Magnesium, which is the second most abundant intracellular cation after potassium, has an essential role in the regulation of numerous cellular functions and enzymes, including ion channels, metabolic cycles and signaling pathways. Approximately 60% of magnesium ions (1215 mg/dL) are stored in tissues and approximately 40% of magnesium ions (972 mg/ dL) contribute to intermediary metabolism. Among these ions, approximately 70% (680 mg) exist in the free form, Mg2+, whereas the other 30% of ions (292 mg/dL) are bound to proteins (especially albumin), citrate, phosphate and other complexes. The magnesium serum levels are kept constant within very narrow limits (0.65- 1.05 mmol/dL; 1.58-2.25 mg/dL). Regulation occurs in the kidneys, especially via the ascending loop of Henle.