Transient expression of myosin heavy chain MHCIα in rabbit muscle during fast-to-slow transition

The expression of an α-cardiac-like myosin heavy chain, MHCIα, was investigated at both the mRNA and protein levels in rabbit tibialis anterior muscle undergoing fast-to-slow transition by continuous chronic low-frequency stimulation (CLFS). According to sequence analyses of the PCR product, the MHCIα isoform was found to be identical to the α-cardiac MHC expressed in rabbit atrium. In muscles at different degrees of transformation, the upregulation of MHCIα mRNA preceded that of the MHCIβ mRNA. At more advanced stages of the transformation, MHCIα mRNA decayed while MHCIβ mRNA persisted at high levels. The expression of MHCIα, therefore, was transitory. Studies at the protein level were based on immunoblotting using a monoclonal antibody (F88 12F8,1), characterized to be specific to MHCIα in rabbit muscle. These studies revealed a similar relationship between initial increase and successive decline of the MHCIα protein as seen at the␣mRNA level. Immunohistochemistry of 30-day stimulated muscle revealed that up to 65% of the fibres expressed the MHCIα isoform in combination with other adult MHC isoforms. The most frequent patterns of coexistence were MHCIIa+MHCIα + MHCIβ (28%), MHCIα+MHCIβ (18%), and MHCIIa + MHCIα (11%). According to these combinations, the upregulation of MHCIα may be assigned as an intermediate step in the transformation of existing fibres during the␣MHCIIa → MHCIβ transition. A small fraction of fibres contained, in addition to the MHCIα + MHCIβ and MHCIIa + MHCIα combinations, developmental myosin, suggesting that MHCIα was also expressed in regenerating fibres originating from satellite cell-derived myotubes.