1. The effect of sodium intake on the natriuresis and hormonal changes induced by head-out water immersion was studied in seven normal subjects during head-out water immersion and on a control day while successively on 20 mmol of sodium/day and 100 mmol of sodium/day diets. The effects of head-out water immersion were compared with those seen on the control day for both diets.
2. The natriuresis on the 100 mmol of sodium/day diet was significantly greater than on the 20 mmol of sodium/day diet (natriuretic peak: 10.3 ± 2.2 versus 3.9 ± 1 mmol of sodium/h; P < 0.01). The total sodium excretion during the 3 h of head-out water immersion was 26.2 ± 2.0 mmol on the 100 mmol of sodium/day diet and 9.9 ± 0.9 mmol on the 20 mmol of sodium/day diet (P < 0.01). In contrast, the increase in the plasma atrial natriuretic factor level was similar on both diets (peak plasma atrial natriuretic factor level 23.1 ± 1.9 versus 26.2 ± 1 pg/ml; not significant). As expected, the baseline serum aldosterone level was higher on the 20 mmol of sodium/day diet and, despite a significant suppression, remained significantly higher at the end of the third hour of head-out water immersion (peak serum aldosterone level: 495 ± 130 versus 197 ± 26 pmol/l, P < 0.06). Furthermore, there was an inverse relationship between the serum aldosterone level and the urinary sodium excretion at the time of peak natriuresis (r = −0.59, P < 0.01).
3. We conclude that the effect of sodium intake on the natriuresis induced by head-out water immersion is more dependent upon anti-natriuretic agents, such as aldosterone, than on natriuretic factors, such as atrial natriuretic factor.