Expression and functional behavior of troponin C in soleus muscle fibers of rat after hindlimb unloading

Expression and functional behavior of troponin C in soleus muscle fibers of rat after hindlimb unloading. J Appl Physiol 90: 1095-1101, 2001.-Troponin C (TnC) plays a key role in the regulation of muscle contraction, thereby modulating the Ca2+-activation characteristics of skinned muscle fibers. Th is study was performed to assess the effects of a 15-day hindlimb unloading (HU) period on TnC expression and its functional behavior in the slow post ural muscles of the rat. We investigated the TnC isoform expression in whol e soleus muscles and in single fibers. The latter were also checked for the ir Ca2+ activation characteristics and sensitivity to bepridil, a Ca2+ sens itizer molecule. This drug has been described as exerting a differential ef fect on slow and fast fibers, depending on the TnC isoform. With regard to TnC expression, three populations were found in control muscle fibers: slow , hybrid slow, and hybrid fast fibers, with the TnC fast being always coexp ressed with TnC slow. In the whole muscle, TnC fast expression increased af ter HU because of the increase in the proportion of hybrid fast fibers. The HU hybrid fast fibers had properties similar to those of control hybrid fa st fibers. The fibers that remained slow after HU exhibited similar bepridi l and Sr2+ properties as control slow fibers. Therefore, in these fibers, t he changes could not be related to the TnC molecule.