Experimental studies on hypokinesis of skeletal muscle with different functions. VI.

In rabbits, the right hind limb was immobilized by means of plaster cast for 1, 2, 4 or 6 weeks and the activities of some metabolic enzymes: GOT, GPT, LDH, aldolase and acid phosphatase (in part of lysosomal origin), were examined in the slow m. soleus, and in the fast m. gastrocnemius. The former muscle is known to have mainly an oxidative, while the latter mainly a glycolytic type of metabolism. The activities of enzymes highly involved in the metabolism of the muscle diminished for a certain time during atrophy, then a relative rise occurred. Acid phosphatase activity likewise decreased after an initial relative increase. Reduction of enzymatic activities is explained by the activation of proteolytic enzymes, on the basis of measurements performed in these experiments and of results published by others. The decrease of enzymatic activity was more marked in the muscle which in normal state exhibits higher activity than in the other type of muscle studied. Thus, in the gastrocnemius a high rate of degradation of glycolytic enzymes was observed, while in the soleus degradation of oxidative enzymes prevailed. This phenomenon leads to the dedifferentiation of the muscle cell during immobilization.