PARASYMPATHETIC INHIBITION OF SYMPATHETIC EFFECTS ON ATRIOVENTRICULAR-CONDUCTION IN ANESTHETIZED DOGS

To investigate-the selective parasympathetic control of atrioventricul ar (AV) conduction during sympathetic activation, we studied the effec ts of cervical vagus nerve stimulation on the positive dromotropic res ponses to sympathetic interventions before and after surgical dissecti on 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 junct ions of the right pulmonary veins in open-chest anesthetized dogs. In atrial-paced hearts, vagus stimulation at low frequencies prolonged at rio-His (A-H) interval and at high frequencies induced second-and thir d-degree AV blocks. Vagus stimulation additively prolonged B-H interva l shortened by stimulation of the ansae subclaviae or isoproterenol in fusion. After dissection of dual fatty tissues, vagus stimulation prol onged A-H interval by only SE. However, during sympathetic stimulation but not during isoproterenol infusion, vagus stimulation prolonged th e shortened A-H interval. Atropine abolished the responses to vagus st imulation. These results suggest that even during sympathetic activati on, regional vagus inputs selectively control A-H interval, and even a fter denervation of the regional parasympathetic nerves, presynaptic p arasympathetic inhibition of the positive cardiac responses to sympath etic activation works in the heart in situ.