Alpha-cardiac-like myosin heavy chain as an intermediate between MHCIIa and MHCI beta in transforming rabbit muscle.

To elucidate the sequence of myosin heavy chain (MHC) transitions in fast-to-slow transforming rabbit muscle, direct reverse transcriptase-polymerase chain reaction was applied for detecting mRNAs specific to five MHC isoforms in single fibers from control and low-frequency-stimulated tibialis anterior muscles. The detection of MHCIIb, MHCIId(x), MHCI alpha, and MHCI beta mRNAs was based on previously published methods. The RT-PCR assay for MHCIIa mRNA was based on the identification of a cDNA sequence in the 3'-region from which specific primers were derived. Comparisons between rat, rabbit, and human MHCIIa sequences revealed high degrees of sequence identities. MHC mRNA isoform patterns in single fibers from stimulated muscles showed hybrid fibers expressing the following combinations: MHCIId(x) + MHCIIa, MHCIId(x) + MHCIIa + MHCI alpha, MHCIId(x) + MHCIIa + MHCI alpha + MHCI beta, MHCIIa + MHCI alpha, MHCIIa + MHCI alpha + MHCI beta, and MHCI alpha + MHCI beta. The combination MHCIIa + MHCI beta without MHCI alpha was never seen. These coexpression patterns suggest that the fast-to-slow fiber transition results from sequential isoform expressions in the order MHCIId(x)--> MHCIIa-->MHCI alpha-->MHCI beta. The allocation of MHCI alpha between MHCIIa and MHCI beta seems to be in line with graded differences in sequence identity of the 3'-regions of these mRNA isoforms.