Paxilline stabilizes closed states with only weak, if any, binding to open states. (A) Curves of concentration-dependent inhibition of BK current at four equilibration conditions were fit with Scheme 1, with the assumption that the current activated at positive potentials reflects channels initially either in C, O, or OB, i.e., any open channels with bound paxilline are conducting. Each panel from left to right corresponds to fits with Scheme 1 (Eq. 7) in which either one, two, or four paxilline molecules can bind to inhibitory positions. Best-fit values are given in the figure. The model with n = 1 best accounts for the shape of the inhibition by paxilline. (B; left) Scheme 1 was again used, while constraining E_p to 0.01 (left), 0.1 (middle), or 1.0 (right), showing that increasing open-state affinity fails to account for the data. Right-hand panel corresponds to state-independent paxilline interaction with C and O. (C) E_p was set to 0, thereby approximating strictly closed-channel block. (D) Data were fit with Scheme 3 (Eq. 3) in which the paxilline-bound open state is nonconducting. On the left, the value for E_p drifted toward 0, similar to a strictly closed-channel block situation. On the right, E_p was set to 1 (state-independent binding of paxilline). SSQN in A and C corresponds to the sum-of-squares for a given fit normalized to the sum-of-squares for the left panel in A.