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 (V1x) (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 V1x 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 V1x 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.

Static contraction causes a reflex-induced release of arginine vasopressin in anesthetized cats

We tested the hypothesis that brief static contraction of the triceps surae muscle causes reflex-induced increases in plasma arginine vasopressin (AVP) in anesthetized cats. Arterial blood samples, for measurement of plasma AVP, were taken before and after 30 s of electrically stimulated static contraction performed at a low intensity (<20% of maximal; n = 5), a high intensity (>70% of maximal; n = 7), and a high intensity after denervation of the triceps surae (n = 5). The low intensity contraction protocol was repeated during alpha-adrenergic blockade (n = 7) to minimize potential baroreflex-induced inhibition of AVP release. Passive stretch of the triceps surae was conducted (n = 5) to determine effects of muscle mechanoreceptor stimulation on the release of AVP. Low intensity contraction had no effect on plasma AVP. During alpha-adrenergic blockade, this same contraction intensity caused this peptide to increase from 12.8 +/- 2.1 to 17.7 +/- 2.6 pg/ml. High intensity contraction caused an increase in AVP (13.2 +/- 3.5 to 26.1 +/- 6.6 pg/ml) that was abolished by denervation (14.4 +/- 3. 7 vs. 17.1 +/- 6.6 pg/ml). Passive stretch had no effect on plasma AVP. These findings suggest that brief static contraction causes increases in plasma AVP that are reflex in nature, intensity dependent, opposed by the arterial baroreflex, and probably unrelated to muscle mechanoreceptor activation.