Exercise myopathy: selectively enhanced proteolytic capacity in rat skeletal muscle after prolonged running.

The proteolytic capacity of rat skeletal muscle was analyzed during the repair of fiber injuries after strenuous exercise. A single bout of prolonged exercise (8 hr running at a speed of 17 m X min-1) caused a slight fiber necrosis and a selective response in the proteolytic activity of rat skeletal muscle. Acid proteolytic capacity (cathepsin D and acid autolysis) was considerably increased on the 4th day after exertion and partially decreased by the 10th day. The acid hydrolytic response was more prominent in red than in white skeletal muscle. Alkaline proteolytic capacity (alkaline and myofibrillar proteases), increased in several atrophic myopathies, was not affected in exercise myopathy. The rate of neutral autolysis slightly increased after exertion. The protein content of skeletal muscle was decreased on the 4th day after exertion. We suggest that the proteolytic responses to acute injuries, as well as to chronic atrophies, are highly selective in skeletal muscle fibers.