Circulating antidiuretic hormone in rats: effects of dietary electrolytes and protein

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Abstract
Plasma antidiuretic activity, believed to be antidiuretic hormone (ADH), has been studied in undisturbed rats: 1) fed diets containing normal amounts of protein but varying concentrations of sodium and potassium salts; 2) fed high-protein diets with and without supplementary urea and NaCl; or 3) dehydrated for periods up to 3 days. Plasma ADH in control rats was generally below the assay sensitivity of 4 µU/ml. With 24 hr dehydration ADH averaged 6.8 µU/ml plasma, and after 48–72 hr dehydration, 18.1 µU/ml. Of animals fed 27% protein diets, only in those whose diet contained very high salt (6% NaCl) was the average ADH level significantly elevated (about 5.5 µU/ml) and considerable variation from animal to animal was observed. Rats fed high-protein diets showed a more consistent rise in circulating ADH (range 4–8 µU/ml); many were above 6 µU/ml, calculated to produce maximal antidiuresis in the rat. High plasma ADH in dehydrated rats, or in rats fed high-salt diets may be explained by changes in plasma osmolality. The mechanisms producing increased ADH with high-protein intake are less clear. The possible effect of chronically elevated endogenous ADH on the renal concentrating mechanism is briefly discussed.

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