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

We discuss the activation of thin-filament-regulated muscles by calcium ion in terms of a qualitative model based on nearest-neighbor lattice statistics. For the most part, the model takes into account only the essential features of the phenomenon--that there must be an interaction between calcium adsorption to troponin and crossbridge reaction with actin for calcium ion to activate contraction and that the relevant stationary states are nonequilibrium ones. Even so, the model predicts the following features which are seen experimentally but have generally not been considered in previous models: (i) the relative activations of stationary-state isometric force and ATPase are not equal; (ii) in general, neither activation of force nor that of ATPase is proportional to calcium adsorption to the activating sites; and (iii) the slopes of the relations between the activations and the logarithm of the calcium ion concentration generally depend on the necessary interaction between calcium ion adsorption and crossbridge reaction with actin. Thus, these relations show cooperative effects even if these is no interaction between calcium adsorption sites.