Water balance and lung fluids in rats at high altitude

A disturbed water and electrolyte homeostasis is not generally held to be a primary mechanism in the pathogenesis of acute mountain sickness (AMS) and high altitude pulmonary edema (HAPE), but the association of oliguria and weight gain with AMS and HAPE has led to the hypothesis that water retention may be a facilitative mechanism, possibly caused by an effect of hypoxia to release antidiuretic hormone (ADH). To examine the problem, normal Long-Evans rats (N) and the strain with congenital diabetes insipidus (DI) were exposed to hypobaric hypoxia (0.5 atm) for 4 days, and fluid balance in the whole animals and in their lungs was studied. Both strains reduced water intake and were oliguric on acute exposure, but the N rats gained body weight and increased lung water, while the DI rats increased neither body weight nor lung water. Neither strain increased lung blood at high altitude. The oliguria in the DI rats could not have been due to a release of antidiuretic hormone, and was attributed to the diminished water intake in both strains. The protection against HAPE in the DI rats was probably due to their more severe dehydration that exists already in normoxia, and its further increase in hypoxia, compared with N rats.