Effects of unweighting and clenbuterol on myosin light and heavy chains infast and slow muscles of rat

To investigate the plasticity of slow and fast muscles undergoing slow-to-f ast transition, rat soleus (SOL), gastrocnemius (GAS), and extensor digitor um longus (EDL) muscles were exposed for 14 days to 1) unweighting by hindl imb suspension (HU), or 2) treatment with the beta(2)-adrenergic agonist cl enbuterol (CB), or 3) a combination of both (HU-CB). In general, HU elicite d atrophy, CB induced hypertrophy, and HU-CB partially counteracted the HU- induced atrophy. Analyses of myosin heavy (MHC) and light chain (MLC) isofo rms revealed HU- and CB-induced slow-to-fast transitions in SOL (increases of MHCIIa with small amounts of MHCIId and MHCIIb) and the upregulation of the slow MHCIa isoform. The HU- and CB-induced changes in GAS consisted of increases in MHCIId and MHCIIb ("fast-to-faster transitions"). Changes in t he MLC composition of SOL and GAS consisted of slow-to-fast transitions and mainly encompassed an exchange of MLC1s with MLC1f. In addition, MLC3f was elevated whenever MHCIId and MHCIIb isoforms were increased. Because the E DL is predominantly composed of type IID and IIB fibers, HU, CB, and HU- CB had no significant effect on the MHC and MLC patterns.