Strontium-induced rat egg activation

Parthenogenetic agents that evoke cytosolic calcium concentration ([Ca²⁺]_i) oscillations similar to those evoked by sperm, mimic fertilization more faithfully than agents that trigger a single [Ca²⁺]_i transient. Strontium chloride (SrCl₂) binds to and activates the Ca²⁺-binding site on the inositol 1,4,5-trisphosphate receptor and evokes [Ca²⁺]_i oscillations. Although SrCl₂ has been reported to activate mouse eggs, little is known regarding the pattern of the [Ca²⁺]_i oscillations it evokes in rat eggs and their effect on the early events of egg activation: cortical granule exocytosis (CGE) and completion of meiosis (CM). In the current study we investigated the effect of various concentrations of SrCl₂ (2, 4 or 6 mM) on [Ca²⁺]_i, by monitoring [Ca²⁺]i, oscillations in fura-2-loaded rat eggs. Treatment with 2 mM SrCl₂ was optimal for inducing the first [Ca²⁺]_i transient, which was similar in duration to that triggered by sperm. However, the frequency and duration of the subsequent [Ca²⁺]_i oscillations were lower and longer in SrCl₂-activated than in sperm-activated eggs. The degree of CGE was identical in eggs activated by either sperm or SrCl₂, as assessed by semi-quantitative immunohistochemistry combined with confocal microscopy. Evoking 1, 2 or 10 [Ca²⁺]_i oscillations (8, 15 or 60 min in SrCl₂ respectively) had no effect on the intensity of fluorescent CGE reporter dyes, while 60-min exposure to SrCl₂ caused a delay in CM. Our results demonstrate that SrCl₂ is an effective parthenogenetic agent that mimics rat egg activation by sperm, as judged by the generation of [Ca²⁺]_i oscillations, CGE and CM.