Potassium-sparing diuretics are one of those Step “small topics” that show up everywhere: hypertension add-ons, heart failure regimens, cirrhosis/ascites, and classic adverse-effect questions (hyperK, gynecomastia, metabolic acidosis). The trick is to sort them by where they act and what they block, then you can predict everything else.
The “Big Picture” in One Sentence
K-sparing diuretics act in the collecting duct to reduce Na reabsorption and reduce K and H secretion → mild diuresis + risk of hyperkalemia (and metabolic acidosis).
Step-by-Step Flowchart (Exam-Style)
Step 1 — You see K-sparing: think Collecting duct
- Site: Late distal tubule / cortical collecting duct
- Key cell: Principal cell (Na and K handling)
- Also affected: -intercalated cell (H secretion)
Step 2 — Decide: Aldosterone problem or ENaC problem?
A) Aldosterone receptor antagonists
If the stem hints at high aldosterone states (HF, cirrhosis, Conn syndrome, resistant HTN):
- Spironolactone
- Eplerenone
Mechanism one-liner:
Block aldosterone receptor → ↓ ENaC + ↓ Na/K-ATPase expression → ↓ Na reabsorption, ↓ K secretion, ↓ H secretion.
Classic uses (high-yield):
- HFrEF (mortality benefit; blocks aldosterone remodeling)
- Hyperaldosteronism (primary or secondary)
- Resistant HTN
- Cirrhosis/ascites
- PCOS / hirsutism (spironolactone is anti-androgen)
B) ENaC inhibitors
If the stem hints at lithium toxicity prevention or Liddle syndrome:
- Amiloride
- Triamterene
Mechanism one-liner:
Directly block ENaC in principal cells → ↓ Na entry → ↓ lumen-negative potential → ↓ K and H secretion.
Classic uses (high-yield):
- Liddle syndrome (gain-of-function ENaC → HTN, hypokalemia, metabolic alkalosis, low renin/aldo)
- Lithium-induced nephrogenic DI prevention (amiloride blocks Li entry via ENaC)
Super-Compact “If/Then” Decision Tool
| If the question emphasizes… | Pick this class | Drugs | Extra clue |
|---|---|---|---|
| Hyperaldosteronism, HF mortality benefit, anti-androgen effects | Aldosterone antagonists | Spironolactone, Eplerenone | Spiro → gynecomastia; Eplerenone more selective |
| Liddle syndrome or lithium nephrogenic DI | ENaC blockers | Amiloride, Triamterene | Amiloride is the lithium one |
Visual/Mnemonic Device (Shareable)
“A-TEAM vs E-LOCK” (Collecting duct edition)
- A = Aldosterone blockers: Aldo receptor antagonists
- Spironolactone, Eplerenone
- E = ENaC blockers: ENaC locked shut
- Amiloride, Triamterene
One-liner memory:
“A blocks Aldo, E blocks ENaC—both spare K.”
Why They Cause Hyperkalemia (Mechanism You Can Explain in 10 Seconds)
Normally:
- ENaC pulls Na from lumen into principal cell
- This makes the lumen relatively negative, encouraging K secretion into lumen (via ROMK)
With K-sparing diuretics:
- Less Na reabsorption via ENaC → less lumen negativity → less K secretion → hyperkalemia
Acid-Base: The Sneaky USMLE Detail
K-sparing diuretics → metabolic acidosis (specifically a normal anion gap / hyperchloremic metabolic acidosis).
Why?
They reduce H secretion by -intercalated cells (directly/indirectly via aldosterone effects and lumen potential).
Adverse Effects You Must Know
Shared (all K-sparing)
- Hyperkalemia (most tested)
- Metabolic acidosis (non–anion gap)
Spironolactone-specific (endocrine effects)
- Gynecomastia
- Decreased libido / impotence
- Menstrual irregularities
- Why: anti-androgen + progesterone receptor effects
Eplerenone
- Less gynecomastia (more selective for mineralocorticoid receptor)
Triamterene
- Can cause kidney stones (rare but classic board association)
- Think: Triamterene crystals → nephrolithiasis
High-Yield Drug Combos & Safety Pearls
The “danger combo” for hyperK
Be cautious when K-sparing diuretics are combined with:
- ACE inhibitors
- ARBs
- Direct renin inhibitors (aliskiren)
- NSAIDs (↓ renin via afferent constriction)
- K supplements
Exam vibe: patient with CKD + ACEi + spironolactone → new weakness/arrhythmia → hyperkalemia.
Rapid Fire USMLE-Style One-Liners
- Spironolactone: “HF med that blocks aldosterone and can cause gynecomastia + hyperK.”
- Eplerenone: “Like spiro but fewer endocrine side effects.”
- Amiloride: “ENaC blocker used for Liddle and lithium nephrogenic DI.”
- Triamterene: “ENaC blocker—think hyperK and rare kidney stones.”
15-Second Summary (What You Should Say Out Loud)
K-sparing diuretics act at the collecting duct. Spiro/eplerenone block aldosterone; amiloride/triamterene block ENaC. They cause hyperkalemia and non–anion gap metabolic acidosis; spironolactone causes gynecomastia; amiloride treats Liddle and prevents lithium nephrogenic DI; triamterene can cause stones.