title: Elevated extracellular K(+) concentrations inhibit N-methyl-D-aspartate-induced Ca(2+) influx and excitotoxicity.
abstract: Although extracellular [K(+)] ([K(+)](E)) is highly elevated during brain ischemia, in vitro studies aimed at explaining the mechanisms of excitotoxicity have been conducted at low [K(+)](E). Whether high [K(+)](E) affects excitotoxicity has not been formally addressed. Therefore this study, using digital fluorescence microscopy, tested how the elevation of [K(+)](E) from 5.6 to 60 mM affects N-methyl-D-aspartate (NMDA)-induced Ca(2+) and Na(+) influx, plasma membrane (PM) potential, mitochondrial Ca(2+) load, and viability of primary cultures of rat cerebellar granule cells. High [K(+)](E) curtailed the NMDA-induced Ca(2+) and Na(+) influx and mitochondrial Ca(2+) overload, and prevented neuronal death. Surprisingly, the inhibitory effect of high [K(+)](E) on the NMDA-induced Ca(2+) influx could not be linked to depolarization of the PM. Apparently, the PM of cerebellar granule cells exposed to NMDA was more depolarized at low than at high [K(+)](E), probably because the NMDA-induced Na(+) influx was greatly enhanced when the extracellular [Na(+)]/[K(+)] ratio was increased. When this ratio was small, i.e., at high [K(+)](E), the NMDA-induced increase in cytoplasmic [Na(+)] was suppressed, preventing Ca(2+) influx via the reverse operation of the Na(+)/Ca(2+) exchanger, which may explain the inhibitory effect of high [K(+)](E) on NMDA-induced Ca(2+) influx and excitotoxicity.
authors.affiliation: The Psychiatric Institute, Departments of Psychiatry and Pharmacology, The Psychiatric Institute, Department of Psychiatry and Pharmacology, College of Medicine, The University of Illinois at Chicago, Chicago, Illinois 60612, USA. lkiedr@psych.uic.edu
authors.forename: L
authors.initials: L
authors.lastname: Kiedrowski
mesh_terms: D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D002118:Calcium / Q000378:metabolism*; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D002478:Cells, Cultured; D018690:Excitatory Amino Acid Agonists / Q000494:pharmacology*; D018698:Glutamic Acid / Q000494:pharmacology; D008928:Mitochondria / Q000187:drug effects / Q000378:metabolism; D016202:N-Methylaspartate / Q000378:metabolism*; D009474:Neurons / Q000187:drug effects* / Q000378:metabolism; D011188:Potassium / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D012964:Sodium / Q000378:metabolism; D019831:Sodium-Calcium Exchanger / Q000378:metabolism
chemical_list: D018690:Excitatory Amino Acid Agonists; D019831:Sodium-Calcium Exchanger; D018698:Glutamic Acid; D016202:N-Methylaspartate; D012964:Sodium; D011188:Potassium; D002118:Calcium