Skeletal muscle weakness due to deficiency of CuZn-superoxide dismutase is associated with loss of functional innervation

An association between oxidative stress and muscle atrophy and weakness in vivo is supported by elevated oxidative damage and accelerated loss of muscle mass and force with aging in CuZn-superoxide dismutasedeficient (Sod1 ^-/-) mice. The purpose was to determine the basis for low specific force (N/cm ²) of gastrocnemius muscles in Sod1 ^-/- mice and establish the extent to which structural and functional changes in muscles of Sod1 ^-/- mice resemble those associated with normal aging. We tested the hypothesis that muscle weakness in Sod1 ^-/-mice is due to functionally denervated fibers by comparing forces during nerve and direct muscle stimulation. No differences were observed for wild-type mice at any age in the forces generated in response to nerve and muscle stimulation. Nerve- and muscle-stimulated forces were also not different for 4-wk-old Sod1 ^-/- mice, whereas, for 8- and 20-mo-old mice, forces during muscle stimulation were 16 and 30% greater, respectively, than those obtained using nerve stimulation. In addition to functional evidence of denervation with aging, fiber number was not different for Sod1 ^-/-and wild-type mice at 4 wk, but 50% lower for Sod1 ^-/- mice by 20 mo, and denervated motor end plates were prevalent in Sod1 ^-/- mice at both 8 and 20 mo and in WT mice by 28 mo. The data suggest ongoing denervation in muscles of Sod1 ^-/- mice that results in fiber loss and muscle atrophy. Moreover, the findings support using Sod1 ^-/- mice to explore mechanistic links between oxidative stress and the progression of deficits in muscle structure and function.