SN5 SHORT NOTE 5M ★ High-yield

Describe the functions and disorders of magnesium.

Biochemistry · 1st Professional — Pre-Clinical

Definition

Magnesium = the second most abundant intracellular cation after potassium, and an essential cofactor for over 300 enzymes.

Parameter	Value
Total body	25 g; 60% in bone, 39% intracellular, 1% extracellular
Serum	1.8–2.4 mg/dL (0.75–1.0 mmol/L)
Requirement	350–400 mg/day
Sources	Green leafy vegetables (chlorophyll), nuts, whole grains, pulses

Serum magnesium reflects body stores poorly, since only 1% is extracellular — deficiency may exist with a normal level

Functions

Cofactor for all reactions involving ATP — the true substrate of kinases is the Mg-ATP complex
Over 300 enzymes — hexokinase, phosphofructokinase, adenylate cyclase, Na+/K+-ATPase, DNA and RNA polymerase
Neuromuscular excitability — it is a physiological calcium antagonist, blocking calcium channels and stabilising membranes
Bone structure
Required for PTH secretion and for PTH action on target tissues
Ribosome and nucleic acid stability; muscle relaxation

Hypomagnesaemia

Causes	Features
Chronic alcoholism — the commonest	Tetany, tremor, weakness
Malabsorption, chronic diarrhoea	Trousseau and Chvostek signs
Diuretics (loop and thiazide), aminoglycosides, cisplatin	Arrhythmias — torsades de pointes, prolonged QT
Diabetic ketoacidosis, refeeding syndrome	Seizures, confusion, apathy
Prolonged parenteral nutrition; hyperaldosteronism	Refractory hypocalcaemia and hypokalaemia

CLINICAL PEARL

The clinically vital point: hypomagnesaemia causes hypocalcaemia and hypokalaemia that will not correct until the magnesium is replaced. It impairs both PTH secretion and PTH action, and promotes renal potassium wasting. Always check magnesium in refractory electrolyte disturbance.

Hypermagnesaemia

Almost always from renal failure, or from magnesium-containing antacids and purgatives, or treatment of eclampsia
Features are progressive with level — loss of deep tendon reflexes (earliest sign), then hypotension, respiratory depression, bradycardia and cardiac arrest
Treated with intravenous calcium gluconate, which antagonises it directly

Therapeutic Uses of Magnesium

Indication	Rationale
Eclampsia and pre-eclampsia	Magnesium sulphate is the drug of choice for preventing and treating seizures
Torsades de pointes	Stabilises the myocardium
Severe asthma	Bronchodilatation through calcium antagonism
Constipation	Magnesium salts as osmotic laxatives
Antacid	Magnesium hydroxide

Applied Aspects

Deep tendon reflexes are monitored during magnesium therapy in eclampsia — their loss is the earliest warning of toxicity, and calcium gluconate must be kept at the bedside
Suspect magnesium deficiency in any alcoholic, and in refractory hypocalcaemia or hypokalaemia

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