Digoxin Na/K ATPase — FRCA Primary MCQ

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ModeratePharmacology - CardiovascularDigoxin Na/K ATPaseFRCA Primary

Digoxin increases myocardial contractility by altering cardiomyocyte ion gradients and secondarily reducing calcium extrusion. Which membrane enzyme is directly inhibited by digoxin?

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Correct answer: D — Sarcolemmal Na+/K+-ATPase

Digoxin directly inhibits sarcolemmal Na+/K+-ATPase. Intracellular Na+ therefore rises, reducing the transmembrane Na+ gradient that drives Ca2+ extrusion through the Na+/Ca2+ exchanger. More Ca2+ remains available for sarcoplasmic-reticulum storage and subsequent release, increasing myocardial contractility. Low extracellular K+ enhances digoxin's effect and predisposes to toxicity. Sarcoplasmic-reticulum Ca2+-ATPase transports Ca2+ into the sarcoplasmic reticulum but is not digoxin's target. Phosphodiesterase III is inhibited by drugs such as milrinone. Guanylyl cyclase participates in nitric oxide signalling, while angiotensin-converting enzyme is inhibited by ACE inhibitors.

Reference: Electronic Medicines Compendium, Digoxin 0.25 mg Tablets, Summary of Product Characteristics, section 5.1 Pharmacodynamic properties, revised February 2026. https://www.medicines.org.uk/emc/product/5465/smpc