Myosin heavy chain profiles in regenerated fast and slow muscles innervated by the same motor nerve become nearly identical

Plasticity of mature muscles exposed to different activation patterns is li mited, probably due to restricted adaptive range of their muscle fibres. In this study, we tested whether satellite cells derived from slow muscles ca n give rise to a normal fast muscle, if transplanted to the fast muscle bed . Marcaine-treated rat soleus and extensor digitorum longus (EDL) muscles w ere transplanted to the EDL muscle bed and innervated by the 'EDL' nerve. S ix months later expression of myosin heavy chain isoforms was analysed by a real densities of fibres, binding specific monoclonal antibodies, and by SD S gel electrophoresis. Both regenerated muscles closely resembled each othe r. Their myosin heavy chain profiles were similar to those in fast muscles although they were not identical to that in the control EDL muscle. Since n ot even regenerated EDL was able to reach the myosin heavy chain isoform pr ofile of mature EDL muscle, our experimental model did not permit studying the adaptive capacity of satellite cells in different muscles in its whole extent. However, the results favour the multipotential myoblast stem cell p opulation in rat muscles and underline the importance of the extrinsic regu lation of muscle phenotype.