Two affinity states of N-methyl-D-aspartate recognition sites: modulation by cations.
Previous studies have indicated that inorganic and organic cations can markedly affect parameters of the function of the N-methyl-D-aspartate receptor ionophore complex. As these effects may involve modulation of agonist binding, the purpose of our study was to investigate the stimulatory effect of mono- and divalent cations on binding properties of glutamate/N-methyl-D-aspartate recognition sites on the N-methyl-D-aspartate receptor complex, using [3H]CGP 39653 as the specific ligand for these sites. In well-washed membranes from rat brain, [3H]CGP 39653 binding sites were present at two affinity states when assayed at 10 mM HEPES-KOH buffer. About 75% of these sites were in a low-affinity state (Kd = 210 +/- 30 nM) although 25% were in a high-affinity state (Kd = 6.4 +/- 0.4 nM). Addition of mono- or divalent cations to the incubation medium stimulated [3H]CGP 39653 binding, measured at a radioligand concentration of 4 nM. Maximal increases in binding were to approximately 230 and 400% of control, in the presence of mono- and divalent cations, respectively. Values of EC50 for stimulation were 5 to 7 mM for monovalent cations and 0.2 to 0.4 mM for divalent cations. At these concentrations, cations increased the Bmax for the high-affinity population of [3H]CGP 39653 sites and decreased the Bmax for low-affinity ones. These findings suggest that, like spermidine, inorganic cations stimulate binding by converting [3H]CGP 39653 binding sites from the low- to high-affinity state.
Mukhin
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A
Brain Imaging Section, Intramural Research Program, National Institute on Drug Abuse, National Institutes of Health, Baltimore, Maryland 21224, USA.
Kovaleva
E S
ES
London
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ED
D015763:2-Amino-5-phosphonovalerate / Q000031:analogs & derivatives / Q000378:metabolism; D000818:Animals; D002412:Cations; D018691:Excitatory Amino Acid Antagonists / Q000378:metabolism; D008297:Male; D016202:N-Methylaspartate / Q000378:metabolism*; D011485:Protein Binding; D011869:Radioligand Assay; D051381:Rats; D011916:Rats, Inbred F344; D016194:Receptors, N-Methyl-D-Aspartate / Q000378:metabolism; D014316:Tritium
D002412:Cations; D018691:Excitatory Amino Acid Antagonists; D016194:Receptors, N-Methyl-D-Aspartate; D014316:Tritium; C069052:CGP 39653; D016202:N-Methylaspartate; D015763:2-Amino-5-phosphonovalerate