Acid proteolytic capacity in mouse cardiac and skeletal muscles after prolonged submaximal exercise

Acid proteolytic capacity in mouse cardiac muscle and in predominantly white (distal head of m. vastus lateralis) or predominantly red (proximal red heads of m. vastus lateralis, m. v. medialis, and m. v. intermedius) skeletal muscle was estimated 5 days after 3 h, 6 h or 9 h prolonged running at a speed of 13.5 m/min. The activities of acid protease and beta-glucuronidase together with the rate of acid autolysis considerably increased in both skeletal muscle types, especially in red muscle, but did not increase in cardiac muscle. Acid proteolytic capacity and beta-glucuronidase activity increased in relation to the duration of running. Protein content and oxidative capacity (the activities of citrate synthase and malate dehydrogenase) decreased in red skeletal muscle after 6 h and 9 h running. In white muscle only protein content slightly decreased after 9 h running. No corresponding changes were observed in cardiac muscle. Histopathological changes were traced in mixed skeletal muscle (m. rectus femoris). Necrotic lesions were observed in the red superficial area of m. rectus femoris after 6 h and, in particular, after 9 h running. The results show that prolonged submaximal running also produces lethal and sublethal skeletal muscle fibre injuries, as well as exhaustive exercise or temporary ischaemia as reported earlier. It is suggested that sublethal injuries precede lethal ones and that acid proteolytic capacity increases especially in the sublethally injured muscle fibres.