Extracellular Ca2+ Requirement For Serotonin-Induced Release And Meiosis Reinitiation From Prophase In Oocytes Of The Scallop Argopecten Purpuratus

The requirement for Ca2+ in the external medium for the release and reinitiation of meiosis in the scallop Argopecten purpuratus was examined. In vitro experiments demonstrated that the serotonin-induced release of oocytes is inhibited in a calcium-free medium. Caffeine (10 and 20 mM), a known activator of ryanodine channels, increased oocyte release by gonadal pieces and 50 mu M ryanodine inhibited this release and blocked the serotonin-induced effect. The number of oocytes released under control conditions was not affected when ryanodine was used in concentrations in the nanomolar range. Germinal vesicle breakdown, indicative of meiosis reinitiation, was inhibited in gonadal pieces incubated in calcium-free normal or artificial seawater and serotonin does not overcome this inhibition in the same conditions. An increase in fluorescence on adding serotonin (10(-5) M, final concentration) was detected under the confocal microscope when oocytes charged with the calcium fluorescent dye Fluo-3 AM were studied. We postulate that there is a rise in cellular calcium in the serotonin-induced spawning process and that the source for the increase is influx from the external medium and release from internal stores. We suggest that this calcium rise plays a role in germinal vesicle breakdown and in the contraction of gonadal smooth muscle.