Thyroxine (T(4)) Transfer from Blood to Cerebrospinal Fluid in Sheep Isolated Perfused Choroid Plexus: Role of Multidrug Resistance-Associated Proteins and Organic Anion Transporting Polypeptides

Thyroxine (T(4)) enters the brain either directly across the blood–brain barrier (BBB) or indirectly via the choroid plexus (CP), which forms the blood–cerebrospinal fluid barrier (B-CSF-B). In this study, using isolated perfused CP of the sheep by single-circulation paired tracer and steady-state techniques, T4 transport mechanisms from blood into lateral ventricle CP has been characterized as the first step in the transfer across the B-CSF-B. After removal of sheep brain, the CPs were perfused with (125)I-T(4) and (14)C-mannitol. Unlabeled T(4) was applied during single tracer technique to assess the mode of maximum uptake (U(max)) and the net uptake (U(net)) on the blood side of the CP. On the other hand, in order to characterize T(4) protein transporters, steady-state extraction of (125)I-T(4) was measured in presence of different inhibitors such as probenecid, verapamil, BCH, or indomethacin. Increasing the concentration of unlabeled-T(4) resulted in a significant reduction in U(max)%, which was reflected by a complete inhibition of T(4) uptake into CP. In fact, the obtained U(net)% decreased as the concentration of unlabeled-T(4) increased. The addition of probenecid caused a significant inhibition of T(4) transport, in comparison to control, reflecting the presence of a carrier mediated process at the basolateral side of the CP and the involvement of multidrug resistance-associated proteins (MRPs: MRP1 and MRP4) and organic anion transporting polypeptides (Oatp1, Oatp2, and Oatp14). Moreover, verapamil, the P-glycoprotein (P-gp) substrate, resulted in ~34% decrease in the net extraction of T(4), indicating that MDR1 contributes to T(4) entry into CSF. Finally, inhibition in the net extraction of T(4) caused by BCH or indomethacin suggests, respectively, a role for amino acid “L” system and MRP1/Oatp1 in mediating T(4) transfer. The presence of a carrier-mediated transport mechanism for cellular uptake on the basolateral membrane of the CP, mainly P-gp and Oatp2, would account for the efficient T(4) transport from blood to CSF. The current study highlights a carrier-mediated transport mechanism for T4 movement from blood to brain at the basolateral side of B-CSF-B/CP, as an alternative route to BBB.