In the thick ascending limb (TAL) of Henle's loop, the Na-K-2Cl cotransporter (NKCC2) is electrogenic. What maintains the driving force for NKCC2 and what unique property of the TAL is explained?
- A H⁺-ATPase maintains lumen-negative potential; TAL freely absorbs water with solute
- B Na-K-ATPase creates lumen-positive potential; TAL has high aquaporin-1 density
- C Back-diffusion of K⁺ via ROMK channels creates a lumen-positive potential; TAL is impermeable to water explaining single-effect of concentration ✓
- D Cl⁻ recycling via ClC-Ka channels creates lumen-negative potential; TAL reabsorbs only Na⁺
Correct answer: C. Back-diffusion of K⁺ via ROMK channels creates a lumen-positive potential; TAL is impermeable to water explaining single-effect of concentration
Explanation
NKCC2 transports Na⁺, K⁺, and 2Cl⁻ into the cell. K⁺ recycling back into the lumen via ROMK channels (and Cl⁻ exit via basolateral ClC-Ka/Kb) creates a lumen-positive transepithelial potential (~+8 mV), which drives additional paracellular reabsorption of Na⁺, Ca²⁺, and Mg²⁺. Critically, the TAL lacks aquaporin water channels and is impermeable to water, so solute removal without water dilutes the tubular fluid (diluting segment) while concentrating the medullary interstitium — the single effect that powers the countercurrent multiplier.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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