Chelation of endogenous membrane calcium inhibits γ‐aminobutyric acid uptake in synaptosomes

In a previous work, we have demonstrated that calcium chelators induce the release of γ‐aminobutyric acid (GABA) from synaptosomes in a Na + ‐dependent manner and that this release is blocked by cations such as Mg 2+ , La 3+ , and ruthenium red. In the present study, we show that treatment of synaptosomes with 0.1 mM EGTA in the absence of both Ca 2+ and Mg 2+ inhibits the sodium‐dependent high‐affinity uptake of [ 3 H]GABA by about 50%. This inhibition increased to about 65% with 1.5 mM EGTA, and it was completely prevented by an excess of Ca 2+ or by 1.2 mM Mg 2+ . In contrast, when EDTA was used as a chelator, Mg 2+ was unable to reverse the inhibition. The inhibitory effect of 0.1 mM EGTA was also prevented by 250 μM La 3+ or by 20 μM ruthenium red. In the absence of chelators and the presence of Ca 2+ and Mg 2+ , 50 μM and 200 μM La 3+ inhibited GABA uptake by about 20 and 50%, respectively, whereas 20 μM ruthenium red produced a nonsignificant 25% inhibition and nifedipine was without effect. It is concluded that the membranesurface 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 Ca 2+ or by another cation in order to permit the adequate function of the transporter. The inhibition by La 3+ in the absence of the chelators could be explained by a binding of this cation to the Na + sites on the GABA carrier.