title: Aquaporin-9 and urea transporter-A gene deletions affect urea transmembrane passage in murine hepatocytes.
abstract: In mammals, the majority of nitrogen from protein degradation is disposed of as urea. Several studies have partly characterized expression of urea transporters (UTs) in hepatocytes, where urea is produced. Nevertheless, the contribution of these proteins to hepatocyte urea permeability (P(urea)) and their role in liver physiology remains unknown. The purpose of this study was to biophysically examine hepatocyte urea transport. We hypothesized that the water, glycerol, and urea channel aquaporin-9 (AQP9) is involved in hepatocyte urea release. Stopped-flow light-scattering measurements determined that the urea channel inhibitors phloretin and dimethylurea reduced urea permeability of hepatocyte basolateral membranes by 70 and 40%, respectively. In basolateral membranes isolated from AQP9(-/-) and UT-A1/3(-/-) single-knockout and AQP9(-/-):UT-A1/3(-/-) double-knockout mice, P(urea) was decreased by 30, 40, and 76%, respectively, compared with AQP9(+/-):UT-A1/3(+/-) mice. However, expression analysis by RT-PCR did not identify known UT-A transcripts in liver. High-protein diet followed by 24-h fasting affected the concentrations of urea and ammonium ions in AQP9(-/-) mouse liver and plasma without generating an apparent tissue-to-plasma urea gradient. We conclude that AQP9 and unidentified UT-A urea channels constitute primary but redundant urea facilitators in murine hepatocytes.
authors.affiliation: The Water and Salt Research Center, Department of Biomedicine, Aarhus University, Aarhus, Denmark.
authors.forename: Sabina
authors.initials: S
authors.lastname: Jelen
authors.forename: Patrizia
authors.initials: P
authors.lastname: Gena
authors.forename: Janne
authors.initials: J
authors.lastname: Lebeck
authors.forename: Aleksandra
authors.initials: A
authors.lastname: Rojek
authors.forename: Jeppe
authors.initials: J
authors.lastname: Praetorius
authors.forename: Jørgen
authors.initials: J
authors.lastname: Frøkiaer
authors.forename: Robert A
authors.initials: RA
authors.lastname: Fenton
authors.forename: Søren
authors.initials: S
authors.lastname: Nielsen
authors.forename: Giuseppe
authors.initials: G
authors.lastname: Calamita
authors.forename: Michael
authors.initials: M
authors.lastname: Rützler
mesh_terms: D000818:Animals; D020346:Aquaporins / Q000172:deficiency*; D004044:Dietary Proteins / Q000008:administration & dosage; D017353:Gene Deletion; D022781:Hepatocytes / Q000378:metabolism*; D026901:Membrane Transport Proteins / Q000235:genetics*; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D014508:Urea / Q000378:metabolism*; D000096982:Urea Transporters
chemical_list: D020346:Aquaporins; D004044:Dietary Proteins; D026901:Membrane Transport Proteins; D014508:Urea; D000096982:Urea Transporters; C491638:Aqp9 protein, mouse