Gramicidin-mediated currents at very low permeant ion concentrations

Abstract

Current-voltage relations have been measured for the fluxes of caesium ions through pores formed by gramicidin in lipid bilayer membranes. The ionic currents have been separated from capacitative currents using a bridge circuit with an integrator as null-detector. The conductances during brief voltage pulses were small enough to avoid the effects of diffusion polarization and the ionic strength was raised using choline chloride or magnesium sulfate to reduce the effects of double-layer polarization. Under these conditions the current-voltage relations have the same shape at 0.1 and 1 mM, but different shapes for higher concentrations. These data demonstrate that the fluxes do not obey independence for concentrations above 10 mM, but they cannot be used in isolation to support a particular value of the binding constant. The shape observed at low concentrations suggests that entry of ions into the pore remains weakly potential dependent even at 300 mV.