Effects of hypotension and fluid depletion on central angiotensin-induced thirst and salt appetite.

We examined the effects of hypotension and fluid depletion on water and sodium ingestion in rats in response to intracerebroventricular infusions of ANG II. Hypotension was produced by intravenous infusion of the vasodilator drug minoxidil (25 microg x kg(-1) x min(-1)) concurrently with the angiotensin-converting enzyme inhibitor captopril (0.33 mg/min) to prevent endogenous ANG II formation. Hypotension increased water intake in response to intracerebroventricular ANG II (30 ng/h) but not intake of 0.3 M NaCl solution and caused significant urinary retention of water and sodium. Acute fluid depletion was produced by subcutaneous injections of furosemide (10 mg/kg body wt) either alone or with captopril (100 mg/kg body wt sc) before intracerebroventricular ANG II (15 or 30 ng/h) administration. Fluid depletion increased water intake in response to the highest dose of intracerebroventricular ANG II but did not affect saline intake. In the presence of captopril, fluid depletion increased intakes of both water and saline in response to both doses of intracerebroventricular ANG II. Because captopril administration causes hypotension in fluid-depleted animals, the results of the two experiments suggest that hypotension in fluid-replete animals preferentially increases water intake in response to intracerebroventricular ANG II and in fluid-depleted animals increases both salt and water intake in response to intracerebroventricular ANG II.