alpha-cardiac-like myosin heavy chain MHCI alpha is not upregulated in transforming rat muscle

The expression of MHCI alpha, an alpha-cardiac-like myosin heavy chain isof orm, 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 MHCI alpha in rabbit muscle. In order to enhance the transformation process, CLF S 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 pape r to be specific for MHCI alpha. In total RNA preparations from slow- and f ast-twitch muscles, MHCI alpha mRNA was present at minute levels, at least three orders of magnitude lower than in cardiac atrium. As verified immunoh istochemically, MHCI alpha is present only in intrafusal fibres of rat musc le. Moreover, MHCI alpha is not expressed in extrafusal fibres and, contrar y 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 CL FS. Another antibody reported to be specific to MHCI alpha, BA-G5, was also investigated by immunoblot and immunohistochemical analyses. Its specifici ty could not be validated for skeletal muscles of the rat. BG-A5 was shown to cross-react with MHCIIb and MHCI beta. These results question an upregul ation of MHCI alpha in transforming rat muscles as reported in studies base d on the use of this antibody.