Involvement of a Novel Organic Cation Transporter in Verapamil Transport Across the Inner Blood-Retinal Barrier

To clarify the transport and inhibition characteristics involved in verapamil transport across the inner blood-retinal barrier (inner BRB). The transport of [H-3]verapamil across the inner BRB was investigated using retinal uptake index and integration plot analyses in rats. The detailed transport characteristics were studied using TR-iBRB2 cells, a conditionally immortalized rat retinal capillary endothelial cell line that is an in vitro model of the inner BRB. The apparent influx permeability clearance of [H-3]verapamil was 614 mu L/(min center dot g retina), which is 4.7-fold greater than that of brain. The retinal uptake of [H-3]verapamil was slightly increased by 3 mM verapamil and 10 mM qunidine and inhibited by 40 mM pyrilamine, supporting the carrier-mediated efflux and influx transport of verapamil across the inner BRB. TR-iBRB2 cells exhibited a concentration-dependent uptake of [H-3]verapamil with a K (m) of 61.9 mu M, and the uptake was inhibited by several cations, such as pyrilamine, exhibiting a different profile from the identified transporters. These transport properties suggest that verapamil transport at the inner BRB takes place via a novel organic cation transporter. Our findings suggest that a novel organic cation transporter is involved in verapamil transport from the blood to the retina across the inner BRB.