Evaluation of blood-brain barrier transporters by co-culture of brain capillary endothelial cells with astrocytes.

To investigate the transport function of the blood-brain barrier (BBB), we employed an in vitro model of the BBB, consisting of a co-culture of porcine brain capillary endothelial cells (BCECs) with rat astrocytes. Porcine BCECs were cultured on a filter insert with rat astrocytes on the underlying plastic well. Rat astrocytes induced characteristic BBB properties of porcine BCECs, such as gamma-glutamyl-transpeptidase activity and intercellular adhesion of porcine BCECs. Next, the transport properties of P-glycoprotein (P-gp) substrate and several anionic compounds across the co-cultured porcine BCECs were characterized. Expression of P-gp was detected by immunocytochemistry, and efflux-directed transport of the P-gp substrate [(3)H]daunomycin was observed. Luminal-to-abluminal transport of the monocarboxylic acid transporter 1 (MCT1) substrate [(14)C]benzoic acid was saturable, and the K(m) value (3.05 mM) was similar to that for brain uptake observed in vivo. Abluminal-to-luminal transport of [(14)C]benzoic acid was also saturable, indicating that the monocarboxylic acid transporter of the BBB contributes to the efflux from the brain as well as to blood-to-brain influx. Abluminal-to-luminal transport of organic anions, [(3)H]dehydroepiandrosterone sulfate, [(3)H]estrone sulfate and [(3)H]estradiol 17beta-D-glucuronide was significantly higher than the corresponding luminal-to-abluminal transport. These results demonstrate the presence of multiple efflux transport pathways in this in vitro model.