Metabolic and phenotypic adaptations of diaphragm muscle fibers with inactivation

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Abstract
Zhan, Wen-Zhi, Hirofumi Miyata, Y. S. Prakash, and Gary C. Sieck. Metabolic and phenotypic adaptations of diaphragm muscle fibers with inactivation. J. Appl. Physiol. 82(4):1145–1153, 1997.—We hypothesized that metabolic adaptations to muscle inactivity are most pronounced when neurotrophic influence is disrupted. In rat diaphragm muscle (Diam), 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 Diam 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 Diam. These results suggest that disruption of neurotrophic influence has a greater impact on muscle fiber metabolic properties than inactivity per se.