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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.
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