Evidence for vagal involvement in the electrophysiologic actions of exogenous adenosine and adenosine triphosphate in the canine heart.

A closed chest canine model was used to study the electrophysiologic effects of adenosine and adenosine triphosphate (ATP) on the sino-atrial (SA) and atrioventricular (AV) nodes. Exogenous adenosine and ATP, when rapidly administered into the right atrium, caused transient negative chronotropic and dromotropic effects on the SA and AV nodes, respectively. These effects reached a maxima in 10-15 sec and dissipated in less than 120 sec. The electrophysiologic effects of ATP were more pronounced than those of adenosine. Physostigmine enhanced the chronotropic effects of adenosine and ATP and the dromotropic effects of ATP. Atropine abolished the enhancement of the electrophysiologic effects of adenosine and ATP caused by physostigmine. In the pentobarbital anaesthetized dog, ATP triggers a vagal reflex which plays a major role in mediating its electrophysiologic effects. Vagal involvement in the electrophysiologic action of adenosine is more modest, but does not appear to be mediated via reflex mechanisms.