Characterization of a novel cationic drug transporter in human retinal pigment epithelial cells

Retinal pigment epithelial (RPE) cells transport a variety of solutes, but the capacity of human RPE cells to transport drugs and xenobiotics is not w ell understood. As an initial step to address this issue, we have examined human RPE transport of verapamil. Transport of [H-3] verapamil was measured in two human RPE cell lines (RPE/Hu and ARPE-19) grown to confluence on 12 -well culture plates. Verapamil uptake by RPE/Hu cells was highly concentra tive, reaching cell-to-medium ratios as high as 42 by 1 h. Uptake was satur able, with an apparent K-m of 7.2 muM. Verapamil uptake decreased in the pr esence of metabolic inhibitors, low temperature, and organic cations, inclu ding quinidine, pyrilamine, quinacrine, and diphenhydramine. However, other organic cations, including tetraethylammonium and cimetidine failed to inh ibit. Verapamil uptake was also inhibited by the cationic antiglaucoma drug s diltiazem, timolol, and propranolol. Verapamil uptake was insensitive to changes in membrane potential. However, transport was markedly altered by c hanges in pH. Decreasing external pH inhibited uptake, whereas efflux was s timulated. Intracellular acidification via NH4Cl prepulse also stimulated u ptake. Identical findings were obtained using the commercially available ce ll line ARPE-19. In view of its unique specificity, the RPE cell verapamil transporter described above is a novel, heretofore undescribed, organic cat ion transporter, distinct from the known members of the OCT family of organ ic cation transporters.