Polarity of 11-cis retinal release from cultured retinal pigment epithelium.

PURPOSE Fetal bovine retinal pigment epithelium (RPE) was grown on porous supports to investigate the polarity of 11-cis retinal (RAL) release from these cells and the influence that the interphotoreceptor retinoid-binding protein (IRBP) has on this process. METHODS [3H]all-trans retinol (ROL) was delivered to the basal surface of the cultured RPE by serum retinol-binding protein (RBP). Apo IRBP was added to either the apical or basal medium, or was absent from the incubation entirely. RESULTS The greatest level of [3H]11-cis RAL was detected in the apical medium but only when apo IRBP was present there. When apo IRBP was present only in the basal medium, or was absent from the incubation entirely, low levels of [3H]11-cis RAL were released apically and basally. CONCLUSIONS If 11-cis RAL release were constitutive, one would expect to find elevated levels of this retinoid in the apical and basal media in the absence of apo IRBP. Instead, the enhancement of [3H]11-cis RAL release into the apical, but not the basal, medium in the presence of apo IRBP suggests that [3H]11-cis RAL release is polarized and dependent on the presence of apo IRBP. It is postulated, therefore, that a mechanism such as an IRBP membrane receptor in the apical plasma membrane may be responsible for this polarity.