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

Abstract

Increases in core temperature (T _c) augment stress hormones and neurotransmitters; however, the effect of different T _c 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 ($\dot{V}{\text{O}}_{{2{\text{peak}}}} = 70 \pm 2$ and 50 ± 1 mL kg of lean body mass⁻¹ min⁻¹) walked on a treadmill to exhaustion (EXH) in 40°C (dry), dressed in protective clothing. PRE and 0.5°C T _c increments from 38.0–40.0°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°C) than UT (39.0°C). Epinephrine, Growth Hormone (GH), and Aldosterone increased initially, with a plateau above 38.5°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.