Effects of age and prolonged running on proteolytic capacity in mouse cardiac and skeletal muscles

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Abstract
Male NMRI‐mice, aged 3, 6, 9, and 12 months, were made to run for a period of 4 h at a speed of 13.5 m/min on a motor‐driven treadmill. 5 days after exertion, selected enzymatic estimates of acid and alkaline proteolytic as well as energy metabolic capacities were analyzed from the cardiac muscle and from the red and white parts of m. quadriceps femoris (MQF). The activities of alkaline and myofibrillar proteases increased most considerably in skeletal muscles with age. Cathepsin D and β‐glucuronidase activities were less affected in both muscles. Prolonged running increased the activities of cathepsin D, dipeptidyl aminopeptidase I and β‐glucuronidase in the white and, especially, in the red part of MQF. This stimulation of acid hydrolytic capacity was more prominent at the ages of 3 and 6 months than in the older animals. The estimates of alkaline proteolytic or energy metabolic capacities were not affected by prolonged running. In cardiac muscle, no significant changes were recorded in acid hydrolytic or energy metabolic capacity. Histological observation showed no necrosis or other pathological phenomena in the proximal part of m. rectus femoris after exertion. We suggest that the increased acid proteolytic capacity is involved in subcellular regenerative processes of skeletal muscle fibres. The smaller lysosomal response of older mice may indicate a reduced potential capacity for cellular repair.