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

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 Ca2+ concentration. In the experiments reported here, we investigated the possible role of Ca2+ in static stiffness by comparing static stiffness measurements in the presence of Ca2+ release inhibitors (D600, Dantrolene, (H2O)-H-2) and cross-bridge formation inhibitors [2,3-butanedione monoxime (BDM), hypertonicity]. Both series of agents inhibited tension; however, only D600, Dantrolene, and 2H2O decreased at the same time static stiffness, whereas BDM and hypertonicity left static stiffness unaltered. These results indicate that Ca2+, 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.