MITOCHONDRIAL-ENZYMES INCREASE IN MUSCLE IN RESPONSE TO 7-10 DAYS OF CYCLE EXERCISE

Endurance exercise training induces a significant increase in the resp iratory capacity of skeletal muscle. This is reflected by a training-i nduced increase in mitochondrial enzyme activity. One consequence of t his adaptation is that there is a decreased reliance on carbohydrate u tilization with a concomitant increase in fat utilization, resulting i n an improvement in endurance capacity. Recently it has been reported that 7-14 days of cycle ergometer-exercise training does not induce an increase in mitochondrial enzyme levels in skeletal muscle but, never theless, results in smaller decreases in phosphocreatine and glycogen and smaller increases in P-i and lactate in muscle in response to the same exercise after compared with before training. However, previous s tudies in rats have shown that an adaptive increase in mitochondrial e nzymes is already evident after only 2 days of exercise training. In v iew of this discrepency, the present study was performed to reevaluate the effect of short-term training (7-10 days) on mitochondrial enzyme s in skeletal muscle of humans. Twelve subjects [6 men and 6 women, 27 +/- 5 (SE) yr old] underwent 7 (n = 5) or 10 days (n = 7) of cycle er gometer exercise for 2 h/day at 60-70% of peak O-2 consumption. Peak O -2 consumption was increased by 9% (from 2.97 +/- 0.16 to 3.24 +/- 0.1 7 l/min) in response to training. Blood lactate levels were lower at t he same absolute work rates after than before training. The activities of citrate synthase, beta-hydroxyacyl-CoA dehydrogenase, mitochondria l thiolase, and carnitine acetyltransferase were increased by similar to 30% in response to training. The results of the present-study provi de evidence that-in humans, as in rats, the adaptive increase in mitoc hondrial enzymes in skeletal muscle occurs fairly rapidly in response to exercise training. They provide no support for the claim that this adaptive response is delayed for >2 wk after the onset of training.