Hormonal disturbances of fluid-electrolyte metabolism under altitude exposure in man.

Early alterations in fluid, electrolytes, and their regulating hormones were investigated in men exposed to 6,000 m simulated altitude (2 h-ascent, 2 h-sojourn, 2 h-return). Hematocrit and serum protein rose with elevated serum osmolality and reduced urine flow upon arrival at 6,000 m, suggesting decreased plasma volume probably due to hypotonic fluid shift to intracellular spaces. Serum K declined reflecting respiratory alkalosis. The exposure raised plasma antidiuretic hormone (ADH), plasma renin activity (PRA), serum cortisol and aldosterone. Increases both in ADH and aldosterone showed close correlations with that in cortisol, suggesting that ADH may be elevated by hypoxic stress in addition to elevated serum osmolality and decreased plasma volume, and that increased secretion of adrenocorticotropin may be the main cause of increased aldosterone, though PRA involvement cannot be excluded. These rises in ADH and aldosterone may act to retain body water, and the latter may exaggerate alkalosis; thus, these hormonal changes may be related to acute mountain sickness.