Cardiac and arterial baroreceptor influences in release of vasopressin and renin during hemorrhage

We studied the role of arterial and cardiac baroreceptors on mean arterial pressure (MAP) and release of arginine vasopressin (AVP) and plasma renin activity (PRA) during hemorrhage in conscious rabbits. Each rabbit was bled at 2% of its blood volume (BV) per minute until 35% had been removed, after which the blood was reinfused. Each rabbit was studied on three occasions, 7 days apart, and in each experiment, BV-MAP and BV-hormone response curves were constructed. The response to hemorrhage was examined when the input from 1) arterial and cardiac baroreceptors were both intact; 2) arterial baroreceptors only were intact (cardiac receptors were blocked with intrapericardial procaine); 3) cardiac receptors only were intact (after sinoaortic denervation); 4) neither receptor was intact. Resting AVP and PRA levels were unaffected by the various deafferentation procedures. AVP steeply increased only after more than 25% BV had been removed; this response was entirely mediated by cardiac baroreceptors. Increases in PRA occurred at BV loss >15% and were largely independent of baroreceptor input. Maintenance of MAP during hemorrhage was mostly due to drive from the arterial baroreceptors. Thus AVP secretion during hemorrhage contributes little to the maintenance of MAP, and the hypovolemic stimulus to AVP release comes entirely from the cardiac baroreceptors.