Role of arginine vasopressin on fluid and electrolyte balance in rats exposed to high altitude.

Normal rats (N) and rats with hereditary hypothalamic diabetes insipidus (DI) were employed to examine the role of arginine vasopressin (AVP) in the reduction of water intake and urine output during hypoxic exposure. The pattern of reduced water intake followed by recovery was similar in both N and DI rats during 7 days of hypobaric hypoxia (inspired O2 pressure of 75 Torr). Water intake was markedly reduced during the first 6 h of hypoxic exposure in both groups, whereas urine output did not decrease significantly until after 6 h in DI rats and after 18 h in N rats. Total urinary excretion of AVP in N rats decreased and remained depressed during 7 days of hypoxia. (AVP excretion corrected for osmolar clearance was unchanged.) Plasma AVP of conscious N rats was 2.7 +/- 0.40 pg/ml plasma during normoxia and 2.4 +/- 0.74 pg/ml plasma after 2 h of exposure to inspired O2 fractional concentrations of 0.105 (paired samples). We conclude that AVP is not a major factor in the decreased water intake and urine output observed during hypoxia, and that the initial disturbance is a reduced water intake that leads to negative water balance, reduced urine volume, increased urine concentration, and hyperosmotic volume contraction. The reduced or unchanged AVP release in normal rats during hypoxia appears to be inappropriate.