Transient expression of myosin heavy chain MHCI alpha in rabbit muscle during fast-to-slow transition.

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