The distribution of BK channel puncta that can be activated by Ca2+ sparks (Fig. 6) also provides evidence that the average punctum consists of multiple BK channels based on the following calculation. Mean STOC amplitude at 0 mV (Ek = −80 mV) was 95 ± 14 pA (n = 28 cells). This current could result from the opening of ∼15 BK channels, assuming a unitary conductance of 80 pS under the K+ gradient used in this study. Because one Ca2+ spark could activate only ∼50% of BK channels in two to three puncta, each punctum could be made up of 10–15 BK channels.
