抄録
We investigated how hindlimb immobilization affected the contractile function of single soleus fibers from the rat. Hindlimb immobilization with casting tape reduced wet weight of soleus muscle by ∼40%. We found by using Triton X-100-treated single fibers that Ca2+ sensitivity and maximal Ca2+-activated force were reduced after disuse. A similar result was obtained with saponin-treated single fibers with intact sarcoplasmic reticulum function; that is, Ca2+ sensitivity as well as maximal force was reduced after disuse. Then, in order to investigate whether or not the reduction in Ca2+ sensitivity is due to possible isoform switches of troponin molecules, we reconstituted thin filaments of control and immobilized fibers with the identical troponin complex. It was found that Ca2+ sensitivity was still lower in immobilized fibers after troponin reconstitution, suggesting that troponin isoform switches, if at all, can not account for functional changes of immobilized fibers. Instead, our small-angle X-ray diffraction experiments revealed that interfilament lattice spacing was expanded by ∼6% in immobilized fibers. Dextran T-500 (MW, ∼500,000) 2% (w/v) reduced the lattice spacing to a level similar to that observed in control fibers and almost completely restored Ca 2+sensitivity. These results favor the interpretation that lattice expansion, but not troponin isoform switches, underlies the disuse-induced reduction in Ca2+ sensitivity. [J Physiol Sci. 2007;57 Suppl:S96]
