[Effect of increased functional activity on the contractile function of skeletal muscle]

The adaptation of rats to the exercise program the intensity of which corresponded to metabolic power output of 190% VO2max, did not produce any significant changes in Mg2+ and Ca2+ activated ATPase activity in the different types of skeletal muscle fibers. The increase in the cross-sectional area of glycolytic muscle fibers was due to the hypertrophy of all cellular structures, especially the srcoplasmic reticulum. The sytochrome aa3 concentration increased by 23%. The submaximal exercise program (130% VO2max) elicited a significant elevation of Mg2+ activated actomyosin ATPase activity in the oxydative-glycolytic muscles and a decrease of the latter enzyme activity in glycolytic muscles. The endurance exercise (70% VO2max) caused a small decrease in Mg2+ activated actomyosin ATPase activity in the oxydative-glycolytic muscles and an increase in the oxydative ones. The character of the adaptive response of the actomyosin ATPase in different types of skeletal muscle fibers depends on the intensity and duration of the running program.