Suppression of cAMP-induced pigment granule aggregation in RPE by organic anion transport inhibitors.

PURPOSE To investigate the mechanism(s) by which intracellular cAMP levels are elevated to induce pigment granule aggregation in teleost retinal pigment epithelium (RPE). METHODS Pigment granule migration was studied in vitro using RPE sheets isolated from dark-adapted green sunfish, Lepomis cyanellus. After preculture to allow pigment granule dispersion, RPE sheets were incubated with various agents to test their ability to induce pigment granule aggregation. RPE sheets were then fixed, and pigment granule position was assessed microscopically. RESULTS Pigment granule aggregation was induced by nonderivatized cAMP. At maximally effective concentration (1 mM), cAMP was as effective as its more membrane-permeant analogs dbcAMP and 8-Br-cAMP. Forskolin (1 or 10 microM), a stimulator of adenylyl cyclase, was also effective at inducing pigment aggregation. Two inhibitors of organic anion transport, probenecid and sulfinpyrazone, inhibited cAMP-induced aggregation by approximately 80% but had no effect on forskolin-induced aggregation. Several agents shown to stimulate RPE adenylyl cyclase in other species failed to induce pigment aggregation in isolated RPE sheets. CONCLUSIONS Our observations strongly suggest that exogenously applied, nonderivatized cAMP can gain access to the cytoplasm of isolated RPE cells via organic anion transporters. Thus, if cAMP were secreted by retinal cells into the subretinal space, it could be taken up by RPE cells and subsequently act as an intracellular messenger to activate dark-adaptive physiological processes such as pigment granule aggregation.