Transepithelial transport of organic anions across the choroid plexus: possible involvement of organic anion transporter and multidrug resistance-associated protein.

Transport characteristics of 17beta-estradiol 17beta-D-glucuronide (E217betaG), a dual substrate of the transporters for cellular uptake (organic anion-transporting polypeptide 1 or oatp1) and cellular excretion (multidrug resistance-associated protein 1or MRP1), in the rat choroid plexus were studied in vivo and in vitro. The uptake of E217betaG into isolated choroid plexus was mediated by an energy-dependent system with a Km of 3.4 microM. Together with the previous finding that oatp1 is localized on the apical membrane of choroid plexus, these results suggest that oatp1 is responsible for the uptake of this ligand. After intracerebroventricular administration, elimination of E217betaG from cerebrospinal fluid was probenecid sensitive and much more rapid than that of inulin; less than 2% of the administered E217betaG and 40 to 50% of inulin remained in the cerebrospinal fluid 20 min after intracerebroventricular administration. In addition, the amount of E217betaG associated with choroid plexus at 20 min was negligible, suggesting the presence of an efficient excretion system on the basolateral membrane of choroid plexus. Expression of MRP1 was detected in choroid plexus. Semiquantitative reverse transcription-polymerase chain reaction and Western blot analyses indicated that the expression level of MRP1 in choroid plexus is about four or five times higher than that in the lung, one of the tissues exhibiting high expression of MRP1. Together with the in vivo vectorial transport of E217betaG, these results can be accounted for by assuming that there is basolateral localization of MRP1 in choroid plexus. Combined, oatp1 and MRP1 may synergistically mediate the efficient transcellular transport of E217betaG across choroid plexus.