Effects of age and nerve-repair grafts on reinnervation and fiber type distribution of rat medial gastrocnemius muscles.

Aging is associated with a decline in skeletal muscle function. Previous research suggests that this decline in skeletal muscle function may, in part, be explained by an age-associated decline in the ability of skeletal muscle to reinnervate and/or age-associated changes in fiber types and distribution during reinnervation. This study used a nerve-repair graft model to investigate age-associated changes in the ability to reinnervate via expression of neural cell adhesion molecule (NCAM), a marker of denervated and recently reinnervated muscle fibers and changes in fiber type and Type I fiber grouping in medial gastrocnemius (MG) muscles of 6-, 12- and 24-month-old male Fischer 344 rats. Age had no effect on total MG muscle fiber number. Aging and nerve-repair grafting led to an increase in percent Type I and a decrease in percent Type IIB fibers. Aging and nerve-repair grafting led to an increase in NCAM positive fibers and an increase in the percentage of enclosed Type I muscle fibers, which was greatest in the 24 month nerve-repair grafted group. Thus, we conclude that diminished contractile function of aged and/or nerve-repair grafted MG muscle may be explained, in part, by an increase in the percentage of denervated fibers.