Mouse retinal pigment epithelial cells exhibit a thiocyanate-selective conductance

The basolateral membrane anion conductance of the retinal pigment epithelium (RPE) is a key component of the transepithelial Cl- transport pathway. Although multiple Cl- channels have been found to be expressed in the RPE, the components of the resting Cl- conductance have not been identified. In this study, we used the patch-clamp method to characterize the ion selectivity of the anion conductance in isolated mouse RPE cells and in excised patches of RPE basolateral and apical membranes. Relative permeabilities (PA/PCl) calculated from reversal potentials measured in intact cells under bi-ionic conditions were as follows: SCN- >> ClO4- > NO3 > I- > Br- > Cl- >> gluconate. Relative conductances (GA/GCl) followed a similar trend of SCN- >> ClO4- > NO3- > I- > Br- approximate to Cl- >> gluconate. Whole cell currents were highly time-dependent in 10 mM external SCN-, reflecting collapse of the electrochemical potential gradient due to SCN- accumulation or depletion intracellularly. When the membrane potential was held at (-)120 mV to minimize SCN- accumulation in cells exposed to 10 mM SCN-, the instantaneous current reversed at -90 mV, revealing that PSCN/PCl is approximately 500. Macroscopic current recordings from outside-out patches demonstrated that both the basolateral and apical membranes exhibit SCN- conductances, with the basolateral membrane having a larger SCN- current density and higher relative permeability for SCN-. Our results suggest that the RPE basolateral and apical membranes contain previously unappreciated anion channels or electrogenic transporters that may mediate the transmembrane fluxes of SCN- and Cl-.