title: Water movement across rat bile duct units is transcellular and channel-mediated.
abstract: In recent studies using freshly isolated rat cholangiocytes, we established that water crosses the cholangiocyte membrane by a channel-mediated mechanism involving aquaporins, a family of water-channel proteins. Our goal was to address the importance of channel-mediated water transport in ductal bile formation by employing a physiologic experimental model, the enclosed, polarized rat intrahepatic bile duct unit (IBDU). Expansion and reduction of luminal areas as a reflection of water movement into and out of IBDUs prepared from livers of normal rats were measured by quantitative computer-assisted image analysis. When enclosed IBDUs were exposed to inward or outward osmotic gradients, their luminal area rapidly increased (approximately 25%) or decreased (approximately 20%) reflecting net water secretion or absorption, respectively. These effects were specifically inhibited by 2 water channel blockers, DMSO and HgCl2. In both instances, beta-mercaptoethanol reversed the inhibitory effects. In the absence of an osmotic gradient, choleretic agents (secretin and forskolin) and a cholestatic hormone (somatostatin) induced a significant increase or decrease of IBDU luminal area by 21% and 22%, respectively. These effects were also inhibited by DMSO and reversed by beta-mercaptoethanol. Under our experimental conditions, DMSO did not interfere with either forskolin-induced cAMP synthesis or the generation of osmotic driving forces via the apical chloride-bicarbonate exchanger. Protamine, an inhibitor of the paracellular pathway, had no effect on hypotonic or forskolin-induced water secretion in IBDUs. These results in a physiologically relevant model of ductal bile formation provide additional support for the concept that osmotically driven and agonist-stimulated water movement into (secretion) and out of (absorption) the biliary ductal lumen is transcellular and water channel-mediated.
authors.affiliation: Center for Basic Research in Digestive Diseases, Division of Gastroenterology and Hepatology, Mayo Medical School, Clinic and Foundation, Rochester, MN 55905, USA.
authors.forename: E
authors.initials: E
authors.lastname: Cova
authors.forename: A
authors.initials: A
authors.lastname: Gong
authors.forename: R A
authors.initials: RA
authors.lastname: Marinelli
authors.forename: N F
authors.initials: NF
authors.lastname: LaRusso
mesh_terms: D000818:Animals; D001653:Bile Ducts, Intrahepatic / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D005576:Colforsin / Q000494:pharmacology; D004848:Epithelium / Q000187:drug effects / Q000378:metabolism; D007473:Ion Channels / Q000502:physiology*; D008297:Male; D009995:Osmosis; D051381:Rats; D011916:Rats, Inbred F344; D012633:Secretin / Q000494:pharmacology; D013004:Somatostatin / Q000494:pharmacology; D014867:Water / Q000378:metabolism*
chemical_list: D007473:Ion Channels; D014867:Water; D012633:Secretin; D005576:Colforsin; D013004:Somatostatin