Heart rate control in awake dog after selective SA-nodal parasympathectomy.

Selective surgical sinoatrial (SA)-nodal parasympathectomy (PSX) was used to distinguish the role of the cardiac autonomic nerves in heart rate (HR) control in awake dogs (n = 8) during rest and behavioral arousal. Resting HR increased from 85 +/- 9 beats (mean +/- SE) before surgery to 114 +/- 6 beats after denervation (P less than 0.05). Atrioventricular (AV)-nodal block occurred during the first 1-3 wk post-PSX, but subsequently resolved. Dogs were behaviorally conditioned by following a 30-s tone (CS+) by a 0.5-s shock. Before denervation the CS+ evoked an initial, rapidly developing tachycardia (phase 1), which was followed by a more slowly developing, but larger, phase 2 tachycardia. The selective SA-nodal parasympathectomy essentially abolished the phase 1 conditional HR response (magnitude: 23 +/- 5 vs. 5 +/- 2 beats, pre- vs. postdenervation, respectively). The phase 2 HR increase was similar before and after the denervation (magnitude: 44 +/- 6 vs. 33 +/- 6 beats; rate of increase: 5 +/- 1 vs. 6 +/- 1 beats/s, pre- vs. post-PSX). Beta-Blockade (propranolol, 1 mg/kg) after PSX decreased phase 2 (magnitude: 7 +/- 3 beats; rate of increase: 1 +/- 0.3 beats/s). These data reveal a sterotypic pattern of change in cardiac autonomic nervous drive during a sudden arousal from rest. Phases 1 and 2 appear to be selective and specific indexes of changes in SA-nodal parasympathetic and sympathetic tone, respectively. The selective denervation unmasks during stress a component of HR control that occurs in the absence of adrenergic or cholinergic mechanisms. These data suggest that multiple interactions occur within the intrinsic ganglion plexuses of the heart with respect to HR control.