IP₃ activates BK_Ca channels in excised inside-out membrane patches. (A) Original recordings from the same inside-out patch illustrating concentration-dependent BK_Ca channel activation by IP₃ at −40 mV. (B) Mean data fit with a Boltzmann function. IP₃ increased BK_Ca channel P_o with an apparent K_d of 4.1 ± 1.3 µM, a slope of 2.8 ± 1.9, and a maximal P_o of 0.38 ± 0.02. Experimental numbers are ([IP₃]): 0.01 µM, 4; 0.1 µM, 4; 1 µM, 4; 3 µM, 5; 10 µM, 5; 30 µM, 5. (C) Mean data illustrating that buffering Ca²⁺ with BAPTA or elevating pipette (extracellular) Ca²⁺ from 10 µM to 2 mM does not alter IP₃ (10 µM)-induced BK_Ca channel activation (n: 10 µM [Ca²⁺]_o/10 µM [Ca²⁺]_i (EGTA/EGTA); control, 4; IP₃, 4; 10 µM [Ca²⁺]_o/10 µM [Ca²⁺]_i (BAPTA/BAPTA); control, 4; IP₃, 4; 2 mM [Ca²⁺]_o/10 µM [Ca²⁺]_i (none/EGTA) control, 3; IP₃, 3). *, P < 0.05.