In the thick ascending limb (TAL) of Henle's loop, the apical NKCC2 cotransporter reabsorbs Na⁺, K⁺, and Cl⁻ in a 1:1:2 ratio. Loop diuretics block this transporter. Beyond diuresis, why do loop diuretics also impair urinary concentrating ability?

• A They directly block ADH receptors in the collecting duct, preventing aquaporin-2 insertion
• B Loop diuretics increase medullary blood flow (vasa recta washout) that dilutes the medullary gradient
• C By blocking NKCC2, they prevent solute accumulation in the medullary interstitium, abolishing the corticomedullary osmotic gradient needed for countercurrent concentration ✓
• D Reduced Na⁺ delivery to the collecting duct prevents osmotic water reabsorption regardless of ADH

Explanation

Urine concentration depends on the hyperosmotic medullary interstitium (up to ~1200 mOsm/kg at papillary tip) generated by the countercurrent multiplier in the TAL. The TAL actively reabsorbs NaCl via NKCC2 (with Na⁺/K⁺-ATPase on the basolateral side) without water (impermeable to water), creating the solute gradient. Loop diuretics (furosemide, bumetanide, torasemide) block NKCC2, preventing NaCl accumulation in the medullary interstitium. Without the gradient, even maximum ADH activity cannot drive water reabsorption from the collecting duct. This is why loop diuretics produce isotonic urine regardless of ADH status — they are most potent diuretics precisely because they destroy the concentrating gradient.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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