Effects of vasopressin and spironolactone on excretion of water and solutes by the pregnant rat

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Abstract
Urinary excretion of water, Na, K, and total solutes was measured in conscious rats 6 hr. after hydration with isotonic saline solution. Saline was administered by stomach tube under light ether anesthesia to rats when nonpregnant and again when 20 days pregnant. Normal pregnant rats showed significant decreases in excretion of water, Na, and total solutes in response to saline loading near term. Injection of 0.5 unit of vasopressin tannate into nonpregnant females prior to loading did not cause decreased urine flow nor did pregnant rats injected with 1.0 unit of vasopressin retain administered water to a significantly greater degree than uninjected normal pregnant rats. Pregnant rats injected with 20 mg spironolactone (aldactone) alone, 1.0 unit of vasopressin alone, or both substances in combination failed to show decreased excretion of Na in response to saline loading near term. Reductions of maternal inulin space were seen in all groups of rats at term. The antidiuresis observed in normal pregnant rats after saline loading cannot be accounted for solely on the basis of increased supply of endogenous vasopressin, and that aldosterone and related salt-active steroid hormones actively function in the conservation of Na late in gestation.

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