長期非荷重に伴う筋萎縮とミオシン重鎖分子種の発現変化

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  • Atrophy and Expression of Myosin Heavy Chain Isoforms in Antigravity Muscles after Long-term Hindlimb Unloading.
  • チョウキ ヒカジュウ ニ トモナウ キン イシュク ト ミオシン ジュウサ ブンシシュ ノ ハツゲン ヘンカ

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The purpose of this study was to clarify time-course of adaptations in antigravity muscles after hindlimb unloading (HU) in adult (6 months old) female rats. Muscle wet weight and myofibrillar protein (MP) content were measured to evaluate muscle atrophy, and myosin heavy chain (MHC) isoform profiles were examined electrophoretically to define type-shift in soleus (SOL), plantaris (PLA), and medial gastrocnemius (MG) muscles. After 8-weeks of HU, the percentage of atrophy (80%) evaluated by MP content (mg/muscle) was greater than that (49%) evaluated by wet weight in slow-twitch SOL, since MP concentration (mg/g muscle) decreased after HU. However, the percentages of atrophy evaluated by both parameters were similar in the fast-twitch PLA and MG because of unchanges in MP concentration with HU. These different responses among muscles for MP are likely to be caused by the differences in fiber type composition. Therefore, it was suggested that functional deficit for force-generating capacity could be underestimated when muscle atrophy was evaluated by wet weight in SOL. So that the decrease in MP concentration of SOL was observed after 3-weeks of HU, muscle atrophy should be evaluated by MP content after 3-weeks or more of HU. Analysis of MHC isoforms with sodium-dodecyl sulfate polyacrylamide gel electrophoresis revealed a shift toward the faster characteristic phenotype with HU. Indeed, MHC IIx increased with a decrease MHC I in SOL, MHC IIb increased with decreases in MHC IIa and I in PLA, and MHC IIb increased with a decrease in MHC IIx in MG. These indicate slow to fast shift in SOL and PLA and fast to faster shift in MG. In PLA and MG, MHC isoform profiles did not change after 1- and 3-weeks of HU, in contrast, MHC isoform profile in SOL changed after 3-weeks of HU (increase in MHC IIx with a decrease in IIa). Based on these results, it could be conclude that 1) HU-induced shift of MHC isoforms precedes in slow-twitch SOL compared with fast-twitch PLA and MG, and 2) a decrease in MHC I expression follows an increase in MHC IIx expression as an initial shift in SOL.

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