Activation of εPKC reduces reperfusion arrhythmias and improves recovery from ischemia: optical mapping of activation patterns in the isolated guinea-pig heart.

UNLABELLED: Pervious biochemical and hemodynamic studies have highlighted the important role of εPKC in cardioprotection during ischemic preconditioning. However, little is known about the electrophysiological consequences of εPKC modulation in ischemic hearts. Membrane permeable peptide εPKC selective activator and inhibitor were used to investigate the role of εPKC modulation in reperfusion arrhythmias. METHODS: Protein transduction domain from HIV-TAT was used as a carrier for peptide delivery into intact Langendorff perfused guinea pig hearts. Action potentials were imaged and mapped (124 sites) using optical techniques and surface ECG was continuously recorded. Hearts were exposed to 30 min stabilization period, 15 min of no-flow ischemia, followed by 20 min reperfusion. Peptides (0.5 µM) were infused as follows: (a) control (vehicle-TAT peptide; TAT-scrambled ψεRACK peptide); (b) εPKC agonist (TAT-ψεRACK); (c) εPKC antagonist (TAT-εV1). RESULTS: Hearts treated with εPKC agonist ψεRACK had reduced incidence of ventricular tachycardia (VT, 64%) and fibrillation (VF, 50%) compared to control (VT, 80%, P<0.05) and (VF, 70%, P < 0.05). However, the highest incidence of VT (100%, P < 0.05) and VF (80%) occurred in hearts treated with εPKC antagonist peptide εV1 compared to control and to εPKC agonist ψεRACK. Interestingly, at 20 min reperfusion, 100% of hearts treated with εPKC agonist ψεRACK exhibited complete recovery of action potentials compared to 40% (P < 0.05) of hearts treated with εPKC antagonist peptide, εV1 and 65% (P < 0.5) of hearts in control. At 20 min reperfusion, maps of action potential duration from εPKC agonist ψεRACK showed minimal dispersion (48.2 ± 9 ms) compared to exacerbated dispersion (115.4 ± 42 ms, P < 0.05) in εPKC antagonist and control (67 ± 20 ms, P<0.05). VT/VF and dispersion from hearts treated with scrambled agonist or antagonist peptides were similar to control. CONCLUSION: The results demonstrate that εPKC activation by ψεRACK peptide protects intact hearts from reperfusion arrhythmias and affords better recovery. On the other hand, inhibition of εPKC increased the incidence of arrhythmias and worsened recovery compared to controls. The results carry significant therapeutic implications for the treatment of acute ischemic heart disease by preconditioning-mimicking agents.

Electrophysiological mechanism of enhanced susceptibility of hypertrophied heart to acquired torsade de pointes arrhythmias: tridimensional mapping of activation and recovery patterns.

BACKGROUND: Cardiac hypertrophy is associated with increased incidence of sudden death and susceptibility to proarrhythmic effects of antiarrhythmic agents. However, the in vivo electrophysiologic mechanism of the arrhythmias has not been investigated in detail. METHODS AND RESULTS: Dose-dependent susceptibility to Torsade de Pointes (TdP) by class III drug dofetilide, 3, 10, and 30 microg/kg, was compared in 6 control dogs (C) and in 5 dogs 6 to 8 weeks after induction of complete atrioventricular block (AVB) that resulted in ventricular hypertrophy (H). Tridimensional ventricular activation and repolarization (R) patterns were simultaneously analyzed from unipolar extracellular electrograms, and local R was measured from activation recovery intervals. Both R and transmural dispersion of R (TDR) were significantly greater in dogs with H compared with C. Dofetilide resulted in cycle length--dependent and dose-dependent prolongation of R, which was more marked in left ventricular endocardium/midmyocardium compared with epicardium, resulting in significant increase of TDR. These changes were more accentuated in dogs with H compared with C. All 5 dogs with H developed TdP at a dose of 3 to 10 microg/kg, whereas only 1 of 6 C dogs developed TdP at 30 microg/kg. TdP was initiated by subendocardial focal activity that infringed on TDR, resulting in functional conduction block and reentrant excitation. CONCLUSIONS: Enhanced susceptibility of hypertrophied heart to class III drugs is attributable to baseline increased TDR and greater dose-related accentuation of TDR compared with nonhypertrophied heart. This provides the electrophysiologic substrate for drug-induced TdP.

Spatial dispersion of repolarization is a key factor in the arrhythmogenicity of long QT syndrome.

INTRODUCTION: The occurrence of significant spatial dispersion of repolarization in vivo as it relates to the mechanism of arrhythmia formation in the long QT syndrome (LQTS) continues to be questioned. METHODS AND RESULTS: We investigated a guinea pig model of LQT3 using anthopleurin-A (AP-A) to study the contribution of rate-dependent spatial dispersion of repolarization in the intact heart to the arrhythmogenicity of LQTS. Optical action potentials were measured using potentiometric fluorescent dye di-4ANEPPS in Langendorff-perfused hearts with induced AV block. AP-A exacerbated the normal uniform epicardial apex-base action potential duration (APD) gradient, resulting in rate-dependent increased APD dispersion and nonuniform APD gradient. Spontaneous focal premature beats induced functional conduction block along boundaries where large nonuniform APD gradient occurred setting the stage for circulating wavefronts and ventricular tachyarrhythmia (VT). Endocardial ablation abolished spontaneous VT, but nonuniform epicardial APD gradient persisted and could be challenged by a stimulated premature stimulus to induce VT. CONCLUSION: The study shows that in LQT3, spatial variations in steady-state properties result in zones of nonuniform APD gradients. These provide a substrate for functional conduction block and reentrant excitation when challenged by subendocardial "early afterdepolarization-triggered" premature beats. The study emphasizes the key importance of spatial dispersion of repolarization, whether located in epicardial or intramyocardial layers, in arrhythmia formation in LQTS.