An immunohistochemical stain for aquaporin water channels in the kidney would stain cells most intensely in which structure?

• A. Distal convoluted tubules
• B. Proximal convoluted tubules
• C. Collecting ducts
• D. Glomeruli

Answer: (C)

Water movement in the kidney is controlled by aquaporin channels, and different segments use different aquaporins. The proximal tubule and descending limb of the loop of Henle express aquaporin-1 constitutively, giving high baseline water permeability. But the collecting ducts rely on aquaporin-2, which is inserted into the apical membrane of principal cells in response to vasopressin, dramatically increasing water reabsorption when needed. Because aquaporin-2 is so heavily expressed and dynamically regulated in the collecting ducts, staining for aquaporin channels shows the strongest signal there. The collecting ducts also have aquaporin-3 and aquapor-4 on the basolateral side to complete water movement out of the cells, reinforcing the intense staining in this region.