Key result
ATP largely binds intracellular ionized magnesium in muscle cells and dictates its accurate 31P-NMR estimation.
Highlights the importance of the equilibrium between free ATP, ionized magnesium, and Mg-ATP in the cytosol for accurately estimating intracellular ionized magnesium concentrations using 31P-NMR.
It is now generally accepted that the intracellular ionized magnesium concentration ([Mg2+]i) in muscle cells is around 1 mmol l−1; in heart muscle this means that from the total some 90‐95% is bound (see McGuigan et al. 1991a). Although binding will include sequestration by intracellular organelles, a large part of the binding is by ATP in the cytosol and an equilibrium exists in the cytosol between free ATP, ionized magnesium and Mg‐ATP. The extend of this equilibrium depends on the equilibrium constant of the reaction, which is a function of pH, temperature and ionic strength. This equilibrium constant is also important in the estimation of [Mg2+]i using 31P‐NMR. In this method the difference between the α and β peaks of ATP is measured and from this shift and the equilibrium constant between Mg2+ and ATP in the cytosol, the [Mg2+]i can be calculated (Vink, 1993).
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Lüthi et al. (1999) reported a review. The intracellular ionized magnesium concentration in muscle cells is largely bound by ATP, and the equilibrium constant of this reaction is critical for estimating [Mg2+]i using 31P-NMR.
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