EFFECT OF CHRONIC EXPOSURE TO COLD ON VASCULAR RESPONSIVENESS TO PHENYLEPHRINE AND ANGIOTENSIN-II

Chronic exposure of rats to cold (5-degrees-C, 3-4 weeks) results in the development of hypertension. To assess potential mechanisms by which this may occur, the vascular responsiveness to administration of phenylephrine (an alpha-adrenergic agonist) and angiotensin II (AII) was studied in unanesthetized rats at 1, 3 and 5 weeks of exposure to cold (5-degrees-C). Vascular responsiveness to intravenous administration of graded doses of phenylephrine was reduced in cold-treated rats, the earliest effect being observed within 1 week of exposure. With respect to AII, vascular responsiveness to graded intravenous doses increased maximally within 1 week of exposure to cold and returned toward the level of the control group at 3 weeks. After 5 weeks of exposure to cold, it had returned to the level of the control group. These results suggest that vascular responsiveness to alpha-adrenergic stimulation appears to be directed toward prevention of an elevation of blood pressure in cold-treated rats. In contrast, vascular responsiveness to administration of AII is increased during the first 3 weeks of exposure to cold, at a time when plasma renin activity is also increased, and may thus play an important role in the initiation of cold-induced elevation of blood pressure.