Activation of thin-filament-regulated muscle by calcium ion: considerations based on nearest-neighbor lattice statistics.

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Abstract
The activation of thin-filament-regulated muscles by Ca ion (is discussed) in terms of a qualitative model based on nearest-neighbor lattice statistics. The model took into account only the essential features of the phenomenon: that there must be an interaction between Ca adsorption to troponin and crossbridge reaction with actin for Ca ion to activate contraction and that the relevant stationary states are nonequilibrium ones. The model predicted the following features which were seen experimentally but had generally not been considered in previously models: the relative activations of stationary-state isometric force and ATPase are not equal; neither activation of force nor that of ATPase is proportional to Ca adsorption to the activating sites; and the slopes of the relations between the activations and the log of the Ca ion concentration generally depend on the necessary interaction between Ca ion adsorption and crossbridge reaction with actin. These relations show cooperative effects even if there is no interaction between Ca adsorption sites.