title: External Ca2+ is predominantly used for cytoplasmic and nuclear Ca2+ increases in fertilized oocytes of the marine bivalve Mactra chinensis.
abstract: 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 Ca(2+). We have clarified here the spatiotemporal patterns and sources of the intracellular Ca(2+) changes at fertilization. Shortly after insemination, increased Ca(2+) simultaneously appeared at the whole cortical region of the oocyte and spread inwardly to the center, attaining the maximal Ca(2+) levels throughout the oocyte, including the cytoplasm and nucleus. The initial maximal Ca(2+) peak was followed by a submaximal plateau phase of cytoplasmic and nuclear Ca(2+) elevations, which persisted for several minutes. The nuclear envelope began to break down shortly before the termination of the plateau phase. These sperm-induced Ca(2+) changes were inhibited by suppression of the influx of external Ca(2+) from seawater but not by disturbance of the release of internal Ca(2+) from inositol 1,4,5-trisphosphate [Ins(1,4,5)P(3)]-sensitive stores, suggesting that the increased Ca(2+) 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 Ca(2+) transients, each of which started from one cortical region and propagated across the oocyte as a Ca(2+) wave. The 5-HT-induced Ca(2+) transients persisted even in the absence of external Ca(2+). Experiments with caged Ins(1,4,5)P(3) revealed that Ca(2+) 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 Ca(2+)-mobilizing pathways: Ca(2+) influx producing a centripetal Ca(2+) wave from the whole cortex and Ca(2+) release from Ins(1,4,5)P(3)-sensitive stores producing a point-source propagating Ca(2+) wave. However, it seems likely that the Ca(2+) influx pathway is predominantly activated at fertilization.
authors.affiliation: Department of Biology, Miyagi University of Education, Aoba-ku, Sendai, Miyagi 980-0845, Japan. deguchi@staff.miyakyo-u.ac.jp
authors.forename: Ryusaku
authors.initials: R
authors.lastname: Deguchi
authors.forename: Masaaki
authors.initials: M
authors.lastname: Morisawa
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism*; D020013:Calcium Signaling / Q000187:drug effects / Q000502:physiology*; D002454:Cell Differentiation / Q000187:drug effects / Q000502:physiology*; D002467:Cell Nucleus / Q000187:drug effects / Q000378:metabolism*; D003593:Cytoplasm / Q000187:drug effects / Q000378:metabolism*; D005260:Female; D005306:Fertilization / Q000502:physiology; D015544:Inositol 1,4,5-Trisphosphate / Q000378:metabolism / Q000494:pharmacology; D008297:Male; D008974:Mollusca / Q000166:cytology / Q000196:embryology* / Q000378:metabolism; D009865:Oocytes / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D012701:Serotonin / Q000378:metabolism / Q000494:pharmacology; D015854:Up-Regulation / Q000187:drug effects / Q000502:physiology
chemical_list: D012701:Serotonin; D015544:Inositol 1,4,5-Trisphosphate; D002118:Calcium