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Differential Actions of Neuronal and Hormonal Vasopressin on Blood Pressure and Baroreceptor Reflex Sensitivity in Rats

The central cardiovascular actions of arginine vasopressin (AVP) and the role of this peptide in the modulation of baroreceptor reflex (BRR) sensitivity were investigated in conscious, chronically instrumented rats. Intracerebroventricular (i.c.v.) injections of AVP (1–100 ng) produced dose-dependent pressor effects together with increases in heart rate, splanchnic, and renal sympathetic nerve activity and a reduction of mesenteric blood flow. These responses were prevented by i.c.v. pretreatment with a V1-AVP receptor antagonist. Furthermore, the central pressor effects of AVP were abolished by peripheral α-adrenoceptor blockade with phentolamine. In contrast to the i.c.v. injections, intravenous (i.v.) or intracarotid injections of AVP produced pressor responses accompanied by bradycardia and a decrease in sympathetic nerve activity. Intracerebro-ventricular pretreatment with a V1-AVP receptor antagonist shifted the slope of the BRR curve (increases in pulse interval plotted against increases in systolic blood pressure in response to i.v. methoxamine) toward higher sensitivity, whereas i.v. pretreatment with a V2-AVP receptor antagonist shifted the slope of the BRR curve toward lower sensitivity of the reflex. These findings suggest that central (neuronal) AVP, by acting on V1-AVP receptors in the brain, participates in central pressor mechanisms, activates the sympathetic nervous system, and desensitizes the BRR. Conversely, circulating (hormonal) AVP can sensitize the BRR by acting on V2-AVP receptors accessible from the blood. Our data provide evidence pointing to a direct interaction between the neuronal and the hormonal axis of the AVP system within cardiovascular control.