Role of vasopressin on cardiovascular changes during hemorrhage in conscious rats

Hypotensive hemorrhage decreases heart rate (HR) and renal sympathetic nerve activity (RSNA). Hemorrhage is a potent stimulus for arginine vasopressin (AVP) release; therefore, AVP may contribute to such inhibitory action of HR and RSNA during hemorrhage. We evaluated the roles of vasopressin on the regulation of blood pressure (BP), HR, and RSNA during hemorrhage using nonpeptide and selective V1- and V2-receptor antagonists (OPC-21268 and OPC-31260) in conscious rats. After hemorrhage (20 ml/kg body wt) BP decreased by 62 +/- 10 mmHg along with bradycardia (-110 +/- 15 beats/min) and renal sympathoinhibition (-50 +/- 8). Pretreatment of V1-receptor antagonist (5 mg/kg iv) did not affect the initial fall of BP but attenuated subsequent BP recovery. Bradycardic and renal sympathoinhibitory responses following hemorrhage were abolished (-14 +/- 24 beats/min and -7 +/- 9) by V1-receptor antagonist. Pretreatment of V2-receptor antagonist (1 mg/kg iv) did not affect the response of BP; however, it did slightly strengthen bradycardia and prolong renal sympathoinhibition. Hemorrhage increased the plasma AVP concentration more than 50-fold. These results indicate that when the plasma concentration of AVP is extremely high during hemorrhage, vasopressin via V1 receptor contributes to BP recovery by the peripheral vasoconstriction and exerts an inhibitory action on RSNA, and vasopressin via V2 receptor exerts opposite stimulatory action on RSNA.