Effects of bite-opening and cyclosporin A on the mRNA levels of myosin heavy chain and the muscle mass in rat masseter.

To gain more insight into the mechanism of muscle plasticity in response to mechanical overload, we analyzed the effects of bite -opening (BO, 3 mm increase in the vertical dimension for 2 weeks) and/or a calcineurin (CaN) inhibitor, cyclosporin A (CsA, 10 mg/kg body weight, once daily for 2 weeks, ip) treatment on the myosin heavy chain (MHC I, IIa, IId/x, IIb) mRNA levels, using real-time RT-PCR with specific primers and on the muscle mass in rat masseter. As compared with normal control (n = 6), the BO treatment (n = 6) significantly increased the MHC I (p < 0.05) and the IIa mRNA levels (p < 0.01), and the CsA treatment (n = 6) significantly decreased the MHC I mRNA level (p < 0.01) in association with the significant decrease in the MHC IIb mRNA level (p < 0.05). The BO + CsA treatment (n = 6) significantly increased the MHC IIa mRNA level (p < 0.01) in association with the significant decrease in the MHC IIb mRNA level (p < 0.01), as compared with control. The masseter muscle mass was significantly decreased by either the CsA (p < 0.05) or the BO + CsA treatment (p < 0.001), but slightly increased by the BO treatment. These results suggest that in rat masseter the BO treatment produces not only the up-regulation of MHC IIa mRNA independently of CaN-signaling pathways, but also the MHC mRNA transition from IIa to I and the muscle mass maintenance mainly of type IIb fiber through the CaN-signaling pathways.