| Abstract | Serotonergic and dopaminergic activity in the brain is associated with increases in core temperature (Tre). Although the development of fatigue and increases in blood-brain barrier (BBB) permeability have been shown to be more pronounced in the heat than in thermoneutral environments, the influence of Tre on the indicators of central fatigue are not well understood. Central fatigue is believed to be dependent upon transport of Tryptophan (TRP), the precursor of Serotonin, into the brain, which is blocked at the BBB by Branch Chain Amino Acids (BCAA). Given that repeated exertional heat stress (EHS) induces a lower resting and exercise Tre, reduced physiological strain, and increased Tre tolerated, it would be of interest to know whether repeated EHS induces a reduced level of central fatigue.
PURPOSE: To examine the amino acids and neuroendocrine indicators related to central fatigue during 9 days of exposure to EHS in untrained (UT) males.
METHODS: Nine UT males (Mean ± SE: VO2peak = 51.7 ± 1.9 mL·kgLBM-1·min-1, 17 ± 2% fat) walked to exhaustion (EXH) on 9 days (over 2 consecutive weeks) on a treadmill (4.5 km·h-1, 2% incline) in 40°C and 30% R.H., wearing combat clothing and a protective hooded overgarment. Venous blood samples were obtained at rest (PRE; prior to heat) and at a Tre of 38, 38.5, 39°C, and EXH. Amino acids were determined by gas chromatography/mass spectrometry and cortisol (COR) by chemiluminescence.
RESULTS: Tre EXH was not different (p < 0.06) from Days 1 (38.8 ± 0.1°C) to 9 (39.1 ± 0.1°C), yet time to EXH increased from Days 1 (95 ± 8 min) to 9 (145 ± 13 min). Free-TRP was attenuated on Day 9 and BCAA values were elevated on Day 9 and at PRE following Day 1. The f-TRP/BCAA ratio was reduced on Day 9 compared to Day 1. No changes were observed for Tyrosine and Phenylalanine from Days 1 to 9 or PRE to EXH. COR responded to absolute thermal strain with greater values at EXH on Day 9 (600.5 ± 71.4 nmol·L-1) than Day 1 (472.5 ± 83.7 nmol·L-1).
CONCLUSION: Attenuation of f-TRP and elevated BCAAs on Day 9 of EHS are consistent with reduced central fatigue via increased f-TRP vs. BCAA competition at the BBB, and thus less f-TRP transport into the brain, increased protection of BBB integrity by COR and/or an increase in binding/re-binding of f-TRP to circulating albumin. |
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