Effects of calcium-antagonistic coronary vasodilators, nifedipine and verapamil, on ventricular automaticity of the dog.

In isolated, blood-perfused canine papillary muscles intra-arterial injection of calcium-antagonistic coronary vasodilators, nifedipine and verapamil, produced a dose-related decrease in force of contraction. The ventricular rate of about 40 beats/min was not significantly changed by nifedipine even in doses which profoundly decreased the force of contraction. Verapamil changed the ventricular rate in a biphasic manner, but the changes remained as small as about 10% of the basal rate in doses which markedly suppressed the force of contraction. Calcium chloride elicited an increase in force of contraction but depressed automaticity. The present results show that in response to nifedipine, verapamil and calcium ions, ventricular automaticity has characteristics different from those of the sinus node.