SUMMARY1. The influence ofADH and the state ofpotassium balance on the renin-angiotensin system was studied in rats with hereditary diabetes insipidus (DI rats).2. Plasma renin concentration in DI rats was higher than in control Long-Evans rats.3. Spontaneous reversal of the hypokalaemia normally found in DI rats did not reduce plasma renin concentration (p.r.c.), suggesting that potassium deficiency does not contribute significantly to the elevation of p.r.c. in DI rats. Similarly, a low potassium diet failed to further increase p.r.c. in DI rats. 4. In contrast, the p.r.c. of DI rats was significantly diminished by a high potassium intake both in the presence and absence of ADH. A highly significant inverse correlation was found between p.r.c. and urinary potassium excretion in both ADH-treated and untreated DI rats on low, normal and high potassium diets.5. Plasma renin concentration was significantly lower in ADH-treated than in untreated DI rats on a high potassium intake, suggesting that the inhibitory effects of ADH and potassium are additive.6. ADH consistently reduced p.r.c. in DI rats independent ofthe state ofpotassium balance.7. ADH and potassium may inhibit renin secretion via different mechanisms of action.

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