Sources of calcium in sea urchin eggs during the fertilization response

Shortly after sperm-egg interaction the sea urchin egg is traversed by a Ca2+ wave, which is necessary for metabolic activation of the quiescent cell. Several sources including influx across the egg plasma membrane and release from intracellular stores may contribute to the rise in cytoplasmic Ca2+ activity. Inositol 1,4,5-trisphosphate (InsP3), cyclic ADP ribose (cADPR), and ryanodine have been reported to induce intracellular Ca2+ release. We used confocal laser scanning microscopy to image the Ca2+ transient during fertilization and parthenogenetic activation by microinjection of Ca2+ release agonists. A near instantaneous rise in Ca2+ localized to the egg cortex occurred near the time of sperm-egg binding, followed by a distinctive delay before the onset of the Ca2+ wave. Since the rise in cortical Ca2+ activity was absent when Ca2+ influx was prevented, it appeared that this change in Ca2+ activity was due to the opening of membrane Ca2+ channels. Blocking the influx did not alter the onset of the Ca2+ wave. The Ca2+ wave during the fertilization response seemed to require Ca2+ release mediated by InsP3-, cADPR-, and ryanodine-sensitive mechanisms. Parthenogenetic activation by microinjection of these three agents had different spatiotemporal patterns of Ca2+ release. Most significantly the injection of either InsP3 or cADPR, but not ryanodine, induced an enhanced pronucleus-associated Ca2+ release, which was similar to the Ca2+ response during fertilization.