Estimation of intracellular free magnesium using ion-selective microelectrodes: evidence for an Na/Mg exchange mechanism in skeletal muscle.

Mg-selective microelectrodes allow continuous direct recording of intracellular free magnesium concentration ([Mg]i). Mg-selective microelectrodes utilizing the neutral carrier ETH 1117 have been used to determine [Mg]i in a wide variety of tissues. However, this type of electrode suffers from considerable interference from other cations (primarily potassium and sodium when used for intracellular studies). The recently developed Mg sensor ETH 5214 reveals a much improved selectivity and sensitivity as compared to ETH 1117. For intracellular measurements of [Mg]i the interference from Na+, K+, Ca2+ and pH can be neglected. Measurements of resting [Mg]i in ferret ventricular muscle and frog skeletal muscle gave values of 0.85 and 0.93 mM, respectively. These values indicate that [Mg]i is not in a thermodynamic equilibrium and Mg2+ ions have to be extruded continuously from the cytoplasm against their electrochemical gradient. In skeletal muscle, measurements of [Mg]i and [Na]i under experimental conditions, which included changes of the membrane potential and the transmembrane gradients for Mg2+ and Na+, provided evidence that an Na/Mg exchange mechanism participates in maintaining [Mg]i at low cytoplasmic levels.