Abstract
The retinal pigment epithelium (RPE) is a transporting epithelial monolayer that regulates the composition of the subretinal space bathing the photoreceptor outer segments. Calcium concentration in the subretinal space varies with illumination of the retina. In this study we characterized the transepithelial calcium transport by the mammalian RPE. By immunocytochemistry we localized plasma membrane calcium transport proteins PMCA, TRPV5 and NCX to both apical and basolateral RPE membranes in bovine or human native tissues. Calcium transport activity was assessed as transepithelial 45‐Ca flux in voltage‐clamped bovine RPE/choroid Ussing preparations. Apical‐to‐basal and basal‐to‐apical Ca fluxes were measured in both short and open circuit conditions. We found that bovine RPE/choroid catalyzes a net, active calcium transport in the basal‐to‐apical (choroid‐to‐retina) direction. In normal open circuit condition, the positive apical transepithelial potential drives a paracellular calcium flux that roughly balances this active transport. Application of low sodium Ringer solution to the apical surface decreased net basal‐to‐apical flux by almost 90%, suggesting that NCX activity contributes to the active transepithelial calcium transport in bovine RPE.Supported by American Health Assistance Foundation, Macular Degeneration Research
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