Circulating free-tryptophan to tyrosine as a marker of central fatigue during heat stress

One theory of central fatigue during exertional heat stress involves the influence on central nervous system activation of the changing ratio of serotonin and dopamine in key thermoregulatory centres in the brain. Circulating concentrations of the precursors, free-tryptophan (f-TRP) and tyrosine (TYR), respectively, compete for the same active carrier across the blood-brain-barrier. Thus, changes in the circulating ratio of f-TRP/TYR may be indicative of changing neurotransmitters in the brain and may help explain the different thermotolerance between endurance trained (TR) and untrained (UT). PURPOSE: The purpose was to examine the use of f-TRP/TYR as an indicator of impending fatigue in TR and UT males during acute uncompensable heat stress to exhaustion (EXH). METHODS: Twelve TR and eleven UT (Mean\ub1SE: VO2peak= 70\ub12 and 50\ub11 mL\ub7kgLBM\u207b \ub9\ub7min\u207b \ub9, 11\ub11 and 16\ub11% fat, respectively) walked to EXH (Tre= 39.7 and 39.0\ub0C, respectively) on a treadmill (4.5 km\ub7h\u207b \ub9, 2% grade) in 40\ub0C and 30% R.H., wearing combat clothing and a protective hooded overgarment. Before and at a Tre of 38.0, 38.5, 39.0\ub0C, and EXH, venous blood samples were obtained. Serum was analyzed for prolactin (PRL) using a chemiluminescent immunoassay, and plasma analyzed for f-TRP and TYR using gas chromatography/mass spectrometry.Peer reviewed: YesNRC publication: N