The interaction of the Bainbridge and Bezold-Jarisch reflexes in the conscious dog

Experiments were undertaken to determine the efferent path of the Bainbridge reflex and to investigate the interaction of the Bainbridge reflex with the Bezold-Jarisch reflex in conscious, chronically instrumented dogs. The Bainbridge reflex was elicited by distending the left atrium by inflating a chronically implanted balloon catheter. The Bezold-Jarisch reflex was elicited using chemical stimulation of left ventricular receptors with infusions of veratridine (0.1-0.8 μg/kg/min) into the left circumflex coronary artery. Heart-rate responses to left atrial balloon inflation were compared before and after either β-1 antagonism with metoprolol or cholinergic antagonism with atropine, and before and during left ventricular receptor stimulation with intracoronary veratridine. Left atrial balloon inflation alone caused a significant increase in heart rate (70.1 ± 5 bpm), left atrial pressure (14 ± 3 mm Hg) and mean arterial blood pressure (10 ± 2 mm Hg). Heart-rate responses to left atrial distension were inhibited, but not abolished by either cholinergic or β-1 antagonism. Left atrial distension after both cholinergic and β-1 antagonism abolished the heart-rate response to balloon inflation. These results indicate that the efferent component of the Bainbridge reflex has both a vagal and a sympathetic component in conscious dogs. Left atrial distension during simultaneous left ventricular receptor stimulation resulted in a significantly decreased tachycardia than did left atrial distension alone (26 ± 3 bpm compared to 68 ± 8 bpm in the control experiments). In addition, the slope of the heart rate vs left atrial pressure relationship was significantly inhibited by left ventricular receptor stimulation (1.8 ± 0.2 bpm/mm Hg compared to 5.7 ± 0.3 bpm/mm Hg in the control experiments). There were no significant differences in either the left atrial pressure or arterial blood pressure changes between the two groups. These data suggest an interaction between these two reflexes that may be occurring in the central nervous system.