Repetitive calcium transients and the role of calcium in exocytosis and cell cycle activation in the mouse egg

Abstract

The role of calcium in cortical granule exocytosis and activation of the cell cycle at fertilization was examined in the mouse egg using the calcium chelator BAPTA (1,2-bis( o-aminophenoxy)ethane- N,N,N′,N′-tetraacetic acid) and the fluorescent calcium indicator fluo-3. BAPTA and fluo-3 were introduced into zona-free mouse eggs by a 30-min incubation with 0.01–50 μ M BAPTA acetoxymethyl ester (AM) and/or 1–20 μ M fluo-3 AM prior to in vitro fertilization. Incubation of eggs in ⩾5.0 μ M BAPTA AM inhibited cortical granule exocytosis in all cases. Introduction of the calcium chelator into the egg blocked second polar body formation at ⩾1.0 μ M BAPTA AM. Sperm entry occurred in all eggs regardless of the BAPTA AM concentration. Sperm induce a large transient increase in calcium lasting 2.3 ± 0.6 min, followed by repetitive transients lasting 0.5 ± 0.1 min and occurring at 3.4 ± 1.4-min intervals. Incubation with ⩾5.0 μ M BAPTA AM inhibited all calcium transients. Introduction of BAPTA also inhibited calcium transients, exocytosis, and the resumption of meiosis following application of the calcium ionophore A23187 or SrCl 2, which activate eggs. These results demonstrate that the calcium increase at fertilization is required for cortical granule exocytosis and resumption of the cell cycle in a mammalian egg.