Aquaporin-9 and Urea Transporter-A gene deletions affect urea transmembrane passage in murine hepatocytes

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 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 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, urea permeability (Purea) was decreased by 30 %, 40 %, and 76 %, respectively, compared to 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 24h 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.