Role of renin-angiotensin system in hypotension-evoked thirst: studies with hydralazine

Injection of rats either with diazoxide (25 mg/kg iv), isoproterenol (0.33 mg/kg sc), or hydralazine (HDZ) (10 mg/kg ip) decreased arterial blood pres sure from similar to 120 to 70-80 mmHg and stimulated renin secretion. Howe ver, diazoxide and isoproterenol treatments each stimulated water ingestion , whereas HDZ treatment did not. HDZ treatment did not reduce water intake evoked by systemic injection of hypertonic saline or 20% polyethylene glyco l solution or by 24-h water deprivation, suggesting that HDZ treatment did not interfere with drinking behavior. In contrast, HDZ treatment markedly r educed water intake evoked by injection of diazoxide or isoproterenol or by intravenous infusion of renin. Furthermore, a highly significant correlati on was observed when plasma ANG II levels were plotted as a function of pla sma renin activity after intravenous infusion of renin and after diazoxide and isoproterenol treatments. However, values obtained after HDZ treatment alone or in combination with intravenous infusion of renin did not fall nea r the 99% confidence interval of the regression line, suggesting that HDZ t reatment blocks ANG II production and/or promotes its clearance. Thus rats apparently do not increase water intake after HDZ treatment, because this d rug interferes with the renin-angiotensin system. These results provide fur ther evidence that arterial hypotension evokes thirst in rats predominantly by activation of the renin-angiotensin system.