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Male Fischer rats of four age groups were subjected to a 3-mo exercise training program that consisted of a gradual increase to 1 h/day of forced swimming. Exercise was initiated at 1, 6, 12, or 17-22 mo of age. After the training period there was an increase in the heart weight relative to body weight in all groups, but heart weight was increased only in the two oldest groups. The specific activities of both actomyosin ATPase and creatine kinase isolated from cardiac muscle decreased with age. In animals that started exercise training at 6 mo of age the activities of both enzymes were higher than that of the age-matched sedentary controls, but the oldest animals (17-22 mo) responded negatively (i.e., a decreased enzymatic activity compared with sedentary individuals of the same age). These results suggest that, after a certain age, the initiation of endurance exercise may not result in the same adaptive response as occurs in younger animals. In the case of actomyosin ATPase this may be a consequence of a different distribution of myosin isozymes.

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