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Induced Release of γ‐Aminobutyric Acid by a Carrier‐Mediated, High‐Affinity Uptake of L‐Glutamate in Cultured Chick Retina Cells
Author(s) -
Nascimento Jose Luiz M.,
Mello Fernando G.
Publication year - 1985
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.1471-4159.1985.tb10539.x
Subject(s) - glutamate receptor , efflux , biophysics , gamma aminobutyric acid , incubation , retina , biochemistry , biology , chemistry , intracellular , glutamic acid , amino acid , neuroscience , receptor
[ 3 H]γ‐aminobutyric acid (GABA) was taken up by cultured embryonic retina cells during the initial stages of cell differentiation. The accumulated GABA was released in the bathing medium and a transient increase in the efflux of GABA was observed when cultures were pulse‐stimulated (2 min) with 0.1 m M L‐glutamate but not with D‐glutamate. The EC 50 for L‐glutamate to evoke [ 3 H]GABA release was approximately 15 μ M. This value is close to the K m for high‐affinity uptake of L‐glutamate by retina cells. When Na + ions were replaced by Li + ions, L‐glutamate‐induced release of GABA was abolished. Moreover. L‐[ 14 C]glutamate uptake by retina cells was significantly reduced when NaCl was replaced by LiCl in the incubation medium. L‐Glutamate elicited release of GABA was Ca 2+ independent, and was observed when Ca 2+ was replaced by Co 2+ or when Mg 2+ ions were increased to 10 m M concentration. D‐Aspartate, which is taken up by the same high‐affinity uptake mechanism as L‐glutamate, induced an increase in [ 3 H]GABA efflux comparable to L‐glutamate. The addition of unlabeled GABA to the medium also promoted the release of accumulated [ 3 H]GABA. However, GABA was twofold less effective than L‐glutamate in eliciting [ 3 H]GABA release. The addition of both GABA and L‐glutamate to the incubation medium indicated that [ 3 H]GABA efflux due to L‐glutamate and GABA was additive. L‐Aspartate also promoted an increase in the efflux of [ 3 H]GABA accumulated by retina cells. However, L‐aspartate effect was significantly decreased in the absence of Ca 2+ or when Na + ions were replaced by Li + . Our results indicate that at least three releasable pools of GABA are present in the chick embryo retina cells: (a) a GABA‐promoted GABA release‐homoexchange, (b) a Ca 2+ ‐dependent L‐aspartate‐promoted release, and (c) a Ca 2+ ‐independent, Na + ‐dependent L‐glutamate‐evoked release. In addition, our data strongly suggest that the L‐glutamate‐promoted GABA release is due to a process of exchange of L‐glutamate with GABA, which may play a fundamental role in the fine control of the excitability of local circuits in the retina.