Magnesium Distribution

TOTAL BODY MAGNESIUM (MG) DISTRIBUTION

Location

Percent of Total

Mg Content, mmol*

Mg Content, mg

Bone

53

530

12720

Muscle

27

270

6480

Soft tissue

19.2

192

4608

Erythrocyte

0.5

5

120

Serum

0.3

3

72

Total

1000

Total distribution of magnesium (Mg) in the body. Mg (molecular weight, 24.305 D) is predominantly distributed in bone, muscle, and soft tissue. Total body Mg content is about 24 g (1 mol) per 70 kg. Mg in bone is adsorbed to the surface of hydroxy-apatite crystals, and only about one third is readily available as an exchangeable pool. Only about 1% of the total body Mg is in the serum and interstitial fluid [1,2,8,9,11,12].

*data typical for a 70 kg adult

Intracellular magnesium (Mg)

Intracellular magnesium (Mg)

FIGURE 4-2

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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