Determination of [Mg2+]i -: an update on the use of Mg2+-selective electrodes

Since their invention, ion-selective microelectrodes have become an indispensable tool for investigations of intracellular ion regulation and transport. While highly selective sensors for all major intracellular monovalent ions have been available for decades, the development of sensors for divalent cations seems to have presented more difficulties. As ion-selective microelectrodes typically have time-constants in the range of 0.5 to several seconds they turned out to be inapt for the investigation of intracellular Ca2+. The development of sensors for Mg2+-selective electrodes has made its most striking progress only over the past few years. While the first Mg2+ sensor, ETH 1117, was barely able to detect physiological Mg2+ concentrations in the presence of other mono- and divalent cations, the newest sensors allow measurements in the micromolar range. When used in macroelectrodes, the most recent developments, ETH 5506 and ETH 5504, have even been reported to do so in the presence of millimolar Ca2+ concentrations. Although there is still room for improvement to make these sensors applicable in microelectrodes, some preliminary data look extremely promising and indicate that a new era for Mg2+-selective microelectrodes is about to start.