Hybrid fibres under slow-to-fast transformations: expression is of myosin heavy and light chains in rat soleus muscle.

The aim of this study was to examine the expression pattern of myosin heavy chain (MHC) and myosin light chain (MLC) isoforms in single fibres from the rat soleus muscle under control (Cont) conditions and under conditions inducing slow-to-fast phenotype transitions. Two models of muscle phenotype modification, namely 2 weeks clenbuterol (CB) administration or hindlimb unloading (HU), were chosen to achieve a full range of appearance of hybrid fibres, i.show $132#e. fibres co-expressing slow and fast myosin isoforms. MHC and MLC compositions were analysed in parallel by one-dimensional-gel electrophoresis. We showed that (i) the slow-to-fast fibre type transitions at the MHC level were accompanied by exchanges of slow with fast MLC isoforms and (ii) that these transitions were characterized by increased proportions of hybrid profiles of both MHC and MLC isoforms, under both CB (27.5%) and HU (18%) conditions when compared with Cont (7%). This suggested a MHC-MLC coordinated program for myosin regulation during fibre type transitions. However, mismatched hybrid co-expression of MHC and MLC was also observed, probably resulting from differences in post-transcriptional regulation. Finally, in all the muscle fibre groups, specific favourable correlations between one MHC (IIa, IId or IIb) and one MLC (regulatory or essential) type were found.