Changes in myosin heavy chain mRNA and protein isoforms of rat muscle during forced contractile activity.

A quantitative reverse transcriptase-polymerase chain reaction was established to determine absolute amounts of mRNAs specific to four myosin heavy chain isoforms [MHCIIb, MHCIId(x), MHCIIa, and MHCI beta] in rat extensor digitorum longus muscle during forced contractile activity by chronic (10 h/day) low-frequency stimulation (CLFS). The induced changes in absolute and relative mRNA amounts were similar. MHCIIb mRNA decreased rapidly after 1 day, and MHCIIa mRNA increased after 3 days. MHCIId(x) started to decrease at day 7. After 42 days, the MHCIIb, MHCIId(x), MHCIIa, and MHCI beta mRNAs amounted to 2, 6, 90, and 2% of total MHC mRNAs, respectively. Changes at the protein level were studied in a second experimental series increasing CLFS (24 h/day, up to 100 days). Also under these conditions, MHCI beta reached only a fraction of 12% (2-fold elevation). The changes at the protein level remained restricted to the MHCIIb to MHCIIa transition, which agrees with the notion that the induced changes in MHC isoform expression primarily resulted from altered pretranslational activities. Rat fast-twitch muscle thus exhibits a restricted capacity for fast-to-slow conversion.