By combining the derivative of the signal mass with knowledge of the cell’s Ca buffering capacity, reaction-diffusion simulations, and fluo-3 fluorescence intensity, we can derive ICa(spark) (ZhuGe et al., 2000). On average, ICa(spark) has an amplitude of 2.8 ± 0.4 pA with a linear rise peaking at 11 ± 1.1 ms and a single exponential decay with a τ of 12 ± 1.4 ms (n = 13) when cells were held at 0 mV. With signal mass and ICa(spark), we further analyzed the relationship between Ca2+ sparks and STOCs. As shown in Fig. 1, there exist weak, yet significant, correlations between Ca2+ spark signal mass and STOC amplitude (r = 0.404, P = 0.005; Fig. 1 B, b), between peak ICa(spark) and STOC amplitude (r = 0.4451, P = 0.002; Fig. 1 B, c), and between rise time of signal mass and STOC amplitude (r = 0.4816, P = 0.001; Fig. 1 B, d). The weakness of the correlations implies that either BK channels are so close to RYRs that they are saturated by Ca2+ or that the coupling between
