Plasma Adrenocorticotrophin and Cortisol Responses to Acute Hypoxia at Rest and During Exercise

Plasma adrenocorticotrophin (ACTH) and cortisol (F) concentrations were studied in six male subjects under normoxic (N) and acute hypoxic (H) conditions (altitude 3000 m) in a hypobaric chamber. Comparisons were made at rest, at 15, 30, and 60 min of exercise (65% VO2max), and after a 10 min recovery period. Mean (+/- SE) resting plasma ACTH levels were significantly higher in H (18.6 +/- 5.7 pmol.l-1) than in N (9.6 +/- 1.6 pmol.l-1) but no difference in resting plasma cortisol was observed between the two conditions. Both plasma ACTH and F concentrations were significantly increased at 60 min of exercise and during the recovery period under normoxic conditions. Hypoxia did not affect the ACTH response to exercise but reduced cortisol elevation. The changes in plasma cortisol concentration from rest to exercise were significantly correlated to ACTH under normoxic (r = 0.89, p less than 0.001) but not under hypoxic (r = 0.43, NS) conditions. Plasma lactate concentration was higher at the end of exercise in hypoxia (p less than 0.01), and no correlation existed between plasma lactate and ACTH levels. These observations provide further evidence that at sea level the increase in plasma cortisol levels during exercise is the result of ACTH-induced steroidogenesis. The responses observed at rest and during exercise in hypoxia suggest that adrenal sensitivity for ACTH may be altered.