Diuretics Part 2: Potassium-Sparing Diuretics, Carbonic Anhydrase Inhibitors, Osmotic Agents, and Vasopressin Antagonists

Spironolactone is a steroidal mineralocorticoid receptor antagonist that competitively blocks aldosterone at the collecting duct principal cell, reducing epithelial sodium channel and sodium-potassium-ATPase expression. The natriuretic effect is modest — the collecting duct handles only 2 to 3% of filtered sodium — but the potassium-sparing effect is clinically powerful. The RALES trial demonstrated a 30% reduction in mortality when spironolactone was added to standard heart failure therapy in patients with severe heart failure with reduced ejection fraction, establishing mineralocorticoid receptor antagonism as a pillar of heart failure management.

Spironolactone's limited selectivity generates endocrine adverse effects. Its active metabolite canrenone and the parent compound both bind androgen and progesterone receptors, causing gynecomastia and breast tenderness in men and menstrual irregularities in women. These effects are dose-dependent and represent the primary reason for switching to more selective agents in patients who cannot tolerate them.

Eplerenone is a steroidal mineralocorticoid receptor antagonist with approximately 40-fold lower affinity for androgen and progesterone receptors than spironolactone, substantially reducing gynecomastia and menstrual irregularities. The EPHESUS trial demonstrated mortality reduction with eplerenone in patients with heart failure with reduced ejection fraction complicating acute myocardial infarction, and it carries an indication for this post-infarction heart failure population.

Finerenone is a nonsteroidal mineralocorticoid receptor antagonist with greater receptor selectivity than either steroidal agent and a distinct tissue distribution profile that concentrates in cardiac and renal tissue. The FIDELIO-DKD and FIGARO-DKD trials demonstrated that finerenone reduced progression of chronic kidney disease and cardiovascular events in patients with type 2 diabetes and chronic kidney disease, establishing it as the first mineralocorticoid receptor antagonist with outcome data in diabetic kidney disease.

All three agents carry a risk of hyperkalemia proportional to the degree of aldosterone blockade. They are contraindicated when glomerular filtration rate falls below 30 milliliters per minute or when baseline potassium exceeds 5.0 milliequivalents per liter, and potassium monitoring is mandatory after initiation.

In the proximal convoluted tubule, carbonic anhydrase on both the luminal brush border and inside the tubular cell drives sodium-hydrogen exchanger-mediated sodium reabsorption by regenerating the hydrogen ions needed for exchange. Acetazolamide blocks this enzyme, causing sodium bicarbonate to remain in the tubular lumen and reach the urine. The resulting metabolic acidosis progressively reduces the filtered bicarbonate load, eventually removing the substrate for the diuretic effect — explaining why acetazolamide produces a self-limiting natriuresis rather than the sustained fluid loss of loop diuretics or thiazides.

At high altitude, hypoxia drives hyperventilation that lowers arterial carbon dioxide and raises arterial pH, producing a respiratory alkalosis that paradoxically blunts the hypoxic ventilatory response and worsens altitude sickness. Acetazolamide induces a compensatory metabolic acidosis that restores the ventilatory drive, allowing more effective acclimatization. It is used for prophylaxis and treatment of acute mountain sickness when initiated 24 to 48 hours before ascent.

In open-angle glaucoma, acetazolamide reduces aqueous humor production by inhibiting carbonic anhydrase in the ciliary body epithelium, lowering intraocular pressure through a mechanism independent of its renal diuretic effect. It is used when topical agents fail to achieve adequate intraocular pressure control. In patients with metabolic alkalosis from loop or thiazide diuretics, acetazolamide provides a targeted correction by promoting bicarbonate excretion. The ADVOR trial demonstrated that adding acetazolamide to standard loop diuretic therapy in acute decompensated heart failure accelerated decongestion and increased the rate of successful decongestion at 3 days compared with placebo.

Tolvaptan is a selective oral vasopressin type 2 receptor antagonist approved for euvolemic and hypervolemic hyponatremia in hospitalized patients when the serum sodium is below 125 milliequivalents per liter or below 130 milliequivalents per liter with symptoms. It produces a predictable rise in serum sodium by promoting free water excretion. The rate of sodium correction must be monitored carefully — overcorrection exceeding 10 to 12 milliequivalents per liter per 24 hours risks osmotic demyelination syndrome, a devastating and largely irreversible neurological complication.

Tolvaptan carries a black box warning for hepatotoxicity with use beyond 30 days, limiting its long-term use. It also carries an indication for autosomal dominant polycystic kidney disease to slow cyst growth and preserve kidney function, based on the TEMPO 3:4 trial data. Patients must have free access to water during tolvaptan therapy — fluid restriction is contraindicated because thirst-driven water consumption is the safety buffer against overcorrection.

Conivaptan blocks both vasopressin type 1a receptors on vascular smooth muscle and vasopressin type 2 receptors in the collecting duct. It is available only as an intravenous formulation for in-hospital use in euvolemic hyponatremia. The vasopressin type 1a blockade adds a vasodilatory effect that makes it less suitable for patients with hemodynamic instability. It is a potent inhibitor of cytochrome P450 3A4 and carries significant drug interaction risk in the inpatient setting.