Ionic regulation of glutamate binding sites

Cl^- and Ca²⁺ increase glutamate binding to rat synaptic plasma membranes (SPMs) by revealing a distinct class of L-glutamate (L-Glu) binding sites. The present study was conducted to examine both the anion specificity of this response and the nature of the interaction between Cl^- and Ca²⁺. Of the anions tested, Br^- was the most effective in increasing the levels of L-Glu binding. Other effective anions were Cl^-, NO₃^- and formate while F^-, HCO₃^-CIO₄^-, propionate, SO₄^2- and PO₄^3- were ineffective. The anion specificity was similar to that observed for the Cl^- membrane channel, suggesting that this binding site and the ion channel may be related. In the absence of Cl^-, Ca²⁺ has little effect on L-Glu binding. Increasing the Cl^- concentration increased the apparent affinity (decreased K_Ca²⁺) of the Ca²⁺-stimulated, L-Glu binding component and also decreased the maximal amount of the enhancement. Conversely, increasing Ca²⁺ levels increased the maximal enhancement of L-Glu binding brought about by Cl^- without affecting the K_Cl^- of the effect. Prior incubation of membranes with Ca²⁺ did not raise the level of L- Glu binding. Furthermore, EGTA was able to reverse the stimulation of L- Glu binding due to Ca²⁺. The results indicate that Ca²⁺ acts ionically to enhance L-Glu binding to rat SPMs.