Raiteri L, Stigliani S, Zedda L, Raiteri M, Bonanno G. 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

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.

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