In depolarized and glucose-deprived neurons, Na+ influx reverses plasmalemmal K+-dependent and K+-independent Na+/Ca2+ exchangers and contributes to NMDA excitotoxicity.
Cerebellar granule cells (CGCs) express K+-dependent (NCKX) and K+-independent (NCX) plasmalemmal Na+/Ca2+ exchangers which, under plasma membrane-depolarizing conditions and high cytosolic [Na+], may reverse and mediate potentially toxic Ca2+ influx. To examine this possibility, we inhibited NCX or NCKX with KB-R7943 or K+-free medium, respectively, and studied how gramicidin affects cytosolic [Ca2+] and 45Ca2+ accumulation. Gramicidin forms pores permeable to alkali cations but not Ca2+. Therefore, gramicidin-induced Ca2+ influx is indirect; it results from fluxes of monovalent cations. In the presence of Na+, but not Li+ or Cs+, gramicidin induced Ca2+ influx that was inhibited by simultaneous application of KB-R7943 and K+-free medium. The data indicate that gramicidin-induced Na+ influx reverses NCX and NCKX. To test the role of NCX and/or NCKX in excitotoxicity, we studied how NMDA affects the viability of glucose-deprived and depolarized CGCs. To assure depolarization of the plasma membrane, we inhibited Na+,K+-ATPase with ouabain. Although inhibition of NCX or NCKX reversal failed to significantly limit 45Ca2+ accumulation and excitotoxicity, simultaneously inhibiting NCX and NCKX reversal was neuroprotective and significantly decreased NMDA-induced 45Ca2+ accumulation. Our data suggest that NMDA-induced Na+ influx reverses NCX and NCKX and leads to the death of depolarized and glucose-deprived neurons.
Czyz
Aneta
A
Departments of Psychiatry and Pharmacology, The Psychiatric Institute, The University of Illinois at Chicago, Chicago, Illinois 60612, USA.
Kiedrowski
Lech
L
D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000378:metabolism; D002132:Calcium Radioisotopes; D021961:Cell Membrane Structures / Q000378:metabolism; D002470:Cell Survival / Q000187:drug effects; D002478:Cells, Cultured; D002586:Cesium / Q000494:pharmacology; D005947:Glucose / Q000172:deficiency / Q000378:metabolism*; D006096:Gramicidin / Q000494:pharmacology; D008094:Lithium / Q000494:pharmacology; D016202:N-Methylaspartate / Q000633:toxicity*; D009474:Neurons / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D011188:Potassium / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D012964:Sodium / Q000378:metabolism*; D019831:Sodium-Calcium Exchanger / Q000037:antagonists & inhibitors / Q000378:metabolism*; D000254:Sodium-Potassium-Exchanging ATPase / Q000037:antagonists & inhibitors; D013890:Thiourea / Q000031:analogs & derivatives* / Q000494:pharmacology
C101670:2-(2-(4-(4-nitrobenzyloxy)phenyl)ethyl)isothiourea methanesulfonate; D002132:Calcium Radioisotopes; D019831:Sodium-Calcium Exchanger; D006096:Gramicidin; D002586:Cesium; D016202:N-Methylaspartate; D000255:Adenosine Triphosphate; D008094:Lithium; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D013890:Thiourea; D005947:Glucose; D011188:Potassium; D002118:Calcium