Figure 4.

Number and affinities of Na⁺ binding sites. The Mg²⁺ association rate constant was used to probe the occupancy of the channel by Na⁺. Data obtained in 100 mM [Na⁺]_in are shown to the left, and data obtained in 5 mM [Na⁺]_in are shown to the right. (A) Fits of experimental Mg²⁺ association rate constants assuming one Na⁺ binding site (). The apparent Mg²⁺ association rate constant obtained from different [Na⁺]_ex is plotted as a function of the membrane potential. A model with a single Na⁺ binding site does not provide an adequate description of the experimental results. (B) Fits of the same experimental data by an “n-site” scheme (). The solid lines are the predicted curves from the model (parameters for the best fit are shown in Table ). The model with n = 2 independent Na⁺ sites is better (MSC = 5.5) than the model with a single Na⁺ site (MSC = 4.0).