Are calcium antagonists proarrhythmic?

Clinical and experimental studies demonstrate that calcium (Ca2+) overload in myocardial cells is an important factor in the genesis of various serious arrhythmias. Calcium antagonists block voltage-dependent channels and thus reduce entry of Ca2+ into heart cells. Because of their specificity for atrioventricular nodal cells, verapamil and diltiazem are used clinically to treat supraventricular arrhythmias involving transmission in the atrioventricular node. These two drugs and the dihydropyridine (DHP) calcium antagonists have been shown to prevent ventricular ischemic and reperfusion arrhythmias in the laboratory. Despite these data indicating that calcium antagonists are antiarrhythmic, a recent controversy has raised the possibility that certain calcium antagonists are unsafe to use, especially for patients with coronary heart disease. Proarrhythmia has been proposed to be a mechanism contributing to potentially adverse outcomes. Although excessive concentrations of verapamil and diltiazem may cause sino-atrial nodal asystole and varying degrees of atrioventricular block, there is little direct evidence that this contributes to significant proarrhythmia, for example, ventricular tachyarrhythmias. Nonetheless, although it appears paradoxical that agents which block the entry of Ca2+ into heart cells may be considered arrhythmogenic, there are circumstances under which dosage with certain calcium antagonists potentially leads to myocardial Ca2+ overload. For example, bouts of neurohormonal activation brought about by calcium antagonist-induced abrupt reductions in blood pressure may be accompanied each time by significant beta-adrenergic-enhanced influx of Ca2+ through the L-type cardiac calcium channels. This elevates the intracellular Ca2+ concentration and disturbs Ca2+ regulation, especially in diseased hearts whose intracellular Ca2+ regulation has already been compromised, and might induce alterations in cardiac electrical activity. In the present article, interactions among cardiac calcium channels, classes of calcium antagonists, and specific formulations of certain antagonists are considered with respect to directly induced ventricular arrhythmogenesis. Indirect potentially proarrhythmic actions of the calcium antagonists are also discussed. We outline some of the many questions that remain to be answered with respect to the actions of DHP on the heart including that of whether beta-adrenergic stimulation modifies the degree of cardiac Ca2+ channel inhibition by DHP-type calcium antagonists.