Multiple mechanisms of transmitter release evoked by "pathologically" elevated extracellular [K+]: involvement of transporter reversal and mitochondrial calcium.
The release of [3H]GABA evoked by depolarization with various concentrations of KCl was studied using superfused rat cerebrocortex synaptosomes. Elevating [K+] produced release of [3H]GABA over basal which was increasingly less dependent on external Ca2+ but more sensitive to the GABA transporter blocker SKF 100330 A. Accordingly, the sensitivity to clostridial toxins of the depolarization-evoked amino acid release was inversely correlated to the concentration of KCl used. However, at 50 mM K+, one-third of the stimulated release remained which was external Ca2+-independent but insensitive to SKF 100330 A. This release was prevented by BAPTA, thapsigargin or dantrolene; it also was inhibited by blocking in mitochondria the ATP production with oligomycin, the H+-dependent Ca2+ uniporter with RU 360, the Na+/Ca2+ exchanger with CGP 37157 or by lowering extraterminal [Na+]. In fluorescence experiments with fura-2/AM, 50 mM K+ (in Ca2+ free medium) caused elevation of cytosolic [Ca2+] that was sensitive to thapsigargin or CGP 37157; these compounds produced partially additive effects. When exocytosis was monitored with the fluorescent dye acridine orange, the fluorescence elicited by 50 mM K+ was sensitive to thapsigargin or CGP 37157, which produced additive effects, and to low-Na+ media. To conclude, extracellular K+ concentrations occurring in the CNS in certain pathological conditions provoke GABA release by mechanisms different from classical exocytosis. These include carrier-mediated release and internal Ca2+-dependent exocytosis; in the latter, mitochondrial Ca2+ seems to play a primary role.
Raiteri
Luca
L
Department of Experimental Medicine, Pharmacology and Toxicology Section, University of Genova, Genova, Italy.
Stigliani
Sara
S
Zedda
Luca
L
Raiteri
Maurizio
M
Bonanno
Giambattista
G
D000818:Animals; D000927:Anticonvulsants / Q000494:pharmacology; D001923:Brain Chemistry; D002118:Calcium / Q000378:metabolism; D002352:Carrier Proteins / Q000037:antagonists & inhibitors; D002540:Cerebral Cortex / Q000737:chemistry; D002614:Chelating Agents / Q000494:pharmacology; D002998:Clonazepam / Q000031:analogs & derivatives* / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005089:Exocytosis / Q000187:drug effects; D005110:Extracellular Space / Q000737:chemistry*; D005456:Fluorescent Dyes; D050485:GABA Plasma Membrane Transport Proteins; D008297:Male; D008565:Membrane Proteins / Q000037:antagonists & inhibitors; D026901:Membrane Transport Proteins; D009539:Nicotinic Acids / Q000494:pharmacology; D027361:Organic Anion Transporters; D011189:Potassium Chloride / Q000737:chemistry / Q000494:pharmacology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D019831:Sodium-Calcium Exchanger / Q000037:antagonists & inhibitors; D013574:Synaptosomes / Q000737:chemistry / Q000187:drug effects / Q000378:metabolism*; D013841:Thiazepines / Q000494:pharmacology; D014316:Tritium; D005680:gamma-Aminobutyric Acid / Q000737:chemistry / Q000378:metabolism*
D000927:Anticonvulsants; D002352:Carrier Proteins; D002614:Chelating Agents; D004791:Enzyme Inhibitors; D005456:Fluorescent Dyes; D050485:GABA Plasma Membrane Transport Proteins; D008565:Membrane Proteins; D026901:Membrane Transport Proteins; D009539:Nicotinic Acids; D027361:Organic Anion Transporters; D019831:Sodium-Calcium Exchanger; D013841:Thiazepines; D014316:Tritium; D005680:gamma-Aminobutyric Acid; D002998:Clonazepam; D011189:Potassium Chloride; C079032:CGP 37157; C045597:SK&F 100330-A; D002118:Calcium