ADVANTAGES AND PITFALLS WHEN USING MG2-MACROELECTRODES TO MEASURE THEFREE MG2+ CONCENTRATION IN CELL OR MEMBRANE VESICLE SUSPENSIONS, OR IN MG2+-LIGAND BINDING-STUDIES()

The introduction of Mg2+-selective macroelectrodes capable of accurate calibration down to 1 mu mol/l in either intracellular or extracellul ar-like physiological Mg2+-buffer solutions makes continuous on-line m easurements of the ionized Mg2+ concentration ([Mg2+](f)) in small vol umes (0.5 ml) possible. These macroelectrodes can be used to investiga te Mg2+ transport in suspensions of isolated calls or membrane sarcole mmal vesicles, and to study binding to Mg2+ Ligands. However, such mac roelectrodes have their pitfalls. Investigation of Mg2+ transport invo lving the addition of Na+, K+ to the ionic milieu, or changing extrace llular pH could cause an increase in interference with the macroelectr ode. Such interference would produce a change in the measured potentia l and an apparent increase in the [Mg2+](f). In the investigation of M g2+ ligand binding, the macroelectrodes can react to either Ca2+ and M g2+ contamination in the Ligands or to the ligand itself. In this pape r the conditions under which Mg2+-selective macroelectrodes can be use d are defined, their limitations described and methods developed to al low these limitations to be characterised. if the guide lines are adhe red to, then the electrodes can be successfully applied to the problem s of Mg2+-transport in small volumes of either cell or vesicle suspens ions. Since they directly measure the [Mg2+](f) they can also be used in Mg2+ ligand binding studies.