Fig. 1. Simultaneous confocal imaging of the cytoplasmic and nuclear [Ca²⁺] transient (CaT) in an electrically stimulated rabbit atrial myocyte. (A) 2D confocal images of an atrial myocyte. Numbers indicate milliseconds after electrical stimulation. (B, left) CaTs of distinct subcellular regions, as indicated in the schematic representation of the cell (same cell as in A): black trace: whole cytoplasm (wc); green traces: subsarcolemmal regions (ss); red trace: nucleus (nuc); blue traces: central regions adjacent to the nucleus (ct). The vertical dashed line marks the peak of the CaTs of the two central regions. (B, right) Average values of peak systolic [Ca²⁺], time-to-peak and the time constant for decay from 13 atrial myocytes. Colours refer to the subcellular regions as shown in the cell scheme (bar, 5 µm). *, P<0.05 versus subsarcolemmal regions; #, P<0.05 versus central regions. (C) A pair of atrial myocytes (a,b). Left: Fluo-4 fluorescence (Ca²⁺) during the decaying phase of an electrically stimulated CaT. Right: same cells after cessation of stimulation and loading with Syto-16 (DNA). Because of the large increase in fluorescence caused by Syto-16 loading, laser energy was attenuated to 10% of the initial value. Bars, 10 µm. The results in C are representative for a total of seven cells studied.