Quantification of Mg2+ extrusion and cytosolic Mg2+-buffering in Xenopus oocytes

Intracellular Mg2+ buffering and Mg2+ extrusion were investigated in Xenopus laevis oocytes. Mg2+ or EDTA were pressure injected and the resulting changes in the intracellular Mg2+ concentration were measured simultaneously with Mg2+-selective microelectrodes. In the presence of extracellular Na+, injected Mg2+ was extruded from the oocytes with an estimated vmax and KM of 74 pmol cm−2 s−1 and 1.28 mM, respectively. To investigate genuine cytosolic Mg2+ buffering, measurements were carried out in the nominal absence of extracellular Na+ to block Mg2+ extrusion, and during the application of CCCP (inhibiting mitochondrial uptake). Under these conditions, Mg2+ buffering calculated after both MgCl2 and EDTA injections could be described by a buffer equivalent with a concentration of 9.8 mM and an apparent dissociation constant, Kd-app, of 0.6 mM together with an [ATP]i of 0.9 mM with a Kd-app 0.12 mM. Xenopus oocytes thus possess highly efficient mechanisms to maintain their intracellular Mg2+ concentration.