Non-cross-bridge calcium-dependent stiffness in frog muscle fibers

Non-cross-bridge calcium-dependent stiffness in frog muscle fibers. Am J Physiol Cell Physiol 286: C1353-C1357, 2004. First published January 28, 2004; 10.1152/ajpcell.00493.2003.--At the end of the force transient elicited by a fast stretch applied to an activated frog muscle fiber, the force settles to a steady level exceeding the isometric level preceding the stretch. We showed previously that this excess of tension, referred to as 'static tension,' is due to the elongation of some elastic sarcomere structure, outside the cross bridges. The stiffness of this structure, 'static stiffness,' increased upon stimulation following a time course well distinct from tension and roughly similar to intracellular [Ca.sup.2+] concentration. In the experiments reported here, we investigated the possible role of [Ca.sup.2+] in static stiffness by comparing static stiffness measurements in the presence of [Ca.sup.2+] release inhibitors (D600, Dantrolene, [sup.2][H.sub.2]O) and cross-bridge formation inhibitors [2,3-butanedione monoxime (BDM), hypertonicity]. Both series of agents inhibited tension; however, only D600, Dantrolene, and [sup.2][H.sub.2]O decreased at the same time static stiffness, whereas BDM and hypertonicity left static stiffness unaltered. These results indicate that [Ca.sup.2+], in addition to promoting cross-bridge formation, increases the stiffness of an (unidentified) elastic structure of the sarcomere. This stiffness increase may help in maintaining the sarcomere length uniformity under conditions of instability. intact muscle fiber; static stiffness; tension inhibitors; titin