SEROTONIN-INDUCED MEIOSIS REINITIATION FROM THE FIRST PROPHASE AND FROM THE FIRST METAPHASE IN OOCYTES OF THE MARINE BIVALVE HIATELLA-FLACCIDA - RESPECTIVE CHANGES IN INTRACELLULAR CA2+ AND PH

In the marine bivalve Hiatella flaccida, full-grown oocytes in ovaries are arrested at the first prophase (prophase-I) of meiosis, whereas spawned oocytes have reinitiated meiosis from prophase-I and are again arrested at the first metaphase (metaphase-I). The neurohormone serotonin (5-hydroxytryptamine, 5-HT) was able to trigger meiosis reinitiation both from prophase-I and from metaphase-I. Exposure of prophase-I oocytes to 5-HT caused an increase in intracellular Ca2+ ([Ca2+](i)) composed of an initial towering transient and a following lower but sustained elevation. 5-HT-stimulated prophase-I oocytes also showed a gradual rise in intracellular pH (pH(i)), reaching a plateau level. None of these 5-HT-induced responses was affected by the complete absence of external Ca2+. On the other hand, these responses were suppressed by preinjection of heparin, an antagonist of inositol 1,4,5-trisphosphate-sensitive receptors. Metaphase-I oocytes also exhibited a [Ca2+](i) increase in response to 5-HT; the initial [Ca2+](i) transient was larger than that in prophase-I oocytes when stimulated with the same 5-HT concentration. Furthermore, after the initial transient, the elevated [Ca2+](i) was not sustained but sometimes returned to the prestimulus level and then increased again. Metaphase-I oocytes had higher resting pH(i) levels than prophase-I oocytes and showed no significant pH(i) changes after addition of 5-HT. These results suggest that both a [Ca2+](i) increase and a pH(i) rise are responsible for the release from prophase-I arrest, while a [Ca2+](i) increase alone is concerned with the release from metaphase-I arrest. (C) 1995 Academic Press, Inc.