Rp. Gallemore et al., RETINAL-PIGMENT EPITHELIAL TRANSPORT MECHANISMS AND THEIR CONTRIBUTIONS TO THE ELECTRORETINOGRAM, Progress in retinal and eye research, 16(4), 1997, pp. 509-566
The translocation of ions, fluid and macromolecules across epithelia i
s made possible by the asymmetric distribution of transport proteins,
enzymes and receptors in two physically distinct plasma membrane domai
ns that form the apical and basolateral sides of the cell. Each side f
aces a different extracellular environment. In the back of the vertebr
ate eye, the retinal pigment epithelium (RPE) apical membrane receives
a continuous stream of paracrine signals that are generated by a vari
ety of retinal neurons in the light and dark. These signals help regul
ate RPE function, and conversely, alterations in RPE function can modi
fy 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 i
n particular. the photoreceptors. This review provides an integrated s
ummary of the apical and basolateral membrane and intracellular signal
ing mechanisms that mediate the vectorial traffic of ions and fluid ac
ross 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)) tha
t occurs in the intact eye and serves as a paracrine signal for the RP
E. This signal plays a central role in regulating RPE physiology and i
n mediating retina/RPE interactions, following transitions between lig
ht 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.