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Selectivity of ion channels: competitive catalysis versus independent electrodiffusion

In order to express selectivity of ion channels, it is common to determine the reversal voltages, V r , in the presence of different ion species, and to convert these voltages to permeability ratios by constant field equations, which are based on independent electrodiffusion. In the case of individual ion channels, competition rather than independence can be expected. This situation has been described here in terms of a catalytic reaction cycle for a channel which may translocate K + as well as Ca 2+ . An example is given, where the fundamental rate constants for the K + cycle and for the Ca 2+ cycle are the same, and opposing 1 :10 concentration gradients for Ca 2+ and for K + are applied. In this case, the reversal voltage of the resultant total current is so close to the equilibrium voltage of Ca 2+ , that constant field calculation would yield an overestimated permeability ratio P Ca :P K of about 10. Two limiting cases are pointed out. In the case of dilute solution, the ions move independently and the system can obey the constant field theory. At high concentrations, the system approaches complete dependence and will consequently behave as an antiporter.