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

Abstract

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.