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Ischemia‐induced Stimulation of the BBB Na‐K‐Cl Cotransporter: Evidence for a Role of p38 MAP Kinase

Increased blood‐brain barrier (BBB) Na‐K‐Cl cotransporter activity appears to contribute to cerebral edema formation during ischemic stroke. The cotransporter is stimulated by hypoxia and aglycemia, although the mechanisms responsible are unknown. Inhibition of p38MAP kinase (p38MAPK), which is activated by ischemia, has been found to reduce brain infarct in ischemic stroke. In the present study we examined the possibility that p38MAPK participates in ischemia‐induced stimulation of the BBB Na‐K‐Cl cotransporter. Cerebral microvascular endothelial cells (CMEC) were exposed to hypoxia (7.5% O2) or to aglycemia, then subjected to Western blot analysis, using antibodies that recognize phosphorylated (activated) p38MAPK (p‐p38MAPK) or both phosphorylated and non‐phosphorylated (total) p38MAPK. CMEC were also evaluated for cotransporter activity as bumetanide‐sensitive K influx. Hypoxia (5 – 120 min) and aglycemia (30 min) caused 1.9‐ and 2.8‐fold increases in p‐p38MAPK levels without changing total p38MAPK. Exposing CMEC to the p38 MAPK inhibitor SB239063 (20 μM) abolished hypoxia‐ and aglycemia‐induced increases in Na‐K‐Cl cotransporter activity. Confocal immunofluorescence of perfusion‐fixed rat brain slices also revealed the presence of p38MAPK in BBB in situ. These findings suggest that p38 MAPK participates in ischemia stimulation of BBB Na‐K‐Cl cotransporter activity.