EXTRACELLULAR ATP ACTIVATES CALCIUM SIGNALING, ION, AND FLUID TRANSPORT IN RETINAL-PIGMENT EPITHELIUM

The presence of receptors for ATP has not been established in any nati ve preparation of retinal neurons or glia. In the present study, we us ed conventional electrophysiological and [Ca2+](in) fluorescence imagi ng techniques to investigate the effects of ATP added to Ringer's solu tion perfusing the retinal-facing (apical) membrane of freshly isolate d monolayers of bovine retinal pigment epithelium (RPE). ATP (or UTP) produced large, biphasic voltage and resistance changes with a K-d of similar to 5 mu M for ATP and similar to 1 mu M for UTP. Electrical an d pharmacological evidence indicates that the first and second phases of the response are attributable to an increase in basolateral membran e CI conductance and a decrease in apical membrane K conductance, resp ectively. The ATP-induced responses were not affected by adenosine, bu t were reduced by the P-2-purinoceptor blocker suramin. ATP also produ ced a large, transient increase in [Ca2+](in) that was blocked by cycl opiazonic acid, an inhibitor of endoplasmic reticulum Ca2+-ATPases. Th e calcium buffer BAPTA attenuated the voltage effects of ATP. We also found that apical DIDS significantly inhibited the ATP-evoked [Ca2+](i n) and electrical responses, suggesting that DIDS blocked the purinoce ptor, Measurements of fluid movement across the RPE using the capacita nce probe technique demonstrated a significant increase in fluid absor ption by apical UTP. These data indicate the presence of metabotropic P-2Y/P-2U-purinoceptors at the RPE apical membrane and implicate extra cellular ATP in vivo as a retinal signaling molecule that could help r egulate the hydration and chemical composition of the subretinal space .