Effects of age on the role of atrial natriuretic factor in renal adaptation to physiologic variations of dietary salt intake.

The effects of normal, low, and high dietary salt intake on basal atrial natriuretic factor plasma levels, plasma renin activity, and aldosterone were evaluated in seven young (Group 1), seven middle-aged (Group 2), and seven elderly healthy volunteers (Group 3). In all subjects, progressively higher doses of human alpha-atrial natriuretic factor were infused at low-sodium diet conditions to obtain hormone plasma values during infusion similar to those obtained in the same subjects at high-sodium diet conditions. Atrial natriuretic factor plasma values were significantly higher in Group 3 than in the other two groups at both normal- and high-salt diet conditions, and at all steps of the infusion study. At low-sodium diet conditions, peptide concentrations averaged 23.2 +/- 6.2 in Group 3, 26.2 +/- 1.9 in Group 2, and 19.1 +/- 3.9 pg/mL in Group 1 (P = not significant between groups). Hormone plasma values at high-salt diet conditions averaged 47 +/- 6.9 pg/mL in Group 1, 60 +/- 6.5 pg/mL in Group 2, and 136.3 +/- 14.6 pg/mL in Group 3. Each value was not significantly different from the corresponding value gained at an infusion step of 2 ng/min per kg in Group 1 and 2 (57.1 +/- 11.9 and 62.7 +/- 6.5 pg/mL, respectively), and of 1 ng/min per kg (139.1 +/- 22.2 pg/mL) in Group 3. At these infusion steps and at high-salt diet conditions, the urinary sodium excretion rate was, respectively, 0.185 +/- 0.02 and 0.311 +/- 0.02 mEq/min in Group 1, 0.168 +/- 0.01 and 0.300 +/- 0.02 mEq/min in Group 2, and 0.110 +/- 0.01 and 0.256 +/- 0.01 mEq/min in Group 3. Hormone infusion induced a progressive fall of plasma renin activity and aldosterone level in all groups. By experimentally increasing plasma concentrations of atrial natriuretic factor in a low-salt diet condition to the levels occurring physiologically in a high-salt diet condition, a significant rise in urinary sodium excretion rate is evoked, which accounts for 52% in young, 47% in middle-aged, and 30% in older subjects of the rise that is necessary to balance the increased salt intake.