Effect of tryptophan and of glucose on exercise capacity of horses

We hypothesized that central fatigue may have a role in limitingthe endurance capacity of horses. Therefore, we tested the effect ofinfusing tryptophan and/or glucose on endurance time and plasmaconcentrations of free tryptophan and other substrates thought toaffect tryptophan uptake into the brain of seven mares (3-4 yr ofage, 353-435 kg) that ran on a treadmill at 50% of maximalO₂ consumption to fatigue. Withuse of a counterbalanced crossover design, the horses were infused withtryptophan (100 mg/kg in saline solution) or a similar volume of salinesolution (placebo) before exercise. During exercise, horses receivedinfusions of glucose (2 g/min, 50% wt/vol) or a similar volume ofsaline. Thus the treatments were 1)tryptophan and glucose (T & G), 2) tryptophan and placebo (T & P), 3)placebo and glucose (P & G), and 4)placebo and placebo (P & P). Mean heart rate, hematocrit, andconcentration of plasma total solids before and during exercise weresimilar for all trials. Mean time to exhaustion was reduced (P < 0.05) for T & P and T & Gcompared with P & P 86.1 ± 6.9 and 87.1 ± 6.8 vs. 102.3 ± 10.3 (SE) min], whereas endurance for P & G(122.4 ± 11.9 min) was greater than for all other trials (P < 0.05). Compared withnontryptophan trials, during the tryptophan trials plasma prolactinincreased (P < 0.05) nearlythreefold before exercise and almost twofold early in exercise. Muscleglycogen concentrations were reduced(P < 0.05) below preexercise values in the P & G and P & P trials only. However, glucose infusions (P & G)did not affect (P > 0.05)concentrations of plasma free fatty acids or ratios of branched-chainamino acids to free tryptophan. In conclusion, tryptophan infusionreduced endurance time, which was consistent with the central fatiguehypothesis. The failure of glucose infusion to alleviate the effects oftryptophan and the absence of significant muscle glycogen reduction inthe tryptophan trials suggest that the early onset of fatigue in thetryptophan trials is not due to a lack of readily available substrate.