[Na.sup.+] Occupancy and [Mg.sup.2+] Block of the N-methyl-D-aspartate Receptor Channel

The effect of extracellular and intracellular [Na.sup.+] on the single-channel kinetics of [Mg.sup.2+] block was studied in recombinant NR1-NR2B NMDA receptor channels. [Na.sup.+] prevents [Mg.sup.2+] access to its blocking site by occupying two sites in the external portion of the permeation pathway. The occupancy of these sites by intracellular, but not extracellular, [Na.sup.+] is voltage-dependent. In the absence of competing ions, [Mg.sup.2+] binds rapidly ([is greater than] [10.sup.8] [M.sup.-1][s.sup.-1], with no membrane potential) to a site that is located 0.60 through the electric field from the extracellular surface. Occupancy of one of the external sites by [Na.sup.+] may be sufficient to prevent [Mg.sup.2+] dissociation from the channel back to the extracellular compartment. With no membrane potential; and in the absence of competing ions, the [Mg.sup.2+] dissociation rate constant is [is greater than] 10 times greater than the [Mg.sup.2+] permeation rate constant, and the [Mg.sup.2+] equilibrium dissociation constant is ~12 [micro]M. Physiological concentrations of extracellular [Na.sup.+] reduce the [Mg.sup.2+] association rate constant ~40-fold but, because of the 'lock-in' effect, reduce the [Mg.sup.2+] equilibrium dissociation constant only ~18-fold. KEY WORDS: ion binding sites magnesium * channel blockade * permeation selectivity