Cross-linking studies related to the location of the rigor compliance in glycerinated rabbit psoas fibers: is the SII portion of the cross-bridge compliant?

The muscle tension generation model of Huxley and Simmons (1971) postulates an independent elastic element in the cross-bridge. This elastic structure was tentatively placed in the SII portion of the cross-bridge in the model. To check this assumption, we fixed the SII portion onto the surface of the thick filament in glycerinated rabbit psoas fibers in rigor by chemically cross-linking with dimethyl suberimidate, and compared the stiffness of the cross-linked fibers with that of the fibers before cross-linking. The stiffness was determined by measuring the tension increment upon stretching a fiber segment in rigor. The contribution of the end compliance was found to be small. Cross-linking increased the rigor stiffness by 20 to 30%. Almost the same amount of the stiffness increase was also observed at a sarcomere length where there was no overlap between the thin and thick filaments, and in a fiber segment cross-linked in relaxing solution. Therefore, the 20 to 30% increase of the stiffness is not caused by the fixation of the SII portion onto the thick filament but caused by the cross-linking of some parallel elastic components. Since the rigor stiffness before cross-linking is almost proportional to the overlap between thick and thin filaments, we conclude that the muscle stiffness in rigor does not originate in the SII portion but reflects some compliance of the head portion of the cross-bridge.