Contributions of baroreceptor reflex to the hypothermic effect of intraventricular angiotensin II in rats.

The mechanisms of the hypothermic effect of angiotensin II (AII) injected into the lateral ventricle were investigated in unanesthetized rats at an ambient temperature of 18 degrees C. Mean blood pressure (BP), heart rate (HR), metabolic rate (M), colonic temperature (Tcol), and temperatures of the interscapular brown adipose tissue (TBAT), and the tail skin (Tsk) were continuously monitored. AII at a dose of 5 micrograms produced a sharp and marked elevation in BP accompanied by bradycardia, and a decrease of M and Tco1 in the sinoaortic baroreceptor intact rats. The difference between TBAT and Tcol decreased significantly, which suggests a suppression of nonshivering thermogenesis of the BAT. Tsk was not changed by the AII injection. After sinoaortic denervation, however, the decrease in Tcol and M with AII injection was significantly reduced despite a marked elevation in BP. In addition, intravenous arginine-vasopressin antagonist pretreatment suppressed the elevation in BP and the decrease in HR, Tcol, and M after AII injection. From these results, it is concluded that the hypothermia which occurred after AII injection into the lateral ventricle can be largely attributed to the baroreflexive suppression of M, and to some extent to the direct effect on the thermoregulatory center in rats.