Intracellular free magnesium in excitable cells: its measurement and its biologic significance

As a necessary cofactor for hundreds of enzymes, intracellular Mg2+ influences a wide range of cellular functions such as transmembrane transport of other ions, glycolysis, respiration, muscle contraction, and phosphorylation of ion channels. Unlike Ca2+, Mg2+ does not seem to have a "trigger" function. However, the wide range of enzymes requiring Mg2+ to be activated suggests that Mg2+ plays a pivotal role in fine control and coordination of cell activity, determining the "set point" of hundreds of metabolic reactions. In this sense, intracellular Mg2+ might be regarded as a static rather than a dynamic regulator of cell function. Little is known about the mechanisms by which excitable and other cells keep their [Mg2+]i within narrow limits against large electrochemical gradients. Furthermore, the actual basal level of [Mg2+]i has been the subject of recent controversy. In the present paper the roles of intracellular Mg2+ on cell function as well as four current techniques for measuring [Mg2+]i are briefly reviewed. These techniques are (i) metallochromic indicators, (ii) 31P nuclear magnetic resonance, (iii) null point for plasma membrane permeabilization using the ionophore A23187 and, (iv) Mg2+-selective microelectrodes. The relative advantages and disadvantages of each of these techniques are discussed with special emphasis on Mg2+-selective microelectrodes.