抄録
In this study, we investigated whether or not disuse alters sarcomere length (SL) dependence of activation, using single skinned fibers of rat soleus muscle. Disuse was induced by hindlimb immobilization with casting tape, resulting in a reduction in wet weight of soleus muscle by ∼40%. We measured maximal Ca2+-activated force at varying sarcomere lengths (SL), from 2.0 up to 4.0μm. It was found that the descending limb of the relationship was shifted to the left upon disuse, with no change in the ascending limb. Electron microscopic observation demonstrated that the thick filament length was not uniform and varied in disused muscle, with shorter filaments coexisting with normal filaments, resulting in shortening of the average A-band length. The I-band length, on the other hand, tended to be shorter upon disuse, but not significant. These structural changes are likely to decrease the overlap between thick and thin filaments, and consequently reduces active force, especially at longer SLs. Our biochemical studies revealed that titin expression was reduced upon disuse, accompanying a reduction in passive force, and that the expression of other major sarcomere proteins was similar in muscles with and without disuse. It has been reported that titin may function as a molecular scaffold for thick filament formation during myofibrillogenesis. Therefore, a long-term reduction of titin expression may form myofibrils with altered sarcomere structure, resulting in the changes in SL-dependence. [J Physiol Sci. 2007;57 Suppl:S96]
