Bainbridge reflex in conscious, unrestrained, and tranquilized baboons.

The effects of volume loading were examined in conscious, unrestrained, and tranquilized baboons (20-25 kg) 1-3 mo after instrumentation with miniature left ventricular pressure and diameter gauges. The response to volume loading was accompanied by significant increases (P less than 0.01) in cardiac rate (32 +/- 4 beats/min) in tranquilized baboons. When rapid saline infusion was accomplished in conscious unrestrained baboons by means of activating a remote-controlled interrogator, heart rate also rose significantly (41 +/- 8 beats/min, P less than 0.01). After cholinergic blockade with atropine, ganglionic blockade with hexamethonium, bilateral vagotomy, or combined cholinergic and beta-adrenergic blockades, volume loading failed to increase heart rate. In two smaller (12-14 kg) intact conscious baboons with relatively high spontaneous heart rates, volume loading did not alter heart rate. These experiments suggest that the Bainbridge reflex exists in larger primates with relatively low spontaneous heart rates.