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Renal Pharmacology: Diuretics

With Focus on Loop and Thiazide Agents

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1. Overview & Nephron Targets

Urinary electrolyte changes: Loop agents produce ++++NaCl, +K⁺, metabolic alkalosis ↑; Thiazides produce ++NaCl, +K⁺, metabolic alkalosis ↑ — Katzung, Table 15-2

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2. LOOP DIURETICS

Chemistry

• Furosemide — sulfonamide derivative; most widely used
• Bumetanide — sulfonamide; ~40× more potent than furosemide by weight
• Torsemide — sulfonamide; longest duration
• Ethacrynic acid — phenoxyacetic acid derivative (NOT a sulfonamide); the methylene group forms a cysteine adduct which is the active form

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Mechanism of Action

1. Massive natriuresis — the TAL normally reabsorbs 25–30% of filtered NaCl; this is the largest single-segment reabsorptive capacity in the nephron, so its blockade yields the greatest diuresis of any drug class. — Lippincott Pharmacology, p. 337
2. Abolition of the medullary concentrating gradient — TAL is impermeable to water but reabsorbs solute, creating the hyperosmotic medullary interstitium essential for water reabsorption from the collecting duct. Loop diuretics destroy this gradient → dilute urine even in states of ADH excess.
3. Inhibition of tubuloglomerular feedback — Increased NaCl delivery to the macula densa would normally reduce GFR via TGF. Because TGF is mediated by NKCC2 itself (NKCC1 at macula densa), loop diuretics prevent this shutdown, maintaining stable GFR. This contrasts with carbonic anhydrase inhibitors and thiazides, which are limited by TGF-mediated GFR reduction. — Goodman & Gilman, p. 675
4. Hypercalciuria — Normally, the positive lumen potential in the TAL (generated by K⁺ recycling via ROMK into the lumen) drives paracellular reabsorption of Ca²⁺ and Mg²⁺. Loop diuretics abolish this lumen-positive potential → increased urinary Ca²⁺ and Mg²⁺.
5. Secondary K⁺ wasting — Increased Na⁺ delivery to the distal tubule and collecting duct stimulates ENaC-mediated Na⁺ reabsorption in exchange for K⁺ secretion, causing hypokalemia. — Goodman & Gilman, p. 675

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Pharmacokinetics

• All loop diuretics are secreted into the tubular lumen by OAT1/OAT3 in the proximal tubule. They act from the luminal side. Therefore, drugs competing for proximal secretion (e.g., probenecid, NSAIDs) reduce their efficacy. — Katzung, p. 407
• In hypoalbuminemia (nephrotic syndrome), furosemide is highly protein-bound in plasma; albumin-bound drug cannot be filtered or secreted, reducing luminal concentration and efficacy.
• In renal failure: furosemide t½ increases from ~1 h (normal) to >24 h.

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Clinical Indications

1. Acute pulmonary edema / heart failure — Drug of choice for acute decompensated HF; reduces venous pressure, preload, and edema rapidly
2. Chronic heart failure (when GFR < 30–40 mL/min, or thiazide-refractory)
3. Renal insufficiency — Effective even in CKD (unlike thiazides, which lose efficacy at GFR <30)
4. Nephrotic syndrome — First-line for edema management (twice-daily preferred)
5. Hypercalcemia — IV furosemide + IV saline; loop diuretics promote Ca²⁺ excretion (combine saline to prevent compensatory proximal Ca²⁺ reabsorption)
6. Acute hyperkalemia — Promotes K⁺ excretion
7. Hypertensive urgency/emergency — IV furosemide for rapid volume reduction
8. Thiamine deficiency caution — Long-term loop diuretics may worsen thiamine deficiency in HF patients — Katzung, p. 408

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Adverse Effects — Loop Diuretics

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3. THIAZIDE DIURETICS

Chemistry & Key Agents

• Chlorthalidone: ~2× more potent than HCTZ, much longer t½ (~45–60 h vs 6–12 h for HCTZ) → better 24-h BP control. Studies support superior antihypertensive effect but also more adverse effects. — Katzung, p. 417
• Indapamide: significant vasodilatory properties; preferred in some guidelines
• Metolazone: retains efficacy at low GFR; synergistic with loop diuretics

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Mechanism of Action

• Loop diuretics → hypercalciuria (loss of lumen-positive potential in TAL)
• Thiazides → hypocalciuria (Ca²⁺ reabsorption enhanced)
  • Mechanism: (1) Volume depletion → enhanced proximal Na⁺ and passive Ca²⁺ reabsorption; (2) In DCT, blockade of Na⁺ entry lowers intracellular Na⁺ → enhances basolateral Na⁺/Ca²⁺ exchanger (NCX) activity → more Ca²⁺ exits basolaterally. — Katzung, p. 412
• This is the basis for using thiazides in hypercalciuric nephrolithiasis and osteoporosis protection

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Pharmacokinetics

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Clinical Indications

1. Hypertension — First-line agent; large ALLHAT trial confirmed outcomes equivalent to or better than ACE inhibitors and calcium channel blockers in essential HTN. Thiazides enhance efficacy of virtually all other antihypertensives, especially ACE inhibitors. — Katzung, p. 416–417
2. Mild-moderate heart failure (NYHA I–II, preserved renal function)
3. Nephrolithiasis from hypercalciuria — Reduces urinary Ca²⁺; must combine with low-salt diet (excess NaCl overwhelms hypocalciuric effect)
4. Nephrogenic diabetes insipidus (NDI) — Paradoxically reduces urine volume by causing mild volume depletion → ↑ proximal reabsorption → ↓ volume reaching collecting duct
5. Osteoporosis — Modestly increases bone mineral density (from Ca²⁺ retention)
6. Spironolactone add-on — Growing evidence that spironolactone is the most effective agent in drug-resistant hypertension when added to diuretics

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Adverse Effects — Thiazides

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4. Comparison: Loop vs Thiazide

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5. Diuretic Resistance

• Non-compliance / excess dietary Na⁺
• Reduced renal perfusion (GFR ↓ → less secretion of drug into tubular lumen)
• Protein binding in tubular lumen (especially in nephrotic syndrome with hypoalbuminemia)
• NSAIDs — inhibit prostaglandin synthesis → reduce renal blood flow → reduce drug secretion and response (both loop and thiazide)
• "Braking phenomenon" — Compensatory ↑ Na⁺ reabsorption at segments distal to drug's site of action (especially DCT hypertrophy with loop diuretic use)
• Distal tubular remodeling with chronic loop diuretic use

• IV loop diuretics (bolus or continuous infusion)
• Sequential nephron blockade — Add a thiazide/metolazone to a loop diuretic: blocks Na⁺ reabsorption at BOTH TAL AND DCT, preventing compensatory distal reabsorption
• IV albumin + furosemide in nephrotic syndrome
• Switch furosemide → torsemide or bumetanide (more reliable bioavailability)
• Add acetazolamide for metabolic alkalosis-associated resistance
• Ultrafiltration as last resort

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6. Drug Interactions (Both Classes)

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7. Special Clinical Scenarios

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Sources

• Katzung's Basic and Clinical Pharmacology, 16th Edition — Chapter 15 (Diuretics)
• Goodman & Gilman's The Pharmacological Basis of Therapeutics — Chapter 33
• Lippincott Illustrated Reviews: Pharmacology — Chapter 9 (Renal Pharmacology)
• Harrison's Principles of Internal Medicine, 21st Edition — Mechanism diagrams
• Brenner and Rector's The Kidney — Diuretic physiology
• Comprehensive Clinical Nephrology, 7th Edition — Clinical diuretic useThis covers renal pharmacology / diuretics comprehensively, synthesized across all available textbooks. Here's a quick orientation to the content above:

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1. Overview table — All diuretic classes, nephron site, transporter, and potency hierarchy
2. Loop Diuretics — full pharmacology:
   • Chemistry of all 4 agents (furosemide, bumetanide, torsemide, ethacrynic acid)
   • NKCC2 mechanism with Harrison's diagram showing TAL transport
   • Why they're the most potent diuretics (25–30% of filtered NaCl handled by TAL)
   • Sigmoidal dose-response curve with threshold and ceiling (Lippincott)
   • Pharmacokinetics table including furosemide's variable bioavailability vs. torsemide/bumetanide >80%
   • Adverse effects: hypokalemia, ototoxicity (NKCC1 in inner ear), hypercalciuria, hypomagnesemia, hyperuricemia, metabolic alkalosis
3. Thiazide Diuretics — full pharmacology:
   • NCC mechanism with Harrison's DCT diagram
   • The critical Ca²⁺ reabsorption paradox (opposite of loops) explained mechanistically
   • Chlorthalidone vs HCTZ comparison
   • Loss of efficacy at GFR <30 (vs loops)
   • Adverse effects: hyponatremia (> loops), hyperglycemia (dose-dependent), hyperlipidemia, skin cancer risk, gout
4. Side-by-side comparison table
5. Diuretic resistance — mechanisms + "sequential nephron blockade" strategy
6. Drug interactions — NSAIDs, digoxin, lithium, aminoglycosides
7. Special scenarios — nephrogenic DI, acute pulmonary edema, cirrhosis, osteoporosis