DIFFERENT SYMPATHETIC-PARASYMPATHETIC INTERACTIONS ON SINUS RATE AND ATRIOVENTRICULAR-CONDUCTION IN DOG HEARTS

We investigated the sympathetic-parasympathetic interactions involved in SA nodal pacemaker activity and AV conductivity in the anesthetized dog heart. Stimulation of the intracardiac parasympathetic nerves to the SA nodal region (SAPS) and stimulation of the intracardiac parasym pathetic nerves to the AV nodal region (AVPS) induced negative chronot ropic and dromotropic responses, respectively. Cardiac sympathetic sti mulation, aminophylline, 3-isobutyl-1-methylxanthine (IBMX, a relative ly pure nonselective phosphodiesterase inhibitor) and 4-(2-trifluorome thylphenyl)-piridine-5-carboxylate (Bay k 8644, a Ca2+ channel agonist ) increased sinus rate and decreased AV conduction time. Sympathetic s timulation augmented the negative chronotropic response to SAPS but no t the negative dromotropic response to AVPS, IBMX augmented both respo nses, Bay k 8644 augmented the chronotropic response and attenuated th e dromotropic response, and aminophylline did not affect the chronotro pic response to SAPS and inhibited the dromotropic response to AVPS. A dditionally, when Bay k 8644 directly given via the AV node artery dec reased AV conduction time, it attenuated the negative dromotropic resp onse to AVPS and carbachol injected into the AV node artery. These res ults suggest that the differential sympathetic-parasympathetic interac tions on sinus rate and AV conduction are at least partly induced by a n interaction between changes in slow inward Ca2+ current or intracell ular Ca2+ and the cardiac effects of acetylcholine in the heart in sit u. (C) 1997 Elsevier Science B.V.