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Stimulation of [ 3 H]γ‐Aminobutyric Acid Release by Calcium Chelators in Synaptosomes
Author(s) -
Arias Clorinda,
Sitges Maria,
Tapia Ricardo
Publication year - 1984
Publication title -
journal of neurochemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.75
H-Index - 229
eISSN - 1471-4159
pISSN - 0022-3042
DOI - 10.1111/j.1471-4159.1984.tb12736.x
Subject(s) - egta , veratridine , chemistry , calcium , aminobutyric acid , ruthenium red , stimulation , biophysics , depolarization , acetylcholine , tetrodotoxin , synaptosome , sodium , biochemistry , endocrinology , biology , sodium channel , receptor , organic chemistry
Abstract: The effect of EGTA on the release of labeled γ‐aminobutyric acid (GABA), glutamate, acetylcholine, and dopamine was studied in superfused synaptosomes from mouse brain. In the absence of both Ca 2+ and Mg 2+ , EGTA and also EDTA at 50 μ M or higher concentrations induced a 2.5‐5‐fold stimulation of [ 3 H]GABA release, similar to that produced by potassium depolarization, whereas only a slight effect, or no effect at all, was observed on the release of the other transmitters studied. The GABA‐releasing action of EGTA was practically abolished in the presence of Mg 2+ . In contrast, the effect of EDTA was also observed when the medium contained Mg 2+ . Studies on the ionic dependence showed that the stimulation of GABA release by EGTA was abolished in a Na + ‐free medium. Li + did not substitute Na + for the EGTA effect, which was also independent of chloride. This Na + dependence does not seem to involve voltage‐sensitive channels, since tetrodotoxin did not affect the GABA‐releasing action of EGTA, whereas in parallel su‐perfusion chambers it blocked over 80% the stimulation of GABA release by veratridine. In contrast, two calcium channel blockers in synaptosomes, La 3+ and the cationic dye ruthenium red, greatly inhibited the GABA‐releasing effect of EGTA. L‐2,4‐Diaminobutyric acid, an inhibitor of the Na + ‐dependent GABA carrier, did not affect the releasing action of EGTA, whereas in a parallel experiment this drug inhibited by more than 90% the exchange of labeled GABA with unlabeled GABA. It is concluded that the Na + ‐dependent releasing action of EGTA and EDTA on GABA is probably due to a destabilization of the synaptosomal membrane by chelation of endogenous membranal Ca 2+ , which can be prevented by Mg 2+ . Such destabilization results in Na + influx through Ca 2+ channels, and the consequent increase in the intrater‐minal Na + concentration induces the release of GABA by a mechanism probably not involving the amino acid carrier. The possible participation of mitochondrial Na + ‐Ca 2+ exchange is considered improbable in view of the lack of effect of EGTA on the release of other neurotrans‐mitters.