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

Departments of 1 Neurosurgery and 2 Physiology, University of Wisconsin Medical School, Madison, Wisconsin 53792; 3 Department of Molecular Genetics, Biochemistry and Microbiology, University of Cincinnati, Cincinnati, Ohio 45267; and 4 Department of Cardiovascular Sciences, University of Leicester, Leicester LE2 7LX, United Kingdom Submitted 10 December 2003 ; accepted in final form 4 March 2004 ABSTRACT 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 pH i recovery and ischemic injury in astrocytes. In the absence of HCO 3 – , 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 NH 4 Cl 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 80% increase in pH i 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 pH i regulatory mechanism in cortical astrocytes and that ablation of NHE1 in astrocytes attenuates ischemia-induced disruption of ionic regulation and swelling. intracellular pH; cortical astrocytes; sodium/calcium exchange; ischemia; intracellular sodium Address for reprint requests and other correspondence: D. Sun, Dept. of Neurological Surgery, Univ. of Wisconsin Medical School, H4/332 Clinical Sciences Center, 600 Highland Ave., Madison, WI 53792 (E-mail address: sun{at}neurosurg.wisc.edu ).