Decreased neuronal death in Na+/H+ exchanger isoform 1-null mice after in vitro and in vivo ischemia

Na+/H+ exchanger isoform 1 (NHE1) is a major acid extrusion mechanism after intracellular acidosis. We hypothesized that stimulation of NHE1 after cerebral ischemia contributes to the disruption of Na+ homeostasis and neuronal death. In the present study, expression of NHE1 was detected in cultured mouse cortical neurons. Three hours of oxygen and glucose deprivation (OGD) followed by 21 h of reoxygenation (REOX) led to 68 +/- 10% cell death. Inhibition of NHE1 with the potent inhibitor cariporide (HOE 642) or genetic ablation of NHE1 reduced OGD-induced cell death by similar to 40-50% (p < 0.05). In NHE1(+)/(+) neurons, OGD caused a twofold increase in [Na+](i), and 60 min REOX triggered a sevenfold increase. Genetic ablation of NHE1 or HOE 642 treatment had no effects on the OGD-mediated initial Na-i(+) rise but reduced the second phase of Na-i(+) rise by similar to 40-50%. In addition, 60 min REOX evoked a 1.5-fold increase in [Ca2+] i in NHE1(+)/(+) neurons, which was abolished by inhibition of either NHE1 or reverse-mode operation of Na+/Ca2+ exchange. OGD/REOX-mediated mitochondrial Ca2+ accumulation and cytochrome c release were attenuated by inhibition of NHE1 activity. In an in vivo focal ischemic model, 2 h of left middle cerebral artery occlusion followed by 24 h of reperfusion induced 84.8 +/- 8.0 mm(3) infarction in NHE1(+)/(+) mice. NHE1(+)/(+) mice treated with HOE 642 or NHE1 heterozygous mice exhibited a similar to 33% decrease in infarct size (p < 0.05). These results imply that NHE1 activity disrupts Na+ and Ca2+ homeostasis and contributes to ischemic neuronal damage.