Figure 2.

Effects of extracellular Na⁺ on the Mg²⁺ association rate constant. (A, left) Single-channel currents recorded at different [Na⁺]_ex ([Mg²⁺]_ex = 3 μM, [Na⁺]_in = 5 mM; V = −80 mV). (A, right) Open interval duration distributions. Open intervals are longer (i.e., block by Mg²⁺ is slower) at higher [Na⁺]_ex. (B) The inverse open channel lifetime (1/τ_o) versus [Mg²⁺]_ex. Increasing extracellular [Na⁺] reduces the slope (which is equal to the Mg²⁺ association rate constant) about fivefold, from 500.0 ± 35.6 μM⁻¹s⁻¹ in 50 mM [Na⁺]_ex to 109.6 ± 6.9 μM⁻¹s⁻¹ in 150 mM [Na⁺]_ex. (C) The Mg²⁺ association rate constant versus membrane potential, at different [Na⁺]_ex. The solid line is the fit by . The apparent voltage dependence of the Mg²⁺ association rate constant is the same in 50 mM and 150 mM [Na⁺]_ex (43 ± 2 mV and 44 ± 1 mV per e-fold change, respectively). In B and C, the SD is smaller than the symbols for all except one point.