α-cardiac-like myosin heavy chain MHCIα is not upregulated in transforming rat muscle

The expression of MHCIα, an α-cardiac-like myosin heavy chain isoform, was studied in extensor digitorum longus (EDL) and tibialis anterior (TA) rat muscles undergoing fast-to-slow transition by chronic low-frequency stimulation (CLFS), a condition inducing a transient upregulation of MHCIα in rabbit muscle. In order to enhance the transformation process, CLFS was applied to hypothyroid rats. mRNA analyses were performed by RT-PCR, and studies at the protein level by immunoblotting and immunohistochemistry, using the F88 antibody (F88 12F8,1) demonstrated in the accompanying paper to be specific for MHCIα. In total RNA preparations from slow- and fast-twitch muscles, MHCIα mRNA was present at minute levels, at least three orders of magnitude lower than in cardiac atrium. As verified immunohistochemically, MHCIα is present only in intrafusal fibres of rat muscle. Moreover, MHCIα is not expressed in extrafusal fibres and, contrary to the rabbit, was not upregulated at both the mRNA and protein levels by CLFS. These results support our notion of species-specific responses to CLFS. Another antibody reported to be specific to MHCIα, BA-G5, was also investigated by immunoblot and immunohistochemical analyses. Its specificity could not be validated for skeletal muscles of the rat. BG-A5 was shown to cross-react with MHCIIb and MHCIβ. These results question an upregulation of MHCIα in transforming rat muscles as reported in studies based on the use of this antibody.