Peripheral baroreceptor control of neurohypophysial blood flow during hemorrhage

We measured neurohypophysial blood flow (NHBF) in dogs made hypotensive to study the time course of neurohypophysial autoregulation. We found that the neurohypophysis has a unique autoregulatory response in which blood flow increases transiently before establishing a stable perfusion level. During the "hyperemic" phase, the time course of the flow transient correlates with an increased plasma arginine vasopressin (pAVP) neurosecretory transient. However, unlike pAVP, blood flow does not remain elevated. The transitory nature of the flow response led us to evaluate the hypothesis that peripheral baroreceptor activity may be responsible for the hyperemic phase. Bilateral vagotomy and carotid sinus nerve denervation attenuated the increase in neural lobe blood flow (NLBF) to a standardized step to 80 mmHg mean arterial pressure. The pAVP response was enhanced both by vagotomy and carotid sinus denervation. Combined denervation abolished both the pAVP and the NLBF responses. We conclude that NHBF is autoregulated; however, control may be exerted at multiple levels, i.e., via nerves as well as local metabolic mechanisms. The peripheral baroreceptors influence NHBF primarily during periods of changing blood pressure. When blood pressure is stable, local mechanisms appear to be dominant.