A common observation on STOCs from different smooth muscles is that there is a great variation in the amplitude of STOCs (Benham and Bolton, 1986; Nelson et al., 1995; Bolton and Imaizumi, 1996; ZhuGe et al., 1998; Pérez et al., 1999; Kirber et al., 2001; Löhn et al., 2001; Ohi et al., 2001). This is even evident in a single cell as demonstrated in Fig. 2. In the present study, we found that the number of BK channel clusters being activated by Ca2+ sparks and BK channel kinetics affect STOC amplitude, and, thus, these two parameters could contribute to the variation in STOC amplitude. As shown in Fig. 6, the number of BK channel clusters that can be affected by Ca2+ sparks spans from 0 to 15. Assuming each cluster has a fixed number of BK channels that open at the same Po, Ca2+ sparks could produce STOCs with amplitudes that differ by 15-fold (excluding STOC-less events).
