Vasopressin (AVP) acts in the area postrema (AP) to augment baroreflex opposition of the exercise pressor reflex

Circulating AVP can enhance baroreflex function via its action in the AP. In addition, the exercise pressor reflex is enhanced by intravenous injection of an AVP V, receptor antagonist or chemical inhibition of cell bodies in the AP. We tested the hypothesis that AVP acts in the AP during static contraction to enhance baroreflex function. Thus, blood pressure (MAP) and heart rate (HR) responses to 30 s of electrically stimulated hindlimb contraction were compared before and after bilateral microinjections of 200 nl of the selective AVP V, receptor antagonist d(CH ₂) ₅ Tyr(Me)-AVP (1 ng/nl) into the AP of the anesthetized cat (α-chloralose; n=4). This protocol was repeated in 3 other cats where sinoaortic denervation was performed prior to any intervention. Microinjection of the V, receptor antagonist into the AP augmented the MAP and HR responses to static contraction from 47±8 to 69±12 mmHg and from 14±3 to 20±5 beats/min, respectively In the SAD cats, injection of the V, antagonist had no effect on MAP or HR. Intravenous injection of a similar dose of the antagonist (n=2) also had no effect on contraction-induced increases in MAP or HR. These results suggest that baroreflex function is enhanced during static contraction due to the action of AVP in the AP.