The glucose transport in retinal pigment epithelium is via passive facilitated diffusion

The glucose transport across the bovine retinal pigment epithelium (RPE) was studied in a modified Ussing chamber. Unidirectional fluxes were recorded with radioactive tracers L-[14C]-glucose (LG) and 3-O-methyl-D-[3H]-glucose (MDG). There was no significant difference between the unidirectional MDG fluxes (retina to choroid, and choroid to retina directions) with or without ouabain. The effects of two glucose transporter inhibitors, phloretin and cytochalasin B, on the glucose fluxes from choroid to retina cells were also investigated. The MDG flux was found to be inhibited by 45.5% by phloretin (10(-4) M) and 87.4% by cytochalasin B (10(-4) M). These inhibitory characteristics resembled the facilitated diffusion mode of glucose transport. The glucose transporter protein in the plasma membrane of RPE was located by means of photolabeling [3H]-cytochalasin B. The labeled plasma membrane enriched fraction was analysed by SDS-PAGE. The glucose transporter of bovine RPE was found to have a molecular weight range of 46-53 kDa. The molecular weight range of this transporter protein agreed with those of facilitated glucose transporters in other tissues indicating a molecular similarity between them. The results indicated that the glucose transport across the RPE is via passive facilitated diffusion.