Total and myofibrillar protein degradation in isolated soleus muscles after exercise.

The effect of exercise on the rate of total and myofibrillar protein degradation was determined by measuring the rate of release of tyrosine and 3-methylhistidine, respectively, from isolated rat soleus muscle strips after exercise. The rate of tyrosine release was 30-50% greater from the muscles of the exercised rats, whereas the rate of 3-methylhistidine release was unchanged. Thus the exercise-induced increase in the rate of protein degradation is due to increased breakdown of nonmyofibrillar proteins. The rate of protein degradation increases as a function of exercise duration and rapidly returns to the preexercise level during recovery. The exercise-induced increase in the rate of protein degradation is not inhibited by chloroquine. Together these observations suggest that the increase in the rate of protein degradation observed immediately after exercise is due to the breakdown of nonmyofibrillar proteins and occurs via the nonlysosomal pathway of protein degradation.