Autonomic modulation of atrioventricular conduction in ouabain-treated dogs

There is a pronounced interaction between the two autonomic divisions in the control of some cardiac variables, e.g., heart rate and contractility; usually parasympathetic activation suppresses the sympathetic effects. There is controversy about whether a similar interaction occurs for atrioventricular (AV) conduction. We have previously been unable to describe such an interaction in adult dogs, although others found a sympathetic-parasympathetic interaction when the dog heart was paced at very short intervals (200-240 ms). We reasoned that a cardiac glycoside (ouabain 20 micrograms/kg iv), which accentuates the parasympathetic effect, might expose any autonomic interaction. The atrial and ventricular electrograms, heart period, and atrioventricular conduction time (A-A and A-V intervals) were measured in 18 anesthetized, open-chest adult dogs. The sinoatrial (SA) node was suppressed by injection of Nembutal (pentobarbital sodium) through the SA nodal artery, and the hearts were paced at 500- to 700-ms intervals. The sympathetic and vagus nerves were stimulated bilaterally at three levels before and after ouabain or a diluent was given. Data were analyzed by analysis of variance. We found that there was no significant sympathetic-parasympathetic interaction (P greater than 0.05) even after ouabain had potentiated the vagal effect by up to 100%. Thus, in dogs, control of AV conduction time by either autonomic division is independent of activity of the other divisions when the cycle length is not abbreviated.