Parasympathetic inhibition of sympathetic effects on atrioventricular conduction in anesthetized dogs.

To investigate the selective parasympathetic control of atrioventricular (AV) conduction during sympathetic activation, we studied the effects of cervical vagus nerve stimulation on the positive dromotropic responses to sympathetic interventions before and after surgical dissection of dual fatty tissues at the junction of the inferior vena cava and inferior left atrium and at the right atrial side of the atrial junctions of the right pulmonary veins in open-chest anesthetized dogs. In atrial-paced hearts, vagus stimulation at low frequencies prolonged atrio-His (A-H) interval and at high frequencies induced second- and third-degree AV blocks. Vagus stimulation additively prolonged A-H interval shortened by stimulation of the ansae subclaviae or isoproterenol infusion. After dissection of dual fatty tissues, vagus stimulation prolonged A-H interval by only 7%. However, during sympathetic stimulation but not during isoproterenol infusion, vagus stimulation prolonged the shortened A-H interval. Atropine abolished the responses to vagus stimulation. These results suggest that even during sympathetic activation, regional vagus inputs selectively control A-H interval, and even after denervation of the regional parasympathetic nerves, presynaptic parasympathetic inhibition of the positive cardiac responses to sympathetic activation works in the heart in situ.