title: 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 (VO(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.
authors.affiliation: Faculty of Health, Kinesiology and Health Science, York University, Toronto, ON, M3J 1P3, Canada. hewright@yorku.ca
authors.forename: Heather E
authors.initials: HE
authors.lastname: Wright
authors.forename: Glen A
authors.initials: GA
authors.lastname: Selkirk
authors.forename: Tom M
authors.initials: TM
authors.lastname: McLellan
mesh_terms: D000222:Adaptation, Physiological / Q000502:physiology; D000293:Adolescent; D000313:Adrenal Medulla / Q000378:metabolism / Q000502:physiology*; D000328:Adult; D001831:Body Temperature / Q000502:physiology*; D001833:Body Temperature Regulation / Q000502:physiology; D005080:Exercise Test; D017079:Exercise Tolerance / Q000502:physiology; D006339:Heart Rate / Q000502:physiology; D018882:Heat Stress Disorders / Q000209:etiology / Q000378:metabolism / Q000503:physiopathology* / Q000534:rehabilitation; D006801:Humans; D007030:Hypothalamo-Hypophyseal System / Q000378:metabolism / Q000502:physiology*; D008297:Male; D005082:Physical Exertion / Q000502:physiology; D010913:Pituitary-Adrenal System / Q000378:metabolism / Q000502:physiology*; D055070:Resistance Training; D013546:Sweating / Q000502:physiology; D013564:Sympathetic Nervous System / Q000378:metabolism / Q000502:physiology*; D055815:Young Adult