Fig. 2.

BK channel open probabilities and calcium set points. A: representative inside-out patch recordings of BK channel from LTH adult and fetal smooth muscle cells in symmetrical 140 mM KCl. In both cases, command potentials were −30 mV making BK channels experience +30 mV depolarizations. The bath [Ca²⁺] was 1.0 μM. Dotted lines represent the channels in closed (C) and open (O) states. B: voltage activation curves at different membrane potentials in 10-mV increments for various [Ca²⁺]_i. Data are channel open probability (P_o) expressed relative to maximum channel open probability (P_omax). Solid lines indicate best-fit curves to the Boltzmann equation: P_o/P_omax = 1{1 + exp[(V_1/2 − V_m)/K]}, where V_1/2 is the membrane potential (V_m) required for half-maximal activation of the channel and K is the logarithmic voltage sensitivity (change in voltage required for an e-fold increase in activity). The voltage sensitivities estimated from the fitted curves were similar for all concentrations of Ca²⁺ tested and indicated that channel activity increased e-fold (∼2.72 times) for 23.5 ± 1.8 mV (n = 4, adult) and 25.0 ± 2.1 mV (n = 4, fetus) depolarizations. C: estimation of changes in V_1/2 for a 10-fold change in [Ca²⁺]_i (ΔV_1/2) and estimation of the Ca²⁺ axis intercept (calcium set point, Ca₀) for both adult and fetal BK channels. V_1/2 values were obtained from B. The lines represent the best linear regression fits. LTH adult and fetal Ca₀ values were calculated to be 3.6 μM and 3.0 μM, respectively. NX, normoxic.