Reflex effect of vasopressin after blockade of V1 receptors in the area postrema.

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Abstract
This study investigated the effect of micropressure injection of the V1 arginine vasopressin (AVP) receptor antagonist into the area postrema on the ability of circulating AVP to augment baroreflex inhibition of renal sympathetic nerve activity (RSNA) in urethane-anesthetized rabbits. In addition, the effects of micropressure injections of AVP into the area postrema on RSNA, arterial pressure, heart rate, and baroreflex control of RSNA were evaluated. Injection of 100 ng (in a 10-nl volume) of AVP antagonist into the area postrema abolished the ability of AVP to enhance baroreflex inhibition of RSNA compared with phenylephrine (-8.84 +/- 0.89 before antagonist versus -4.83 +/- 0.44 %RSNA/mm Hg after antagonist). Normal baroreflex inhibition to phenylephrine (-3.95 +/- 0.26 versus -4.10 +/- 0.33 %RSNA/mm Hg) was unaltered. This dose of AVP antagonist given intravenously or into the adjacent medial nucleus tractus solitarius was without effect. Micropressure injection of AVP directly into the area postrema produced a dose-dependent decrease in RSNA without significant effects on arterial pressure or heart rate. Local injection of 4 +/- 0.6 ng (in a 4-nl volume) of AVP produced an average 27 +/- 3% decrease in resting RSNA. Continuous injection of AVP into the area postrema using short-duration, low-frequency pressure pulses significantly augmented the baroreflex inhibition of RSNA during phenylephrine infusion (during AVP injection, -7.12 +/- 1.60%RSNA/mm Hg; control, -3.38 +/- 0.55 %RSNA/mm Hg). These data support the hypothesis that circulating AVP acts at the area postrema to augment baroreflex inhibition of RSNA by a V1 receptor mechanism.