Extracellular Mg²⁺ induces an intracellular Ca²⁺ wave during oocyte activation in the marine shrimp Sicyonia ingentis

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 Mg²⁺-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 μm/sec. Extracellular Ca²⁺ was not required for induction of the wave. Treatment with Ca²⁺ ionophore in Mg²⁺-free medium or a localized injection (0.3% oocyte volume) of 3-5 μM Ca²⁺ also initiated the wave; injection of 250 mM Mg²⁺ (up to 1.5% oocyte volume) had no effect. Microinjection of 750 μM EGTA (final) suppressed the Mg²⁺-induced wave, while an identical concentration of EDTA had no inhibitory effect. Subsequent to the initial Mg²⁺-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 Mg²⁺-induced wave was initiated, thus an event associated with spawning may be involved. While oocytes in normal seawater were monospermic, those in Mg²⁺-free seawater were polyspermic, suggesting a role for the Mg²⁺-induced Ca²⁺ wave in regulating sperm entry into the oocyte.