Retinal pigment epithelial transport mechanisms and their contributions to the electroretinogram

The translocation of ions, fluid and macromolecules across epithelia is made possible by the asymmetric distribution of transport proteins, enzymes and receptors in two physically distinct plasma membrane domains that form the apical and basolateral sides of the cell. Each side faces a different extracellular environment. In the back of the vertebrate eye, the retinal pigment epithelium (RPE) apical membrane receives a continuous stream of paracrine signals that are generated by a variety of retinal neurons in the light and dark. These signals help regulate RPE function, and conversely, alterations in RPE function can modify the activity of retinal neurons. At the basolateral surface, there is a continual exchange of nutrients and waste products, along with a flow of hormonal signals from the choroidal blood supply, all of which serve to maintain the health and integrity of the distal retina and in particular. the photoreceptors. This review provides an integrated summary of the apical and basolateral membrane and intracellular signaling mechanisms that mediate the vectorial traffic of ions and fluid across the RPE. These same mechanisms help regulate the chemical milieu within the cell and in the extracellular spaces that surround the cell. They also generate specific components of the electrical (EGG). The last part of this review is focused on the light-induced photoreceptor-dependent decrease in subretinal potassium concentration ([K](o)) that occurs in the intact eye and serves as a paracrine signal for the RPE. This signal plays a central role in regulating RPE physiology and in mediating retina/RPE interactions, following transitions between light and dark; it is mimicked in vitro by a small (3 mM) change in [K](o) on the apical side of the epithelium. The clinical implications are discussed in terms of the transport mechanisms that regulate hydration of the subretinal space and that potentially mediate fluid absorption out of the retina. (C) 1997 Elsevier Science Ltd.