In theory, two types of BK channel distributions could result in the generation of STOCs by Ca2+ sparks. In one scenario, individual BK channels localize randomly or homogeneously in the surface membrane, independent of RYR location. As a result, only the BK channels that localize within a certain distance of the RYRs could be activated during a spark because [Ca2+] decreases steeply as it moves away from RYRs, the Ca2+ source. Alternatively, BK channels could distribute in much higher density near the RYRs, i.e., RYRs and BK channels constitute a physical microdomain. In our previous study of amphibian gastric smooth muscle (ZhuGe et al., 2002), we observed that the conductance of STOCs reaches a plateau at positive potentials, arguing that the alternative scenario could be the structural basis for Ca2+ sparks to induce STOCs. In the present study, we directly tested these two possibilities by visualizing BK channels and RYRs using high spatial resolution immunofluorescence microscopy. We found that both RYRs and BK channels distribute in puncta. Our calculation indicates that each punctum represents a group of clustered channels, which is consistent with our previous hypothesis that BK channels concentrate near RYRs (ZhuGe et al., 2002).
