IP₃ elevates BK_Ca channel apparent Ca²⁺ sensitivity. (A) Mean data illustrating IP₃-induced BK_Ca channel activation over a range of free Ca²⁺ concentrations in inside-out patches at −40 mV. Data are fit with a Boltzmann function constrained to ≥0. Experimental numbers are given after each [Ca²⁺] (µM): control: 1, 8; 3, 6; 10, 6; 30, 6; 100, 8; 300, 8; 10 µM IP₃: 1, 7; 3, 6; 10, 6; 30, 6; 100, 7; 300, 7. (B) Mean data illustrating IP₃-induced change in BK_Ca channel P_o at each [Ca²⁺]. Data are fit with a Gaussian function. In control, the K_d for Ca²⁺ was 20.4 ± 0.9 µM, a slope of 1.2 ± 0.2, and a maximum P_o of 0.82 ± 0.04. 10 µM IP₃ decreased the mean K_d for Ca²⁺ to 12.4 ± 0.8 µM (P < 0.05), but did not alter the slope (1.3 ± 0.3) or the maximum P_o (0.83 ± 0.03; P > 0.05 for each).