EFFECT OF IONIC-STRENGTH ON SKINNED RABBIT PSOAS FIBERS IN THE PRESENCE OF MAGNESIUM PYROPHOSPHATE

The effect of ionic strength on the kinetics of myosin cross-bridges in the presence of the ATP analogue PP(i) has been examined. It was found that increasing ionic strength from moderate values (mu-approximately 100 mM) to high values (mu-approximately 200 mM) has three effects. It causes a big decrease in the half time for the force decay after a small stretch, it causes a significant decrease in the sigmoidicity of the nucleotide analogue concentration dependence of the "apparent rate constant" of force decay after a small stretch, and it causes a big decrease in the range of rate constants necessary to describe the multiexponential force decay. It causes the last of these by causing a much larger increase in the slowest rate constants of the decay than in the fastest rate constants. The results suggest that whereas the behavior of cross-bridges in the presence of ATP is well-described by the simple independent-head equilibrium cross-bridge model of Schoenberg (1985. Biophys. J. 48:467-475), cross-bridges in the presence of the ATP analogue PP(i) require the more complicated double-headed equilibrium cross-bridge model of Anderson and Schoenberg (1987. Biophys. J. 52:1077-1082) to describe their behavior.