Vagal modulation of adrenergic effects on canine sinus and atrioventricular nodes

Vagal stimulation modulates adrenergic neurotransmission at pre- and postjunctional sites. The purpose of this study was to test whether such multilevel cholinergic inhibition depressed sinus nodal automaticity and atrioventricular (AV) nodal conduction (A-H interval) in open-chest dogs anesthetized with chloralose. We compared the inhibitory effects of vagal stimulation when sinus nodal automaticity and AV nodal conduction were enhanced by bilateral stellate ganglia stimulation with comparable degrees of enhanced automaticity and conduction achieved by infusing norepinephrine and isoproterenol. We found that vagal stimulation prolonged in a frequency-dependent fashion sinus nodal cycle length and A-H interval more during stellate ganglion stimulation than during norepinephrine or isoproterenol infusion. Atropine eliminated all effects of vagal stimulation. These data provide functional evidence to support the concept that muscarinic modulation of adrenergic effects on sinus nodal cycle length and AV nodal conduction occurs at site(s) in addition to the postjunctional effector cell.