Passive Transport of Water out of Cell by Aquaporin-4 Aquaporin-4 (AQP4) passively transports water across the plasma membrane according to the osmotic gradient. AQP4 is expressed in the collecting duct of the kidney and in astroglial cells at the blood-brain barrier and ependymal cells lining the ventricles of the brain. Authored: May, B, 2009-08-06 Edited: May, B, 2009-08-06 Pubmed14514735 Pubmed17178220 Pubmed18511455 Pubmed19170255 Pubmed19383790 Pubmed8855281 Reactome Database ID Release 43432067 Reactome, http://www.reactome.org ReactomeREACT_24005 Reviewed: Beitz, E, 2010-06-24 Reviewed: Calamita, G, 2010-07-15 Reviewed: Mathai, JC, MacIver, B, 2010-07-31