Effect of ionic strength on crossbridge kinetics as studied by sinusoidal analysis, ATP hydrolysis rate and X-ray...

In order to identify which steps in the crossbridge are affected by changes in ionic strength, we studied the effect of ionic strength on the rate constants and magnitudes of three exponential processes, the ATP hydrolysis rate and isometric tension during maximal activation (pCa 4.52, 5 mM MgATP). Equatorial X-ray diffraction measurements were also carried out in both relaxing and rigor conditions to examine whether the distance between thick and thin filaments changes with ionic strength (range: 100-300 mM). All experiments were carried out at 20 degrees C and at pH 7.0 on chemically skinned rabbit psoas muscle fibres. Isometric tension and muscle stiffness declined significantly as the ionic strength was increased from 150 mM to 300 mM. The concomitant decrease in the ATP hydrolysis rate was much less than tension, resulting in a large increase in the tension cost. Three rate constants of exponential processes, deduced from sinusoidal analysis, did not change appreciably. The magnitude parameters of all three processes diminished as the ionic strength was increased. During relaxation the filament spacing increased by 5% when the ionic strength was increased from 150 mM to 300 mM. After rigor induction, the spacing did not change with ionic strength. We conclude that a change in ionic strength modifies the rapid equilibrium between the detached state and the 'weakly attached' state, and that this causes considerable effect on isometric tension. We also conclude that other steps in the crossbridge cycle are less sensitive to ionic strength, and that the lattice spacing change is unable to account for the considerable effect of ionic strength on isometric tension.