Changes in Water Metabolism during the Estrous Cycle in Three Species of Rodents

Abstract

Total body water (TBW) and the biological half-life of water (t-½) were estimated by the tritiated water dilution technique using both plasma and urine samples. Values for t-½ did not depend on the source of the sample. Urine sampling overestimated TBW by 2%-3%. Since values for t-½ were identical and those for TBW were similar for plasma and urine sampling, urine sampling could be useful in field studies of water metabolism. Changes in water metabolism during the estrous cycle were different in each of the three species studied. Female rats showed very transient increases in t-½ during the night of proestrus, but no changes in TBW. During estrus, t-½ gradually increased in female spiny mice. Maximum t-½ occurred following ovulation, at the start of the diestrus phase of the cycle. Lowest values were seen at the end of diestrus and in spayed females. No changes in TBW or body mass were seen during the cycle. Food intake was decreased and TBW was increased in female prairie dogs at estrus, but t-½, water intake, and body mass did not change at this time. Increases in TBW could be induced in anestrous prairie dogs by the injection of estradiol benzoate and progesterone but not by estradiol benzoate alone. Rats could not maintain their body mass when given vegetables as a water source, but both spiny mice and prairie dogs could. Spiny mice did not begin to lose mass until their water intake was reduced to 50% of ad lib. values. Prairie dogs began losing mass when their water intake was reduced to 30% of normal. Changes in water metabolism during the estrous cycle in spiny mice were probably a result of changes in water intake that were not sufficient to cause mass loss but did induce water conservation. Increase in TBW at estrus in the prairie dog were probably due to changes in body composition resulting from decreased food intake.