Effect of calcium-channel antagonist on repolarization heterogeneity of ventricular myocardium in an in vitro rabbit model of long QT syndrome.

Intracellular Ca(2+) and Ca(2+)-dependent signaling molecule play an essential role in the genesis of long-QT (LQT) syndrome-related ventricular arrhythmias. The effect of calcium-channel antagonist verapamil on repolarization heterogeneity of ventricular myocardium was assessed in an in vitro rabbit model of LQT syndrome. By using the monophasic action potential (MAP) recording technique, MAPs of epicardium, mid-myocardium and endocardium were simultaneously recorded by specially designed plunge-needle electrodes across the left ventricular free wall in rabbit hearts perfused by Langendorff method with standard Tyrode's solution. Bradycardia was induced by complete ablation of atrioventricular node. A catheter was introduced into the right ventricle to pace at the cycle lengths (CLs) of 1500, 1000, and 500 ms, successively. Quinidine (2 micromol/L) prolonged QT interval and ventricular MAP duration (MAPD), and increased transmural dispersion of repolarization (TDR) in a reverse rate-dependent fashion in isolated rabbit heart. No polymorphic ventricular tachycardias were induced under this condition. The effective free therapeutic plasma concentrations of verapamil (0.01-0.05 micromol/L) used in this experiment had no effect on quinidine-induced changes of QT interval, MAPD and TDR. This study demonstrated that, in this model of LQT syndrome, blockade of calcium-channel with verapamil had no effect on quinidine-induced changes of repolatiation heterogeneity of ventricular myocardium.