[Effect of chronic hypobaric hypoxia on water distribution in the rat]

Rats of both sexes chronically submitted to a simulated sltitude of 4700 m (440 mm Hg) in a hypopressure chamber show a different behavior in relation to total body water. The aim of the present work was to analyze the behavior of body fluids in these conditions in order to precise whether or not the observed change in total body water is accompanied by a difference in its distribution. Male, female and castrated female rats, submitted to 440 mm Hg during nine months and their respective controls maintained at ambient pressure, were used. At the end of that period, and previous insertion of a carotid cannula, total water, extracellular fluid and blood volume were determinated by isotopic dilution techniques (Fig. 1), using tritiated water, radiosulfate and 131I human serum albumin, respectively. Blood samples were obtained without interruption of the hypopressure. Immediately after the experiments, the animals were sacrificed and total body fat was determined. Results, expressed in percentage of lean body mass (Table 1), showed that in intact female rats the increase in total body water would be due to an increase in interstitial water volume and that there was a significant decrease in plasma water volume. On the other hand, in castrated females, exposure to hypoxia produced dehydration, by a decrease in intraellular water, without significant changes in plasma water volume. The results in males showed a tendency in the same direction, but the differences were not statistically significant. The difference found in water distribution can be adscribed to an effect of estrogens, which may be direct or mediated by other endocrine factors.