Disuse-induced changes in Ca²⁺ sensitivity of force in rat soleus muscle

Abstract

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]