Myosin phosphorylation in permeabilized rabbit psoas fibers.

The 18,000-Da myosin light chains in segments of rabbit psoas fibers were stably phosphorylated to assess the mechanical effects of this modification. Before and after phosphorylation of the same fiber, the maximal shortening velocity (Vmax) was measured at 12 degrees C by a quick-release slack test and by extrapolation to Vmax of hyperbolic force-velocity curves from isotonic releases. The experiments were performed at saturating concentrations of Ca2+, as determined from isometric force-pCa curves, under conditions in which the pH and ATP-ADP ratio were buffered. No effect of phosphorylation on isometric force, Vmax, or the shape of the force-velocity curve was detected under conditions of maximal calcium activation. Thus we find no mechanical evidence for a modulation by light chain phosphorylation of actomyosin interaction in these fiber segments.