Abstract
We studied the role of the Na+/Ca2+ exchanger (NCX) in modulating oocyte postovulatory aging by observing changes in NCX contents and activities in aging mouse and rat oocytes. Whereas the NCX activity was measured by observing oocyte activation following culture with NCX inhibitor or activator, the NCX contents were determined by immunohistochemical quantification. Although NCX was active in freshly-ovulated rat oocytes recovered 13 h post hCG injection and in aged oocytes recovered 19 h post hCG in both species, it was not active in freshly-ovulated mouse oocytes. However, NCX became active when the freshly-ovulated mouse oocytes were activated with ethanol before culture. Measurement of cytoplasmic Ca2+ revealed Ca2+ increases always before NCX activation. Whereas levels of the reactive oxygen species (ROS) and the activation susceptibility increased, the density of NCX member 1 (NCX1) decreased significantly with oocyte aging in both species. While culture with H2O2 decreased the density of NCX1 significantly, culture with NaCl supplementation sustained the NCX1 density in mouse oocytes. It was concluded that (a) the NCX activity was involved in the modulation of oocyte aging and spontaneous activation; (b) ROS and Na+ regulated the NCX activity in aging oocytes by altering its density as well as functioning; and (c) cytoplasmic Ca2+ elevation was essential for NCX activation in the oocyte.
Highlights
Mammalian oocytes are arrested at the meiotic metaphase II (MII) stage following ovulation
At the end of culture, while rat oocytes were observed for spontaneous activation immediately, mouse oocytes were treated with 10% ethanol for 10 min, cultured in CZB without supplements and observed for activation 6 h later
Na+/Ca2+ exchanger (NCX) is active in aged oocytes in both mice and rats To test the NCX activity in aged oocytes, mouse and rat oocytes collected 19 h post human chorionic gonadotropin (hCG) injection were cultured for 6 h with supplementation of 50 mM bepridil or different concentrations of NaCl
Summary
Mammalian oocytes are arrested at the meiotic metaphase II (MII) stage following ovulation. If not fertilized in time, the ovulated oocytes undergo a time-dependent process of aging [1,2]. The postovulatory oocyte aging has marked detrimental effects on embryo development [5,7,8,9] and offspring [10,11]. The use of aged oocytes resulted in significant decrease in embryonic development following in vitro fertilization, intracytoplasmic sperm injection [12] or nuclear transfer [13,14,15]. Studies on the mechanisms and control of oocyte aging are important for both normal and assisted reproduction. The mechanisms for oocyte aging are not fully clear
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