EFFECT OF WATER RESTRICTION DURING ADAPTATION TO A LOW-SODIUM DIET INMAN

1. Plasma sodium concentration may influence renal sodium excretion. W e have examined the possibility that the fall in plasma sodium that oc curs 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 r estricted (200ml/day) six normal subjects for the 2 days after the tra nsition from 260 (high-sodium diet, day 3) to 20mmol (low-sodium diet, days 4 and 5) sodium per day. In the control (hydrated) group water i ntake was held constant at 1800ml/day. 3. Plasma sodium fell during th e low-sodium diet in the hydrated group but remained constant in the d ehydrated group (141.3+/-0.2 to 140.2+/-0.2mmol/l versus 141.1+/-0.3 t o 141.3+/-0.3 mmol/l). Plasma arginine vasopressin concentration was s ignificantly 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+/-0.5pmol/l). Weight fell by a similar amount in both groups (hydrated group, 1.23+/-0.17 kg; dehy drated group, 1.45+/-0.19 kg). 4. On the low-sodium diet there were no differences between groups in changes in plasma renin activity (hydra ted group, 1.6+/-0.24 to 4.78+/-0.65nmol angiotensin I h(-1)ml(-1); de hydrated group 1.57+/-0.18 to 5.14+/-0.56nmol angiotensin I h-(1)ml-(1 )) or atrial natriuretic peptide (hydrated group, 23+/-2.3 to 14.7+/-1 .6pg/ml; dehydrated group, 26.8+/-3.6 to 12.7+/-1.3pg/ml). Salivary al dosterone concentration increased further in the dehydrated study, but only on day 5. 5. Sodium excretion fell further in the dehydrated gro up over the first 16h of the low-sodium diet on day 4 (hydrated group, 8.62+/-0.76 mmol/h; dehydrated group, 6.57+/-0.38mmol/h). Creatinine clearance fell on day 5 (low sodium) in the dehydrated group but did n ot change significantly in the hydrated group (hydrated group, 152+/-7 to 137+/-7; dehydrated group, 157+/-7 to 123+/-7ml/min). 6. We conclu de that the fall in plasma sodium during salt restriction is not an im portant trigger for renal sodium conservation in man. The principal di fference between studies was the elevation of arginine vasopressin in the dehydrated study. Differences in salivary aldosterone concentratio n occurred on day 5 only, too late to account for the differences in s odium excretion on day 4. Under the conditions of this study, our find ings could be explained if physiological range elevation of arginine v asopressin has an antinatriuretic effect.