Na+-Ca2+ exchange in mouse oocytes: modifications in the regulation of intracellular free Ca2+ during oocyte maturation.
Increases in intracellular free Ca(2+)+ concentration (Ca(2+)+ oscillations) occur during meiotic maturation and fertilization of mammalian oocytes but little is known about the mechanisms of Ca(2+) homeostasis in these cells. Cells extrude Ca(2+) from the cytosol using two main transport processes, the Ca(2+)-ATPase and the Na(+)-Ca(2+) exchanger. The aim of this study was to determine whether Na(+)-Ca(2+) exchange activity is present in immature and mature mouse oocytes. Na(+)-Ca(2+) exchange can be revealed by altering the Na(+) concentration gradient across the plasma membrane and recording intracellular free Ca(2+) concentrations using Ca(2+)-sensitive fluorescent dyes. Depletion of extracellular Na(+) caused an immediate increase in Ca(2+) concentration in immature oocytes and a delayed increase in mature oocytes. The Na(+) ionophore, monensin, caused an increase in intracellular Ca(2+) in immature oocytes similar to that induced by Na(+)-depleted medium. In mature oocytes, monensin had no effect on intracellular Ca(2+) but the time taken for Ca(2+) to reach a peak value on removal of extracellular Na(+) was significantly decreased. Finally, addition of Ca(2+) to immature oocytes incubated in Ca(2+)-free medium caused an increase in the concentration of intracellular Ca(2+) that was dependent upon the presence of extracellular Na(+). This effect was not seen in mature oocytes. The data show that Na(+)-Ca(2+) exchange occurs in immature and mature mouse oocytes and that Ca(2+) homeostasis in immature oocytes is more sensitive to manipulations that activate Na(+)-Ca(2+) exchange.
Carroll
J
J
Department of Physiology, University College London, Gower Street, London, WC1E 6BT, UK.
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D002478:Cells, Cultured; D004533:Egtazic Acid / Q000494:pharmacology; D005260:Female; D005456:Fluorescent Dyes; D006706:Homeostasis; D007424:Intracellular Fluid / Q000378:metabolism*; D007476:Ionophores / Q000494:pharmacology; D051379:Mice; D008815:Mice, Inbred Strains; D008985:Monensin / Q000494:pharmacology; D009865:Oocytes / Q000378:metabolism* / Q000502:physiology; D009866:Oogenesis / Q000502:physiology*; D010507:Periodicity; D012964:Sodium / Q000378:metabolism; D019831:Sodium-Calcium Exchanger / Q000378:metabolism*
D005456:Fluorescent Dyes; D007476:Ionophores; D019831:Sodium-Calcium Exchanger; D004533:Egtazic Acid; D008985:Monensin; D012964:Sodium; D000252:Calcium-Transporting ATPases; D002118:Calcium