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.