Chelation of endogenous membrane calcium inhibits gamma-aminobutyric acid uptake in synaptosomes.
In a previous work, we have demonstrated that calcium chelators induce the release of gamma-aminobutyric acid (GABA) from synaptosomes in a Na+ -dependent manner and that this release is blocked by cations such as Mg2+, La3+, and ruthenium red. In the present study, we show that treatment of synaptosomes with 0.1 mM EGTA in the absence of both Ca2+ and Mg2+ inhibits the sodium-dependent high-affinity uptake of [3H]GABA by about 50%. This inhibition increased to about 65% with 1.5 mM EGTA, and it was completely prevented by an excess of Ca2+ or by 1.2 mM Mg2+. In contrast, when EDTA was used as a chelator, Mg2+ was unable to reverse the inhibition. The inhibitory effect of 0.1 mM EGTA was also prevented by 250 microM La3+ or by 20 microM ruthenium red. In the absence of chelators and the presence of Ca2+ and Mg2+, 50 microM and 200 microM La3+ inhibited GABA uptake by about 20 and 50%, respectively, whereas 20 microM ruthenium red produced a nonsignificant 25% inhibition and nifedipine was without effect. It is concluded that the membrane-surface negative charges, probably those of the sialic acid molecules that have been implicated in the functioning of the GABA carrier, must be neutralized by endogenous Ca2+ or by another cation in order to permit the adequate function of the transporter. The inhibition by La3+ in the absence of the chelators could be explained by a binding of this cation to the Na+ sites on the GABA carrier.
Tapia
R
R
Departamento de Neurociencias, Universidad Nacional Autónoma de México, México, D.F.
Salazar
C
C
D000818:Animals; D002118:Calcium / Q000378:metabolism / Q000502:physiology*; D002614:Chelating Agents; D004533:Egtazic Acid / Q000494:pharmacology*; D007811:Lanthanum / Q000494:pharmacology; D008297:Male; D051379:Mice; D012430:Ruthenium Red / Q000494:pharmacology; D013574:Synaptosomes / Q000187:drug effects / Q000378:metabolism*; D005680:gamma-Aminobutyric Acid / Q000493:pharmacokinetics*
D002614:Chelating Agents; D012430:Ruthenium Red; D004533:Egtazic Acid; D005680:gamma-Aminobutyric Acid; D007811:Lanthanum; D002118:Calcium