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 ( P_A/ P_Cl) calculated from reversal potentials measured in intact cells under bi-ionic conditions were as follows: SCN^- >> ClO₄^- > [Formula: see text] > I^- > Br^- > Cl^- >> gluconate. Relative conductances ( G_A/ G_Cl) followed a similar trend of SCN^- >> ClO₄^- > [Formula: see text] > I^- > Br^- ≈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 P_SCN/ P_Cl 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^-.