Renal responses to acute angiotensin II inhibition and administered angiotensin II in the aging, conscious, chronically catheterized rat.

Studies of conscious, chronically catheterized, young (3 to 5 months of age) and old (19 to 22 months of age) male Sprague Dawley rats in the baseline state showed that glomerular filtration rate (factored for body weight) was lower and urine flow higher in old compared with young rats. Acute blockade of endogenous angiotensin II (AII) with either converting enzyme inhibitor (CEI) or Losartan (Dupont Merck, Wilmington, DE) caused small variable decreases in blood pressure in both age groups and produced a significant renal vasodilation with increases in renal plasma flow in the older rats. In separate studies using low-dose AII infusion (5 ng/kg body weight/min), an increase in filtration fraction was the only effect seen in both young and old rats with an intact renin/AII system; no effect was seen with CEI. During high-dose AII (20 ng/kg body weight/min), significant and similar increases were seen in blood pressure and renal vascular resistance in old and young rats, and the patterns of blood pressure and renal hemodynamic responses were similar with and without acute CEI. A natriuretic and diuretic response to high-dose AII was seen in young rats with intact endogenous renin/AII, whereas old rats were completely refractory to this action of AII. Against a background of acute CEI, no natriuretic/diuretic response to high-dose AII was seen in either age group. Thus, in the baseline state, renal hemodynamics in the old kidney are controlled by endogenous AII since CEI and Losartan produce renal vasodilation in old but not young rats.(ABSTRACT TRUNCATED AT 250 WORDS)