Renal Pharmacology · Module 2 of 5

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

Four specialized classes targeting distinct physiological problems at the collecting duct, proximal tubule, and vasopressin receptor

Mineralocorticoid receptor antagonists block aldosterone at collecting duct principal cells → ↓ ENaC and Na⁺/K⁺-ATPase expression → potassium-sparing natriuresis. Spironolactone: steroidal; limited MR selectivity → binds androgen and progesterone receptors → gynecomastia/breast tenderness (men) and menstrual irregularities (women), dose-dependent; RALES trial: 30% mortality reduction in severe HFrEF. Eplerenone: steroidal; ~40× more MR-selective → fewer endocrine AEs; EPHESUS trial: mortality benefit in post-MI HFrEF. Finerenone: nonsteroidal, greatest MR selectivity; cardiac/renal tissue distribution; FIDELIO-DKD + FIGARO-DKD: reduced CKD progression and CV events in type 2 diabetes + CKD. All three: hyperkalemia risk; contraindicated when GFR <30 mL/min or K⁺ >5.0 mEq/L; mandatory K⁺ monitoring after initiation.
ENaC blockers (amiloride, triamterene): block the epithelial sodium channel directly at the luminal surface of collecting duct and connecting tubule — aldosterone-independent potassium sparing. Amiloride: reliable oral bioavailability; renal elimination; key niche: lithium-induced nephrogenic diabetes insipidus (lithium enters collecting duct cells via ENaC → accumulates → impairs AQP2 insertion; amiloride blocks ENaC entry → attenuates DI without lithium discontinuation; preferred over thiazides in lithium patients because it does not promote proximal Li⁺ reabsorption); commonly combined with thiazides in fixed-dose formulations for potassium offset. Triamterene: prodrug requiring hepatic activation; hepatic impairment reduces efficacy; nephrotoxic metabolites; triamterene nephrolithiasis (crystallizes in tubule — distinguishing toxicity from amiloride); both agents: hyperkalemia amplified by ACEi/ARBs/NSAIDs.
Acetazolamide: carbonic anhydrase inhibitor; blocks luminal brush border and intracellular CA in the proximal convoluted tubule → impairs NHE3-mediated Na⁺ reabsorption by preventing H⁺ regeneration → NaHCO₃ remains in lumen → bicarbonate diuresis + metabolic acidosis. Self-limiting: the resulting metabolic acidosis reduces filtered bicarbonate load → removes the substrate for the diuretic effect. Clinical uses: altitude sickness prophylaxis/treatment (induces compensatory metabolic acidosis → restores ventilatory drive blunted by hypoxia-driven respiratory alkalosis; start 24–48 h before ascent); glaucoma (inhibits CA in ciliary body epithelium → ↓ aqueous humor production → ↓ intraocular pressure; when topical agents fail); correct diuretic-induced contraction alkalosis; ADVOR trial: acetazolamide added to IV furosemide in acute decompensated HF → accelerated decongestion vs placebo.
Mannitol: six-carbon sugar alcohol; freely filtered, not reabsorbed or secreted; osmotic water retention in lumen → diuresis without transporter target. Biphasic vascular effect: initial IV dose draws water osmotically from intracellular/interstitial → plasma expansion (hazardous in HF/pulmonary edema before diuresis begins); then filtered mannitol in lumen → water retained → diuresis. Elevated ICP: osmotic gradient draws water from cerebral interstitium across intact blood-brain barrier → reduces cerebral edema within 15–30 min. Contraindications: heart failure/pulmonary edema (initial volume expansion → decompensation); anuric renal failure (drug accumulates, cannot be excreted → worsening hyperosmolarity); disrupted blood-brain barrier (mannitol crosses into brain tissue → reverses osmotic gradient → worsens cerebral edema).
Vasopressin antagonists (vaptans): block V2 receptor in collecting duct principal cells → prevents AQP2 insertion → electrolyte-free water excretion (aquaresis — no sodium loss; distinguishes from all other diuretics). Tolvaptan: oral selective V2 antagonist; euvolemic and hypervolemic hyponatremia when Na⁺ <125 mEq/L or <130 with symptoms; also ADPKD (TEMPO 3:4 trial: slows cyst growth); black box warning for hepatotoxicity beyond 30 days. Conivaptan: IV only; blocks V1a (vascular smooth muscle) + V2 → vasodilation (less suitable for hemodynamic instability); potent CYP3A4 inhibitor; in-hospital euvolemic hyponatremia. Overcorrection risk: max 10–12 mEq/L per 24 h; osmotic demyelination syndrome is irreversible; highest risk: Na⁺ <120 mEq/L for >48 h, malnutrition, alcoholism, liver disease; free water access required throughout (fluid restriction contraindicated).

ACEi = angiotensin-converting enzyme inhibitor · ADPKD = autosomal dominant polycystic kidney disease · AQP2 = aquaporin-2 · ARB = angiotensin receptor blocker · CA = carbonic anhydrase · CKD = chronic kidney disease · DI = diabetes insipidus · ENaC = epithelial sodium channel · GFR = glomerular filtration rate · HFrEF = heart failure with reduced ejection fraction · ICP = intracranial pressure · MR = mineralocorticoid receptor · NHE3 = sodium-hydrogen exchanger isoform 3 · ODS = osmotic demyelination syndrome · V2 = vasopressin type 2 receptor

Four Classes — Targets and Primary Uses

MR Antagonists

Spironolactone / Eplerenone / Finerenone

Block aldosterone at collecting duct principal cells → ↓ ENaC + Na⁺/K⁺-ATPase
Potassium-sparing natriuresis
Heart failure (RALES, EPHESUS), diabetic CKD (FIDELIO/FIGARO)
Hyperkalemia — monitor K⁺; avoid if GFR <30 or K⁺ >5.0

ENaC Blockers

Amiloride / Triamterene

Block ENaC directly — aldosterone-independent K⁺ sparing
Amiloride: lithium-induced nephrogenic DI (blocks Li⁺ entry via ENaC)
Triamterene: nephrolithiasis risk (crystallizes in tubule)
Both: hyperkalemia amplified by ACEi/ARB/NSAIDs

CA Inhibitor

Acetazolamide

Block PCT carbonic anhydrase → NaHCO₃ wasting → metabolic acidosis
Self-limiting diuresis (acidosis removes substrate)
Altitude sickness prophylaxis; glaucoma; correct contraction alkalosis
ADVOR trial: accelerates decongestion in HF

Osmotic / Vaptan

Mannitol / Tolvaptan

Mannitol: osmotic water retention in lumen → ↓ ICP; acute glaucoma
Mannitol CI: HF, anuria, disrupted BBB
Tolvaptan: V2 block → aquaresis (water only, no Na⁺ loss)
Monitor Na⁺ overcorrection risk (ODS)

Mineralocorticoid Receptor Antagonists — Selectivity and Indications

	Spironolactone	Eplerenone	Finerenone
Scaffold	Steroidal	Steroidal	Nonsteroidal — distinct binding mode
MR selectivity	Low — binds androgen + progesterone receptors	High — ~40× more MR-selective than spironolactone	Highest — cardiac and renal tissue concentration
Gynecomastia	Common — dose-dependent; primary reason to switch	Rare	Rare
Key trial / indication	RALES: 30% mortality reduction in severe HFrEF; resistant hypertension; cirrhosis with ascites	EPHESUS: mortality reduction in post-MI HFrEF	FIDELIO-DKD + FIGARO-DKD: reduced CKD progression + CV events in type 2 diabetes

ENaC Blockers and Carbonic Anhydrase Inhibitor

ENaC Blocker

Amiloride

Blocks ENaC at luminal surface of collecting duct and connecting tubule — independent of aldosterone levels
Lithium-induced nephrogenic DI: Li⁺ enters principal cells via ENaC → impairs AQP2 → amiloride blocks Li⁺ entry → attenuates DI without lithium discontinuation
Preferred K-sparing diuretic in lithium patients — unlike thiazides, does not increase proximal Li⁺ reabsorption
Commonly co-formulated with thiazides to offset hypokalemia
Hyperkalemia — amplified by ACEi, ARBs, NSAIDs

ENaC Blocker — Prodrug

Triamterene

Same mechanism as amiloride but prodrug requiring hepatic activation
Hepatic impairment reduces efficacy; nephrotoxic metabolites at high concentrations
Triamterene nephrolithiasis — crystallizes in renal tubule; distinguishing toxicity not seen with amiloride
Hyperkalemia — amplified by ACEi, ARBs, NSAIDs

CA Inhibitor

Acetazolamide

Blocks CA in PCT → impairs NHE3-driven Na⁺ reabsorption (no H⁺ to exchange) → NaHCO₃ in urine → metabolic acidosis
Self-limiting: metabolic acidosis ↓ filtered HCO₃⁻ → removes substrate → diuresis stops
Altitude sickness: metabolic acidosis restores ventilatory drive; start 24–48 h before ascent
Glaucoma: ↓ aqueous humor via ciliary body CA inhibition
Contraction alkalosis: promotes HCO₃⁻ excretion; ADVOR trial: accelerates decongestion in HF

Osmotic Diuretics and Vasopressin Antagonists

Osmotic Diuretic

Mannitol

Freely filtered at glomerulus; not reabsorbed or secreted → osmotic water retention in lumen → diuresis without transporter target
Biphasic: initial IV dose draws water from intracellular/interstitial into plasma → volume expansion (hazardous in HF before diuresis begins)
Primary use: elevated ICP from cerebral edema (intact BBB) → osmotic gradient draws water from brain → effect within 15–30 min
Contraindicated: heart failure/pulmonary edema (volume expansion → decompensation before diuresis)
Contraindicated: anuric renal failure (accumulates → worsening hyperosmolarity)
Contraindicated: disrupted BBB (mannitol crosses → reverses osmotic gradient → worsens cerebral edema)

Vasopressin Antagonists

Tolvaptan / Conivaptan

Block V2 receptor → prevent AQP2 insertion → electrolyte-free water excretion (aquaresis) — no sodium loss; unique among diuretics
Tolvaptan: oral; selective V2; euvolemic + hypervolemic hyponatremia; ADPKD (TEMPO 3:4)
Tolvaptan: hepatotoxicity black box warning for use >30 days
Conivaptan: IV only; blocks V1a + V2 → vasodilation (avoid in hemodynamic instability); potent CYP3A4 inhibitor; euvolemic hyponatremia in-hospital
Both: free water access required (fluid restriction contraindicated — thirst is the safety buffer against overcorrection)
Both: max Na⁺ correction 10–12 mEq/L per 24 h — ODS risk

Sequential Nephron Blockade and Combination Strategies

Loop + Metolazone

Most Potent Combination

Loop blocks NKCC2 → ↑ Na delivery to DCT → compensatory NCC hypertrophy over time
Metolazone blocks the compensatory NCC → prevents escape → dramatic additive natriuresis
Profound hypokalemia and hypomagnesemia can develop rapidly — monitor electrolytes within 24–48 h

Loop + MR Antagonist

K-Neutral HF Combination

Loop maximizes Na delivery out of TAL; MRA blocks aldosterone-driven Na recapture in collecting duct
Potassium-neutral: loop-induced hypokalemia offset by MRA potassium sparing
Physiologically rational in HFrEF — addresses both volume and neurohumoral activation

Loop + Acetazolamide

Correct Contraction Alkalosis

Sustained loop/thiazide therapy → metabolic alkalosis (secondary aldosteronism + hypokalemia + NHE3 upregulation)
Alkalosis ↑ serum HCO₃⁻ → impairs respiratory drive; reduces ionized calcium fraction
Acetazolamide promotes HCO₃⁻ excretion → corrects alkalosis + restores loop diuretic responsiveness
ADVOR trial: acetazolamide + IV furosemide → faster, more complete decongestion in acute HF

Vaptan Sodium Overcorrection — Osmotic Demyelination Syndrome

Serum sodium must not rise more than 10–12 mEq/L per 24 hours or 18 mEq/L per 48 hours during vaptan therapy. Osmotic demyelination syndrome is largely irreversible, causing permanent neurological disability. Highest-risk patients: serum sodium below 120 mEq/L for more than 48 hours, malnutrition, alcoholism, or liver disease. Check serum sodium frequently after initiation. Maintain free water access throughout therapy — thirst-driven consumption is the physiological buffer against overcorrection. If correction is proceeding too rapidly, administer free water orally or intravenous dextrose to slow the rate.

Suggested References

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