title: Extracellular Mg2+ induces an intracellular Ca2+ wave during oocyte activation in the marine shrimp Sicyonia ingentis.
abstract: In contrast to most systems in which oocyte activation is triggered by the fertilizing sperm, Sicyonia ingentis oocytes are activated by seawater Mg2+ during spawning. S. ingentis oocytes were spawned into Mg(2+)-free seawater and microinjected with the fluorescent Ca2+ indicator Fluo-3 to study the effects of added Mg2+ on intracellular Ca2+ levels. The Mg2+ induced a wave of fluorescence across the oocyte that traveled at a speed of 13 +/- 3 microns/sec. Extracellular Ca2+ was not required for induction of the wave. Treatment with Ca2+ ionophore in Mg(2+)-free medium or a localized injection (0.3% oocyte volume) of 3-5 microM Ca2+ also initiated the wave; injection of 250 mM Mg2+ (up to 1.5% oocyte volume) had no effect. Microinjection of 750 microM EGTA (final) suppressed the Mg(2+)-induced wave, while an identical concentration of EDTA had no inhibitory effect. Subsequent to the initial Mg(2+)-induced intracellular Ca2+ increase, a second Ca2+ increase was observed at approximately 15 min postspawning; the timing of this second increase appeared to be independent of when the Mg(2+)-induced wave was initiated, thus an event associated with spawning may be involved. While oocytes in normal seawater were monospermic, those in Mg(2+)-free seawater were polyspermic, suggesting a role for the Mg(2+)-induced Ca2+ wave in regulating sperm entry into the oocyte.
authors.affiliation: Bodega Marine Laboratory, University of California, Bodega Bay 94923.
authors.forename: L L
authors.initials: LL
authors.lastname: Lindsay
authors.forename: P L
authors.initials: PL
authors.lastname: Hertzler
authors.forename: W H
authors.initials: WH
authors.lastname: Clark
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism*; D033364:Decapoda; D005260:Female; D005306:Fertilization; D008274:Magnesium / Q000494:pharmacology*; D009865:Oocytes / Q000378:metabolism*
chemical_list: D008274:Magnesium; D002118:Calcium