Comparison of body fluid compartment sizes in Brattleboro homozygous and Long-Evans rats

Because of their hypothalamic diabetes insipidus (DI), Brattleboro homozygous rats exhibit profound polyuria and polydipsia with elevated plasma osmolalities. This combination of symptoms has led to the tacit assumption that these animals are chronically volume contracted. However, the few direct measurements that have been published indicate otherwise. In the present study, total body water (TBW), total body lipid content, and plasma volume were measured in conscious DI rats and compared with corresponding values in weight-matched conscious Long-Evans (LE) rats. Fat content of DI rats (5.0 +/- 0.4 g/100 g body wt) was significantly lower than that of LE rats (7.6 +/- 0.6, P less than 0.005). TBW was not significantly different between the two strains whether expressed as milliliters per 100 g wet wt (DI 69.2 +/- 0.4 vs. LE 67.7 +/- 0.7) or as milliliters per 100 g fat-free wet wt (DI 73.4 +/- 0.5 vs. LE 73.3 +/- 0.4). Plasma volume of DI rats (4.4 +/- 0.1 ml/100 g body wt) was significantly higher than that of LE rats (3.9 +/- 0.2, P less than 0.025). Extracellular fluid volume was similar in the two strains although, for this measurement, the animals were neither weight- nor age-matched. Accordingly, the data indicate that under the conditions of this study the DI rat is not chronically volume contracted. It would appear that, given an adequate water supply, water balance can be achieved via the thirst mechanism, perhaps abetted by vasopressin-independent mechanisms of urine concentration.