Experimental investigations on the hypokinesis of skeletal muscles with different functions, V

The composition of myofibrillar proteins was studied in the soleus and gastrocnemius muscles of rabbit hind limbs immobilized by plaster cast in experiments lasting 1--4 weeks. The amounts of actin, M protein and C protein increased relative to the normal composition. The ratio of the light chain peptides of the fast muscle myosin changed from 1 : 2 : 1 to 1 : 2 : 0.5 as a result of 4 weeks of disuse. The LC-1 : LC-2 ratio of slow myosin did not change considerably while the amount of fast LC-3 peptide, hardly detectable in soleus muscle, increased more than tenfold. The amount of tropomyosin decreased significantly in both muscles. The submolecular composition of troponin changed, mostly in the slow muscle; TN--C and TN--I decreased significantly, whereas there was an increase in the TN--T values. It is concluded that the phenotype of the structural proteins of muscles with different functions is de-differentiated by disuse, while the genetic functions of the muscle cells is reprogrammed to the synthesis of contractile proteins (e.g. myosin) characteristic of the other type of muscle.