Collecting duct

A collecting duct receives tubular fluid from several nephrons and participates in the final adjustment of urine. Collecting ducts merge as they pass through the renal medulla, conducting the remaining fluid toward the renal outlet. Although continuous with the renal tubules, the shared collecting duct system is not technically part of an individual nephron.^[ref]

Its epithelium can change its membrane permeability to water. Vasopressin promotes the presence of aquaporin-2 in the apical membrane facing the tubular lumen. When an appropriate osmotic pressure difference favours outward movement, water crosses toward the surrounding interstitial fluid and can return to the blood. The channel changes access to an osmosis pathway; it does not itself pump water against that driving difference.

A 1993 study by K. Fushimi and collaborators identified a collecting-tubule water-channel Complementary DNA, then tested the encoded channel by gene expression in Xenopus oocytes. Expression increased osmotic water permeability. Localization to the apical region of collecting-tubule biological cells connected that functional result to its site in the kidney.^[ref]

The regulated movement was tested more directly in 1995. Investigators measured osmotic water permeability in isolated perfused rat collecting ducts, then used immunoelectron microscopy to examine channel distribution in the same tubules before, during and after vasopressin exposure. Increased permeability accompanied more channel labelling at the apical membrane relative to intracellular vesicles. Withdrawing vasopressin reversed both changes.^[ref]

This links a physiological change to membrane trafficking: exocytosis can add channel-bearing membrane to the surface, while endocytosis can retrieve it. The amount of channel at the correct surface, as well as its presence somewhere inside the epithelial cell, determines the available water pathway.