Single turnover of cross-bridge ATPase in rat muscle fibers studied by photolysis of caged ATP

A mechanical study on skinned rat psoas muscle fibers was performed at about 16°C with X-ray diffraction and caged-ATP photolysis. The amount of photoreleased ATP was set <0.2 mM for analysis of a `single turnover' of the cross-bridge ATPase. With regard to the phase of activation, the results under the single turn-over condition were generally consistent with previous results obtained with larger amount of photoreleased ATP. Formation of the ADP-rigor state was mechanically monitored by the 90° out-of-phase component of stiffness at 500 Hz, which was elevated on activation and then decreased to zero with a half-time of 0.2–0.3 s. Intensity changes of the X-ray reflections (e.g. equatorial reflections, actin layer lines and a myosin meridional reflection) indicated that a large number of cross-bridges returned to the rigor structure with a half-time of 0.5–0.7 s. During this phase, tension did not increase but slowly decreased with a half-time of about 1.0 s. The in-phase stiffness increased only 20–30% at the most. These results indicate that, even if the number of cross-bridges formed at any moment during full contraction is small, they can interact with actin and form rigor bonds with a rate of 1 s−1. The force developed in the rigor formation is probably lost due to the presence of rigor bridges and compliance in the preparation.