Increased tolerance to oxygen and glucose deprivation in astrocytes from Na+/H+ exchanger isoform 1 null mice

The ubiquitously expressed Na+/H+ exchanger isoform 1 (NHE1) functions as a major intracellular pH (pH(i)) regulatory mechanism in many cell types, and in some tissues its activity may contribute to ischemic injury. In the present study, cortical astrocyte cultures from wild-type (NHE1(+/+)) and NHE1-deficient (NHE1(-/-)) mice were used to investigate the role of NHE1 in pHi recovery and ischemic injury in astrocytes. In the absence of HCO3-, the mean resting pH(i) levels were 6.86+/-0.03 in NHE1(+/+) astrocytes and 6.53+/-0.04 in NHE1(-/-) astrocytes. Removal of extracellular Na+ or blocking of NHE1 activity by the potent NHE1 inhibitor HOE-642 significantly reduced the resting level of pH(i) in NHE1(+/+) astrocytes. NHE1(+/+) astrocytes exhibited a rapid pH(i) recovery (0.33+/-0.08 pH unit/min) after NH4Cl prepulse acid load. The pH(i) recovery in NHE1(+/+) astrocytes was reversibly inhibited by HOE-642 or removal of extracellular Na+. In NHE1(-/-) astrocytes, the pH(i) recovery after acidification was impaired and not affected by either Na+-free conditions or HOE-642. Furthermore, 2 h of oxygen and glucose deprivation (OGD) led to an similar to80% increase in pHi recovery rate in NHE1(-/-) astrocytes. OGD induced a 5-fold rise in intracellular [Na+] and 26% swelling in NHE1(-/-) astrocytes. HOE-642 or genetic ablation of NHE1 significantly reduced the Na+ rise and swelling after OGD. These results suggest that NHE1 is the major pHi regulatory mechanism in cortical astrocytes and that ablation of NHE1 in astrocytes attenuates ischemia-induced disruption of ionic regulation and swelling.