Which transporter is characteristic of the thick ascending limb and reabsorbs ions?

• A. SGLT2 transporter
• B. NKCC transporter
• C. Na+/K+-ATPase on basolateral membrane
• D. Aquaporin-1

Answer: (B)

The thick ascending limb reabsorbs ions mainly through a Na+-K+-2Cl− cotransporter on the apical membrane. This NKCC (specifically NKCC2 in the TAL) brings in sodium, potassium, and chloride from the tubular lumen into the cell, driven by the sodium gradient maintained by the Na+/K+-ATPase on the basolateral side. Because this segment is water-impermeable, water stays in the tubule while solutes are reabsorbed, diluting the tubular fluid downstream. This transporter is also a classic target of loop diuretics, which block its function and reduce ion reabsorption in the TAL. The other options don’t fit: SGLT2 is for proximal tubule glucose reabsorption; Aquaporin-1 is a water channel linked to water reabsorption in segments that are permeable to water; and Na+/K+-ATPase on the basolateral membrane helps drive reabsorption broadly but is not the defining transporter of the thick ascending limb.