1. Plasma sodium concentration may influence renal sodium excretion. We have examined the possibility that the fall in plasma sodium that occurs during salt restriction in man might be an important stimulus for renal sodium conservation.

2. In order to prevent the fall in plasma sodium that usually occurs during dietary salt restriction, we water restricted (200 ml/day) six normal subjects for the 2 days after the transition from 260 (high-sodium diet, day 3) to 20 mmol (low-sodium diet, days 4 and 5) sodium per day. In the control (hydrated) group water intake was held constant at 1800 ml/day.

3. Plasma sodium fell during the low-sodium diet in the hydrated group but remained constant in the dehydrated group (141.3 ± 0.2 to 140.2 ± 0.2 mmol/l versus 141.1 ± 0.3 to 141.3 ± 0.3 mmol/l). Plasma arginine vasopressin concentration was significantly higher and urine flow lower during the low-sodium diet in the dehydrated group (arginine vasopressin on day 5: hydrated group, 0.72 ± 0.1 pmol/l; dehydrated group, 2.18 ± 0.5 pmol/l). Weight fell by a similar amount in both groups (hydrated group, 1.23 ± 0.17 kg; dehydrated group, 1.45 ± 0.19 kg).

4. On the low-sodium diet there were no differences between groups in changes in plasma renin activity (hydrated group, 1.6 ± 0.24 to 4.78 ± 0.65 nmol angiotensin I h−1 ml−1; dehydrated group 1.57 ± 0.18 to 5.14 ± 0.56 nmol angiotensin I h−1 ml−1) or atrial natriuretic peptide (hydrated group, 23 ± 2.3 to 14.7 ± 1.6 pg/ml; dehydrated group, 26.8 ± 3.6 to 12.7 ± 1.3 pg/ml). Salivary aldosterone concentration increased further in the dehydrated study, but only on day 5.

5. Sodium excretion fell further in the dehydrated group over the first 16 h of the low-sodium diet on day 4 (hydrated group, 8.62 ± 0.76 mmol/h; dehydrated group, 6.57 ± 0.38 mmol/h). Creatinine clearance fell on day 5 (low sodium) in the dehydrated group but did not change significantly in the hydrated group (hydrated group, 152 ± 7 to 137 ± 7; dehydrated group, 157 ± 7 to 123 ± 7 ml/min).

6. We conclude that the fall in plasma sodium during salt restriction is not an important trigger for renal sodium conservation in man. The principal difference between studies was the elevation of arginine vasopressin in the dehydrated study. Differences in salivary aldosterone concentration occurred on day 5 only, too late to account for the differences in sodium excretion on day 4. Under the conditions of this study, our findings could be explained if physiological range elevation of arginine vasopressin has an antinatriuretic effect.

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