Addition of phosphate to active muscle fibers probes actomyosin states within the powerstroke

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Abstract
We have measured the effect of phosphate (Pi) on the tension and maximum shortening velocity of permeable rabbit psoas fibers. Work in a number of laboratories has established that addition of phosphate (0–25 mM) to active muscle fibers at physiological MgATP concentrations decreases isometric tension with little effect on the maximum shortening velocity. Here we extend these results to a wider range of Pi concentrations and to low MgATP concentrations. Low levels of Pi (approx. 150 μM – 200 μM) were obtained by using sucrose phosphorylase and sucrose to reduce contaminating Pi in the solutions used to activate the fiber, and high levels (52–73 mM) were obtained by replacing acetate with Pi as the principal anion. In an activating solution containing either 50 μM or 4 mM MgATP, pH 6.2 or 7.0, isometric tension declines linearly with the logarithm of Pi concentration. Although the isometric tension decreases with increasing concentrations of H+ or MgATP, the slope of relative isometric tension as a function of log[Pi] is the same at the two values of pH and [MgATP]. At pH 7 and 4 mM MgATP, the velocity of contraction increased slightly as Pi increased from 0.2 to 52 mM. At 50 μM MgATP the velocity decreased slightly as Pi increased from 10 to 52 mM. These results are discussed in terms of models of cross-bridge energetics. The observation that force declines linearly with the logarithm of [Pi] is compatible with models in which a major force producing state occurs subsequent to Pi release. The inhibition of shortening velocity by Pi at low concentration of MgATP can be explained by a competition between MgATP and Pi at the end of the cross-bridge powerstroke.