K(+) occupancy of the N-methyl-d-aspartate receptor channel probed by Mg(2+) block.
The single-channel kinetics of extracellular Mg(2+) block was used to probe K(+) binding sites in the permeation pathway of rat recombinant NR1/NR2B NMDA receptor channels. K(+) binds to three sites: two that are external and one that is internal to the site of Mg(2+) block. The internal site is approximately 0.84 through the electric field from the extracellular surface. The equilibrium dissociation constant for this site for K(+) is 304 mM at 0 mV and with Mg(2+) in the pore. The occupancy of any one of the three sites by K(+) effectively prevents the association of extracellular Mg(2+). Occupancy of the internal site also prevents Mg(2+) permeation and increases (by approximately sevenfold) the rate constant for Mg(2+) dissociation back to the extracellular solution. Under physiological intracellular ionic conditions and at -60 mV, there is approximately 1,400-fold apparent decrease in the affinity of the channel for extracellular Mg(2+) and approximately 2-fold enhancement of the apparent voltage dependence of Mg(2+) block caused by the voltage dependence of K(+) occupancy of the external and internal sites.
Department of Physiology and Biophysics, State University of New York at Buffalo, Buffalo, New York 14214, USA.
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Zhu
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Auerbach
D000818:Animals; D001665:Binding Sites; D004594:Electrophysiology; D015640:Ion Channel Gating; D007700:Kinetics; D008274:Magnesium / Q000737:chemistry / Q000494:pharmacology*; D009865:Oocytes; D011188:Potassium / Q000737:chemistry / Q000494:pharmacology*; D051381:Rats; D016194:Receptors, N-Methyl-D-Aspartate / Q000502:physiology*; D015398:Signal Transduction; D014981:Xenopus
D016194:Receptors, N-Methyl-D-Aspartate; D008274:Magnesium; D011188:Potassium