Abstract
The effect of hydration status on plasma vasopressin release was studied in six healthy volunteers undergoing a 15-min bicycle ergometer exercise. The hydration states were dehydration, produced by a 14-h abstinence from fluids, euhydration, and hyperhydration caused by drinking 20 ml kg-1 water 2 h before the exercise. The exercise load was individually adjusted to 70% of maximal oxygen uptake. Exercise resulted in a significant increase in plasma vasopressin only during euhydration, in spite of the marked increase in plasma osmolality in all hydration states. The vasopressin concentrations during hyperhydration wee significantly lower than during euhydration. Plasma volume and serum sodium were significantly affected by exercise. The highest responses were obtained in euhydration and the lowest in hyperhydration. On this basis, the body''s drive towards normal homeostasis and to the best ability to respond to the challenges of the environment is rational.

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