Potassium-Sparing Diuretics

Introduction

• Definition — Potassium-sparing diuretics act on the late distal tubule and collecting duct to produce a mild natriuresis while conserving K⁺ and H⁺ — the opposite of the kaliuresis caused by all more proximally acting diuretics.
• Low-efficacy (weak) agents — Maximal fractional Na⁺ excretion is only ~2–3% of the filtered load, because >90% of filtered Na⁺ is reabsorbed upstream of their site of action.
• Two mechanistic sub-classes — Pooled under one label because they share a net urinary signature (↑Na⁺, ↓K⁺, ↑Cl^-, slight ↑HCO₃^-, urine mildly alkalinised): (a) MR (aldosterone) antagonists — spironolactone, eplerenone, finerenone; (b) ENaC blockers — amiloride, triamterene.
• Why they matter — Rarely used alone; principal roles are to offset the K⁺/Mg²⁺ loss of loop/thiazide diuretics, add a small natriuretic/antihypertensive increment, and (MR antagonists) deliver disease-modifying benefit in heart failure, primary aldosteronism, cirrhotic ascites and diabetic CKD.