ADAPTIVE RANGE OF MYOSIN HEAVY-CHAIN EXPRESSION IN REGENERATING SOLEUS IS BROADER THAN IN MATURE MUSCLE

In adult rat muscles experimentally exposed to various patterns of act ivation, expression of myosin heavy chain isoforms changes, but only w ithin a certain adaptive range. It is characteristic and different in fast or slow muscles. This may be due either to different intrinsic pr operties of the myogenic cells of the two types of muscles or to extri nsic factors. To test these assumptions, either rat soleus or extensor digitorum longus muscles were injured and transplanted to the bed of the extensor digitorum longus muscle. They regenerated and were reinne rvated by the extensor digitorum longus nerve. Expression of myosin he avy chain isoforms was demonstrated immunohistochemically and by in si tu hybridization, and analysed by SDS-gel electrophoresis. Three month s after cross-transplantation, regenerated soleus expressed all adult myosin heavy chain isoforms, including the myosin heavy chain-2B. The latter was detected in about 50% of muscle fibres and contributed abou t 10-20% of all myosin heavy chains. The same percentage of myosin hea vy chain-2B was found in regenerated extensor digitorum longus. In thi s regard therefore, the adaptive range of the regenerated soleus muscl e was not significantly different from that of the extensor digitorum longus regenerating under the same conditions. This indicates that res triction of the adaptive range in a mature soleus muscle is not due to intrinsic properties of its myogenic cells. It is probably imposed by an extrinsic factor leading to irreversible shut-down of individual m yosin heavy chain genes. On the other hand, myosin heavy chain-1 expre ssion was significantly greater in the regenerated soleus than in the extensor digitorum longus innervated by the same nerve. Myosin heavy c hain-1 and myosin heavy chain-2B were coexpressed in some regenerated soleus muscle fibres.