CALCIUM-INDEPENDENT REGULATION OF PIGMENT GRANULE AGGREGATION AND DISPERSION IN TELEOST RETINAL-PIGMENT EPITHELIAL-CELLS

In the eyes of teleosts and amphibians, melanin pigment granules of th e retinal pigment epithelium (RPE) migrate in response to changes in l ight conditions, In the light, pigment granules disperse into the cell s' long apical projections, thereby shielding the rod photoreceptor ou ter segments and reducing their extent of bleach, In darkness, pigment granules aggregate towards the base of the RPE cells, In vitro, RPE p igment granule aggregation can be induced by application of nonderivat ized cAMP, and pigment granule dispersion can be induced by cAMP washo ut, In previous studies based on RPE-retina cocultures, extracellular calcium was found to influence pigment granule migration. To examine t he role of calcium in regulation of RPE pigment granule migration in t he absence of retinal influences, we have used isolated RPE sheets and dissociated, cultured RPE cells, Under these conditions depletion of extracellular or intracellular calcium ([Ca2+](0), [Ca2+](i)) had no e ffect on RPE pigment granule aggregation or dispersion, Using the intr acellular calcium dye fura-2 and a new dye, fura-pe3, to monitor calci um dynamics in isolated RPE cells, we found that lCa(2+)](i) did not c hange from basal levels when pigment granule aggregation was triggered by cAMP, or dispersion was triggered by cAMP washout, Also, no change in [Ca2+](i) was detected when dispersion was triggered by cAMP washo ut in the presence of 10 mu M dopamine, a treatment previously shown t o enhance dispersion, In addition, elevation of [Ca2+](i) by addition of ionomycin neither triggered pigment movements, nor interfered with pigment granule motility elicited by cAMP addition or washout, Since o ther studies have indicated that actin plays a role in both pigment gr anule dispersion and aggregation in RPE, our findings suggest that RPE pigment granule migration depends on an actin-based motility system t hat is not directly regulated by calcium.