Expansion
A cofactor for ATP-dependent enzymes and a natural calcium antagonist
Expansion
Magnesium is the second most abundant intracellular cation, with only about 1 per cent extracellular, so serum levels underestimate deficiency.
Roles: cofactor for all ATP-dependent enzymes, stabilisation of the sodium-potassium ATPase, natural calcium antagonist at membranes, and required for parathyroid hormone secretion and action.
Hypomagnesaemia
- Causes: diarrhoea, alcohol, diuretics, proton pump inhibitors, aminoglycosides, ciclosporin, refeeding, poor intake
- Effects: refractory hypokalaemia (magnesium deficiency opens renal potassium channels), hypocalcaemia (impaired parathyroid hormone release), tremor, tetany, seizures, and arrhythmia, notably torsades de pointes
Hypermagnesaemia
- Almost always iatrogenic or with renal failure
- Progressive loss of reflexes, then respiratory depression, then cardiac arrest, which is why deep tendon reflexes are monitored during magnesium infusion in eclampsia
Magnesium’s use in eclampsia and severe asthma reflects its membrane-stabilising and smooth muscle relaxing actions, and in torsades it acts regardless of the baseline level.