CEREBRAL MICROVASCULAR ENDOTHELIAL-CELL NA-K-CL COTRANSPORT - REGULATION BY ASTROCYTE-CONDITIONED MEDIUM

Brain microvascular endothelial cells play an important role in regula tion of ion and fluid movement between the blood and the brain interst itium. Astrocytes have been shown to induce blood-brain barrier proper ties in the endothelial cells, including formation of tight junctions and increased expression and asymmetric distribution of enzymes and io n transport systems. Previous studies have demonstrated that endotheli al cells of bovine aorta possess a highly active Na-K-Cl cotransport s ystem that participates in intracellular volume regulation. The presen t study was conducted to evaluate Na-K-Cl cotransport activity of cere bral microvascular endothelial cells and to determine whether astrocyt e-conditioned medium (CM) influences Na-K-Cl cotransport activity of t hese cells. We found the brain microvascular endothelial cells to exhi bit a robust Na-K-Cl cotransport activity, comprising 50% of the total K influx. Activity of the cotransporter was stimulated by agents that elevate intracellular Ca and by hypertonicity and was inhibited by ag ents that elevate adenosine 3',5'-cyclic monophosphate, guanosine 3',5 '-cyclic monophosphate, or activate protein kinase C. Exposure of the cells to primary astrocyte- or C6 glial cell-CM but not A7r5 or A10 va scular smooth muscle cell-CM also increased cotransport activity. Howe ver, this effect required > 1 h of exposure to CM, was additive with t he effects of vasopressin, calcium ionophore, and hypertonicity, and w as blocked by the protein synthesis inhibitor cycloheximide. These fin dings are consistent with the hypothesis that expression of endothelia l cell Na-K-Cl cotransport is induced and/or maintained by astrocytes and that the cotransporter may be an important component of blood-brai n barrier function.