(A–H) Traces show whole-cell Cal520 fluorescence ratio (smooth colored traces; ΔF/F0) and SD fluctuations (noisy black traces) of HEK cells locally stimulated with CCH locally delivered in a Ca2+-containing bath solution when indicated by the bars. Panels on the left are representative records from individual cells, and panels on the right show mean traces from 7 (B) or 3 (D,F,H) cells. (A, B) Records from HEK WT cells. (C–H) Records from HEK cells solely expressing IP3R1 (C,D), IP3R2 (E,F), or IP3R3 (G,H). (I) Overlaid mean Cal520 fluorescence ratio traces, aligned to their rising phase, in WT HEK cells (cyan; n = 7), and HEK cells solely expressing IP3R1 (gold; n = 3), IP3R2 (green; n = 3), and IP3R3 (pink; n = 3). (J) Scatter plots of SD signal vs. fluorescence ratio during the rising phase of the Ca2+ responses in WT HEK cells (cyan diamonds) and HEK cells solely expressing IP3R1 (gold circles) IP3R2 (green squares) or IP3R3 (pink triangles). Data points are means from the same cells as in I. (K) The same data as in J, after normalizing to the same peak SD values.