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First published online 27 November 2002
doi: 10.1242/jcs.00221

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

¹ Department of Biology, Miyagi University of Education, Aoba-ku, Sendai, Miyagi 980-0845, Japan
² Misaki Marine Biological Station, the University of Tokyo, Miura, Kanagawa 238-0225, Japan

^* Author for correspondence (e-mail: deguchi{at}staff.miyakyo-u.ac.jp)

Accepted 15 October 2002

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)P₃ revealed 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.

Key words: Intracellular Ca²⁺, Fertilization, Ca²⁺ channels, Serotonin, Ins(1,4,5)P₃

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