To mathematically model STOCs reliably using the spatial organization of RYRs and BK channels derived from immunostaining experiments, we validated BK channel current kinetics in response to a [Ca2+] jump. To do so, we first derived functions describing the activation and deactivation time constants of the BK channel using the results from [Ca2+] jump experiments performed by Markwardt and Isenberg (1992) in urinary bladder myocytes (the only study of this sort in smooth muscle). We defined that (a) τdecay = 0 0.025 × exp(E/50 mV), where E is voltage, and (b) τact = mE + b, with both m and b being interpolations between the slope and intercepts, respectively, as seen in Fig. 9 in Markwardt and Isenberg (1992) for the low and high [Ca2+] curves (e.g., at low [Ca2+] we used the m and b values from their low [Ca2+] curve). We then applied these functions to drive the BK channel Po toward the steady-state Po,ss in response to each [Ca2+] jump (see Materials and methods). Fig. S1 A displays the time course of BK channel currents caused by
