Stress hormones after prolonged physical training in normo- and hypobaric conditions in rats.

In order to study the long-term effects of prolonged physical training in hypobaric hypoxia on plasma stress hormone concentrations, male rats (N = 84) were exposed to progressive running training on a motordriven treadmill for 10, 21 or 56 days, twice a day and 5 days a week either in hypobaric hypoxic (O2-pressure 740-770 mbar) or in normobaric conditions. The plasma samples were taken 14-16 hours after the last exercise bout. Plasma corticosterone concentration was higher in animals trained 10 days in hypobaric conditions than in those trained in normobaric conditions (1127 +/- 158 nmol/l and 710 +/- 87 nmol/l, p < 0.05, respectively) while no effect of hypobaric conditions were seen in resting animals. Rats trained for 21 and 56 days in both normo- and hypobaria had significantly higher plasma corticosterone levels than the untrained animals. The changes in plasma ACTH and beta-endorphin concentrations were not significant in any group. According to our corticosterone results, we suggest that hypobaric hypoxia increases stress of training animals at the beginning of long-term training. The absolute work load was the same in both conditions, which probably at the beginning of training causes more stress to hypobaria trained rats than those trained in normobaria. Later when the adaptation to hypobaric hypoxia occurs, physical training itself seems to be responsible for increased plasma corticosterone levels in hypobaria as well as in normobaria.