Change in intracellular Ca²⁺ is not involved in serotonin-induced meiosis reinitation from the first prophase in oocytes of the marine bivalve Crassostrea gigas

In response to the neurohormone serotonin (5-hydroxytryptamine, 5-hT), prophase-arrested oocytes of the marine bivalve Crassostrea gigas (oyster) reinitiate meiosis, undergo germinal vesicle breakdown (GVBD), and are arrested again at metaphase I. We examined the pharmacological characteristics of 5-HT receptors and the signal transduction pathway following 5-HT stimulation in oyster oocytes. Among 5-HT agonists tested, only α-methyl 5-HT,a 5-HT₂ agonist, induced GVBD, although it was 1000 times less sensitive than 5-HT. The rank order of the potency of 5-HT antagonists to inhibit GVBD was propranolol, cyproheptadine > metoclopramide > mianserin. These results are quite different from those reported for other mollusks, suggesting the presence of unique 5-HT receptors on oyster oocytes. Using the fluorescent Ca²⁺ dyes fura2 and calcium green and the pH indicator 1-hydroxypyrene-3,6,8-trisulfonic acid, we examined changes in intracellular Ca²⁺ ([Ca²⁺](i)) and intracellular pH (pH(i)) during 5-HT-induced meiosis reinitiation. 5-HT did not trigger any changes in [Ca²⁺](i). However, an increase in pH(i) was observed during the 5-HT-induced meiosis reinitiation. The increased pH(i) level was rather small before GVBD and not necessary for GVBD, because lowering pH(i) by sodium acetate seawater (pH 7.0) did not prevent 5-HT-induced GVBD. Measurement of the kinase activity toward a peptide substrate specific to cdc2 demonstrated that maturation-promoting factor (MPF) was activated in accordance with the occurrence of GVBD in response to 5-HT. Therefore, it is likely that in oyster oocytes the signal transduction pathways and intracellular effectors participating in 5-HT-induced meiosis reinitiation via the activation of MPF are insensitive to [Ca²⁺](i) and pH(i).