In situ measurements of free cytosolic magnesium ions

A nondestructive technique is described for measuring intracellular free Mg2+ concentrations. It is based on the microinjection of a Mg2+ metallochromic indicator in the cytosol of a large single cell and measuring its differential absorbance changes with sensitive multiple wavelength pulsed microspectroscopy. The most reliable measurements of free intracellular Mg2+ were obtained in two different kinds of cells by a null point method using Eriochrome blue as an indicator of free Mg2+ and intracellular dialysis with solutions of varying Mg2+ concentrations. By using this technique free Mg2+ was found to be between 3 and 3.5 mM in freshly dissected axons from Loligo pealei, the total Mg2+ measured by atomic being 8.9 mM. The total magnesium in freshly isolated fibers of Balanus aquila barnacle muscle was 10.5 mmol/kg wet wt (14 mM). The null point method indicated a free Mg2+ concentration of 6 mM, or 4.2 mmol/kg wet wt. A tentative distribution of magnesium among various intracellular constituents, based on present data and published work, yields (mmol/kg wet wt): free, 4.2; MgATP, 4.2; myofibrillar-bound, 1; residual (bound probably to phosphoarginine), 1.