Verapamil potentiates vagally mediated atrioventricular chronotropic responses in dogs

When the cervical vagus nerve is briefly stimulated, a triphasic cardiac chronotropic response ensues: the cardiac cycle length initially increases, then briefly decreases, and subsequently increases again. Verapamil, a calcium channel blocker, alters these responses to acetylcholine when the heart is in sinoatrial nodal rhythm. We investigated whether verapamil has a similar effect on parasympathetic modulation of the cardiac cycle when the heart is in atrioventricular rhythm. Atrioventricular rhythm was induced in anesthetized, open-chest dogs by infusion of sodium pentobarbital into the sinoatrial nodal artery. Verapamil was administered by bolus injection through a femoral vein. Verapamil accentuated the initial lengthening of the cardiac cycle by approximately 55% (from 301 to 466 ms). The acceleratory phase of this triphasic response and the secondary cardioinhibition were not significantly affected by verapamil. Thus, the initial action of acetylcholine at the atrioventricular node is potentiated by verapamil when the vagus is activated with brief stimuli.