Rapid dissociation and reassociation of actomyosin cross-bridges during force generation: a newly observed facet of cross-bridge action in muscle.

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Abstract
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.