Effects of 3NB, 3PP, and sulfate on CFTR permeation. (A, B, F, G, I, and J) Decaying macroscopic currents of locked-open K1250A CFTR channels after removal of ATP, recorded at membrane potentials of −80 (A, F, and I) or 60 mV (B, G, and J) and responses to brief applications of various concentrations of 3NB (A and B, blue bars), 3PP-sulfate (F and G, red bars), or sulfate (I and J, green bars). Zero-current levels (dotted lines) were estimated from the responses of final current segments to application of 32 mM 3NB (blue bars in all panels). (C and H) Dose–response curves at −80 (cyan-filled symbols) and 60 mV (yellow-filled symbols) of fractional currents as a function of [3NB] (C), [3PP-sulfate] (H, red), and sulfate (H, green). The two alternative abscissae in H were scaled to reflect the presence of 0.45 mol/mol sulfate ions in 3PP-sulfate; i.e., the green abscissa correctly reflects sulfate concentrations for all four plots. The plots in C were fitted by the Michaelis-Menten and the yellow–red plot in H to the Hill equation, and midpoints are printed in the panels. (D) Responses of decaying macroscopic locked-open K1250A CFTR current to brief applications of 32 mM 3NB (blue bars) at various membrane potentials. Zero-current level (dotted line) was estimated from the final current segment after subtraction of the small (∼2 pA/60 mV) linear seal current. (E) Voltage dependence of macroscopic current block by 32 mM 3NB (symbols) and Boltzmann fit (solid line) yielding parameters z = 0.49 ± 0.02 mV and V_1/2 = 41 ± 2 mV (calculated K_d(0) = 14.7 mM). Mean ± SEM is shown.