Fig. 6. Calcium images of fluo-3 fluorescence in a mouse myotube. Confocal imaging allowed us to detect both a fast and a slow rise of intracellular Ca²⁺ concentration ([Ca²⁺]i). Basal fluorescence is shown at the top of the left panel. The next image was taken immediately after the bath solution was quickly changed to 47 mM K⁺; this solution remained in the bath throughout the time of the whole record. The subsequent images were taken every second or at times indicated. After the fast signal faded, a focus of fluorescence appeared in the right extreme of the myotube (8 seconds), and the slow propagation to the left of a Ca²⁺ wave became evident (8-15 seconds). Note that a low fluorescence increase (yellow-green) precedes high fluorescence (red) localized in confined regions, where nuclei are found. Some frames were omitted to make a more concise image. Total length of the myotube section, 173 µm.