Changes in the contractile proteins of skeletal muscles induced by steroid myopathy.

The effect of chronic dexamethasone treatment was investigated on the myofibriller proteins of slow (soleus) and fast (semimembranous) muscles. The relative quantities of actin, protein C and protein M increased, while the myosin, troponin inhibitor and calcium binding components showed a relative decrease in both muscle types. The amount of troponin regulatory component and alpha-actinin decreased only in the fast muscle, while tropomyosin decreased significantly in the slow muscle. The isoenzyme pattern of fast muscle myosin was altered as a consequence of decreased myosin (LC-3 subunit.