EXTRACELLULAR CA2-TRISPHOSPHATE-SENSITIVE STORES FUNCTION AT FERTILIZATION IN OOCYTES OF THE MARINE BIVALVE MYTILUS-EDULIS( ENTRY AND CA2+ RELEASE FROM INOSITOL 1,4,5)

An oocyte of the marine bivalve Mytilus edulis, which is arrested at m etaphase I, reinitiates meiosis at fertilization, The fertilized oocyt e shows increases in intracellular Ca2+ ([Ca2+](i)) comprising three d ifferent phases: an initial large [Ca2+](i) transient, a subsequent lo w but sustained [Ca2+](i) elevation, and repetitive small [Ca2+](i) tr ansients. In this study, we have investigated the sources and mechanis ms of the sperm-induced [Ca2+](i) increases. Application of methoxyver apamil (D-600), an inhibitor of voltage-dependent Ca2+ influx, suppres sed the initial [Ca2+](i) transient but did not affect the following t wo phases of [Ca2+](i) changes. Injection of heparin, an antagonist of the inositol 1,4,5-trisphosphate (IP3) receptor, inhibited the later two phases without much affecting the initial transient. Combined appl ication of D-600 and heparin almost completely abolished the three pha ses of the sperm-induced [Ca2+](i) changes. Furthermore, Ca2+ influx c aused by seawater containing excess K+ was blocked by D-600 but not by heparin, and IP3-induced Ca2+ release caused by photolysis of injecte d 'caged' derivatives of IP3 was blocked by heparin but not by D-600, These results strongly suggest that two types of Ca2+ mobilization sys tems, the extracellular Ca2+ entry responsible for an initial [Ca2+](i ) transient and the IP3 receptor-mediated Ca2+ release responsible for the following two phases of [Ca2+](i) changes, function at fertilizat ion of Mytilus oocytes.