Vasopressin acts in the area postrema to attenuate the exercise pressor reflex in anesthetized cats

Circulating arginine vasopressin (AVP) can enhance baroreflex function via its action in the area postrema (AP). We tested the hypothesis that AVP acts in the AP to enhance baroreflex function during static contraction and, in turn, attenuates the exercise pressor reflex. Thus mean arterial blood pressure (n = 9) and heart rate (HR) (n = 9) during 30 s of electrically stimulated hindlimb contraction were compared before and after bilateral microinjections of 200 nl of the AVP V₁-receptor antagonist d(CH₂)₅Tyr(Me)-AVP (V(1x)) (1 ng/nl) into the AP of the anesthetized cat. This protocol was repeated in three other cats in which sinoaortic denervation (SAD) was performed before any intervention. Injection of V(1x) into the AP had no effect on baseline blood pressure or HR. However, pressor and HR responses to static contraction were augmented by 44 ± 10 and 29 ± 9%, respectively. Static contraction also increased plasma AVP from 15.9 ± 2.0 to 25.5 ± 3.4 pg/ml. In the SAD cats, microinjection of V(1x) had no effect on contraction-induced increases in blood pressure or HR. These results suggest that baroreflex opposition of the reflex cardiovascular response to static contraction is enhanced by the action of AVP in the AP.