Cardiovascular actions of vasopressin: baroreflex modulation in the conscious rat

Mean arterial pressure (MAP), heart rate (HR), cardiac output (CO), and total peripheral resistance (TPR) were recorded during graded infusions of arginine vasopressin (AVP), angiotensin II (ANG II), and phenylephrine (PE) in conscious, unrestrained, sinoaortic-denervated (SAD) and normal rats. Base-line MAP, CO, and TPR values before infusion were not different between groups. HR values were significantly higher in SAD rats. Dose-response curves indicated that there was a similar enhancement in pressor sensitivity to AVP, ANG II, and PE in the absence of the baroreceptors. Pressor responses to AVP were buffered by offsetting decreases of CO. Similar elevations in MAP evoked a 50% greater reduction in CO with AVP, and HR decreased 1.5 times as much with AVP than with ANG II or PE. The dose of AVP required to raise MAP by 25 mmHg in control rats resulted in similar falls of CO in SAD rats, whereas HR responses to AVP were attenuated significantly in SAD rats. We conclude that baroreceptor buffering of AVP-induced pressor responses is due principally to reflex reduction of TPR. Furthermore, CO suppression was not baroreflex-mediated, whereas bradycardia was reflex dependent. Finally, in rats, AVP does not appear to interact with the baroreceptor reflexes in a manner unique from other vasoconstrictor agents to buffer MAP.