| Abstract | Understanding the neuroendocrine stress response to heat stress in trained (TR) versus untrained (UT) individuals may assist in identifying the mechanisms behind improved thermotolerance in TR individuals. Present knowledge suggests that an increase in rectal temperature (Tre) augments the stress response and corresponding hormone and neurotransmitter changes. However, the impact of Tre on the stress hormones in TR and UT individuals is unclear.
PURPOSE: The purpose of this study was to examine the neuroendocrine response to acute uncompensable heat stress in TR compared to UT individuals at the same rectal temperatures.
METHODS: Eleven TR (Mean ± SE: VO2peak= 68 ± 1 mL-kgLBM-l-mm-1, 11 ± 1 % fat, 24.0 ± 1.2y) and nine UT (VO2peak= 50 ± 1 mL-kgLBM-l-min-1, 15 ± 1% fat, 23.6 ± 1.6y) males walked to exhaustion on a treadmill (4.5 km-h-1, 2% grade) in 40°C and 30% R.H. while wearing combat clothing and a protective overgarment with hood. Venous blood samples were obtained at baseline (prior to heat stress), and at a Tre of 38.0, 38.5, 39.0°C, and exhaustion. Blood was centrifuged, separated, and serum/plasma stored frozen until analyzed for dopamine (DA), norepinephrine (NE), epinephrine (E), and serotonin (5-HT) using gas chromatography/mass spectrometry.
RESULTS: Peak Tre reached was 39.7 ± 0.1°C for TR and 39.1 ± 0.1°C for UT, with exercise times of 165 ± 12 min for TR and 106 ± 11 min for UT. At the same absolute intensity of exercise, there were no differences in DA from baseline to a Tre 39°C between TR and UT, however, DA was significantly elevated at exhaustion compared to baseline. In contrast, E and NE increased similarly for both groups with increasing Tre from baseline (Mean ± SE: E=94 ± 17 and 80 ± 21 pg·mL-1, NE=362 ± 33 and 346 ± 31 pg·mL-1) to exhaustion (E=143 ± 28 and 153 ± 11 pg·mL-1, NE=1133 ± 67 and 1204 ± 98 pg·mL-1). Serotonin was significantly greater in TR (57 ± 12 ng·mL-1) versus UT (19 ± 5 ng·mL-1) throughout exercise to 39°C and at exhaustion (TR=51 ± 16 ng·mL-1 and UT=19 ± 4 ng·mL-1).
CONCLUSION: In summary, E and NE increased during acute uncompensable heat stress in proportion to the increases in Tre but changes were independent of fitness level, whereas 5-HT was increased with higher fitness levels. Elevations in DA were only detected at exhaustion in both groups. |
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