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/ P) calculated from reversal potentials measured in intact cells under bi-ionic conditions were as follows: SCN >> ClO > [Formula: see text] > I > Br > Cl >> gluconate. Relative conductances ( G/ G) followed a similar trend of SCN >> ClO > [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/ P 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.