Body fluid volumes in rats with mestranol-induced hypertension

Because estrogens have been reported to produce sodium retention, this study investigated the possibility that hypertension in rats resulting from the ingestion of an estrogen used as an oral contraceptive could be due to increases in body fluid volumes. Female rats were given feed containing mestranol for 1, 3, and 6 mo; control rats were given the feed without mestranol. The mestranol-treated rats had higher arterial pressures than the controls only after 6 mo of treatment. Plasma volume, extracellular fluid volume, and total body water were measured in each rat by the distribution volumes of radioiodinated serum albumin, 35SO4, and tritiated water, respectively. Values for blood volume, interstitial fluid volume, and intracellular fluid volume were derived from these measurements. These body fluid volumes, expressed per 100 g of body weight, were not different between the mestranol-treated rats and their controls at any of the three treatment times. Due to differences in body weight and lean body mass between the mestranol-treated and the control rats, these volumes also were expressed per 100 g of lean body mass. Again, no differences were observed between the mestranol-treated rats and the control rats for any of these body fluid compartments at any of the treatment times. These studies, therefore, were unable to provide evidence that increases in body fluid volumes contributed to the elevated arterial pressure in this rat model of oral contraceptive hypertension.