Na+-Ca2+ exchange in mouse oocytes:: modifications in the regulation of intracellular free Ca2+ during oocyte maturation

Increases in intracellular free Ca2+ concentration (Ca2+ oscillations) occur during meiotic maturation and fertilization of mammalian oocytes but little is known about the mechanisms of Ca2+ homeostasis in these cells. Cells extrude Ca2+ from the cytosol using two main transport processes, the Ca2+-ATPase and the Na+-Ca2+ exchanger. The aim of this study was to determine whether Na+-Ca2+ exchange activity is present in immature and mature mouse oocytes. Na+-Ca2+ exchange can be revealed by altering the Na+ concentration gradient across the plasma membrane and recording intracellular free Ca2+ concentrations using Ca2+-sensitive fluorescent dyes. Depletion of extracellular Na+ caused an immediate increase in Ca2+ concentration in immature oocytes and a delayed increase in mature oocytes. The Na+ ionophore, monensin, caused an increase in intracellular Ca2+ in immature oocytes similar to that induced by Na+-depleted medium. In mature oocytes, monensin had no effect on intracellular Ca2+ but the time taken for Ca2+ to reach a peak value on removal of extracellular Na+ was significantly decreased. Finally, addition of Ca2+ to immature oocytes incubated in Ca2+-free medium caused an increase in the concentration of intracellular Ca2+ that was dependent upon the presence of extracellular Na+. This effect was not seen in mature oocytes. The data show that Na+-Ca2+ exchange occurs in immature and mature mouse oocytes and that Ca2+ homeostasis in immature oocytes is more sensitive to manipulations that activate Na+-Ca2+ exchange.