Meiosis reinitiation from the first prophase is dependent on the levels of intracellular Ca2+ and pH in oocytes of the bivalves Mactra chinensis and Limaria hakodatensis.

Naturally spawned oocytes of the marine bivalves Mactra chinensis and Limaria hakodatensis are arrested at the first prophase (prophase-I) and the first metaphase, respectively, until fertilization. Using the Ca2+ indicator fura-2 and the pH indicator 1-hydroxypyrene-3,6,8-trisulfonic acid, we have examined the respective effects of intracellular Ca2+ ([Ca2+]i) and pH (pHi) on meiosis reinitiation from prophase-I in oocytes of the two species. Shortly after insemination, Mactra oocytes displayed a transient [Ca2+]i increase followed by a period of sustained [Ca2+]i elevation. Removal of external Ca2+ shortly after fertilization immediately decreased the elevated [Ca2+]i to the resting level and inhibited germinal vesicle breakdown (GVBD); 100% GVBD was obtained when elevated [Ca2+]i above the threshold level (F340/F380: approximately 0.55) was kept for at least 5 min. Fertilized Mactra oocytes also showed a gradual pHi rise; sperm-induced GVBD was blocked when pHi was maintained below the threshold level (F450/F380: approximately 0.95) by adding ammonia and acetate to the bath after insemination. In contrast, 2 mM ammonia caused a pHi rise and GVBD in Limaria oocytes without much affecting the [Ca2+]i level. For obtaining 100% GVBD, pHi had to be maintained for at least 5 min above the threshold level (F450/F380: approximately 0.9), which is similar to that in Mactra. Resting [Ca2+]i levels (F340/F380: approximately 0.65) in Limaria prophase-I oocytes were higher than the threshold level for GVBD in fertilized Mactra oocytes. It is possible that maintenance of both [Ca2+]i and pHi above threshold levels are required for GVBD and the levels are about the same in Mactra and Limaria, assuming that spectral characteristics of the indicators are the same in oocytes of the two species.