External Ca2+ is predominantly used for cytoplasmic and nuclear Ca2+ increases in fertilized oocytes of the marine bivalve Mactra chinensis

Oocytes of the marine bivalve Mactra chinensis are spawned and arrested at the germinal vesicle stage (first meiotic prophase) until fertilization, without undergoing a process called oocyte maturation. As is the case of other animals, a fertilized oocyte of the bivalve displays increases in intracellular free Ca2+. We have clarified here the spatiotemporal patterns and sources of the intracellular Ca2+ changes at fertilization. Shortly after insemination, increased, Ca2+ simultaneously appeared at the whole cortical region of the oocyte and spread inwardly to the center, attaining the maximal Ca2+ levels throughout the oocyte, including the cytoplasm and nucleus. The initial maximal Ca2+ peak was followed by a submaximal plateau phase of cytoplasmic and nuclear Ca2+ elevations, which persisted for several minutes. The nuclear envelope began to break down shortly before the termination of the plateau phase. These sperm-induced Ca2+ changes were inhibited by suppression of the influx of external Ca2+ from seawater but not by disturbance of the release of internal Ca2+ from inositol 1,4,5-trisphosphate [Ins(1,4,5)P-3]-sensitive stores, suggesting that the increased Ca2+ is from an external source. In contrast to the situation observed at fertilization, an oocyte artificially stimulated with serotonin (5-hydroxytryptamine, 5-HT) displayed repetitive Ca2+ transients, each of which started from one cortical region and propagated across the oocyte as a Ca2+ wave. The 5-HT-induced Ca2+ transients persisted even in the absence of external Ca2+. Experiments with caged Ins(1,4,5)P-3 revealed that Ca2+ release from Ins(1,4,5)P-3-sensitive stores is another pathway that is sufficient to trigger meiosis reinitiation from the first prophase. These results demonstrate that Mactra oocytes can potentially use two different Ca2+-mobilizing pathways: Ca2+ influx producing a centripetal Ca2+ wave from the whole cortex and Ca2+ release from Ins(1,4,5)P3-sensitive stores producing a point-source propagating Ca2+ wave. However, it seems likely that the Ca2+ influx pathway is predominantly activated at fertilization.