REGENERATED RAT FAST MUSCLE TRANSPLANTED TO THE SLOW MUSCLE BED AND INNERVATED BY THE SLOW NERVE, EXHIBITS AN IDENTICAL MYOSIN HEAVY-CHAIN REPERTOIRE TO THAT OF THE SLOW MUSCLE

The hypothesis that the limited adaptive range observed in fast rat mu scles in regard to expression of the slow myosin is due to intrinsic p roperties of their myogenic stem cells was tested by examining myosin heavy chain (MHC) expression in regenerated rat extensor digitorum lon gus (EDL) and soleus (SOL) muscles. The muscles were injured by bupiva caine, transplanted to the SOL muscle bed and innervated by the SOL ne rve. Three months later, muscle fibre types were determined. MHC expre ssion in muscle fibres was demonstrated immunohistochemically and anal ysed by SDS-glycerol gel electrophoresis. Regenerated EDL transplants became very similar to the control SOL muscles and indistinguishable f rom the SOL transplants. Slow type 1 fibres predominated and the slow MHC-I isoform was present in more than 90% of all muscle fibres. It co ntributed more than 80% of total MHC content in the EDL transplants. A bout 7% of fibres exhibited MHC-2a and about 7% of fibres coexpressed MHC-1 and MHC-2a. MHC-2x/d contributed about 5-10% of the whole MHCs i n re generated EDL and SOL transplants. The restricted adaptive range of adult rat EDL muscle in regard to the synthesis of MHC-1 is not roo ted in muscle progenitor cells; it is probably due to an irreversible maturation-related change switching off the gene for the slow MHC isof orm.