Free Ca²⁺ increases in exponential phases during mouse oocyte activation

Abstract

A dramatic rise in cytoplasmic free Ca²⁺ concentration has been shown to occur during fertilization and artificial activation in the oocytes of both the medaka fish¹ and sea urchin². Indirect evidence has implicated Ca²⁺ in the parthenogenetic activation of mammalian oocytes. Mouse oocytes can be activated by the intracellular injection of Ca²⁺ but not Mg²⁺ (ref. 3), and hamster oocytes by exposure to the calcium ionophore, A23187 (ref. 4). We report here measurements of cytoplasmic free Ca²⁺ during the artificial activation and fertilization of single mouse oocytes injected with the Ca²⁺-sensitive photoprotein aequorin. Free Ca²⁺ rises exponentially from a resting level of below 0.1 µM to >5 µM over a period of 10–30 min. A series of oscillatory Ca²⁺ transients precedes this Ca²⁺ rise during fertilization, but not during artificial activation.