Calcium-potassium selectivity: kinetic analysis of current-voltage relationships of the open, slowly activating channel in the vacuolar membrane of Vicia faba guard-cells
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Single-channel current-voltage (IV ) relationships of the open, slowly activating vacuolar (SV) channel of Vicia faba L. were recorded in solutions with different activities of Ca2+ and K+, and have been analyzed for Ca2+/K+ selectivity. Two models with one binding site have been examined. A rigid-pore model with a main binding site between two energy barriers (nine free parameters) provides fair fits. Slightly better fits are obtained with an alternative, dynamic-pore model, where the selectivity filter is located between two Mitchellian ion wells of the cytoplasmic and luminal pore sections, and where the selectivity filter alternates the orientation of the binding site between the two faces of the pore (ten free parameters). Using sets of IV-relationships with only Ca2+ or only K+ as transportable substrates, both models consistently predict open-channel IV-relationships in the presence of both substrates. Fits of both models to the entire ensemble of␣data yield very similar flux-voltage characteristics for␣Ca2+ and for K+ in experimental conditions, and consistently predict such flux-voltage characteristics over physiologically relevant ranges of voltage and substrate concentrations. In a very general sense, physiological Ca+ fluxes through the open SV channel are predominantly inward and about 50 times smaller than K+ fluxes. The ions Cl−, OH−, and H+, do not pass the SV channel at significant rates. Kinetic details of the SV channel with respect to binding and passage of Ca2+ and K+ are discussed on the basis of the consistent results of the reaction-kinetic analysis of the experimental data by the two models.
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