HPA and SAS responses to increasing core temperature during uncompensable exertional heat stress in trained and untrained males

Increases in core temperature (T-c) augment stress hormones and neurotransmitters; however, the effect of different Tc tolerated with varying fitness levels during uncompensable exertional heat stress (EHS) is unclear. The purpose was to examine the hypothalamic-pituitary-adrenal (HPA) axis and sympathetic-adrenomedullary system (SAS) responses during uncompensable EHS in trained (TR) versus untrained (UT) males. Twelve TR and 11 UT ((V) over dotO(2peak) = 70 +/- 2 and 50 +/- 1 mL kg of lean body mass(-1) min(-1)) walked on a treadmill to exhaustion (EXH) in 40 degrees C (dry), dressed in protective clothing. PRE and 0.5 degrees C T-c increments from 38.0-40.0 degrees C/EXH venous blood was obtained. Cortisol responded to absolute thermal strain, increasing throughout EHS and independent of fitness. Adrenocorticotropic Hormone, Norepinephrine, and Dehydroepiandrosterone- Sulphate responded to relative thermal strain with similar EXH values, despite higher T-c tolerated for TR (39.7 degrees C) than UT (39.0 degrees C). Epinephrine, Growth Hormone (GH), and Aldosterone increased initially, with a plateau above 38.5 degrees C T-c. Findings demonstrate the complexity of the HPA axis, SAS, and T-c relationship, with the stress pathways responding largely to relative thermal strain, although some hormones exhibited a clamping response likely as a protective mechanism. For the TR, evidence existed for a reduced pituitary sensitivity to glucocorticoids and the amplified GH may have contributed to their greater T-c tolerated.