Contents of myosin heavy chains in denervated slow and fast rat leg muscles.

The total content of myosin heavy chain (MHC) and individual MHC isoforms were studied in 14-day denervated rat leg muscles: the slow-twitch (soleus) and fast-twitch (extensor digitorum longus and gastrocnemius) by biochemical methods. The weight of the denervated muscles decreased by about 50%, as compared to the control muscles. In all denervated muscles the total content of MHCs decreased, more so in the slow than in the fast muscles. We have observed that the proportion among the MHC isoforms changed: while MHC-1 and MHC-2B decreased, MHC-2A and MHC-2X increased. Taking into account muscle atrophy, the loss of MHC total content and the shift in pattern of MHC isoforms, the total net changes of the particular MHC isoforms were evaluated. It was found that the muscle content of each of the MHCs decreased after denervation, but their tissue concentration changed variously. The concentration of the MHC-1 and MHC-2B decreased in all denervated muscles, but that of the MHC-2A and MHC-2X changed variously, depending on the muscle. The concentration of MHC-2A decreased in the soleus and increased in the fast muscles, whereas the concentration of the MHC-2X changed inversely. In the denervated soleus a considerable amount of MHC-2X was expressed, while in the contralateral muscles this isoform was undetectable or appeared at trace levels.