LOCALIZATION OF NAK ATPASE ON CULTURED HUMAN RETINAL-PIGMENT EPITHELIUM

Purpose. To localize NaK ATPase sites on cultured human retinal pigmen t epithelium (RPE). Methods. Cultured human RPE from fetal, 2-year-old , and 21-year-old donors was grown to confluence in microporous cultur e wells for 4 months to 2 years, mounted in a small-volume Ussing cham ber, and perfused with growth medium. Ouabain (10(-5)-M) was applied t o the basal and apical sides of the RPE. Changes in transepithelial re sistance (R(t)), transepithelial potential (TEP), and apical and basal membrane potentials were measured. Results. Application of ouabain to the basal side of RPE produced a small sustained increase in TEP afte r 6 minutes and, simultaneously, small depolarizations of both apical and basal membranes. During the continued presence of ouabain on the b asal side, application of ouabain to the apical side produced a signif icantly larger TEP decrease and greater depolarization of both membran es. Significant changes in R(t) were not observed. Conclusions. These results indicate that NaK ATPase sites are present on both the apical and basolateral membranes of cultured human RPE. The greater effect of ouabain when applied to the apical side suggests that functional NaK ATPase sites are more abundant on the apical membrane.