Role of renin-angiotensin system in the impairment of baroreflex control of heart rate in renal hypertension.

Studies of the baroreceptor heart rate reflex were performed in two-kidney, one clip (2K1C) hypertensive rats to evaluate the relative importance of two factors--high blood pressure and high angiotensin II circulating levels--on impairment of the baroreflex, present in the acute phase of this model of hypertension. The sensitivity of baroreceptor reflex was determined by the slope of the relationship between changes in mean arterial pressure (MAP) and changes in heart rate in response to injections of phenylephrine and nitroprusside. Bradycardic and tachycardic responses were analyzed separately. In basal conditions, the slope of the MAP-heart rate relationship in 2K1C hypertensive animals was significantly lower than in control animals, both for tachycardic and bradycardic responses. Lowering of blood pressure with captopril to normotensive levels in the 2K1C animals significantly increased baroreflex gain in bradycardic responses to the level found in normotensive rats. Normalization of blood pressure with nitroprusside did not change baroreflex sensitivity. Infusion of angiotensin II at a dose that did not change MAP, previously normalized with captopril, completely reverted the effect of this agent on baroreflex sensitivity. Our data indicate that, in 2K1C hypertensive rats, decreased baroreflex sensitivity is mediated, at least in part, by high angiotensin II circulating levels. Elevated blood pressure per se is of secondary importance.