筋と運動ニューロンの結合がラット横隔膜の収縮特性に及ぼす影響

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  • Influence of myoneural interactions on contractile properties of rat diaphragm
  • キン ト ウンドウ ニューロン ノ ケツゴウ ガ ラット オウカクマク ノ シュウシュク トクセイ ニ オヨボス エイキョウ

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We hypothesize that the adaptation of rat diaphragm muscle to inactivity is affected by myoneural interactions. We compared two models of hemidiaphragm inactivity; 1) Denervation (DV) of phrenic nerve, where the interaction between motoneuron and muscle is disrupted; and 2) Spinal transection (ST) by hemisection of the spinal cord at the C2 level, where the interaction between motoneuron and muscle remains intact. Following 2 weeks of inactivity, in vitro isometric and isotonic contractile properties were assessed using direct supramaximal muscle stimulation. Maximal tetanic force (Po) was normalized for cross-sectional area, and maximal unloaded shortening velocity (Vo) was determined using a slack test. Isometric fatigue resistance was assessed by repetitive stimulation at 40 Hz, and a fatigue index (Fl) was then calculated as the ratio of force after 2 min to initial force. Expression of different myosin heavy chain (MHC) isoforms was determined by SDS-PAGE gel. Compared to control (CT) group, there was a reduction in Po in both experimental groups, but this decrease was greater in the DV group than in the ST group. Vo also decreased in the DV group, but was unchanged in the ST group. FI was higher in the DV group than in the CT group, but was unchanged in the ST group. These adaptations in the physiologic properties of muscles were attributable to changes in the expression of MHC isoforms. The relative contribution of fast MHC isoforms was decreased in the DV group, but was unchanged in the ST group compared to that in the CT group. Our results suggest that intact interactions between muscle and motoneurons play an important role in regulating physiological adaptations of diaphragm muscle to inactivity.

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