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)P₃]-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)P₃revealed that Ca²⁺ release from Ins(1,4,5)P₃-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.