Mechanism of compensatory hypertrophy in skeletal muscle of the rat

Abstract

Compensatory hypertrophy (CH) of skeletal muscle, induced by functional elimination of synergistic muscles, is shown to be mainly the result of stretch brought about by the action of the antagonistic muscles. It results in an increase in diameter in muscle fibers of different histochemical types and is thus not restricted to certain types of motor units only. In early periods of CH, there is considerable decrease of activities of oxidative enzymes as indicated by histochemical observations. Increase of dry weight and prolongation of full contraction time, half contraction time, maximal rate of tension development, latency period and half relaxation time occurs in both the soleus and extensor digitorum longus (EDL) muscles undergoing CH. These changes are influenced by degree of stretch. Accordingly, if stretch is released by elimination of the antagonist, there is no weight increase and less prolongation of contraction time in the soleus muscle after tenotomy of its synergists. On the other hand, complete release of stretch by tenotomy is accompanied by changes opposite to those in CH, i.e., there is a decrease of weight and a shortening of contraction and relaxation times in both EDL and soleus muscles. The effect of stretch can be shown also in the denervated muscle. Weight decrease is less and prolongation of contraction time is more pronounced in the denervated solcus muscle undergoing CH due to tenotomy of the synergistic muscle: this is apparently due to summation effects of denervation and CH. Contrary to this, pronounced decrease of weight and little prolongation of contraction time is observed in the denervated and tenotomized soleus muscle. Increase of weight of the EDL muscle undergoing CH is still marked after 16 days and 3 months, but no changes in contractile behavior are observed at this period. Also the denervation changes are no longer affected by CH at later periods of time. The variations in changes of contractile behavior in muscles undergoing CH are explained by the varying degree of overlap of the filaments known to be affected by sarcomere length related to lengthening or shortening of the muscle. In interpretation of short-term experiments testing the effect of use and disuse on contractile properties of skeletal muscle peripheral effects of stretch have, theretofore, to be taken into account.