Effects of inorganic phosphate on cross-bridge behavior after photorelease of ATP in permeabilized cells of rat skeletal muscle

Effects of 20 mM inorganic phosphate on the cross-bridge behavior after photorelease of ATP from caged ATP was studied by X-ray diffraction in rat skinned psoas muscle fibers at 16degreesC. In the first 30 ms after the photorelease, tension was similar in the presence and absence of phosphate. The tension development was then suppressed in the presence of phosphate. At 500 ms after the photolysis, it was lower by about 30% in the presence of phosphate. In the presence of phosphate, the intensity of the third meridional reflection from the thick filament at 1/14.4 nm(-1) increased more slowly and was 60% of the level without phosphate at 500 ms after the photolysis. The intensities of the equatorial (1,1) reflection and the actin layer-line at 1/36 nm(-1) decreased to a lower level in the presence of phosphate. These results suggest that phosphate does not affect dissociation of myosin heads from actin, but decreases the number of myosin heads in force-generating conformation and reduces tension. Phosphate may also shift the equilibrium between the detached and attached states towards the former.