EFFECT OF VERAPAMIL AND DILTIAZEM ON CALCIUM-DEPENDENT ELECTRICAL-ACTIVITY IN CARDIAC PURKINJE-FIBERS

The effects of verapamil and diltiazem on normal action potentials, abnormal automaticity at depolarized membrane potential and oscillatory afterpotentials were compared in sheep cardiac Purkinje fibers. Concentrations of verapamil and diltiazem exerting the same action on abnormal automaticity due to slow action potentials, caused different effects on action potential characteristics and on oscillatory afterpotentials. Diltiazem significantly shortened action potential duration whereas verapamil slightly lengthened it (not significant). Diltiazem appeared to be more effective than verapamil in preventing the development of oscillatory afterpotentials induced by Ba or by strophanthidin. In 50% of Ba-treated preparations, verapamil caused the appearance of spontaneous activity due to enhanced normal diastolic depolarization, while diltiazem had no such effect. The differences were explained in terms of the different effects of the 2 drugs on currents other than the slow inward current, since diltiazem was more potent than verapamil in depressing .ovrhdot.Vmax [initial maximal velocity of uptake].