Effect of aging on renin activity and aldosterone excretion.

Twenty or more normal subjects (half of whom were males) in each decade of life from age 20 to 80 years were included in the study. Plasma renin activity (PRA) was measured with the subjects on a normal sodium diet and after sodium restriction and standing. Aldosterone and sodium excretion rate was measured on unrestricted sodium intake and on day 3 of sodium restriction. Plasma renin activity was quantitated by both bioassay and radioimmunoassay of angiotensin-I. Plasma renin activity under both test conditions declined with increasing age, especially those over 40. Values of those in the seventh decade averaged 60 per cent of the value of that of the younger age group. Response of renin activity to sodium restriction and standing was 73 per cent of that of the younger age group. Older females tended to be less responsive to sodium restriction and standing. Aldosterone excretion rate declined with increasing age so that by the seventh decade the values averaged 55 per cent of those in the second decade of life. Aldosterone excretion rate in response to sodium restriction also declined with increasing age, particularly above the fourth decade, so that the values on day 3 of sodium restriction in the older subjects averaged 33 per cent of that of the subjects in the second decade of life. Renin substrate did not decline with increasing age. The decline in renin activity and aldosterone values in the older subjects appeared to be due to factors other than sodium or potassium intake.