The influence of activity on muscle size and protein turnover.

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Abstract
The effects of the loss of normal activity were studied in [rat] muscles held in a shortened position by immobilizing the ankle in a plaster cast. Since the innervation remained intact with this procedure, removal of the restraining cast allowed restoration of normal activity. The soleus and plantaris muscles grew as a function of time after the return of normal activity and these changes in tissue size were explained by changes in the average rates of protein synthesis and protein break-down as measured by sensitive in vitro techniques. Activity stimulated protein synthesis and inhibited protein break-down of the soleus muscle, enabling the tissue to accumulate protein. Blockage of de novo synthesis of RNA, but not DNA, severely restricted the normal, rapid enhancement of protein synthesis after the return of activity. The return of isotonic activity to the extensor digitorum longus muscle after immobilization in a lengthened position failed to fully compensate for the loss of the growth-promoting influence of stretch and the tissue gradually returned to the size of the control muscle. During the recovery process the higher rate of protein turnover and RNA concentrations of the immobilized muscle returned to the lower values of the control.