Fig. 4. The artificial pacemaker (PM3) predominates over the physiological calcium wave pacemaker(PM2). (A) Variations of [Ca²⁺]c in a fertilized egg undergoing meiosis II (1 image every 10 seconds), in response to a global increase in gPtdIns(4,5)P₂ levels (red arrows indicate the four successive 1.5-second UV flashes applied). The flashes produce a sustained calcium increase, then two oscillations that are waves initiated in the animal pole (PM3, green graph). The three subsequent waves emanate from pacemaker PM2 in the contraction pole (pink graph) and precede the extrusion of the second polar body (pb2). (B) Another experiment showing the effect of cgPtdIns(4,5)P₂ uncaging during the series II calcium waves (1 image every 4 seconds). The images displayed in the CG/TR sequence correspond to the yellow dots on the graph. The physiological series II calcium wave pacemaker PM2 is located in the contraction pole (cp) after PM1 activity has ceased (calcium waves shown in i and ii; Bar, 23 µm). This particular experiment shows that global UV-flashes over the whole zygote (uv1, uv2, uv3) can elicit three different types of waves. The first global photorelease of gPtdIns(4,5)P₂ (uv1, flash duration 3 seconds; red arrowhead at 7'19'') triggers an artificial calcium wave whose initiation site (PM3) is in the animal pole (a) cortex. The second UV-flash (uv2, flash duration 1 second; red arrowhead at 11'17'') triggers an artificial calcium wave initiated in the contraction pole in the vegetal hemisphere. The third UV-flash (uv3, flash duration 2 seconds, arrowhead at 13'49'') elicits a calcium wave that is initiated in both the animal and vegetal pole regions. Probabilities for the occurrence of each type of wave are shown in the bar graph.