Ionic Regulation of the Binding of dl-[3H]2-Amino-4-Phosphonobutyrate to l-Glutamate-Sensitive Sites on Rat Brain Membranes

Abstract

The effects of ions on the binding of the excitatory amino acid analog DL-[3H]2-amino-4-phosphonobutyrate to L-glutamate-sensitive sites on rat brain synaptic membranes was investigated. The divalent cations Mg, Mg, Sr, and particularly Ca, produced a marked enhancement in specific binding. This effect was manifest only in the presence of added chloride, or to a lesser extent, with bromide ions. Application of saturation analysis revealed that both chloride and Ca acted to increase the binding site density in a concentration-dependent manner, without affecting the dissociation constant. The only other ionic species found to have a significant effect on 2-amino-4-phosphonobutyrate binding was Na, which produced an apparent reduction in site affinity, without modifying the binding site density. Although the significance of these striking ionic effects is as yet unknown, it seems feasible that chloride (and possibly also Ca) ions may serve a role in regulating the interaction of excitatory amino acids with their physiological receptors.