Mechanisms of glutamate release elicited in rat cerebrocortical nerve endings by 'pathologically' elevated extraterminal K+ concentrations.
Extracellular [K+] can increase during some pathological conditions, resulting into excessive glutamate release through multiple mechanisms. We here investigate the overflow of [3H]D-aspartate ([3H] D-ASP) and of endogenous glutamate elicited by increasing [K+] from purified rat cerebrocortical synaptosomes. Depolarization with [K+] <or= 15 mmol/L provoked [3H] D-ASP and glutamate overflows almost totally dependent on external Ca2+. Consistent with release by exocytosis, the overflow of [3H] D-ASP evoked by 12 mmol/L K+ was sensitive to clostridial toxins. The overflows evoked by 35/50 mmol/L K+ remained external Ca2+-dependent by more than 50%. The Ca2+-independent components of the [3H] D-ASP overflows evoked by [K+] > 15 mmol/L were prevented by the glutamate transporter inhibitors DL-threo-beta-benzyloxyaspartate (DL-TBOA) and dihydrokainate. Differently, the overflows of endogenous glutamate provoked by [K+] > 15 mmol/L were insensitive to both inhibitors; the external Ca2+-independent glutamate overflow caused by 50 mmol/L KCl was prevented by bafilomycin, by chelating intraterminal Ca2+, by blocking the mitochondrial Na+/Ca2+ exchanger and, for a small portion, by blocking anion channels. In contrast to purified synaptosomes, the 50 mmol/L K+-evoked release of endogenous glutamate or [3H]D-ASP was inhibited by DL-TBOA in crude synaptosomes; moreover, it was external Ca2+-insensitive and blocked by DL-TBOA in purified gliosomes, suggesting that carrier-mediated release of endogenous glutamate provoked by excessive [K+] in CNS tissues largely originates from glia.
Department of Experimental Medicine, Pharmacology and Toxicology Section, University of Genoa, Genoa, Italy.
Luca
L
Raiteri
Simona
S
Zappettini
Marco
M
Milanese
Ernesto
E
Fedele
Maurizio
M
Raiteri
Giambattista
G
Bonanno
D000818:Animals; D001224:Aspartic Acid / Q000378:metabolism / Q000494:pharmacology; D015220:Calcium Channels / Q000187:drug effects / Q000378:metabolism; D002540:Cerebral Cortex / Q000737:chemistry / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D027342:Excitatory Amino Acid Transporter 2 / Q000037:antagonists & inhibitors / Q000378:metabolism; D005089:Exocytosis / Q000187:drug effects / Q000502:physiology; D045604:Extracellular Fluid / Q000187:drug effects / Q000378:metabolism; D050491:Glutamate Plasma Membrane Transport Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism*; D018698:Glutamic Acid / Q000378:metabolism*; D006947:Hyperkalemia / Q000378:metabolism / Q000503:physiopathology; D007608:Kainic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D018942:Macrolides / Q000494:pharmacology; D008297:Male; D011188:Potassium / Q000633:toxicity*; D017729:Presynaptic Terminals / Q000187:drug effects / Q000378:metabolism*; D011500:Protein Synthesis Inhibitors / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D009435:Synaptic Transmission / Q000187:drug effects / Q000502:physiology; D013574:Synaptosomes / Q000737:chemistry / Q000187:drug effects* / Q000378:metabolism
D015220:Calcium Channels; D027342:Excitatory Amino Acid Transporter 2; D050491:Glutamate Plasma Membrane Transport Proteins; D018942:Macrolides; D011500:Protein Synthesis Inhibitors; C120673:benzyloxyaspartate; C057620:bafilomycin A; D001224:Aspartic Acid; D018698:Glutamic Acid; C020889:dihydrokainic acid; D011188:Potassium; D007608:Kainic Acid