High ionic strength depresses muscle contractility by decreasing both force per cross-bridge and the number of strongly attached cross-bridges

An increase in ionic strength (IS) lowers Ca2+ activated tension in muscle fibres, however, its molecular mechanism is not well understood. In this study, we used single rabbit psoas fibres to perform sinusoidal analyses. During Ca2+ activation, the effects of ligands (ATP, Pi, and ADP) at IS ranging 150-300 mM were studied on three rate constants to characterize elementary steps of the cross-bridge cycle. The IS effects were studied because a change in IS modifies the inter- and intra-molecular interactions, hence they may shed light on the molecular mechanisms of force generation. Both the ATP binding affinity (K (1)) and the ADP binding affinity (K (0)) increased to 2-3x, and the Pi binding affinity (K (5)) decreased to 1/2, when IS was raised from 150 to 300 mM. The effect on ATP/ADP can be explained by stereospecific and hydrophobic interaction, and the effect on Pi can be explained by the electrostatic interaction with myosin. The increase in IS increased cross-bridge detachment steps (k (2) and k (-4)), indicating that electrostatic repulsion promotes these steps. However, IS did not affect attachment steps (k (-2) and k (4)). Consequently, the equilibrium constant of the detachment step (K (2)) increased by similar to 100 %, and the force generation step (K (4)) decreased by similar to 30 %. These effects together diminished the number of force-generating cross-bridges by 11 %. Force/cross-bridge (T (56)) decreased by 26 %, which correlates well with a decrease in the Debye length that limits the ionic atmosphere where ionic interactions take place. We conclude that the major effect of IS is a decrease in force/cross-bridge, but a decrease in the number of force generating cross-bridge also takes place. The stiffness during rigor induction did not change with IS, demonstrating that in-series compliance is not much affected by IS.