Sodium ions regulate a specific population of acidic amino acid receptors in synaptic membranes.
The regulatory effects of Na+ on C1-/Ca2+-dependent and C1-/Ca2+-independent L-glutamate binding sites were examined. In Tris-C1-/Ca2+ buffer, the binding of L-[3H]-glutamate to rat brain synaptic membranes was 5-fold higher than in Tris-acetate buffer. Low concentrations of Na+ (less than 5 mM) markedly depressed L-glutamate binding when assayed in Tris-C1/Ca2+ buffer, and this effect was attenuated by the selective blocker of C1-/Ca2+-dependent binding sites, DL-2-amino-4-phosphonobutyrate (APB). Scatchard analyses indicated that the effect of Na+ was due to a decrease in the number of C1-/Ca2+-dependent binding sites with no change in affinity. In Tris-acetate buffer, low concentrations of Na+ had little effect on L-glutamate binding. Dose-response curves for the inhibition of L-glutamate binding by DL-APB indicated a predominant high-affinity (Ki 5-10 microM) inhibitory component in Tris-C1-/Ca2+ buffer, but mainly a low-affinity component (Ki 1-2 mM) in Tris-acetate buffer and in Tris-C1-/Ca2+ buffer containing Na+. These data indicate that low concentrations of Na+ regulate specifically the C1-/Ca2+-dependent, APB-sensitive class of L-glutamate binding sites.
D000613:Aminobutyrates / Q000494:pharmacology; D000818:Animals; D001665:Binding Sites; D003864:Depression, Chemical; D004305:Dose-Response Relationship, Drug; D005971:Glutamates / Q000378:metabolism*; D007700:Kinetics; D008297:Male; D051381:Rats; D011956:Receptors, Cell Surface / Q000378:metabolism*; D017470:Receptors, Glutamate; D012964:Sodium / Q000502:physiology*; D013570:Synaptic Membranes / Q000378:metabolism*
D000613:Aminobutyrates; D005971:Glutamates; D011956:Receptors, Cell Surface; D017470:Receptors, Glutamate; D012964:Sodium; C012729:2-amino-4-phosphonobutyric acid