Maintenance of the adaptation of skeletal muscle mitochondria to exercise in old rats.

The purpose of this study was to determine whether the adaptive increase in mitochondrial enzymes in skeletal muscle induced by a constant exercise program is altered by aging. Male pathogen-free Long-Evans rats were exercised by means of swimming for 3h/d, 5d/wk beginning at age 6 months and studied at ages 9 and 24 months. The levels of activity of the four mitochondrial enzymes used as indicators of the capacity for aerobic metabolism (citrate synthase, succinate DH, 3-hydroxyacyl-CoA-DH, and 3-ketoacid-CoA-transferase) were significantly increased in epitrochlearis muscle in both the 9-month and 24-month-old swimmers. The increases ranged from approximately 20% for 3-hydroxyacyl-CoA-DH to 50-60% for 3-ketoacid-CoA-transferase. Neither the 24-month-old sedentary rats nor the 24-month-old swimmers had significantly lower muscle enzyme levels than the comparable 9-month-old rats. Thus, it appears that aging does not result in a progressive decline in the capacity of muscle for aerobic metabolism or a progressive impairment in the ability to maintain an increase in muscle mitochondrial enzymes in response to chronic exercise in healthy rats.