| Abstract | Understanding the neuroendocrine changes that occur during heat stress in trained (TR) versus untrained (UT) individuals may assist in identifying the mechanisms behind improved thermotolerance in trained individuals. Present knowledge suggests that an increase in core temperature is necessary for certain hormonal changes to be evident during acute exercise. However, the impact of core temperature on the neuroendocrine response in TR and UT individuals is unclear.
PURPOSE: The purpose of this study was to examine how the neuroendocrine response to acute heat stress in UT individuals compared to TR individuals at the same core temperature levels.
METHODS: Five UT males (Mean±SE: VO₂peak= 48 ± 2 mL·kgLBM·mm⁻ ¹, 14 ± 3% fat, 20.8 ± 5.8y) and ten TR males (VO₂peak= 68 ± 4 mL-kgLBM⁻ ¹min⁻ ¹, 10 ± 4 % fat, 24.2 ± 4.3y) walked to exhaustion on a treadmill (4.5 km·h⁻ ¹, 2% grade) in 40°C and 30% RH while wearing combat clothing and a protective overgarment with hood. Rectal temperature (Tre) reached 39.1 ± 0.1°C (UT) and 39.7 ± 0.1°C (TR). Venous blood samples were obtained at baseline (prior to the heat stress trial), and at T of 38.0, 38.5, 39.0°C, and exhaustion. Blood was centrifuged, separated, and serum stored frozen until analyzed for prolactin (PRL), growth hormone (GH), dehydroepiandrosterone (DHEA), cortisol (COR), and adrenocorticotrophic hormone (ACTH) using a chemiluminescent immunoassay (Immulite®).
RESULTS: Despite different relative intensities of exercise during the acute heat stress (TR = 27 ± 1, UT = 37 ± 2% of VO₂peak), there were no differences between TR and UT for PRL, DHEA, COR, and ACTH at the same T and at exhaustion. In contrast, GH was elevated at 38.0, 38.5, and 39.0°C in TR (15.9 ± 2.9, 17.3 ±2.2, and 15.7 ±2.6 μg·mL⁻ ¹, respectively) versus UT (8.4 ± 1.9, 6.4 ± 1.3, and 5.5 ± 1.1 μg·mL⁻ ¹, respectively) but there was no difference at exhaustion (12.0 ±3.4 and 7.6 ± 2.5 μg·mL¹, respectively). PRL, DHEA, COR, and ACTH increased significantly across exercise duration, where GH peaked at a Tre of 38.0°C.
CONCLUSION: In summary, PRL, DHEA, COR, and ACTH responses during acute heat stress were related to the level of Tre and independent of fitness level, whereas growth hormone response was influenced by both Tre and fitness level. |
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