Sodium ions regulate a specific population of acidic amino acid receptors in synaptic membranes |
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Authors: | G E Fagg B Riederer A Matus |
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Institution: | Friedrich Miescher-Institut P.O. Box 2543, CH-4002 Basel, Switzerland |
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Abstract: | 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. |
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