Peripheral markers of central fatigue in trained and untrained during uncompensable heat stress

The development of fatigue is more pronounced in the heat than thermoneutral environments; however, it is unclear whether biomarkers of central fatigue are consistent with the higher core temperature (T-c) tolerated by endurance trained (TR) versus untrained (UT) during exertional heat stress (EHS). The purpose of this study was to examine the indicators of central fatigue during EHS in TR versus UT. Twelve TR and 11 UT males (mean +/- SE (V) over dotO(2peak) = 70 +/- 2 and 50 +/- 1 mL kg LBM-1 min(-1), respectively) walked on a treadmill to exhaustion (EXH) in 40 degrees C (dry) wearing protective clothing. Venous blood was obtained at PRE and 0.5 degrees C T-c increments from 38 to 40 degrees C/EXH. Free tryptophan (f-TRP) decreased dramatically at 39.5 degrees C for the TR. Branch chain amino acids decreased with T-c and were greater for UT than TR at EXH. Tyrosine and phenylalanine remained unchanged. Serum S100 beta was undetectable (<5 pg mL(-1)). Albumin was greater for the UT from PRE to 39.0 degrees C and at EXH. Prolactin (PRL) responded to relative thermal strain with similar EXH values despite higher T-c tolerated for TR (39.7 +/- 0.09 degrees C) than UT (39.0 +/- 0.09 degrees C). The high EXH PRL values for both groups support its use as a biomarker of the serotonin and dopamine interplay within the brain during the development of central fatigue.