Prolongation of cardiac cycle length attenuates negative dromotropic response to selective vagal stimuli

We stimulated intracardiac parasympathetic nerve fibers that selectively innervated the atrioventricular (AV) nodal area (AV parasympathetic stimulation), and the sinoatrial (SA) nodal area (SA parasympathetic stimulation), in autonomically decentralized, anesthetized dogs. We then compared these responses to those elicited by stimulation of the cervical vagus nerves. We investigated the interactions between the dromotropic and chronotropic responses to simultaneous AV and SA parasympathetic stimulation. AV parasympathetic stimulation increased the AV interval (AV conduction time) but did not alter the interval between atrial depolarizations (sinus cycle length). SA parasympathetic stimulation increased the sinus cycle length and evoked small changes in the AV interval. Simultaneous AV and SA parasympathetic stimulation, at different combinations of frequencies, induced negative dromotropic and chronotropic responses that were similar to those evoked by cervical vagal stimulation. The greater the increase in sinus cycle length, the less did a given parasympathetic stimulation prolong the AV interval. The prolongation of the AV interval by parasympathetic stimulation did not affect the sinus cycle length. These results suggest that the direct pure negative dromotropic response to parasympathetic nerve stimulation is attenuated by the prolongation of the sinus cycle length, e.g. a concomitant negative chronotropic effect of the parasympathetic stimulation, in the dog heart. This attenuation reflects a mechanism that does not depend on the relative timing of the stimulus impulses in the cardiac cycle, i.e. a phase-independent, as well as the previously reported phase-dependent, mechanism.