To examine effects of aging and endurance training on human muscle metabolism during exercise, 31P magnetic resonance spectroscopy was used to study the metabolic response to exercise in young (21-33 yr) and older (58-68 yr) untrained and endurance-trained men (n = 6/group). Subjects performed graded plantar flexion exercise with the right leg, with metabolic responses measured using a 31P surface coil placed over the lateral head of the gastrocnemius muscle. Muscle biopsy samples were also obtained for determination of citrate synthase activity. Rate of increase in P(i)-to-phosphocreatine ratio with increasing power output was greater (P < 0.01) in older untrained [0.058 +/- 0.022 (SD) W-1] and trained men (0.042 +/- 0.010 W-1) than in young untrained (0.038 +/- 0.017 W-1) and trained men (0.024 +/- 0.010 W-1). Plantar flexor muscle cross-sectional area and volume (determined using 1H magnetic resonance imaging) were 11-12% (P < 0.05) and 16-18% (P < 0.01) smaller, respectively, in older men. When corrected for this difference in muscle mass, age-related differences in metabolic response to exercise were reduced by approximately 50% but remained significant (P < 0.05). Citrate synthase activity was approximately 20% lower (P < 0.001) in older untrained and trained men than in corresponding young groups and was inversely related to P(i)-phosphocreatine slope (r = -0.63, P < 0.001). Age-related reductions in exercise capacity were associated with an altered muscle metabolic response to exercise, which appeared to be due to smaller muscle mass and lower muscle respiratory capacity of older subjects.(ABSTRACT TRUNCATED AT 250 WORDS)

The risk of death appears to be greater in colic-affected horses with high circulating concentrations of epinephrine and cortisol. The correlation of epinephrine with other biochemical markers of illness severity and with heart rate indicates that the degree of sympathetic activation in horses with colic can be inferred from routinely measured variables.

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.

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