Farris J W, Hinchcliff K W, McKeever K H, Lamb D R, Thompson D L
School of Physical Activity and Educational Services, College of Veterinary Medicine, The Ohio State University, Columbus, Ohio 43210, USA.
J Appl Physiol (1985). 1998 Sep;85(3):807-16. doi: 10.1152/jappl.1998.85.3.807.
We hypothesized that central fatigue may have a role in limiting the endurance capacity of horses. Therefore, we tested the effect of infusing tryptophan and/or glucose on endurance time and plasma concentrations of free tryptophan and other substrates thought to affect tryptophan uptake into the brain of seven mares (3-4 yr of age, 353-435 kg) that ran on a treadmill at 50% of maximal O2 consumption to fatigue. With use of a counterbalanced crossover design, the horses were infused with tryptophan (100 mg/kg in saline solution) or a similar volume of saline solution (placebo) before exercise. During exercise, horses received infusions of glucose (2 g/min, 50% wt/vol) or a similar volume of saline. 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, and concentration of plasma total solids before and during exercise were similar for all trials. Mean time to exhaustion was reduced (P < 0.05) for T & P and T & G compared 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 with nontryptophan trials, during the tryptophan trials plasma prolactin increased (P < 0.05) nearly threefold before exercise and almost twofold early in exercise. Muscle glycogen 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-chain amino acids to free tryptophan. In conclusion, tryptophan infusion reduced endurance time, which was consistent with the central fatigue hypothesis. The failure of glucose infusion to alleviate the effects of tryptophan and the absence of significant muscle glycogen reduction in the tryptophan trials suggest that the early onset of fatigue in the tryptophan trials is not due to a lack of readily available substrate.
我们推测中枢性疲劳可能在限制马匹耐力方面起作用。因此，我们测试了输注色氨酸和/或葡萄糖对七匹母马（3 - 4岁，353 - 435千克）耐力时间以及游离色氨酸和其他被认为会影响色氨酸进入大脑的底物血浆浓度的影响。这些母马在跑步机上以最大耗氧量的50%运动至疲劳。采用平衡交叉设计，在运动前给马匹输注色氨酸（100毫克/千克溶解于盐溶液中）或相同体积的盐溶液（安慰剂）。运动期间，马匹接受葡萄糖输注（2克/分钟，50%重量/体积）或相同体积的盐溶液。因此，处理方式有1）色氨酸和葡萄糖（T & G），2）色氨酸和安慰剂（T & P），3）安慰剂和葡萄糖（P & G），4）安慰剂和安慰剂（P & P）。所有试验中运动前和运动期间的平均心率、血细胞比容以及血浆总固体浓度相似。与P & P相比，T & P和T & G的平均疲劳时间缩短（P < 0.05）[分别为86.1 ± 6.9和87.1 ± 6.8分钟，而P & P为102.3 ± 10.3（标准误）分钟]，而P & G的耐力（122.4 ± 11.9分钟）比所有其他试验都长（P < 0.05）。与非色氨酸试验相比，在色氨酸试验中，运动前血浆催乳素增加（P < 0.05）近三倍，运动早期增加近两倍。仅在P & G和P & P试验中，肌肉糖原浓度降至运动前值以下（P < 0.05）。然而，葡萄糖输注（P & G）对血浆游离脂肪酸浓度或支链氨基酸与游离色氨酸的比值没有影响（P > 0.05）。总之，输注色氨酸会缩短耐力时间，这与中枢性疲劳假说一致。葡萄糖输注未能减轻色氨酸的影响，且在色氨酸试验中肌肉糖原没有显著减少，这表明色氨酸试验中疲劳的早期发生并非由于缺乏现成的底物。
