Persistence of myosin heavy chain-based fiber types in innervated but silenced rat fast muscle.

Myosin heavy chain (MHC) profile and size of fibers in deep and superficial regions of the adult rat medial gastrocnemius (MG) were determined after 4, 15, 30, and 60 days of inactivity induced by spinal cord isolation (SI). After 4 days, fiber size decreased by 33 to 50% and 36 to 46% in deep and superficial regions, whereas MHC composition was unaffected. By 15 days, these values were 45 to 78% and 51 to 69%, and MHC composition was shifting toward faster isoforms. By 60 days, there were no pure type I MHC fibers and increases from 1 to 18% and 78 to 93% in pure type IIb fibers in deep and superficial regions. The percentage of type I MHC (gel electrophoresis) was approximately 10 and approximately 3%, and of type IIb approximately 40 and approximately 60% in control and 60-day SI rats. Thus, adaptations in the MHC molecule occurred at a slower rate and for a longer duration than the atrophic response.