Hypothalamic-pituitary-adrenal axis and the sympathetic-adrenomedullary system responses during heat acclimation in sedentary males

Previous research suggests that the release of HPA and SAS stress hormones are functionally linked to the increase in core temperature during exercise and heat stress exposure. Yet, heat acclimation is known to increase tolerance time and the core temperature tolerated during uncompensable heat stress (UHS), and reduce physiological strain. It would be of interest, therefore, to know whether the HPA and SAS hormone response following heat acclimation is reduced at any given level of thermal strain. PURPOSE: To examine the HPA and SAS stress hormone responses in sedentary males during 9 days of exposure to UHS. METHODS: Nine sedentary males (Mean\ub1SE: VO2peak= 51.7\ub11.9 mL\ub7kgLBM\u207b\ub9\ub7min\u207b\ub9, 17\ub12% fat) walked to exhaustion (EXH) on 9 days (over 2 consecutive weeks) on a treadmill (4.5 km\ub7h\u207b\ub9, 2% incline) in 40\ub0C and 30% R.H., wearing combat clothing and a protective hooded overgarment. At rest (PRE; prior to heat) and at a Tre of 38, 38.5, 39\ub0C, and EXH venous blood samples were obtained. COR, GH, and ACTH were determined by chemiluminescence and NE and E by gas chromatography/mass spectrometry. RESULTS: Although Tre EXH was not different from Day 1 (38.8\ub10.1\ub0C) to 9 (39.1\ub10.1\ub0C), time to EXH increased from 95\ub18 to 145\ub113 min. Significant increases were observed from PRE to EXH for COR (357.0\ub18.5 to 491.3\ub118.8 nmol\ub7L\u207b\ub9), ACTH (4.7\ub10.2 to 21.7\ub11.9 pmol\ub7L\u207b\ub9), NE (1.9\ub10.0 to 4.7\ub10.0 nmol\ub7L\u207b\ub9), and E (208.4\ub13.5 to 547.5\ub118.9 pmol\ub7L\u207b\ub9) that were unaffected by heat acclimation. No differences also were observed between Day 1 and 9 from Pre to 39\ub0C for COR, ACTH, NE, and E (p 640.1). However, GH EXH on Day 1 (9.9\ub11.7 mIU\ub7mL\u207b\ub9) was significantly greater than Days 5, 6, and 9 (6.5\ub10.9, 6.4\ub10.9, and 7.7\ub10.2 mIU\ub7mL\u207b\ub9, respectively). CONCLUSION: Nine days of exertional heat acclimation improved the GH stress response, yet COR, ACTH, NE, and E remained temperature drivenPeer reviewed: YesNRC publication: N