Figure 1.

Decreasing permeant ion concentrations enhances Mg_o²⁺-blocking rate. (a) Single-channel currents activated by 10 μM NMDA + 10 μM glycine during block by 10 μM Mg_o²⁺ at −50 mV with [Na⁺]_o = 140 mM and the indicated [Cs⁺]_i. (b) Open-time histograms corresponding to the currents shown in a to the left of each histogram. Lines are double-exponential (upper two histograms) or single-exponential (lower histogram) fits to the histograms. Number of events in each histogram is more than 2,000. The principal mean open time (τ_o) is shown above each histogram. (c and d) Dependence of 1/τ_o on [Mg²⁺]_o under the indicated conditions. ○, 140 mM Na_o⁺/130 mM Cs_i⁺; ▵, 140 mM Na_o⁺/25 mM Cs_i⁺; □, 140 mM Na_o⁺/8 mM Cs_i⁺; ▴, 105 mM Na_o⁺/25 mM Cs_i⁺; ▾, 70 mM Na_o⁺/25 mM Cs_i⁺. Lines are regression fits to the plotted points.