within the Ca2+ microdomain, there are, on average, two to three BK channel clusters. With these new results, we propose that a Ca2+ spark site is composed of a single cluster of RYRs and a variable number of clustered BK channels (Fig. 8 D). Considering that the distribution of the distance between RYR clusters and the closest BK channel clusters can be fit with the probability distribution of distance to the closest random point (Fig. 5 B), we suggest that BK channel clusters randomly distribute inside the Ca2+ spark microdomains (note that they are not randomly distributed along the entire plasma membrane). Our findings on the spatial arrangement of BK channels and RYR are not only significant for understanding the activation of STOCs by Ca2+ sparks, but they also raise new possibilities for further exploration. For instance, it would be interesting to investigate what determines the number of channels in a cluster and cluster density and whether BK channels form clusters as a result of the involvement of scaffolding proteins or localize in specialized structural domains such as caveolae or lipid rafts in the plasma membrane (Shmygol et al., 2007; Lu et al., 2010).
