Potassium-Sparing Diuretics

These drugs act at the collecting duct, the final segment, where the hormone aldosterone normally fine-tunes the last of the sodium. Aldosterone tells the tubule’s principal cells to reabsorb sodium through the epithelial sodium channel (ENaC) — and because that sodium reabsorption makes the tubular lumen electrically negative, it pulls potassium out into the urine. This is exactly the step loops and thiazides rely on to dump potassium; it is also the step these drugs block, which is why they keep potassium in the body instead of wasting it. Only about 3% of filtered sodium is handled here, so they are weak diuretics — their value is potassium balance and specific disease effects, not raw fluid removal.

There are two mechanisms at the same segment. The aldosterone (mineralocorticoid-receptor) antagonists — spironolactone and eplerenone — competitively block the aldosterone receptor, removing the signal that drives sodium-for-potassium exchange. The ENaC blockers — amiloride and triamterene — simply plug the sodium channel directly. Either way, sodium stays in the tubule (mild diuresis) and the lumen loses its negative pull, so potassium is retained. They are frequently paired with a loop or thiazide precisely to offset that drug’s potassium loss (e.g., triamterene/HCTZ).

If loops and thiazides risk hypokalemia, potassium-sparing diuretics risk the opposite — hyperkalemia — and it is the defining danger of the class, because a high potassium can cause fatal cardiac arrhythmias. The risk climbs sharply in exactly the situations these drugs are often used: alongside an ACE inhibitor or ARB (which also raise potassium), with potassium supplements or salt substitutes (salt substitutes are potassium chloride), with NSAIDs, and in renal impairment, diabetes, and the elderly. This drives the nursing approach: monitor the potassium and renal function, and do not stack potassium sources.

The practical rules follow directly. Teach patients to avoid potassium supplements and salt substitutes, and be cautious combining these drugs with an ACEi/ARB. Teach the warning signs of hyperkalemia — muscle weakness or cramps, palpitations or an irregular heartbeat, numbness/tingling — and to report them. And set expectations that, especially for spironolactone, the effect builds over days rather than producing the instant diuresis of a loop. Of the group, amiloride’s and triamterene’s labels carry a boxed warning for hyperkalemia — a reminder of how seriously this risk is taken.

A key point about this class is that its biggest benefits have little to do with urine output. Blocking aldosterone does more than spare potassium — aldosterone also drives harmful cardiac and vascular remodeling in heart failure, so the mineralocorticoid antagonists reduce mortality in heart failure with reduced ejection fraction: spironolactone in the landmark RALES trial (~30% relative mortality reduction) and eplerenone after MI in EPHESUS (~15%). They are also key add-ons for resistant hypertension and primary hyperaldosteronism, and spironolactone is first-line for the ascites of cirrhosis (which is driven by high aldosterone).

The catch is spironolactone’s endocrine side effects. Because it isn’t perfectly selective, it also blocks androgen and progesterone receptors, causing gynecomastia and breast tenderness in men and menstrual irregularities in women (plus reduced libido). The elegant fix is eplerenone, which is far more selective for the aldosterone receptor and largely spares these effects — so a patient troubled by gynecomastia on spironolactone is often switched to eplerenone. (A historical note: spironolactone once carried a boxed warning about tumors from very-high-dose rat studies; that boxed warning has since been removed from the current label.) As with the other two classes, the Cardiovascular section’s Diuretics class frames these agents around heart failure — here the emphasis is the nephron and the potassium.