Renal Clearance of Water and Solutes by Pregnant Rats Treated with Spironolactone ∗

Abstract

Summary

Rates of filtration and urinary excretion of water, sodium, potassium, and total osmols were determined in renal clearance studies with nonpregnant and 20-day pregnant albino rats. Excretion of radiosodium-22 (²²Na) was determined as the percentage of dose excreted in the clearance urine sample begun 1 hr after a single intravenous dose of 100,000 cpm. Twenty-three rats were given no drugs and 25 were injected intramuscularly with 20 mg of spironolactone (Aldactone) just prior to initial hydration with saline solution. Significant reductions in the rates of excretion and in the percentage excretion of filtered loads of water, sodium, and total osmols were seen in pregnant rats as compared to nonpregnant females, and excretion of ²²Na was reduced by approximately 50%. Aldactone did not prevent these decreases in excretion of water, sodium, total osmols, and ²²Na. Under the conditions of this experiment water, sodium, osmols, and labeled sodium were retained by pregnant rats despite unchanged or increased GFR and a diminished concentration of effective salt-saving mineralocorticoids. These findings suggest increased proximal reabsorption of sodium as a mechanism for reduced urinary excretion of sodium late in gestation.