Experiments were performed to examine the potential role of circulating arginine vasopressin (AVP) on baroreflex sensitivity during hypotensive and nonhypotensive hemorrhage in the conscious rat. Animals were chronically instrumented for measurement of cardiac output, blood pressure, and heart rate (HR). Three potential stimuli for release of AVP were utilized: 1) rapid 20% arterial hemorrhage that resulted in hypotension, 2) nonhypovolemic hypotension induced by intravenous infusion of nitroprusside, and 3) nonhypotensive hemorrhage (rapid 10% arterial blood withdrawal). Hypotensive hemorrhage was associated with significant reductions in blood pressure, cardiac output, HR, and calculated total peripheral resistance, an increase in baroreflex (BRR) bradycardia in response to pressor infusions of phenylephrine, and a moderate elevation in circulating AVP. Prior intravenous administration of a specific V1-vasopressinergic antagonist augmented the hypotensive response to hemorrhage; however, neither V1- nor V2-blockade affected hemorrhage-induced augmentation of the BRR. Inducement of hypotension by infusion of nitroprusside did not alter subsequent BRR sensitivity. Finally, nonhypotensive hemorrhage was associated with an increase in resting HR and augmented BRR sensitivity. However, in contrast to hypotensive hemorrhage, either V1- or V2-antagonism attenuated the increase in BRR sensitivity seen with 10% hemorrhage. These data suggest that, although AVP may play a role in blood pressure maintenance via its direct vasoconstrictor actions during hypotensive hemorrhage, the observed augmentation of BRR sensitivity associated with severe blood loss is not attributable to a vasopressinergic mechanism activated by circulating AVP. However, blood-borne AVP may contribute to BRR sensitivity alterations in response to mild blood loss.
