Fig. 5. Collecting ducts in the kidney, characterized by non-regulated tight junctions with high paracellular resistances do not express Kir6.1-SUR2A complexes. (a) Staining of consecutive sections of the rat kidney cortex for Kir6.1 visualizes tight junctions in proximal and distal tubules, leaving collecting ducts (CDs) negative. (b) CDs are easily identified by the presence of aquaporin 2 in their principal cells. Tight junctions in CDs are not regulated and, therefore, exhibit high paracellular resistance. (c) They are identified together with those in proximal and distal tubules by staining for ZO-1 immunoreactivity. Bar, 20 µm.