In this study, using a combination of functional and immunocytochemical analyses, we uncovered the spatial arrangements of RYRs and BK channels that underlie the activation of STOCs by Ca2+ sparks in ASM cells. We found that both RYRs and BK channels distribute in the form of clusters, and the opening of a single cluster of RYRs can act on two to three clusters of BK channels on average that reside randomly in an area within ∼600 nm from the RYRs. Integrating this spatial relationship with BK channel kinetics and the spatial-temporal profile of [Ca2+] generated by Ca2+ sparks, we can model IBK with characteristic features of STOCs from experiments in the physiological membrane potentials, thus providing a mechanistic understanding of Ca2+ spark signaling in smooth muscle.
