Na⁺-independent choline transport in rat retinal capillary endothelial cells

The purpose of this study was to clarify the mechanism of the inner blood-retinal barrier (inner BRB) transport of choline and examine the choline uptake ability of rat choline transporter-like protein (CTL) 1. The transcript level of CTL1 in a conditionally immortalized rat retinal capillary endothelial cell line (TR-iBRB2) is more than 100-fold greater than that of CTL3 and CTL4, and no expression of organic cation transporter (OCT) mRNA was detected. The apparent influx permeability clearance of [³H]choline in the rat retina was found to be 271 μl/(min · g retina). The [ ³H]choline uptake by TR-iBRB2 cells was Na⁺-independent, potential-dependent, and concentration-dependent with Michaelis-Menten constants of 6.44 μM and 99.7 μM, and inhibited by several organic cations but not tetraethylammonium. The inhibition of CTL1 mRNA by small interfering RNA had little effect on the [³H]choline uptake by TR-iBRB2 cells. Rat CTL1-expressing Xenopus laevis oocytes exhibited an increase in the [ ³H]choline uptake by 45% compared with a control. In conclusion, our findings are consistent with Na⁺-independent choline transport being the mechanism for blood-to-retina transport of choline at the inner BRB. Although rat CTL1 expression is associated with the choline uptake, CTL1 does not play a major role in the choline uptake at the inner BRB.