Effects of injecting calcium chloride into in vitro-matured porcine oocytes

In vitro-matured porcine oocytes were given injections of 0.1 M CaCl₂ and after 6 h evaluated for signs of early and late activation events. CaCl₂ injection caused cortical granule exocytosis in 75% (3 of 4) of the oocytes tested. It also induced cell cycle resumption as monitored by the histone H1 kinase assay: the phosphorylation rate of histone H1 decreased to 36.7% of the original value. Treated oocytes completed meiosis, extruded the second polar body, and progressed to first interphase: 79.4% of them formed one or more pronuclei. The elevated intracellular Ca²⁺ level resulted in activation-related changes in the protein synthetic profile in 90% (9 of 10) of the oocytes. Furthermore, 14.7% (9 of 61) of the treated oocytes developed to the compact morula/early blastocyst stage after a 7-day culture in ligated porcine oviduct, and one blastocyst hatched from the zona pellucida. Control oocytes given injections of 0.1 M MgCl₂ or carrier medium (10 mM Hepes) did not show the changes mentioned. The results strengthen the idea that Ca²⁺ is a cell messenger that plays a central part in oocyte activation; it is concluded that elevated intracellular Ca²⁺ level caused by a single injection of CaCl₂ leads to both early and late events of porcine oocyte activation.