Magnesium, metabolism, biological functions and drugs
The radius of the Mg2+ ion is 0.65 Angström, quite similar to that of lithium which is 0.60 Angström. But since the magnesium ion is doubly charged, it has a polarizing effect much more important than lithium. The atomic mass of magnesium is 24.
The diet daily supplies 10 to 20 mmol of magnesium (approximately 250 to 500 mg) of which a third is absorbed from the intestine. One knows little about the mechanisms involved in the digestive absorption of magnesium. It is probably a secondary active transport because the absorption is saturable and inversely proportional to the amount ingested.
The daily intake considered as desirable for an adult is 300 to 400 mg. Whole wheat bread is rich in magnesium.
When calcium and magnesium are taken simultaneously in large doses, calcium could, according to certain studies, limit the digestive absorption of magnesium.
The body of an adult of 70 kg contains approximately 1 000 mmol of magnesium i e approximately 24 g . About 50% of magnesium is in bone in partly exchangeable form with the other compartments. The remaining magnesium is primarily intracellular.
In blood, most magnesium is in blood cells, particularly in erythrocytes where its average normal level is 50 mg/L. The normal plasma concentration (magnesemia) is approximately 20 mg/L, corresponding to 0.82 mmol/L. In plasma, two thirds of magnesium is in a free form and a third bound to plasma proteins.
The magnesium concentrations in the cells and the extracellular medium can vary independently, i.e hypomagnesemia is not necessarily accompanied by a decrease of intracellular magnesium. The intramyocardial free magnesium concentration is very high.
Magnesium is eliminated by the kidney. Being two thirds in free form, it is filtered by the glomerulus and 95% of the filtered magnesium are reabsorbed from the proximal tubule and from the ascending limb of Henle' s loop. There is no tubular magnesium secretion.
Many diuretics (thiazides, loop diuretics such as furosemide) increase the urinary elimination of magnesium, whereas amiloride, triamterene and aldosterone antagonists decrease it and are magnesium sparing.
Other drugs, aminoglycosides, platinum derivatives, cyclosporine, methotrexate, increase the urinary elimination of magnesium.
Disorders of magnesium metabolism
Hypomagnesemia, often combined with hypokalemia and hypocalcemia, is observed especially in patients with digestive disorders, in particular diarrhea, and with insufficient intake by oral or parenteral route. Hypomagnesemia is frequent in alcoholic patients in which it increases the risk of delirium tremens.
Hypomagnesemia is frequently observed in neonates where it causes various symptoms, nervousness, hyperreflexia, muscular hypertonicity and sometimes seizures, symptoms which regress after magnesium administration.
Hypomagnesemia predisposes to heart rate disorders and to coronary artery spasms.
A concomitant deficiency of potassium and magnesium and calcium is frequently observed. In this case, the preliminary correction of the magnesium deficiency is necessary to obtain a correction of the other deficiencies, perhaps because magnesium is necessary for the Na+/K+-ATPase pump to function and create cellular polarization.
Hypermagnesemia is observed in patients with renal impairment, generally combined with an increase of intake. When the magnesium concentration exceeds 50 mg/L in plasma, hyoactive tendon reflexes are observed and, at very high concentrations, a respiratory failure can appear.
The mechanisms responsible for the transfer of magnesium between cells and the extracellular medium are not well known.