of ∼1 µM (Fill and Copello, 2002; Laver, 2007), the probability that activation of RYRs in one punctum leads to the activation of RYRs in a neighboring punctum should be <0.5 because the maximal [Ca2+] at 600 nm from the sparks reaches approximately ≤2 µM (see Fig. 7 in Bao et al., 2008). Also, spontaneous Ca2+ transients always appear in the form of Ca2+ sparks but not in the form of regional or global changes in these cells (Fig. 1; ZhuGe et al., 1998; Bao et al., 2008). Therefore, we conclude that Ca2+ sparks result from the opening of RYRs in a single punctum rather than multiple puncta, and so a single RYR punctum consists of multiple RYRs.
