Central fatigue is associated with the brain's increased synthesis of serotonin (5-HT) from tryptophan (TRP) during exercise. The central fatigue hypothesis proposes that the transportation of TRP across the blood-brain barrier increases during endurance exercise, due to a rise in the ratio of free TRP to Branched Chain Amino Acids (BCAA) which results from increased lipolysis. Serotogenic neurons are also sensitive to proton variations, and endogenous production of 5-HT during exercise may increase ventilation in order to maintain pH homeostasis. PURPOSE: These findings imply that up-regulation of central (brain) 5-HT occurs via two pathways: (1) ratio of free TRP/BCAA (2) blood acidosis. This study compared the relationship between plasma TRP and blood pH during high intensity, short duration or low intensity prolonged endurance exercise. METHODS: Fifteen male cyclists (mean age 26.1 ± 6.0) completed two exercise trials: 20-min at 85 % of peak power output or PPO (T20) and 90-min at 75 % PPO (T90) within a 14- day period. Variations in plasma TRP, pH, HCO3- and VE in relation to work duration and intensity were recorded at 0, 20, 40, 60, 80 and 100 % of T20 and T90. RESULTS: TRP and pH were not statistically different within trials, and better correlations between these variables were observed in T20 (r2 = 0.33, p<0.01) than in T90 (r2 = 0.28, p <0.05). VE and pH correlated in both trials (r2 = 0.87 and r2 = 0.65 respectively, both p < 0.01). HCO3- and TRP only correlated in T90 (r2 = 0.34, p<0.01). Exercise induces serotogenic modulation of autonomic function to restore pH homeostasis, the latter being more significant at T20. When the difference in intensity and duration was sufficient to induce changes of blood pH and HCO3-, no changes in total TRP were observed. CONCLUSIONS: These preliminary results demonstrate low blood pH may be a primary cause for increased brain 5-HT synthesis and fatigue. The processes involved in central fatigue still require further investigation by considering 5-HT in relation to acidosis and BCAA. |