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 f ast-to-slow transforming rabbit muscle, direct reverse transcriptase-p olymerase chain reaction was applied for detecting mRNAs specific to f ive MHC isoforms in single fibers from control and low-frequency-stimu lated tibialis anterior muscles. The detection of MHCIIb, MHCIId(x), M HCI alpha, and MHCI beta mRNAs was based on previously published metho ds. The RT-PCR assay for MHCIIa mRNA was based on the identification o f a cDNA sequence in the 3'-region from which specific primers were de rived. Comparisons between rat, rabbit, and human MHCIIa sequences rev ealed high degrees of sequence identities. MHC mRNA isoform patterns i n single fibers from stimulated muscles showed hybrid fibers expressin g the following combinations: MHCIId(x) + MHCIIa, MHCIId(x) + MHCIIa MHCI alpha, MHCIId(x) + MHCIIa + MKCI alpha + MHCI beta, MHCIIa + MHC I 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 transi tion results from sequential isoform expressions in the order MHCIId(x ) --> MHCIIa --> MHCI alpha --> MHCI beta. The allocation of MHCI alph a between MHCIIa and MHCI beta seems to be in line with graded differe nces in sequence identity of the 3'-regions of these mRNA isoforms.