Biochemical adaptations in muscle. Effects of exercise on mitochondrial oxygen uptake and respiratory enzyme activity in skeletal muscle.

The capacity of the mitochondrial fraction from gastrocnemius muscle to oxidize pyruvate doubled in rats subjected to a strenuous program of treadmill running. Succinate dehydrogenase, reduced diphosphopyridine nucleotide dehydrogenase, DPNH cytochrome c reductase, succinate oxidase, and cytochrome oxidase activities, expressed per g of muscle, increased approximately Z-fold in hind limb muscles in response to the training. The concentration of cytochrome c was also increased 2-fold, suggesting that the rise in respiratory enzyme activity was due to an increase in enzyme protein. The total protein content of the mitochondrial fraction increased approximately 60%. These changes in the concentration of cytochrome c and total mitochondrial protein are of special interest because they suggest that exercise could serve as a useful tool for studying the biosynthesis of mitochondrial proteins. Mild exercise, such as that used in previous studies, was found to have no effect on the level of succinate dehydrogenase in muscle, suggesting that the failure of earlier studies to show an increase in respiratory enzyme activity resulted from the use of an insufficient exercise stimulus. Mitochondria from muscles of the exercised animals exhibited a high level of respiratory control and tightly coupled oxidative phosphorylation. Thus, the increase in electron transport capacity was associated with a concomitant rise in the capacity to produce adenosine triphosphate. This adaptation may partially account for the increase in aerobic work capacity that occurs with regularly performed, prolonged exercise.