Elevated Extracellular K+ Concentrations InhibitN-Methyl-d-Aspartate-Induced Ca2+ Influx and Excitotoxicity
- The Psychiatric Institute, Departments of Psychiatry and Pharmacology, College of Medicine, The University of Illinois at Chicago, Chicago, Illinois
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+]Efrom 5.6 to 60 mM affectsN-methyl-d-aspartate (NMDA)-induced Ca2+ and Na+ influx, plasma membrane (PM) potential, mitochondrial Ca2+ load, and viability of primary cultures of rat cerebellar granule cells. High [K+]E curtailed the NMDA-induced Ca2+ and Na+ influx and mitochondrial Ca2+ overload, and prevented neuronal death. Surprisingly, the inhibitory effect of high [K+]E on the NMDA-induced Ca2+ 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 Ca2+ influx via the reverse operation of the Na+/Ca2+ exchanger, which may explain the inhibitory effect of high [K+]Eon NMDA-induced Ca2+ influx and excitotoxicity.
Footnotes
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Send reprint requests to: Lech Kiedrowski, Ph.D., The Psychiatric Institute, 1601 W. Taylor St., Chicago, IL 60612. E-mail:lkiedr{at}psych.uic.edu
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This work was supported by National Institutes of Health Grant NS 37390 and was presented in part in abstract form, Soc Neurosci Abst 895.4, 1997 and 300.5, 1998.
- Abbreviations:
- PM
- plasma membrane
- CaDF
- electrochemical force for Ca2+ influx
- CGCs
- cerebellar granule cells
- CM
- conditioned medium
- DiBAC4(3)
- bis(1,3-dibutylbarbituric acid)trimethine oxonol
- Em
- plasma membrane potential
- F334/F380
- ratio of fluorescence intensities emitted after 334 nm and 380 nm excitation
- [K+]E and [Na+]E
- extracellular concentration of K+ and Na+, respectively
- [Na+]C
- [K+]C and [Ca2+]C, cytoplasmic concentration of Na+, K+, and Ca2+, respectively
- NaCaX
- Na+/Ca2+exchanger
- nF488
- normalized fluorescence intensity emitted after excitation at 488 nm
- NMDA
- N-methyl-d-aspartate
- SBFI
- Na+-binding benzofuran isophthalate
- R/NaCaX
- reverse operation of the Na+/Ca2+ exchanger
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- Received May 18, 1999.
- Accepted June 29, 1999.
- The American Society for Pharmacology and Experimental Therapeutics
