Rigor tension development in glycerinated rabbit psoas fibers at high salt concentrations

We attempted to measure the rigor tension development by glycerinated fibers of rabbit psoas at high salt concentrations such as 0.5 M KCl. The measurements were made feasible by covalently crosslinking the rod-portion of thick filaments in the fibers in the rigor state with a water-soluble carbodiimide (EDC) so that the thick filaments are not dissolved even at 0.5 M KCl. EDC crosslinks, though with much a slower rate, the myosin cross-bridge heads to the thin filaments. At high salt concentrations, the fibers developed no active tension but developed rigor tension when they were put into a rigor solution from a contracting or relaxing solution. Removal of only Mg++ from a MgATP-containing solution induced similar rigor tension development. The magnitude of the rigor tension was proportional to the fraction of the cross-bridge heads that were crosslinked to the thin filaments. The results suggest that the rigor tension at high salt concentrations is generated by structural changes in the cross-bridge heads that are crosslinked to the thin filaments, when these heads release MgATP or Mg++ (with ATP retained) from their active sites, but not generated by re-formation of the rigor complexes of uncrosslinked myosin heads with the thin filaments. Extrapolation to 100% crosslinked heads gave an estimate of the rigor tension development of more than 1 kg wt/cm2 at high salt concentrations.