Spontaneous calcium release initiates cytosolic calcium oscillations in embryonic cardiac myocytes. (A) Laser scanning confocal image from a Fluo-4–loaded, isolated E10 spontaneously active cardiomyocyte (left) with corresponding line scan (yellow line) through the cytosol surrounding the dark nuclear area (red n). Red and blue arrowheads in the line scan image denote near SR area and near SL area, respectively. Graphs above the line scan image show relative calcium signals (Fluo-4 emission, F/F₀) from near SR (red line) and near SL (blue line). Insert (gray background) shows both graphs in an expanded timescale from selected area below (gray). (B) Effects of inhibitors of SR Ca²⁺-ATPase (10 μM thapsigargin; top), RyRs (50 μM ryanodine; middle), and IP₃Rs (50 μM 2-APB; bottom) on spontaneous calcium signals from isolated E10 cardiomyocytes.