Figure 9.

Apparent reduction in unitary BK channel current due to fast block by external Ba²⁺ and TEA⁺ compared in PE and PS bilayers. Ordinate values (I/I₀) correspond to single-channel current at the indicated Ba²⁺ (or TEA⁺) concentration (I) divided by the unitary current of the same channel (I₀) in the absence of Ba²⁺ (or TEA⁺). Data point are the mean ± SE for 4–9 experiments. Fast block by external Ba²⁺ (A) or external TEA⁺ (B) measured as the apparent reduction in unitary current at 20 mV. Solid symbols, PE; open symbols, PS; circles, symmetrical 100 mM KCl; squares, symmetrical 20 mM KCl. Solid lines in A and B are fits to the empirical Hill equation (Eq. 9) as described in the text. Dotted lines in A are theoretical curves using the K_B parameter obtained for the fit of the PE data at 20 mM KCl and the local concentration of Ba²⁺ at various distances (in Å) from a bilayer containing 80% PS as calculated from GCS theory outlined in materials and methods. Dotted lines in B show similar theoretical curves based on the K_B parameter for the fit to the TEA⁺ titration in PE bilayers and the calculated local concentration of TEA⁺ at various distances from a PS bilayer.