Biochemical adaptation of cardiac and skeletal muscle to physical activity

1. 1. Female Wistar rats were randomly assigned to control (C) or exercising (T) groups and subsequently portioned into 1, 3, 5 and 10 day T and C groups. The T groups completed a progressive endurance running program. Biochemical indices of adaptation were measured in cardiac muscle and in plantaris and soleus muscles of C and T animals after their last exercise bout. 2. 2. In cardiac muscle, myofibrillar ATPase activity was significantly elevated in the 3T (0.241 ± 0.031) and 5T (0.242 ± 0.013) groups ( P ⩽ 0.05) compared to their respective controls (3C = 0.187 ± 0.015 and 5C = 0.190 ± 0.007). 3. 3. After 10 days of training cardiac myofibrillar ATPase activity was elevated by 17% but this was not significant ( P ⩽ 0.05). 4. 4. No changes in myofibrillar ATPase activity were seen in skeletal muscle ( P ⩾ 0.05), however, hexokinase activity progressively increased and was significantly elevated in the 3T, 5T and 10T soleus and plantaris muscles of rats over controls ( P ⩽ 0.05). 5. 5. Minimal nonsignificant changes were noted in the hexokinase activity of the hearts of all T groups ( P ⩾ 0.05). 6. 6. These results indicate that metabolic adaptation of the heart and skeletal muscles takes place after as little as three training sessions. 7. 7. Although the adaptation of the skeletal muscles continually progresses, the adaptation of the heart appears to be transitory. 8. 8. The reason for the decline in the cardiac adaptation could be related to the increased respiratory capacity of skeletal muscle and/or to a decrease in the stimulus specific to the myocardium.