Rapid propagation of endothelial cell activation within a confluent cell population follows laser-induced activation in vitro. HUVECs were loaded with Fluo-4 AM. Cells were observed for 1 minute prior to activation to confirm a stable baseline then subjected to a single pulse from a nitrogen laser and continuous imaging. (A) Images from different time points of a representative cell population. The first frame shows the cell monolayer in its basal state just prior to activation and the X marks the point upon which the laser is focused. The subsequent time points show activation of the target cell (within 1 second of laser firing), closely followed by activation of surrounding cells and then steady return toward a basal cytoplasmic Ca²⁺ concentration. (B) The mean pixel intensity of the 4 cells is plotted versus time to yield the corresponding 4 kinetic traces shown on the right. Cell 1, solid line; Cell 2, -.-.; Cell 3, - - -; Cell 4, ⋯.