Cross-bridge state, detached

Huxley s model (1957) for muscle contraction states that the mechanical and enzymatic characteristics can be described by the overall apparent attachment and detachment rates of the cross-bridge. Contraction was described as a transition between free and attached states (Huxley, 1957). This simple yet elegant two-state model produces several specific predictions about the relationships between force production, ATPase rates, and shortening as a function of these two rate constants. Although this model is somewhat oversimplified given the actual number of biochemical states, and several of its assumptions may not strictly hold in... 

SM, it implies that the cross-bridge theoretically can be regulated in two basic ways by regulating attachment (or perhaps more appropriately the transition to a force-producing state) and by regulating detachment. 

Four-state models incorporating a constitutively active MLCP imply that a significant fraction of the increase in ATP consumption associated with contraction would be used for cross-bridge phosphorylation/-dephosphorylation (Walker et al., 1994 Butler et al, 1994). This is counterintuitive, given the extraordinary economy of force maintenance by smooth muscle. Paul (1990) suggested that a very slow cross-bridge detachment rate (where k4 = ky) would provide for the necessary economy observed in SM while reducing... 

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