Antidiuretic hormone responses to eucapnic and hypocapnic hypoxia in humans

Urinary excretion rate of antidiuretic hormone (UADHV) was studied in male volunteers in response to hypobaric hypoxia. The first series consisted of three groups. The chamber was decompressed to 465, 495, and 438 Torr during high-altitude (HA) exposure for groups I (n = 5), II (n = 5), and III (n = 4), respectively. In group I, the chamber air contained 3.77% CO2 to prevent alkalosis. The level of hypoxemia was similar in groups I and II. Mean 24-h UADHV was unchanged in group I, but increased 96% (P less than 0.05) and 180% (P less than 0.05) in groups II and III, respectively, on day 1 at HA and was normal during subsequent days at HA regardless of symptoms of acute mountain sickness. Shorter sampling intervals employed in a second series of experiments conducted at 495 Torr revealed a twofold increase in UADHV (P less than 0.05) 8-12 h after ascent in eight asymptomatic subjects; UADHV returned to base line within 9 h and remained low. The symptomatic subjects both had increased UADHV (3- and 8-fold from base line) between 2 and 4 h after ascent. Increased UADHV in asymptomatic subjects may be a result of the concomitant decrease in plasma volume, both of which appeared to be eliminated by CO2 supplementation.