Active transport of fentanyl by the blood-brain barrier

Previous studies have shown that uptake of the lipophilic opioid, fentanyl, by pulmonary endothelial cells occurs by both passive diffusion and carrie r-mediated processes. To evaluate if the latter mechanism also exists in br ain endothelium, transport of [H-3]fentanyl was examined in primary culture d bovine brain microvessel endothelial cell (BBMEC) monolayers. Uptake of f entanyl appears to occur via a carrier-mediated process as uptake of [H-3]f entanyl by BBMECs was significantly inhibited in a dose-dependent manner by unlabeled fentanyl, Fentanyl uptake was also significantly inhibited by ei ther 4 degrees C or sodium azide/2-deoxyglucose, suggesting that carrier-me diated uptake of fentanyl was an active process. Fentanyl was also tested t o determine whether it might be a substrate of the endogenous blood-brain b arrier efflux transport system, P-glycoprotein (P-gp), Release of [H-3]fent anyl or rhodamine 123, a known substrate of P-gp, previously loaded in the BBMECs was studied in the presence or absence of either fentanyl or verapam il, a known competitive inhibitor of P-gp. Both fentanyl (10 mu M) and vera pamil (100 mu M) decreased release of rhodamine 123 from BBMECs, indicating that fentanyl is a substrate of P-gp in the BBMECs, This was further suppo rted by the observation that uptake of [H-3]fentanyl was significantly incr eased in Mg2+-free medium, a condition known to reduce P-gp activity. Howev er, release of [H-3]fentanyl was significantly increased when incubated wit h either unlabeled fentanyl or verapamil. These results suggest that the ac tive P-gp-mediated extrusion of fentanyl in these cells is overshadowed by an active inward transport process, mediated by an as yet unidentified tran sporter, In addition, verapamil was shown to be a substrate of both P-gp an d the fentanyl uptake transporter.