Characterization of BK F380A mutant. (A) IbTx blocks the BK-F380A mutant channel. Currents elicited by a 150-mV pulse were recorded in outside-out patches and symmetrical K⁺ 110 mM. The bath solution contained 100 nM IbTx, [Ca²⁺]_i = 100 µM. Currents were recorded immediately before (t₁), 1 min after (t₂), and 5 min after IbTx addition (t₃). (B) I_g and leak currents were recorded in symmetrical 110 mM K⁺ and internal 0 Ca²⁺, and the pipette solution contained 500 nM IbTx. Currents were recorded immediately after gigaseal formation (t₁), after 1 min (t₂), and after 5 min (t₃). (C) Single-channel recordings of WT BK, F380A, and F380W in the absence of internal Ca²⁺ and upon depolarization at 50 mV. To elicit F380A mutant current, we used a huge pipette, ∼30–40 µm. (D–F) Nonstationary noise analysis of BK-WT, BK-F380A, and BK-F380W. Inside-out patches were held to 0 mV and pulsed 200 times from −100 to 100 mV at steps of 2-ms, 10-ms, and 10-ms duration for F380A, F380W, and WT, respectively. [Ca²⁺]_i = 100 µM. (G) Voltage dependence of deactivation time constants (mean ± SEM) for the WT and F380A channels. Voltage dependence of deactivation rates were calculated from the slope of the straight lines (solid lines) at very negative potentials: z_γ = 0.159 for WT; z_γ = 0.138 for F380A. Deactivation rates at 0 mV were estimated by extrapolating the straight lines: γ₀ = 1,494/s for WT; γ₀ = 7,243/s for F380A.