Blood-to-retina transport of riboflavin via RFVTs at the inner blood-retinal barrier

Riboflavin (vitamin B2) supply to the retina across the inner blood-retinal barrier (BRB) was investigated. In rats, the apparent influx permeability clearance of [3H]riboflavin (62.8 μL/(min·g retina)) was much higher than that of a non-permeable paracellular marker, suggesting the facilitative influx transport of riboflavin across the BRB. The retinal uptake index (RUI) of [3H]riboflavin was 59.0%, and significantly reduced by flavin adenine dinucleotide (FAD), but not by l-ascorbic acid, suggesting the substrate specificity of riboflavin transport. TR-iBRB2 cells, an in vitro model of the inner BRB, showed a temperature- and concentration-dependent [3H]riboflavin uptake with a Km of 113 nM, suggesting that the influx transport of riboflavin across the inner BRB involves a carrier-mediated process. [3H]Riboflavin uptake by TR-iBRB2 cells was slightly altered by Na+- and Cl−-free buffers, suggesting that riboflavin transport at the inner BRB is preferentially Na+- and Cl−-independent. [3H]Riboflavin uptake by TR-iBRB2 cells was significantly reduced by riboflavin analogues while the uptake remained unchanged by other vitamins. The function and inhibition profile suggested the involvement of riboflavin transporters (SLC52A/RFVT) in riboflavin transport at the inner BRB, and this is supported by expression and knockdown analysis of rRFVT2 (Slc52a2) and rRFVT3 (Slc52a3) in TR-iBRB2 cells.