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 th at AVP acts in the AP to enhance baroreflex function during static con traction and, in turn, attenuates the exercise presser reflex. Thus me an arterial blood pressure (n = 9) and heart rate (HR) (n = 9) during 30 s of electrically stimulated hindlimb contraction were compared bef ore and after bilateral microinjections of 200 nl of the AVP V-1-recep tor antagonist d(CH2)(5)Tyr(Me)-AVP (V-1x) (1 ng/nl) into the AP of th e anesthetized cat. This protocol was repeated in three other cats in which sinoaortic denervation (SAD) was performed before any interventi on. Injection of V-1x into the AP had no effect on baseline blood pres sure or HR. However, presser and HR responses to static contraction we re augmented by 44 +/- 10 and 29 +/- 9%, respectively. Static contract ion 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 oft he reflex cardiovascular response to static contraction is enhanced by the action of AVP in the AP.