Blood-brain barrier (BBB) Na transporters are essential for brain water and
electrolyte homeostasis. However, they also contribute to
edema formation during the early hours of
ischemic stroke by increased transport of Na from blood into brain across an intact BBB. We previously showed that a
luminal BBB
Na-K-Cl cotransporter is stimulated by
hypoxia, aglycemia, and AVP and that inhibition of the cotransporter by intravenous
bumetanide significantly reduces
edema and
infarct in the rat
middle cerebral artery occlusion (MCAO) model of
stroke. More recently, we found evidence that intravenous
cariporide (HOE-642), a highly potent Na/H exchange inhibitor, also reduces
brain edema after MCAO. The present study was conducted to investigate which Na/H exchange
protein isoforms are present in BBB endothelial cells and to evaluate the effects of ischemic factors on BBB Na/H exchange activity. Western blot analysis of bovine cerebral microvascular endothelial cells (
CMEC) and immunoelectron microscopy of perfusion-fixed rat brain revealed that
Na/H exchanger isoforms 1 and 2 (NHE1 and NHE2) are present in BBB endothelial cells. Using microspectrofluorometry and the pH-sensitive
dye BCECF, we found that
hypoxia (2% O(2), 30 min), aglycemia (30 min), and AVP (1-200 nM, 5 min) significantly increased
CMEC Na/H exchange activity, assessed as Na-dependent, HOE-642-sensitive H(+) flux. We found that AVP stimulation of
CMEC Na/H exchange activity is dependent on intracellular Ca concentration and is blocked by V(1), but not V(2),
vasopressin receptor antagonists. Our findings support the hypothesis that a BBB
Na/H exchanger, possibly NHE1 and/or NHE2, is stimulated during
ischemia to participate in
cerebral edema formation.