J Neurochem 2002 Feb;80(4):706-714
Multiple mechanisms of transmitter release evoked by 'pathologically' elevated extracellular [K+]: involvement of transporter reversal and mitochondrial calcium.
Raiteri L, Stigliani S, Zedda L, Raiteri M, Bonanno G.
Department of Experimental Medicine, Pharmacology and Toxicology Section, and Center of Excellence Cell to Cell Communication, University of Genova, Genova, Italy.
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[blank]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.