Metabolic and phenotypic adaptations of diaphragm muscle fibers with inactivation

Zhan, Wen-Zhi, Hirofumi Miyata, Y. S. Prakash, and Gary C. Sieck. Metabolic and phenotypic adaptations of diaphragm musclefibers with inactivation. J. Appl.Physiol. 82(4):1145-1153, 1997.We hypothesizedthat metabolic adaptations to muscle inactivity are most pronouncedwhen neurotrophic influence is disrupted. In ratdiaphragm muscle(Dia_m), 2 wk ofunilateral denervation or tetrodotoxin nerve blockade resulted in areduction in succinate dehydrogenase (SDH) activity of type I, IIa, andIIx fibers (~50, 70, and 24%, respectively) and a decrease in SDHvariability among fibers (~63%). In contrast, inactivity induced byspinal 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 SDHvariability among fibers. Actomyosin adenosinetriphosphatase (ATPase)activities of type I, IIx, and IIb fibers in denervated andtetrodotoxin-treated Dia_m werereduced by ~20, 45, and 60%, respectively, and actomyosin ATPasevariability among fibers was ~60% lower. In contrast, onlyactomyosin ATPase activity of type IIb fibers was reduced (~20%) inST Dia_m. These results suggestthat disruption of neurotrophic influence has a greater impact onmuscle fiber metabolic properties than inactivity per se.