Myosin heavy chain profile of cat soleus following chronic reduced activity or inactivity

To determine the role that normal neuromuscular activity plays in maintaining the myosin heavy chain (MHC) profile of adult cat soleus muscles, the spinal cords of 4 cats were transected (ST) and 8 cats were spinal isolated (SI) for 6 months. Nine nonoperated cats served as controls. Electrophoresis demonstrated that the soleus from control cats contained 98% type I, and 2% IIa MHCs. Both ST and SI resulted in decreased type I and increased IIa MHC, as well as de novo expression of IIb MHC. Immunohistochemistry with MHC-specific antibodies demonstrated that the soleus from control cats contained 99% type I, 1% IIa, and < 1% hybrid fibers (containing both type I and II MHCs). Following ST there were 67% type I, 17% IIa, 3% IIb, and 13% hybrid fibers. After SI, 48% of the fibers were type I, 11% were IIa, 1% were IIb, 25% were hybrid, and 15% contained embryonic MHC. Thus, normal levels of neuromuscular activity appear to be necessary for maintenance of the normal adult MHC profile in some fibers. Complete inactivation results in developmental MHC isoform expression in some fibers. Therefore, the dependence of a fiber on activity as a source of MHC modulation differs substantially among fibers even in a relatively homogeneous muscle.