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 anteri or muscle undergoing fast-to-slow transition by continuous chronic low-freq uency 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 transformat ion, the upregulation of MHCI alpha mRNA preceded that of the MHCI beta mRN A. At more advanced stages of the transformation, MHCI alpha mRNA decayed w hile MHCI beta mRNA persisted at high levels. The expression of MHCI alpha, therefore, was transitory. Studies at the protein level were based on immu noblotting using a monoclonal antibody (F88 12F8,1), characterized to be sp ecific to MHCI alpha in rabbit muscle. These studies revealed a similar rel ationship between initial increase and successive decline of the MHCI alpha protein as seen at the mRNA level. Immunohistochemistry of 30-day stimulat ed muscle revealed that up to 65% of the fibres expressed the MHCI alpha is oform in combination with other adult MHC isoforms. The most frequent patte rns of coexistence were MHCIIa+MHCI alpha+MHCI beta (28%), MHCI alpha+MHCI beta (18%), and MHCIIa+MHCI alpha (11%). According to these combinations, t he upregulation of MHCI alpha may be assigned as an intermediate step in th e transformation of existing fibres during the MHCIIa --> MHCI beta transit ion. A small fraction of fibres contained, in addition to the MHCI alpha+MH CI beta and MHCIIa+MHCI alpha combinations, developmental myosin, suggestin g that MHCI alpha was also expressed in regenerating fibres originating fro m satellite cell-derived myotubes.