Strontium-induced rat egg activation

Abstract

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+]itransient. Strontium chloride (SrCl2) binds to and activates the Ca2+-binding site on the inositol 1,4,5-trisphosphate receptor and evokes [Ca2+]ioscillations. Although SrCl2has been reported to activate mouse eggs, little is known regarding the pattern of the [Ca2+]ioscillations 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+]ioscillations in fura-2-loaded rat eggs. Treatment with 2 mM SrCl2was optimal for inducing the first [Ca2+]itransient, which was similar in duration to that triggered by sperm. However, the frequency and duration of the subsequent [Ca2+]ioscillations 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+]ioscillations (8, 15 or 60 min in SrCl2respectively) had no effect on the intensity of fluorescent CGE reporter dyes, while 60-min exposure to SrCl2caused a delay in CM. Our results demonstrate that SrCl2is an effective parthenogenetic agent that mimics rat egg activation by sperm, as judged by the generation of [Ca2+]ioscillations, CGE and CM.