Fast spreading of calcium signals within a compartment and across the nuclear envelope. Cells were imaged within a narrow band to increase the image acquisition rate. A and C, the image frame was cut (indicated by grey boxes) to a field of view of 512 × 32 pixels. The 5 μm2 areas in the nucleus (A) and in the cytosol (C) that were illuminated with UV light for photolysis of NP-EGTA are indicated with white circles; the positioning of the uncaging spot was guided by MitoTracker staining shown in red. The positions of calcium measurements at the 3 μm and 7 μm distances from the uncaging spot are indicated with white filled squares. B and D, calcium signals were determined using calibrated Fluo-4 fluorescence measurements in squares of 6 × 6 pixels at distances of 3 μm and 7 μm from the uncaging spot in the nucleus (continuous line) and the cytoplasm (dotted line). UV light (t
= 10.0 msec) was switched on as indicated by the dashed line. Images were taken every 86 msec. Scale bar is 5 μm.