Cross-bridge attachment during high-speed active shortening of skinned fibers of the rabbit psoas muscle: Implications for cross-bridge action duringmaximum velocity of filament sliding

To characterize the kinetics of cross-bridge attachment to actin during unl oaded contraction (maximum velocity of filament sliding), ramp-shaped stret ches with different stretch-velocities (2-40,000 nm per half-sarcomere per s) were applied to actively contracting skinned fibers of the rabbit psoas muscle. Apparent fiber stiffness observed during such stretches was plotted versus the speed of the imposed stretch (stiffness-speed relation) to deri ve the rate constants for cross-bridge dissociation from actin. The stiffne ss-speed relation obtained for unloaded shortening conditions was shifted b y about two orders of magnitude to faster stretch velocities compared to is ometric conditions and was almost identical to the stiffness-speed relation observed in the presence of MgATP gamma S at high Ca2+ concentrations, i.e ., under conditions where cross-bridges are weakly attached to the fully Ca 2+ activated thin filaments. These data together with several control exper iments suggest that, in contrast to previous assumptions, most of the fiber stiffness observed during high-speed shortening results from weak cross-br idge attachment to actin. The fraction of strongly attached cross-bridges d uring unloaded shortening appears to be as low as some 1-5% of the fraction present during isometric contraction. This is about an order of magnitude less than previous estimates in which contribution of weak cross-bridge att achment to observed fiber stiffness was not considered. Our findings imply that 1) the interaction distance of strongly attached cross-bridges during high-speed shortening is well within the range consistent with conventional cross-bridge models, i.e., that no repetitive power strokes need to be ass umed, and 2) that a significant part of the negative forces that limit the maximum speed of filament sliding might originate from weak cross-bridge in teractions with actin.