Clenbuterol Antagonizes Rapamycin-Induced Atrophy in Rat Masseter Muscle

Objectives: It has been well known that in skeletal muscle clenbuterol (CB, a beta-2-adrenergic agonist) promotes muscle hypertrophy as well as a fiber type transition, whereas rapamycin (RAPA, a inhibitor of the mammalian target of rapamycin, mTOR) induces atrophy. To our knowledge, the interaction between the two drugs at the cellular level has not been studied. To gain more insight into the molecular mechanism of muscle growth and fiber-type transformations, we analyzed the effects of CB and/or RAPA on the muscle mass as well as on the mRNA levels of myosin heavy chains (MHC I, IIa, IId/x, IIb) in rat masseter. Methods: Wistar rats, aged 8 weeks, were divided into four groups: control (n=7), CB treatment (via the drinking water containing 30 mg/l, n=7), RAPA treatment (injected i.p. with 1.2 mg/kg RAPA once daily, n=7) and CB+RAPA treatment (n=7). After 2 weeks of each treatment, we measured both the muscle mass and the MHC mRNA levels using a real-time RT-PCR with specific primers in masseter. Results: As compared with control, the CB treatment significantly decreased the mRNA levels of MHC I (p<0.05), IIa (p<0.05) and IIb (p<0.05) but increased the MHC IId/x mRNA level (p<0.05), while the RAPA treatment did not change the mRNA levels of these MHCs. The CB+RAPA treatment significantly decreased the MHC I mRNA level (p<0.05) but increased the MHC IId/x mRNA level (p<0.05), as observed in the CB-treated preparations. The masseter muscle mass was significantly increased by either the CB (p<0.01) or the CB+RAPA treatment (p<0.01) but decreased by the RAPA treatment (p<0.05). Conclusion: These results suggest that in rat masseter CB induces not only the MHC transition (I→IIa→IId/x) quite independently of mTOR signaling pathways, but also the muscle hypertrophy via mTOR signaling pathways.