Slow-to-fast transitions in myosin expression of rat soleus muscle by phasic high-frequency stimulation.

Denervated soleus muscles of euthyroid and hyperthyroid rats were exposed to phasic high-frequency stimulation for periods of up to 40 days and analysed for their myosin heavy chain (MHC) composition. Denervation alone induced appreciable amounts of the fast MHCIId/x and minute amounts of MHCIIb. However, the effects of phasic high-frequency stimulation exceeded by far those of denervation, leading to marked increases of these two isoforms, as well as to pronounced decreases in slow MHCI. In addition, the present study suggested a greater impact of neural activity on myosin expression than thyroid hormone.