CHANGES IN ENZYME ACTIVITIES OF VARIOUS MUSCLE FIBER TYPES IN RAT INDUCED BY DIFFERENT EXERCISES

Mutability of enzyme activities (phosphorylase, succinic dehydrogenase, cytochrome oxidase) of red and white fibers was studied in rat quadriceps subjected to normal physiologic chronic exercises. A rise in phosphorylase activity was found in weight lifting and to a greater extent in running rats when muscle was taken as a whole, but both exercises resulted in equal increments for succinic dehydrogenase and cytochrome oxydase activities. Intraregional comparisons, however, revealed the greatest relative rise of succinic dehydrogenase activity in those fibers that were regarded as predominantly anaerobic in type. This effect was seen only with running and not with weight lifting. Statistically unproved but frequently observed after running in some of the preparations was a rise of phosphorylase activity in red fibers, although to a lesser degree than in white. Thus, in contrast to the dichotomy apparent in electrophysiologic events in nerve and its dependent muscle, metabolic demands may alter what is regarded normally as fixed fiber enzyme patterns. Whole muscle cannot be studied as a biochemical entity because of diverse regional responses to the same stimuli. Endurance exercise (running) and brief, forceful exercise (weight lifting) produced quantitatively different regional changes in succinic dehydrogenase and probably in phosphorylase activities.