Figure 4.

Probability that the channel is not blocked as a function of external Ba²⁺ concentration in PE and PS bilayers. Inhibition of single-channel activity due to discrete block by external Ba²⁺ was computed from the time-averaged reduction in open-state probability from long recordings at 20 mV. The ordinate value is a normalized ratio corresponding to the probability that the channel was not blocked by Ba²⁺ at a given Ba²⁺ concentration divided by the corresponding open state probability in the absence of Ba²⁺. Conditions: symmetrical 10 mM HEPES-KOH, pH 7.2, 100 mM KCl (squares) or 20 mM KCl (circles), internal 200 mM CaCl₂, for PE (filled symbols) or PS bilayers (open symbols). The solid and dashed lines are fits to the empirical Hill equation (Eq. 9) with parameters given in the text. The dotted lines correspond to simulations computed according to Eq. 9 using K_B = 0.46 mM based on the fit to PE data at 20 mM KCl, n = 1, and [Ba²⁺]_local calculated from GCS theory (materials and methods) at various distances (0, 5, 10, 20, and 30 Å) from a PS bilayer at 20 mM KCl, and plotted as a function of bulk Ba²⁺ concentration.