Afferent vascular resistance control during hemorrhage in normal and autonomically blocked rabbits

We examined the role of the arterial and cardiac baroreceptors on the hindquarter conductance and heart rate responses of conscious rabbits bled at approximately 3% blood volume (BV)/min to 80% BV (i.e., 20% BV removed). We used rabbits with both sets of baroreceptors working and when only one or neither sets was working. Each animal was studied with normal effector function and during autonomic blockade (hormonal + local effectors), where release of arginine vasopressin (AVP) and renin (angiotensin II, ANG II) were enhanced. The local response (LR) to hemorrhage was determined in a separate group of neurohumorally blocked rabbits. The estimated constrictor response (ECR) was the difference between the LR and net conductance response. In normal rabbits, the ECR was 49 units, with the estimated arterial-to-cardiac barorecptor drive ratio approximately 2.8:1 and with the two receptor groups acting by simple addition. Both barorecptors contributed to the rise in heart rate, with the relative arterial-to-cardiac baroreceptor drive ratio approximately 4:1. When hemorrhage was performed during autonomic blockade, ECR was 84 units (compared with normal rabbits, P less than 0.01), but blood pressure was poorly maintained and the constrictor effect was not under baroreceptor control. Although the baroreceptors were critical for AVP release during autonomic blockade, they played no role in renin release (ANG II production); the latter was released in large amounts, producing near-maximum constriction, which was unrelated to the afferent input. Thus neurally mediated regulation during hemorrhage has substantial advantages over that mediated primarily through the pressor hormones.