Intracellular distribution of magnesium (Mg). Only 1% to 3% of the total intracellular Mg exists as the free ionized form of Mg, which has a closely regulated concentration of 0.5 to 1.0 mmol. Total cellular Mg concentration can vary from 5 to 20 mmol, depending on the type of tissue studied, with the highest Mg concentrations being found in skeletal and cardiac muscle cells. Our understanding of the concentration and distribution of intracellular Mg has been facilitated by the development of electron microprobe analysis techniques and fluorescent dyes using microfluorescence spectrometry. Intracellular Mg is predominantly complexed to organic molecules (eg, adenosine triphosphatase [ATPase], cell and nuclear membrane-associated proteins, DNA and RNA, enzymes, proteins, and citrates) or sequestered within subcellular organelles (mitochondria and endoplasmic reticulum). A heterogeneous distribution of Mg occurs within cells, with the highest concentrations being found in the perinuclear areas, which is the predominant site of endoplasmic reticulum. The concentration of intracellular free ionized Mg is tightly regulated by intracellular sequestration and complexation. Very little change occurs in the concentration of intracellular free Mg, even with large variations in the concentrations of total intracellular or extracellular Mg [1,3,11]. ADP— adenosine diphosphate; ATP—adenosine triphosphate; Ca+—ion-ized calcium.

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