Metabolic and phenotypic adaptations of diaphragm muscle fibers with inactivation

We hypothesized that metabolic adaptations to muscle inactivity are most pronounced when neurotrophic influence is disrupted. In rat diaphragm muscle (Dia(m)), 2 wk of unilateral denervation or tetrodotoxin nerve blockade resulted in a reduction in succinate dehydrogenase (SDH) activity of type I, IIa, and IIx fibers (~50, 70, and 24%, respectively) and a decrease in SDH variability among fibers (~63%). In contrast, inactivity induced by spinal cord hemisection at C₂ (ST) resulted in much less change in SDH activity of type I and IIa fibers (~27 and 24%, respectively) and only an ~30% reduction in SDH variability among fibers. Actomyosin adenosinetriphosphatase (ATPase) activities of type I, IIx, and IIb fibers in denervated and tetrodotoxin-treated Dia(m) were reduced by ~20, 45, and 60%, respectively, and actomyosin ATPase variability among fibers was ~60% lower. In contrast, only actomyosin ATPase activity of type IIb fibers was reduced (~20%) in ST Dia(m). These results suggest that disruption of neurotrophic influence has a greater impact on muscle fiber metabolic properties than inactivity per se.