Essential Role of GLUT1 in the Inner Blood-retinal Barrier Transport of Vitamin C

Abstract

Abstract: : Purpose: Vitamin C (ascorbic acid) plays a key role in protecting the retina from light-induced oxidative stress. Dehydroascorbic acid (DHAA), which is an oxidized form of vitamin C, is a substrate for glucose transporter 1 (GLUT1), which is expressed at the inner blood-retinal barrier (iBRB). The purpose of this study was to elucidate the DHAA transport mechanism at the iBRB. Methods:[14C]DHAA transport to the rat retina was evaluated using an in vivo integration plot analysis. The conditionally immortalized rat retinal capillary endothelial cell line (TR-iBRB), which expresses GLUT1, was used as an in vitro model for the iBRB. [14C]DHAA was generated in all experiments by incubating [14C]ascorbic acid (AA) with ascorbate oxidase. Results: In the rat retina, the apparent influx clearance of [14C]DHAA was 38-fold greater than that of [14C]AA. [14C]DHAA uptake by TR-iBRB cells was Na+-independent and concentration-dependent with a Michaelis constant of 93 uM. The initial uptake rate of [14C]DHAA by TR-iBRB cells was 37-fold greater than that of [14C]AA. [14C]DHAA uptake was inhibited by more than 50% in the presence of D-glucose, 3-O-methyl-D-glucose, 2-deoxyglucose, cytochalasin B, and phloretin, which are substrates and/or inhibitors of GLUT1. Conclusions:These findings suggest that transport of vitamin C into the retina is mediated as DHAA via GLUT1 at the iBRB. It allows a better understanding of the function of the iBRB with regard to the protecting neural retina.

Keywords: retina • oxidation/oxidative or free radical damage • vascular cells