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 (approximately 50, 70, and 24%, respectively) and a decrease in SDH variability among fibers (approximately 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 (approximately 27 and 24%, respectively) and only an approximately 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 approximately 20, 45, and 60%, respectively, and actomyosin ATPase variability among fibers was approximately 60% lower. In contrast, only actomyosin ATPase activity of type IIb fibers was reduced (approximately 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.