MONOCARBOXYLATE TRANSPORTER MCT1 IS LOCATED IN THE APICAL MEMBRANE AND MCT3 IN THE BASAL MEMBRANE OF RAT RPE

The retinal pigment epithelium (RPE) forms the outer blood-retinal bar rier and regulates the movement of nutrients, water, and ions between the choroidal blood supply and the retina. The transport properties of the RPE maintain retinal adhesion and regulate the pH and osmolarity in the space surrounding the photoreceptor cell outer segments. In thi s report we identify two monocarboxylate transporters, MCT1 and MCTS, expressed in rat RPE. On the basis of Northern and Western blot analys es, MCT1 is expressed in both the neural retina and the RPE, whereas t he expression of MCT3 is restricted to the RPE. Using indirect immunol ocalization we show that the two transporters are polarized to distinc t membrane domains. MCT1 antibody labels the apical surface and the ap ical processes of the RPE. A polyclonal antibody produced against the carboxy terminus of rat MCTS labels only the basolateral membrane of t he RPE. The demonstration of MCT1 on the apical membrane and MCT3 on t he basal membrane identifies specific proteins involved in the discrim inate and critical regulation of water and lactate transport from the retina to the choroid.