVB than control myocytes: for LV, 0.19±0.02 versus 0.40±0.09 pA/pF, and for RV, 0.21±0.03 versus 0.51±0.08 pA/pF, respectively (both P<0.05). For the same comparison, I_Kr was similar in LV, 0.34±0.03 versus 0.42±0.05 pA/pF (P=NS), but smaller in RV myocytes, 0.34±0.02 versus 0.48±0.02 pA/pF, in chronic AVB versus control, respectively (P<0.05). To elaborate further on the contribution of I_Ks to total delayed rectifier K⁺ current, we performed envelope-of-tails tests at baseline and in the presence of almokalant. These experiments revealed that I_Ks made up a significant portion of the total current, even for depolarizations as short as 300 ms. However, contributions were much smaller in chronic-AVB than control cells (data not shown).

View larger version (11K): [in this window] [in a new window]   Figure 5. Activation and deactivation of delayed rectifier K+ current. Activation during a 3-second depolarization to +30 mV, followed by deactivation phases at 0 mV for 4.5 seconds and at -50 mV in 2 LV cells (capacitances: chronic AVB, 292 pF and control, 299 pF). In both panels, top current trace is baseline recording, and bottom trace is current during almokalant a few intervals later (interval 20 seconds). Tail currents at first repolarization step (left arrow) are unaltered during almokalant, whereas at second repolarization step (right arrow), they are decreased significantly. In chronic AVB vs control, tail-current amplitudes at first repolarization step are markedly lower in chronic AVB, indicating lower IKs. Similar results were obtained for LV in nChronic AVB=8 and nControl=11 and for RV in nChronic AVB=11 and nControl=16.

Voltage-dependent activation of I_Ks was evaluated from tail currents on repolarization to -25 mV in 0 [K⁺]_o in the presence of almokalant. Examples of families of current traces and pooled data are given in Figure 6. There was no saturation of tail-current amplitudes. Voltage-dependence of I_Ks activation was similar in the 4 cell types. However, I_Ks density was significantly smaller in chronic AVB than in control. Overall, tail currents were reduced by 50% in LV and by 55% in RV myocytes (ie, average percentage of tail-current differences for the conditioning voltages indicated by asterisks [P<0.05] in Figure 6). Whereas interventricular differences of I_Ks exist in normal canine hearts (Figure 6),⁹ because of the small current amplitudes in chronic AVB, no differences could be discerned. Voltage dependence of I_Ks deactivation was not different between chronic AVB and control. Likewise, the time course of deactivation proved similarly fast in all 4 cell types.

View larger version (13K): [in this window] [in a new window]   Figure 6. IKs in chronic AVB. Shown are currents for depolarizations to -20 to +60 mV (3 seconds; 20-mV increments; 20-second intervals), followed by repolarizations to -25 mV in 0 [K+]o superfusate with 2 µmol/L almokalant in 2 LV myocytes (capacitances: 213 and 251 pF in chronic AVB and control, respectively). Horizontal bars on left indicate 0-pA level. Bottom, Activation for IKs tail currents on repolarization to -25 mV in LV (nChronic AVB=12, nControl=5) and RV (nChronic AVB=9, nControl=9). In both ventricles, * indicates significant difference (P<0.05).

I_Kr was quantified as the almokalant-sensitive tail-current portion measured by digital subtraction on single-step repolarizations to -50 mV in 4.0 mmol/L [K⁺]_o (Figure 7). Activation occurred after depolarizations -10 mV and showed saturation at conditioning voltages (V_cond) > +20 mV. Boltzmann fits to the data revealed half points of 0.3±1.1 and -1.1±3.1 mV in LV and of -2.5±2.1 and 1.1±1.7 mV in RV for chronic AVB and control, respectively (both P=NS). Corresponding slope factors were 5.8±1.0 and 5.7±2.8 mV in LV and 5.5±1.7 and 5.5±1.5 mV in RV (both P=NS). I_Kr density was similar in LV myocytes but smaller in RV myocytes of chronic AVB versus control (reduction of 45%; Figure 7). There were no interventricular differences. In both ventricles, voltage dependence and time course of I_Kr deactivation were similar for chronic AVB and control.

View larger version (14K): [in this window] [in a new window]   Figure 7. IKr in chronic AVB. Top, IKr is evaluated as almokalant-sensitive difference current after digital subtraction of baseline current and current remaining during treatment with 2 µmol/L almokalant. Tail currents are measured on repolarization to -50 mV (arrows) and are plotted as a function of Vcond in bottom panels. In LV, nChronic AVB=18 and nControl=8. In RV, nChronic AVB=14 and nControl=13; * indicates significant difference (P<0.05). Horizontal bars on left, 0-pA level.

	Discussion

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We have demonstrated that acquired QT prolongation in dogs with chronic AVB and documented TdP is associated with significant reductions of I_Ks in the LV and RV. In the RV of these animals, I_Kr is also downregulated. Of the other K⁺ currents, I_TO1 is unaltered, and so is I_K1 in the physiological range of voltages.

Reproducible Induction of TdP in Chronic AVB
The dog with chronic AVB is a very suitable large-animal model for the study of TdP. In vivo experiments indicate the critical importance of regional dispersion of repolarization, early afterdepolarizations, and multiple ectopic beats for the initiation of TdP.⁸ Interventricular dispersion during arrhythmogenesis probably reflects the existence of significant repolarization gradients in closely adjacent areas, possibly the septum.¹¹ Our most recent results demonstrate that electrical remodeling of the myocardium is the substrate for the enhanced susceptibility to TdP.^{5 7} The in vivo part of the present study confirmed that the dogs used for cellular experiments were a representative population with significant QT prolongation and a low threshold for TdP.

Contribution of Reduced I_Ks and I_Kr to Action Potential Prolongation
In a previous study, at serial testing in vivo, it was found that chronic (versus acute) AVB leads to increases of endocardial monophasic APDs of +30% in the LV and +20% in the RV.⁵ On the basis of our recent microelectrode study,⁷ the relative increase of the APD in chronic-AVB myocytes is 10% to 30% in the LV (depending on the pacing cycle length) and maximally 10% in the RV under baseline conditions. In these cells, the class III agents almokalant and d-sotalol cause marked action potential prolongation and the occurrence of early afterdepolarizations, which uncovers the importance of I_Kr for ventricular repolarization in chronic AVB. Whether a 50% to 55% reduction of I_Ks can account for the action potential characteristics in LV and RV can only be answered indirectly. In a recent study on canine LV midmyocardial tissue, the I_Ks blocker chromanol 293B at 30 µmol/L increases the APD from 284±13 to 357±25 ms at a pacing cycle length of 2000 ms (+25%).¹² This concentration reduces I_Ks by 80% in guinea pig myocytes.¹³ In a theoretical model of the normal human ventricular action potential,¹⁴ adapted from the Luo-Rudy model,¹⁵ 50% inhibition of I_Ks increases the APD from 374 to 425 ms (+15%; Figure 12 of Reference ¹⁴ ). Thus, at rough approximation, the 50% to 55% decrease of I_Ks observed in our present study could account for a 10% to 30% action potential prolongation, at least in the LV. The RV exhibits only minor action potential alterations under baseline conditions, despite its additional downregulation of I_Kr (-45%). To explain why the alterations are less pronounced in the RV than the LV, we have to consider that the basic determinants of repolarization are different between the 2 ventricles. In the normal canine heart, I_TO1 and I_Ks are much larger in the RV, indicating a larger repolarization reserve.⁹ In chronic AVB, the difference in I_TO1 is maintained, and it is likely that other membrane currents and/or their remodeling are also responsible for the amplification of interventricular action potential inhomogeneities. We are currently investigating 2 candidates: the L-type Ca²⁺ current and Na⁺-Ca²⁺ exchange.¹⁶

Ionic Remodeling in Chronic AVB
Cardiac function of dogs with AVB of 9 weeks’ duration is unimpaired, which is confirmed at the myocyte level.^{5 16} Most animals have an enhanced contractile performance at the imposed bradycardia and lack signs of heart failure. Significant growth responses (only of cell length) are observed in both RV (+23%) and LV (+13%) myocytes,⁷ whereas autopsy findings reveal increased RV and LV tissue weights.⁵ These data strongly support the contention that dogs with long-standing (weeks to months) AVB have a compensated form of biventricular hypertrophy.¹⁷

In many other animal models and in humans with cardiac hypertrophy or failure, downregulation of K⁺ currents has been implicated in (inhomogeneous) ventricular action potential prolongation¹⁸ and the increased risk of ventricular arrhythmias and sudden cardiac death.¹⁹ Reduction of I_TO1 is probably most often observed in the spectrum of early compensated hypertrophy to terminal heart failure²⁰ and has been linked to action potential prolongation.^{21 22} However, it has been questioned whether downregulation of this current alone can cause increased APDs in large mammals, including humans.¹⁴ I_TO1 downregulation as the basis for action potential prolongation was also challenged by Antzelevitch and coworkers (eg, see Reference ²³ ). Reduction of I_TO1 is clearly absent in dogs with chronic AVB, which corresponds to the finding of marked notch amplitudes in midmyocardial action potentials (RV > LV).⁷ There have been only a few reports on the downregulation of delayed rectifier K⁺ current in cardiac hypertrophy induced by pressure overload in cats,^{24 25} but these investigations did not discriminate between I_Kr and I_Ks. Our data do make this distinction for the hypertrophied cells of dogs with chronic AVB and indicate that changes of these relatively small currents can have major impact on the course of repolarization, as noted before.²⁶

Clinical Perspectives
The importance of I_Kr and I_Ks for normal human cardiac repolarization has been established in cellular electrophysiological studies,^{27 28} and the characteristics of both components resemble those found in the dog. Differential downregulation of RV and LV delayed rectifier K⁺ currents could possibly contribute to repolarization abnormalities and arrhythmogenesis in patients with (this or other forms of) cardiac hypertrophy or failure, which is also indicated in a recent modeling of the human ventricular action potential.¹⁴ Experimental data on the possible changes of I_Kr and I_Ks in human ventricular hypertrophy or failure are not yet available, but the importance of these currents can be underscored by the congenital long-QT syndromes.

The combined findings of an enhanced susceptibility to acquired TdP, the (supposed) adrenergic dependence of TdP, the typical T-wave patterns during prolonged QT intervals, and the reduction of I_Ks in the dog with chronic AVB closely resemble the clinical characteristics of the LQT1 or LQT5 form of the human congenital long-QT syndrome. Approaches to a basic electrophysiological and molecular understanding of QT prolongation and T-wave abnormalities in chronic AVB could be derived from information currently obtained in the long-QT syndrome.

Limitations of the Study
We found a differential baseline contribution and hypertrophy-related remodeling of K⁺ currents in the 2 ventricles of the chronic-AVB dog. Similar current alterations could affect the transmural layers of the LV free wall and/or the septum and thus increase local dispersion of repolarization, which would facilitate the induction of TdP. However, this was not investigated.

The studies on I_Ks were performed with the Ca²⁺ buffer EGTA in the pipette solution, and thus, Ca²⁺-modulated I_Ks was not recorded. In addition, the response of I_Ks to stimulation of protein kinase A was not evaluated. The important questions of Ca²⁺-dependent and protein kinase A–mediated (dys)function of I_Ks in chronic AVB are currently being addressed in ongoing studies.

Conclusions
Significant downregulation of delayed rectifier K⁺ current contributes to the repolarization abnormalities in the LV and RV of dogs with chronic AVB. The low functional expressions of I_Ks and I_Kr, in combination with the maintained interventricular difference in I_TO1 and possibly other membrane currents, are responsible for the amplification of interventricular action potential inhomogeneities at control. The resultant increased regional dispersion of repolarization constitutes the substrate for an enhanced susceptibility to acquired TdP.

	Acknowledgments

 
Dr Volders is supported by the Wynand M. Pon Foundation, Leusden, Netherlands. Dr Sipido is supported by the National Fund for Scientific Research, Belgium. The authors wish to thank Jérôme G.M. Jungschleger, MD, and Ferenc F. van der Hulst, BS, for helpful assistance.

Received January 25, 1999; revision received July 15, 1999; accepted July 15, 1999.

	References

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