Plasma-electrolytes in natives to hypoxia after marathon races at different altitudes

Purpose: It is well known that altitude natives differ from sea level nativ es in aspects of fluid and electrolyte homeostasis, Methods: To evaluate ex ercise and environmental influences on the electrolyte and water status in hypoxia adapted subjects, we investigated 11 well-trained marathon runners (33.7 +/- 0.7 yr, 60.5 +/- 1.9 kg), native to an altitude above 2600 m, bef ore and after two marathon races. One competition was held at moderate alti tude (AM, 2650 m, 14 degrees C, 55% RH, running time 3 h 6 min +/- 22 min) and another under tropical conditions (HM, 470 m, 28degrees C, 70% RH, runn ing time 2 h 54 min +/-30 min). Blood samples were taken 3 d before, immedi ately after, 1 h after, and 24 h after the races, Results: The loss in body fluid was calculated to be 2.15 L during AM and 5.05 L during HM, respecti vely. It was compensated mostlyby ingested fluids without electrolyte conte nt and by metabolically produced water, which led to hyponatremia during AA I (plasma [Na+] from 144.3 +/- 0.7 to 131.7 +/- 2.1 mmol.L-1). Severe dehy dration without significant changes in plasma [Na+] could be detected after HM. Serum antidiuretic hormone concentrations and serum aldosterone concen trations significantly increased during both races and remained at a high l evel for at least 1 h after both competitions. Serum atrial natriuretic pep tide (ANP) concentrations were at a high level at rest, increasing during H M, and decreasing during AM. Conclusion: Under tropical conditions, we foun d a severe state of dehydration characterized by an extended ANP-response, which was not prevented by water intake during the race. Under hypoxic cond itions, however, we Found thathyponatremia had developed. This can be partl y explained by pure water intakeand metabolically produced water, and also, possibly, by ii special hypoxia-induced effect.