CALCIUM-POTASSIUM SELECTIVITY - KINETIC-ANALYSIS OF CURRENT-VOLTAGE RELATIONSHIPS OF THE OPEN, SLOWLY ACTIVATING CHANNEL IN THE VACUOLAR MEMBRANE OF VICIA-FABA GUARD-CELLS

Single-channel current-voltage (IV) relationships of the open, slowly activating vacuolar (SV) channel of Vicia faba L. were recorded in sol utions with different activities of Ca2+ and K+, and have been analyze d for Ca2+/K+ selectivity. Two models with one binding site have been examined. A rigid-pore model with a main binding site between two ener gy barriers (nine free parameters) provides fair fits. Slightly better fits are obtained with an alternative dynamic-pore model, where the s electivity filter is located between two Mitchellian ion wells of the cytoplasmic and luminal pore sections, and where the selectivity filte r alternates the orientation of the binding site between the two faces of the pore (ten free parameters). Using sets of IV-relationships wit h only Ca2+ or only K+ as transportable substrates, both models consis tently predict open-channel IV-relationships in the presence of both s ubstrates. Fits of both models to the entire ensemble of data, yield v ery similar flux-voltage characteristics Ca2+ and for K+ in experiment al conditions, and consistently predict such flux-voltage characterist ics over physiologically relevant ranges of voltage and substrate conc entrations, In a very general sense, physiological Ca2+ fluxes through the open SV channel are predominantly inward and about 50 times small er than K+ fluxes. The ions Cl-, OH-, and H+, do not pass the SV chann el at significant rates, Kinetic details of the SV channel with respec t to binding and passage of Ca2+ and K+ are discussed on the basis of the consistent results of the reaction-kinetic analysis of the experim ental data by the two models.