IL-6 SECRETED BY ASTROGLIAL CELLS REGULATES NA-K-CL COTRANSPORT IN BRAIN MICROVESSEL ENDOTHELIAL-CELLS

We showed previously that cerebral microvessel endothelial cells (CMEC ) exhibit a prominent Na-K-CI cotransporter that functions to regulate intracellular volume and may also mediate vectorial ion transport acr oss the blood-brain barrier (BBB). Astrocytes and their conditioned me dia induce BBB properties of the endothelium. Our previous studies dem onstrated that exposure of CMEC to astroglial cells markedly increases cotransport activity, upregulates cotransporter protein expression, a nd also increases cotransporter protein phosphorylation. In the presen t study, we evaluated the possibility that astroglial effects an the N a-K-Cl cotransporter are mediated by astroglial cell-secreted interleu kin-6 (IL-6). Cotransporter activity was assessed as bumetanide-sensit ive K influx, and protein expression was evaluated by Western blot ana lysis. Exposing CMEC to IL-6 for 48 h caused a dose-dependent stimulat ion of Na-K-Cl cotransport activity. Both Cg glial cell-conditioned me dium (C6CM)-induced and IL-6-induced increases in cotransport activity were neutralized by anti-IL-6 antibodies. A 48-h exposure of cells to IL-6 or C6CM also resulted in increased cotransporter protein express ion. Furthermore, using an enzyme-linked immunosorbent assay for IL-6, we found significant amounts of IL-6 in C6CM. These data suggest that astroglia-secreted IL-6 may mediate the observed astroglial cell effe cts on CMEC Na-K-Cl cotransport and further support the hypothesis tha t astrocytes participate in maintenance of cerebral ionic homeostasis by regulating Na-K-Cl cotransporter function at the BBB.