Two measures of Ca2+ sparks were used: (1) the conventional fluorescence ratio, ΔF/F0, within a restricted volume, and (2) the change in total fluorescence, F − Fo, over a larger volume (i.e., 40 × 40 pixels, 333 nm/pixel), also designated as the Ca2+ signal mass, which is proportional to the total quantity of Ca2+ released into the cytosol. These measurements have been previously described (ZhuGe et al., 2000, 2002). The endogenous fixed Ca2+ buffer as estimated in Bao et al. (2008) was taken into account to calculate [Ca2+].
