抄録
To gain more insight into the molecular mechanism of muscle growth and fiber-type transformations, we analyzed the effects of β2-adrenergic agonist, clenbuterol (CB) and/or cyclosporin A (CsA), a potent inhibitor of calcineurin (CaN), on the muscle mass as well as on the mRNA levels of myosin heavy chains (MHC I, IIa, IId/x, IIb) using a real-time RT-PCR with specific primers in rat masseter. As compared with control, the CB treatment significantly decreased the MHC I mRNA level (p < 0.01) but increased the MHC IId/x mRNA level (p < 0.01), while the CsA treatment significantly decreased the MHC I mRNA level (p < 0.05) in association with the significant decrease in MHC IIb mRNA level (p < 0.05). The CB+CsA treatment significantly decreased the levels of MHC I (p < 0.01) and IIa mRNAs (p < 0.05) but increased the MHC IId/x mRNA level (p < 0.001) in association with a significant decrease in MHC IIb mRNA level (p < 0.01), as compared with control. The masseter muscle mass was significantly (p < 0.001) increased by either the CB or the CB + CsA treatment but decreased with the CsA treatment (p < 0.01). These results suggest that in rat masseter muscle CB has an anabolic action accompanying MHC mRNA I → IIa → IId/x sequence transition independently of CaN-signaling pathways, and CaN is involved in the type I fiber gene expression and the muscle mass maintenance of type IIb fiber.
