Na+-Ca 2+ Exchanger Mediates z 2+ Ca Idux During Anoxia in Mmdm Central Nervous System nte Matter Peter K. Stys, MD, FRCP(C), Stephen G. Waxman, MD, PhD, and Bruce R. Ransom, MD, PhD White matter of the mammalian central nervous system suffers irreversible injury after prolonged anoxia, which can result in severe neurological impairment. This type of injury zyxwv is critically dependent on Ca2+ influx into cells. We present evidence that the Na+,Ca2+exchanger mediates the majority of the damaging Ca2' influx into cells during anoxia in white matter. Anoxic injury was studied in the isolated rat optic nerve, and functional recovery was moni- tored using the compound action potential. Blockers of voltage-gated Na+ channels (tetrodotoxin and saxitoxin) significantly improved recovery, zyxwvut as did perfusion with zero-Na+ solution; both maneuvers would prevent intracellular "a+} from rising and thus prevent Ca2+ influx by inhibiting reverse operation of the Na+,Ca*+exchanger. Direct pharmacological blockade of the Na+,Ca2+ exchanger during anoxia with bepridil or benzamil also significantly improved recovery. These findings suggest that reverse operation of the Na+,Ca2+ exchanger during anoxia is a critical mechanism of Ca2+influx and subsequent white matter injury. Stys PK, Waxman SG, Ransom BR. Na+-Ca'+ Exchanger mediates Ca2+ influx during anoxia in mammalian central nervous system white matter. Ann Neurol 1991;30:375-380 In the mammalian central nervous system (CNS), both gray matter and white matter (WM) are susceptible to irreversible anoxic/ischemic injury. Axons, as well as neurons, are essential to the functional integrity of the nervous system; damage to WM can result in serious disability as evidenced by diseases such as stroke (par- ticularly lacunar infarcts [l, 23, spinal cord injury, and some forms of vascular dementia zyxwvutsr [3, zyxwvutsrq 41. A critical mechanism of anoxic injury in gray matter involves excessive influx of Ca2 zyxwvutsr through excitotoxin-gated channels (5-81. In contrast, the basic mechanism or mechanisms of anoxic/ischemic injury in mammalian central WM is unknown. We have begun to study the pathophysiology of irre- versible anoxic injury in WM using the isolated rat optic nerve, a representative CNS WM tract {9-11). We have shown that this type of injury is critically dependent on extracellular Ca2+; removal of this ion from the perfusing solution during 60 minutes of an- oxia allows virtually complete recovery of function (10, 12). Moreover, Ca2+ appears to enter the intracellular compartment only gradually during anoxic conditions, in spite of its large electrochemical gradient. The mech- anism of entry of this ion is not fully understood, but it does not seem to occur through conventional voltage-gated Ca2 channels because Ca" channel blockers, such as polyvalent cations (Mn2+, Co2+, or La3+) or dihydropyridines (nifedipine or nimodipine), do not protect the optic nerve from anoxia (131. The Na+,Ca2+ exchanger is a membrane protein, found in many cell types, that extrudes Ca2+ out of the cytoplasm in exchange for Na+ using the electro- chemical gradient of Na' (14-171. Under normal con- ditions, the Na',Ca2+ exchanger functions to maintain intracellular Ca2 + homeostasis. With a diminished or reversed transmembrane N a + gradient, however, the exchanger can operate in a reverse mode, transporting Ca2+ into the cytoplasm while extruding Na+ (18- 203. Here we present evidence that in WM, the damag- ing Ca" influx induced by anoxia is mediated by re- verse operation of the Naf,Ca2+ exchanger. Methods Long Evans rats aged 50 to 70 days were anesthetized with CO, and decapitated. Following a previously described pro- tocol [lo], optic nerves were dissected free, placed in an interface brain slice chamber (Medical Systems Corp, Greenvale, NY), and perfused at 2 ml/min with artificial cerebrospinal fluid (CSF) containing, in mM: NaCl 126, KCI 3.0, MgSO, 2.0, NaHC03 26, NaH2POd 1.25, CaC12 2.0, From the Department of Neurology, Yale University School of Medicine, New Haven, and Neuroscience Research Center (127A), Veterans Administration Hospital, West Haven, CT. Received Nov 19, 1990, and in revised form Jan 23, 1991. Accepted for publication Feb 23, 1991. Address correspondence to D r Stys, Yale University School of Med- icine, Department of Neurology, 710 LCI, 333 Cedar Street, New Haven, zyxwvu CT 06510. Copyright zyxwv 0 1991 by the American Neurological Association 375