Na⁺ Occupancy and Mg²⁺ Block of the N-Methyl-d-Aspartate Receptor Channel

The effect of extracellular and intracellular Na⁺ on the single-channel kinetics of Mg²⁺ block was studied in recombinant NR1-NR2B NMDA receptor channels. Na⁺ prevents Mg²⁺ 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⁺ is voltage-dependent. In the absence of competing ions, Mg²⁺ binds rapidly (>10⁸ M⁻¹s⁻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⁺ may be sufficient to prevent Mg²⁺ dissociation from the channel back 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 dissociation constant is ∼12 μM. Physiological concentrations of extracellular Na⁺ reduce the Mg²⁺ association rate constant ∼40-fold but, because of the “lock-in” effect, reduce the Mg²⁺ equilibrium dissociation constant only ∼18-fold.