Reduced Aldosterone Secretory Response to Acute ACTH Stimulation in Sodium‐Restricted Elderly Subjects

The plasma aldosterone (PA) response to ACTH stimulation (5 μg/kg, im) under conditions of varied sodium intake was studied in 10 young (20–35 yr), 9 middle‐aged (41–56 yr) and 11 elderly (66–73 yr) normotensive subjects. No differences in serum sodium, serum potassium, plasma Cortisol and urinary sodium excretion were observed among the three groups during both a 130 mEq (normal) intake and a 25 mEq (low) sodium intake for three days. Basal plasma renin activity (PRA) and PA levels were lower in the elderly than in the young or middle‐aged groups. These age‐related decreases were slight with a normal sodium intake and pronounced with a low sodium intake. ACTH administration induced increases in PA in the three groups. However, the maximal net increment of PA above the baseline level after ACTH did not differ among the three groups with a normal sodium intake, but was lower in the elderly than in the young or middle‐aged groups with a low sodium intake. The ACTH‐induced maximal percentage increment of PA did not differ among the three groups during both the normal and the low sodium intakes. A significant positive correlation was found between the maximal net increment of PA and either the basal PRA or PA level in the combined group of young, middle‐aged and elderly subjects. Thus, aging causes a reduced aldosterone secretory response to ACTH, in parallel with decreases in basal PRA and PA levels, under conditions of low sodium intake.