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MUSCLE CELL BIOLOGY AND CELL MOTILITY

Slow myosin heavy chain expression in the absence of muscle activity

¹EA300, Université Paris Diderot, Paris; ²Institut National de la Santé et de la Recherche Médicale, U787, Paris; ³Faculté de Médecine, Université Pierre et Marie Curie, Unité Mixte de Recherche S787, Paris; ⁴Institut de Myologie, Paris; ⁵Généthon, Évry; and ⁶Université Paris Descartes, Paris, France

Submitted 6 August 2008 ; accepted in final form 20 October 2008

Innervation has been generally accepted to be a major factor involved in both triggering and maintaining the expression of slow myosin heavy chain (MHC-1) in skeletal muscle. However, previous findings from our laboratory have suggested that, in the mouse, this is not always the case (30). Based on these results, we hypothesized that neurotomy would not markedly reduced the expression of MHC-1 protein in the mouse soleus muscles. In addition, other cellular, biochemical, and functional parameters were also studied in these denervated soleus muscles to complete our study. Our results show that denervation reduced neither the relative amount of MHC-1 protein, nor the percentage of muscle fibers expressing MHC-1 protein (P > 0.05). The fact that MHC-1 protein did not respond to muscle inactivity was confirmed in three different mouse strains (129/SV, C57BL/6, and CD1). In contrast, all of the other histological, biochemical, and functional muscle parameters were markedly altered by denervation. Cross-sectional area (CSA) of muscle fibers, maximal tetanic isometric force, maximal velocity of shortening, maximal power, and citrate synthase activity were all reduced in denervated muscles compared with innervated muscles (P < 0.05). Contraction and one-half relaxation times of the twitch were also increased by denervation (P < 0.05). Addition of tenotomy to denervation had no further effect on the relative expression of MHC-1 protein (P > 0.05), despite a greater reduction in CSA and citrate synthase activity (P < 0.05). In conclusion, a deficit in neural input leads to marked atrophy and reduction in performance in mouse soleus muscles. However, the maintenance of the relative expression of slow MHC protein is independent of neuromuscular activity in mice.

skeletal muscle; tenotomy; force; slow myosin heavy chain; power; velocity of shortening; oxidative capacity; atrophy

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