Figure 3

Estimation of the Ca²⁺ binding rate constants of BK-type channels.

(A) Trace of mSlo1 currents was evoked by a UV flash in the whole-cell mode at 30 mV. The red box shows the fast rising current of mSlo1 with a time constant τ = 0.15 ms by a fit to eq. (3). (B1) The Ca²⁺-releasing time course by the UV flash was calculated from the 10-state mSlo1 model. The algorithm is described in sFig. 3. (B2) The details of blue box in (B1). Here [Ca²⁺]_p denotes the peak value of [Ca²⁺]_i. (B3) The averaged rising calcium concentration in the blue box of (B2) is calculated by a formula as shown in inset. (C) Top, the current traces of mSlo1 were evoked by uncaged Ca²⁺ at −30, 10 and 50 mV as indicated. The details in red box are shown in inset as indicated by arrows. The values of τ were derived from a fit to eq. (3). Trace is black and fit red. Bottom show the detailed Ca²⁺-releasing time course in the top blue boxes, indicated by blue arrows. (D) The P_o-[Ca²⁺]_i dose-response curves of BK channels at 30 mV were plotted for mSlo1, 5D5N, D362AD367A and D369G, respectively. Their dissociattion equilibrium constant K_d values were listed in sTable2. (E) Based on Eq. 4, k_b = 0.18 ± 0.04 (n = 6) for mSlo1, 0.057 ± 0.003 (n = 5) for 5D5N, 0.014 ± 0.01 (n = 4) for D362AD367A and 0.26 ± 0.02 (n = 5) for D369G, respectively, at 30 mV (sTable 2).