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

To investigate the plasticityof slow and fast muscles undergoing slow-to-fast transition, rat soleus(SOL), gastrocnemius (GAS), and extensor digitorum longus (EDL) muscleswere exposed for 14 days to 1) unweighting by hindlimbsuspension (HU), or 2) treatment with the₂-adrenergic agonist clenbuterol (CB), or 3)a combination of both (HU-CB). In general, HU elicited atrophy, CBinduced hypertrophy, and HU-CB partially counteracted the HU-induced atrophy. Analyses of myosin heavy (MHC) and light chain (MLC) isoformsrevealed HU- and CB-induced slow-to-fast transitions in SOL (increasesof MHCIIa with small amounts of MHCIId and MHCIIb) and theupregulation of the slow MHCIa isoform. The HU- and CB-induced changesin GAS consisted of increases in MHCIId and MHCIIb("fast-to-faster transitions"). Changes in the MLC composition ofSOL and GAS consisted of slow-to-fast transitions and mainlyencompassed an exchange of MLC1s with MLC1f. In addition, MLC3f waselevated whenever MHCIId and MHCIIb isoforms were increased. Becausethe EDL is predominantly composed of type IID and IIB fibers, HU, CB,and HU-CB had no significant effect on the MHC and MLC patterns.