Ca2+ oscillations stimulate an ATP increase during fertilization of mouse eggs

Mammalian eggs and embryos rely upon mitochondrial ATP production to survive and proceed through preimplantation development. Ca2+ oscillations at fertilization have been shown to cause a reduction of mitochondrial NAD+ and flavoproteins, suggesting they might also cause changes in cytosolic ATP levels. Here, we have monitored intracellular Ca2+ and ATP levels in fertilizing mouse eggs by imaging the fluorescence of a Ca2+ dye and luminescence of firefly luciferase. At fertilization an initial increase in ATP levels occurs with the first Ca2+ transient, with a second increase occurring about 1 h later. The increase in cytosolic ATP was estimated to be from a prefertilization concentration of 1.9 mM to a peak value of 3 mM. ATP levels returned to prefertilization values as the Ca2+ oscillations terminated. An increase in ATP also occurred with other stimuli that increase Ca2+ and it was blocked when Ca2+ oscillations were inhibited by BAPTA injection. Additionally, an ATP increase was not seen when eggs were activated by cycloheximide, which does not cause a Ca2+ increase. These data suggest that mammalian fertilization is associated with a sudden but transient increase in cytosolic ATP and that Ca2+ oscillations are both necessary and sufficient to cause this increase in ATP levels.