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.