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 − >> ClO 4 − > [Formula: see text] > I − > Br − > Cl − >> gluconate. Relative conductances ( G A / G Cl ) followed a similar trend of SCN − >> ClO 4 − > [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 − .