Na+ occupancy and Mg2+ block of the N-methyl-D-aspartate receptor channel

The effect of extracellular and intracellular Na+ on the single-channel kin etics of Mg2+ block was studied in recombinant NR1-NR2B NMDA receptor chann els. Na+ prevents Mg2+ access to its blocking site by occupying two sites i n the external portion of the permeation pathway. The occupancy of these si tes by intracellular, but not extracellular, Na+ is voltage-dependent. In t he absence of competing ions, Mg2+ binds rapidly (>10(8) M(-1)s(-1), with n o membrane potential) to a site that is located 0.60 through the electric f ield from the extracellular surface. Occupancy of one of the external sites by Na+ may be sufficient to prevent Mg2+ dissociation from the channel bac k to the extracellular compartment. With no membrane potential; and in the absence of competing ions, the Mg2+ dissociation rate constant is >10 times greater than the Mg2+ permeation rate constant, and the Mg2+ equilibrium d issociation constant is similar to 12 muM. Physiological concentrations of extracellular Na+ reduce the Mg2+ association rate constant similar to 40-f old but, because of the "lock-in" effect, reduce the Mg2+ equilibrium disso ciation constant only similar to 18-fold.