Abstract
We investigated the effects of hindlimb immobilization (HI) on contractile properties of single skinned soleus fibers in rats. HI (6 weeks) resulted in reduced wet weight of soleus muscle (–40%). In immobilized fibers, maximal Ca2+-actiavated force was reduced by ∼40% and the force-pCa curve was shifted to the lower pCa side by ∼0.15 pCa units (sarcomere length, 2.20 μm). Our EM observation revealed thinner myofbrils in immobilized muscle, with the width of Z-line similar in control and immobilized muscles during relaxation and contraction (pCa 4.5). We reconstituted thin filaments of control and immobilized fibers with the identical troponin complex (from rabbit psoas muscle). It was found that Ca2+ sensitivity was still lower in immobilized fibers by ∼0.15 pCa units after troponin reconstitution, suggesting that troponin isoform switches, if at all, can not account for functional changes of immobilized fibers. We then induced Ca2+-independent active force by lowering the MgATP concentration (= -log [MgATP], from 5 to 7) to investigate whether or not the decreases in Ca2+-activated force are the result of reduced cross-bridge formation. We found that in immobilized fibers, maximal Ca2+-independent active force was reduced by ∼40%, with a leftward shift of the force-pMgATP curve. These results suggest that cross-bridge recruitment is suppressed in immobilized muscle, via e.g., structural changes of the sarcomere, resulting in reduced active force production. [J Physiol Sci. 2006;56 Suppl:S146]
