Uptake of 3H-cAMP by retinal pigment epithelium isolated from bluegill sunfish (Lepomis macrochirus)

Here we show isolated RPE takes up cyclic adenosine monophosphate (cAMP) in a saturable manner, exogenously applied cAMP induces pigment granule aggregation in retinal pigment epithelium isolated from bluegill, and aggregation induced in this manner is inhibited by treatment with probenecid, an organic anion transport inhibitor.Our results raise the possibility that cAMP functions as a messenger secreted from the neural retina to signal darkness to the RPE, which takes it up. It further suggests that organic anion transport systems are the route by which cAMP crosses RPE cell membranes since probenecid inhibits extracellular cAMP from causing pigment granule aggregation.In the eyes of teleosts and many other vertebrates, the retinal photoreceptors (rods and cones) and the retinal pigment epithelium (RPE) interact continuously during normal physiological conditions to optimize light capture for vision. The RPE lies posterior and adjacent to the retinal photoreceptors; therefore, light entering the eye through the pupil strikes the photoreceptors before the RPE. The apical surface of RPE is directly adjacent to the photoreceptor outer segments, while the basal surface lies next to the choroid coat. From the apical surface of RPE, delicate apical processes interdigitate with the photoreceptors. In green sunfish (Lepomis cyanellus) the apical projections are often greater than 100 μm long [1]Pigment granules inside the RPE cells migrate into the apical projections from the cell body to shade the rods when ambient light increases in intensity or as a response to circadian cues [2]. The movements of the pigment granules are one element of retinomotor movements; additional elements include cell shape changes manifested by rod and cone photoreceptors [3]. In teleosts and other lower vertebrates, pupils are often fixed in diameter; thus, instead of pupillary constriction, the coordinated movements of the photoreceptors and RPE pigment granules are thought to regulate the amo