Figure 5.

Effect of extracellular Na⁺ on the Mg²⁺ off rate constant. (A) Single-channel currents of Mg²⁺ block recorded with different extracellular Na⁺ concentrations (5 μM extracellular Mg²⁺, 5 mM [Na⁺]_in, V = −80 mV). Closed interval duration histograms are shown below. The Mg²⁺ off rate decreases with increasing [Na⁺]_ex. (B) Separating the Mg²⁺ off rate into a dissociation rate constant and a permeation rate constant. The solid lines are the fits using . Each point is the mean ± SD of at least three patches (SD may be smaller than the symbol). (C) Analyses of the inhibition of the Mg²⁺ dissociation rate constant by extracellular Na⁺. The same four sets of data as in B were fitted simultaneously by . The solid lines are the predicted curves from the model. The parameters are κ⁰ _−Mg = 8,944 ± 205 s⁻¹, ɛ = 0.36 (fixed), k⁰ _pMg = 624 ± 5 s⁻¹, λ = 0.03 (fixed), and K_Naex = 251 ± 12 mM. Additional results are summarized in Table and Fig. 6.