Magnesium also affects calcium homeostasis through two mechanisms. 1This can lead to a hypokalaemia which only responds to magnesium replacement. 2 Loss of intracellular potassium also occurs in the renal tubules.

Magnesium is needed for the proper functioning of the Na +/K +-ATPase pump, so a deficiency causes an increase in intracellular sodium and allows potassium to leak out of cells. It is needed in energy metabolism, glucose utilisation, protein synthesis, fatty acid synthesis and breakdown, muscle contraction, all ATPase functions, for almost all hormonal reactions, and in the maintenance of cellular ionic balance. Magnesium is involved in over 300 enzymatic reactions. The reabsorption of magnesium and calcium parallel each other in this segment, but the hormonal regulation of magnesium homeostasis is incompletely understood.

Situations that abolish the positive luminal charge (for example loop diuretics, hypercalcaemia) will reduce the reabsorption of magnesium. 1, 4 The amount reabsorbed is inversely related to tubular flow. Most reabsorption occurs in the thick ascending limb of the loop of Henle where 65-75% of filtered magnesium is reabsorbed passively down an electrochemical gradient which is actively maintained. 1, 4 The amount reabsorbed can vary, however, from nearly zero to 99.5% depending on the individual's magnesium balance. To maintain homeostasis, the nephrons normally reabsorb more than 96% of the filtered magnesium. 3, 6 Ionised and complexed magnesium are freely filtered at the glomerulus (70% of circulating magnesium). There is a circadian excretory rhythm with more magnesium excretion occurring at night. The kidney's handling of magnesium is more complicated. Approximately 1 mmol is lost in gastrointestinal secretions daily.

4Absorption can range from 24% of ingested magnesium in magnesium replete states to 76% in deficient states. The main site of absorption is the small intestine with smaller amounts absorbed in the colon.

Homeostasis depends on the balance between intestinal absorption and renal excretion, with kidney tubules having primary control. Under normal conditions the body maintains constant circulating concentrations of magnesium in the blood.

Mag sulfate antidote