Potassium-sparing diuretics show up everywhere on USMLE—CHF regimens, resistant HTN, and the classic “patient on a loop develops hypokalemia.” The trick is not just knowing the names, but instantly recalling where they work, what they do to labs, and the side effects that love to get tested. Here’s a quick memory-palace you can “walk through” in seconds.
The 10-second big picture (what you must know)
K-sparing diuretics act in the collecting tubule → they decrease Na reabsorption and decrease K and H secretion.
One-liner:
They prevent potassium loss by blocking aldosterone signaling or ENaC in the collecting duct → risk: hyperkalemia + metabolic acidosis.
Memory Palace: “The Collecting Duct VIP Lounge”
Picture the Collecting Duct as a velvet-rope VIP club. Two ways to shut down the club’s Na-in / K-out party:
Room 1: “ALDO’s Office” (Aldosterone antagonists)
You walk into an office with ALDO (aldosterone) signing contracts.
- Drugs on the desk: Spironolactone, Eplerenone
- What they do: Block aldosterone receptor → ↓ ENaC and ↓ Na/K-ATPase expression
- Mnemonic image: ALDO’s contract gets “SPIRALed” and “EPLoded” (spironolactone/eplerenone) so the Na-in/K-out plan never happens.
USMLE hook: great for conditions with high aldosterone effect (CHF, hyperaldosteronism).
Room 2: “The ENaC Door” (ENaC blockers)
Next to the club entrance is the ENaC door—the bouncer only lets Na in through this channel.
- Drugs guarding the door: Amiloride, Triamterene
- What they do: Block ENaC directly → ↓ Na entry into principal cells → less lumen-negative potential → ↓ K secretion
- Mnemonic image: “A-MI-Lock” (amiloride) and a “TRI-Arm Turn” (triamterene) physically block the ENaC doorway.
USMLE hook: used to counteract thiazide/loop hypokalemia; amiloride is a favorite in lithium-related questions.
Quick table: Know these cold
| Class | Drugs | Site | Mechanism | Net effect on K | Acid–base | Classic exam side effects / uses |
|---|---|---|---|---|---|---|
| Aldosterone antagonists | Spironolactone, Eplerenone | Collecting tubule (principal cells) | Block aldosterone receptor → ↓ ENaC & Na/K-ATPase expression | ↑ K (hyperkalemia risk) | Metabolic acidosis | Spironolactone: antiandrogen effects (gynecomastia, ↓ libido, impotence) • Eplerenone: fewer endocrine effects • Uses: HFrEF, hyperaldosteronism, resistant HTN |
| ENaC blockers | Amiloride, Triamterene | Collecting tubule (principal cells) | Directly block ENaC | ↑ K (hyperkalemia risk) | Metabolic acidosis | Uses: with loops/thiazides to prevent hypokalemia • Amiloride: Liddle syndrome, lithium-induced nephrogenic DI |
High-yield physiology (why K goes up)
In principal cells, Na reabsorption via ENaC makes the lumen relatively negative, which “pulls” K out into urine via ROMK.
Block ENaC (or block aldosterone → less ENaC) → less lumen negativity → less K secretion → hyperkalemia.
Also: decreased H secretion in intercalated cells → non–anion gap metabolic acidosis.
Rapid-fire USMLE associations (the ones that get tested)
- Hyperkalemia risk is the headline: especially dangerous when combined with ACE inhibitors/ARBs, CKD, or high baseline K.
- Spironolactone endocrine effects (because it hits steroid receptors):
- Gynecomastia, decreased libido, impotence, menstrual irregularities
- Eplerenone is more selective → fewer hormonal side effects.
- Liddle syndrome (gain-of-function ENaC) → treat with amiloride or triamterene (not spironolactone because aldosterone is already low).
- Lithium-induced nephrogenic diabetes insipidus → amiloride helps by reducing lithium entry through ENaC in collecting duct cells.
- CHF/HFrEF: spironolactone/eplerenone improve outcomes via anti-remodeling effects from aldosterone blockade.
Shareable micro-mnemonic (one-liner)
“In the Collecting Duct club: either fire ALDO (spirono/eplerenone) or lock ENaC (amiloride/triamterene) → keep K in, acid stays in (NAGMA).”