Effects of dietary sodium on circadian rhythm and physiological responses of 18-hydroxycorticosterone.

1. The effects of dietary sodium intake on plasma 18-hydroxycorticosterone (18-OHB) responses to physiological stimuli and recumbent 24-h-plasma 18-OHB levels have been examined in nine normal male subjects. 2. Basal supine levels of 18-OHB during a 40 mmol of sodium intake period (62.5 +/- 6.0 ng/dl) were considerably greater (P less than 0.0001) than the levels during a 200 mmol of sodium intake period (9.8 +/- 1.2 ng/dl). Further incremental and percentage changes of 18-OHB in response to graded dose infusions of angiotensin II and adrenocorticotropic hormone (ACTH) were greater during the 40 mmol of sodium intake period. 3. Although the mean 24 h levels of plasma 18-OHB during the 40 mmol of sodium intake period (43.9 +/- 4.0 ng/dl) were greater (P less than 0.001) than those during the 200 mmol of sodium intake period (9.4 +/- 1.2 ng/dl), the circadian rhythm of 18-OHB secretion was similar under the two extremes of sodium intake. 4. Factors which increase angiotensin II levels, such as sodium restriction, isometric exercise and angiotensin infusion, selectively increase 18-OHB and aldosterone, suggesting that angiotensin II increases 18-OHB and aldosterone secretion, in part, by modulation of the 18-hydroxylation reaction involved in conversion of corticosterone into 18-OHB.