Furosemide causes hypomagnesaemia in addition to hypokalaemia. The mechanism is:

• A Inhibition of NKCC2 in the thick ascending limb abolishes the lumen-positive voltage that drives paracellular Mg2+ reabsorption ✓
• B Inhibition of ROMK channels in the collecting duct reduces Mg2+ reabsorption
• C Furosemide chelates Mg2+ in tubular fluid, preventing its paracellular reabsorption
• D Secondary hyperaldosteronism from volume contraction increases Mg2+ secretion in the collecting duct

Explanation

In the thick ascending limb of Henle, NKCC2 co-transport generates a lumen-positive transepithelial voltage that drives paracellular reabsorption of divalent cations including Mg2+ and Ca2+. Furosemide inhibits NKCC2, abolishing this lumen-positive potential, thereby eliminating the electrochemical gradient for paracellular Mg2+ reabsorption. This is why loop diuretics cause hypomagnesaemia and hypercalciuria (unlike thiazides, which cause hypocalciuria).

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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