Astrocytes increase the functional expression of P-glycoprotein in an in vitro model of the blood-brain barrier

Purpose: To investigate the influence of astrocytes on P-glycoprotein (Pgp) expression and intracellular accumulation of Pgp substrates, separate from their net transcelluIar transport across the blood-brain barrier (BBB). Methods. An in vitro BBB model was used, comprising of brain capillary endo thelial cells (BCEC) monolayers or BCEC co-cultured with astrocytes. Results. BCEC+astrocyte co-cultures seemed to express a higher level of Pgp compared to BCEC monolayers. Inhibition of Pgp results in an increased int racellular accumulation of Pgp substrates in both BCEC monolayers and BCECastrocyte co-cultures, and increased the sensitivity for vinblastine mediat ed disruption of the in vitro BBB (called the vinblastine exclusion assay). BCEC monolayers were more sensitive to vinblastine mediated disruption com pared to BCEC+astrocyte co-cultures. In the latter, but not in BCEC monolay ers, an inhibitable polar transport of Pgp substrates was only found from t he brain to the blood side of the filter. Conclusions, Astrocytes increase the functional expression of Pgp in our in vitro BBB model. These results also illustrate that an important role for Pgp on the BBB is to protect the barrier against intracellular accumulation of cytotoxic BBB disrupting compounds.