title: Induced release of gamma-aminobutyric acid by a carrier-mediated, high-affinity uptake of L-glutamate in cultured chick retina cells.
abstract: [3H]gamma-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 mM L-glutamate but not with D-glutamate. The EC50 for L-glutamate to evoke [3H]GABA release was approximately 15 microM. This value is close to the Km 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-[14C]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 Ca2+ independent, and was observed when Ca2+ was replaced by Co2+ or when Mg2+ ions were increased to 10 mM concentration. D-Aspartate, which is taken up by the same high-affinity uptake mechanism as L-glutamate, induced an increase in [3H]GABA efflux comparable to L-glutamate. The addition of unlabeled GABA to the medium also promoted the release of accumulated [3H]GABA. However, GABA was twofold less effective than L-glutamate in eliciting [3H]GABA release. The addition of both GABA and L-glutamate to the incubation medium indicated that [3H]GABA efflux due to L-glutamate and GABA was additive. L-Aspartate also promoted an increase in the efflux of [3H]GABA accumulated by retina cells. However, L-aspartate effect was significantly decreased in the absence of Ca2+ 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 Ca2+-dependent L-aspartate-promoted release, and (c) a Ca2+-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.
authors.forename: J L
authors.initials: JL
authors.lastname: do Nascimento
authors.forename: F G
authors.initials: FG
authors.lastname: de Mello
mesh_terms: D000818:Animals; D001224:Aspartic Acid / Q000494:pharmacology; D002118:Calcium / Q000494:pharmacology; D002478:Cells, Cultured; D002642:Chick Embryo; D003035:Cobalt / Q000494:pharmacology; D005971:Glutamates / Q000494:pharmacology*; D018698:Glutamic Acid; D008094:Lithium / Q000494:pharmacology; D008274:Magnesium / Q000494:pharmacology; D012160:Retina / Q000166:cytology* / Q000378:metabolism; D012964:Sodium / Q000378:metabolism; D005680:gamma-Aminobutyric Acid / Q000378:metabolism*
chemical_list: D005971:Glutamates; D001224:Aspartic Acid; D003035:Cobalt; D018698:Glutamic Acid; D005680:gamma-Aminobutyric Acid; D008094:Lithium; D012964:Sodium; D008274:Magnesium; D002118:Calcium