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 abou t 16 degreesC with X-ray diffraction and caged-ATP photolysis. The amount o f 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 result s under the single turn-over condition were generally consistent with previ ous results obtained with larger amount of photoreleased ATP. Formation of the ADP-rigor state was mechanically monitored by the 90 degrees out-of-pha se component of stiffness at 500 Hz, which was elevated on activation and t hen decreased to zero with a half-time of 0.2-0.3 s. Intensity changes of t he X-ray reflections (e.g. equatorial reflections, actin layer lines and a myosin meridional reflection) indicated that a large number of cross-bridge s returned to the rigor structure with a half-time of 0.5-0.7 s. During thi s 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. Thes e 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 f orm rigor bonds with a rate of 1 s(-1). The force developed in the rigor fo rmation is probably lost due to the presence of rigor bridges and complianc e in the preparation.