RAPID DISSOCIATION AND REASSOCIATION OF ACTOMYOSIN CROSS-BRIDGES DURING FORCE GENERATION - A NEWLY OBSERVED FACET OF CROSS-BRIDGE ACTION IN MUSCLE

The force response of skinned fibers of the rabbit psoas muscle to stretches (and releases) was studied. At physiological ionic strength and low experimental temperature (5-degrees-C) the force response to stretches apparently is affected neither by cross-bridges that occupy weak-binding states nor by transitions among various attached force-generating states. Plots of force vs. imposed length change (T plots) recorded during stretches suggest that cross-bridges even in force-generating states dissociate and reassociate rapidly from and to actin as had previously been proposed [Brenner, B. (1986) Basic Res. Cardiol. 81, 1-15]. Plots of fiber stiffness vs. speed of imposed length changes (stiffness-speed relations) imply rate constants for dissociation (k-) in the force-generating states ranging from 50 to 1000 s-1, while the rate constant for reassociation (k+) has to be at least an order of magnitude larger (high actin affinity). Rapidly reversible actin interaction of cross-bridges in force-generating states provides a mechanism for rapid detachment of force-generating cross-bridges during high-speed shortening which, in contrast with the hypothesis of A. F. Huxley [(1957) Prog. Biophys. 7, 255-318], and related cross-bridge models, does not require completion of the ATP-hydrolysis cycle and thus may account for the unexpectedly low ATPase activity during high-speed shortening.