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Ken-ichi Hosoya, Akito Minamizono, Kazunori Katayama, Tetsuya Terasaki, Masatoshi Tomi; Vitamin C Transport in Oxidized Form across the Rat Blood–Retinal Barrier. Invest. Ophthalmol. Vis. Sci. 2004;45(4):1232-1239. doi: https://doi.org/10.1167/iovs.03-0505.
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purpose. To elucidate the mechanisms of vitamin C transport across the blood–retinal barrier (BRB) in vivo and in vitro.
methods. [14C]Dehydroascorbic acid (DHA) and [14C]ascorbic acid (AA) transport in the retina across the BRB were examined using in vivo integration plot analysis in rats, and the transport mechanism was characterized using a conditionally immortalized rat retinal capillary endothelial cell line (TR-iBRB2) as an in vitro model of the inner BRB.
results. The apparent influx permeability clearance (K in) per gram of retina of [14C]DHA and [14C]AA was found to be 2.44 × 103 μL/(min · g retina) and 65.4 μL/(min · g retina), respectively. In the retina and brain, the K in of [14C]DHA was approximately 38 times greater than that of [14C]AA, whereas there was no major difference in the heart. The K in of [14C]DHA in the retina was eight times greater than that in the brain. HPLC analysis revealed that most of the vitamin C accumulated in AA form in the retina. These results suggest that vitamin C is mainly transported in DHA form across the BRB and accumulates in AA form in the rat retina. In an in vitro uptake study in TR-iBRB2 cells, the initial uptake rate of [14C]DHA was 37 times greater than that of [14C]AA, which is in agreement with the results of the in vivo study. [14C]DHA uptake by TR-iBRB2 cells took place in an Na+-independent and concentration-dependent manner with a K m of 93.4 μM. This process was inhibited by substrates and inhibitors of glucose transporters. [14C]DHA uptake was inhibited by d-glucose in a concentration-dependent manner with a 50% inhibition concentration of 5.56 mM. Quantitative real-time PCR and immunostaining analyses revealed that expression of GLUT1 and -3 was greater than that of the Na+-dependent l-ascorbic acid transporter (SVCT)-2 in TR-iBRB2 cells.
conclusions. Vitamin C is mainly transported across the BRB as DHA mediated through facilitative glucose transporters and accumulates as AA in the rat retina.
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