Chapter 21 - Magnesium Homeostasis

Abstract

Magnesium (Mg) is the fourth most abundant cation and the second most abundant intracellular cation in vertebrates. It is critical for a great number of cellular functions, including oxidative phosphorylation, glycolysis, DNA transcription, and protein synthesis. One-third of skeletal Mg is surface limited and exchangeable, and this fraction may serve as a reservoir for maintaining a normal extracellular Mg concentration. The remainder of Mg in bone is an integral component of the hydroxyapatite lattice, which may be released during bone resorption. Intestinal Mg absorption is proportional to the amount ingested. The mechanism(s) for intestinal Mg absorption is unclear but includes passive diffusion, solvent drag, and active transport. Mg is absorbed along the entire intestinal tract, including the large and small bowel, but the sites of maximal Mg absorption appear to be the ilium and distal jejunum. The kidney is the principal organ involved in Mg homeostasis, and during Mg deprivation in normal subjects, the kidney conserves Mg avidly and less than 1–2 mEq is excreted in the urine per day. Despite the close regulation of Mg by the kidney, there has been no hormone or factor described that is responsible for renal Mg homeostasis. Within the cell, Mg is compartmentalized and most of it is bound to proteins and negatively charged molecules such as ATE ADP, RNA, and DNA, and in the cytoplasm, about 80% of Mg is complexed with ATP. Significant amounts of Mg are also found in the nucleus, mitochondria, and endoplasmic and sarcoplasmic reticulum.