Ion channels permeable to monovalent and divalent cations: a single-file two-site channel model.

A cation channel from Tetrahymena cilia is permeable to both monovalent and divalent cations. A single-file two-site channel model was introduced for explaining the single channel currents of the channel in mixed solutions of K(+) and Ca(2+). In the model it was assumed that two potassium ions or one calcium ion can bind to the binding sites, and that the potassium ions between the binding sites are in a fast equilibrium condition. Single channel currents were calculated from the values of rate constants, ionic concentrations on both sides of the membrane, and the membrane voltages. This model could explain all the observed single channel currents of the channel in K(+) or Ca(2+) solution and in mixed solutions of K(+) and Ca(2+). The values of the reversal potential in the bi-ionic condition could distinguish this single-file two-site channel model from the single-site channel model or the model in which each ion permeates through the same channel independently (the Goldman-Hodgkin-Katz equation). Experimental data supported this model.