A comparison of sperm- and IP3-induced Ca2+ release in activated and aging mouse oocytes

Ca2+ release mechanisms in oocytes are highly sensitive and a number of agents including sperm and inositol trisphosphate (IP3) generate Ca2+ transients. Recently it was shown that this sensitivity decreases after fertilization and subsequent entry into the first mitotic cell cycle (Jones et al., Development 121, 3259-3266, 1995). In this study a similar decrease in the ability of IP3 to cause repetitive Ca2+ transients was observed in parthenogenetic embryos following activation with Sr2+, ethanol, or cycloheximide. This indicates that the decline in sensitivity of the Ca2+ releasing mechanism after oocyte activation is not associated with the fertilizing sperm. A similar decline in IP3-induced Ca2+ release was observed in metaphase II oocytes at 24 hr post hCG or later, although repetitive Ca2+ transients were induced in the aged oocytes after in vitro fertilization. Sperm-induced Ca2+ transients in aged oocytes were similar in duration and peak amplitude to younger oocytes, 15-18 hr post hCG. However, they showed a much reduced rate of rise which was also observed in younger oocytes after the intracellular stores had been depleted by thapsigargin. The results suggest that factors within the oocyte, such as store size, are important in enabling sperm to generate repetitive Ca2+ transients. Also, the Ca2+ release processes decline as the oocyte ages as well as after activation.