Urinary excretion of water, sodium, and total solutes by the pregnant rat

¹ Department of Physiology, Stanford University School of Medicine, Stanford, California

Excretory responses were studied before mating and at intervals during gestation in conscious rats given isotonic saline solution by stomach tube and in sodium pentobarbital-anesthetized rats infused with saline intravenously. In conscious rats excretion of water and solutes in relation to the dose given was enhanced on day 13 of gestation in comparison to before mating and significantly depressed on day 20. Depressed excretion of water on day 20 was proportional to the number of newborn pups and was accompanied by equally depressed excretion of sodium and total solutes. In anesthetized rats excretion of water, sodium, and total solutes was also significantly depressed on day 20 of gestation while glomerular filtration rate was only slightly depressed. Tubular reabsorption of water and solutes was significantly higher in 20-day pregnant rats than in nonpregnant rats. It is suggested that this increase in tubular reabsorption may represent a response to depletion of maternal extracellular fluids by the growing fetuses.