title: Identification and localization of sodium-phosphate cotransporters in hepatocytes and cholangiocytes of rat liver.
abstract: Hepatocytes and cholangiocytes release ATP into bile, where it is rapidly degraded into adenosine and P(i). In rat, biliary P(i) concentration (0.01 mM) is approximately 100-fold and 200-fold lower than in hepatocytes and plasma, respectively, indicating active reabsorption of biliary P(i). We aimed to functionally characterize canalicular P(i) reabsorption in rat liver and to identify the involved P(i) transport system(s). P(i) transport was determined in isolated rat canalicular liver plasma membrane (LPM) vesicles using a rapid membrane filtration technique. Identification of putative P(i) transporters was performed with RT-PCR from liver mRNA. Phosphate transporter protein expression was confirmed by Western blotting in basolateral and canalicular LPM and by immunofluorescence in intact liver. Transport studies in canalicular LPM vesicles demonstrated sodium-dependent P(i) uptake. Initial P(i) uptake rates were saturable with increasing P(i) concentrations, exhibiting an apparent K(m) value of approximately 11 muM. P(i) transport was stimulated by an acidic extravesicular pH and by an intravesicular negative membrane potential. These data are compatible with transport characteristics of sodium-phosphate cotransporters NaPi-IIb, PiT-1, and PiT-2, of which the mRNAs were detected in rat liver. On the protein level, NaPi-IIb was detected at the canalicular membrane of hepatocytes and at the brush-border membrane of cholangiocytes. In contrast, PiT-1 and PiT-2 were detected at the basolateral membrane of hepatocytes. We conclude that NaPi-IIb is most probably involved in the reabsorption of P(i) from primary hepatic bile and thus might play an important role in the regulation of biliary P(i) concentration.
authors.affiliation: Division of Clinical Pharmacology and Toxicology, Dept. of Medicine, Univ. Hospital Zürich, Rämistrasse 100, CH-8091 Zürich, Switzerland.
authors.forename: Pascal
authors.initials: P
authors.lastname: Frei
authors.forename: Bo
authors.initials: B
authors.lastname: Gao
authors.forename: Bruno
authors.initials: B
authors.lastname: Hagenbuch
authors.forename: Alfonso
authors.initials: A
authors.lastname: Mate
authors.forename: Jürg
authors.initials: J
authors.lastname: Biber
authors.forename: Heini
authors.initials: H
authors.lastname: Murer
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Meier
authors.forename: Bruno
authors.initials: B
authors.lastname: Stieger
mesh_terms: D000818:Animals; D001648:Bile Canaliculi / Q000378:metabolism; D001692:Biological Transport; D015153:Blotting, Western; D002462:Cell Membrane / Q000378:metabolism; D005455:Fluorescent Antibody Technique; D022781:Hepatocytes / Q000378:metabolism*; D008099:Liver / Q000166:cytology / Q000378:metabolism*; D008297:Male; D010710:Phosphates / Q000378:metabolism; D012333:RNA, Messenger / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D020133:Reverse Transcriptase Polymerase Chain Reaction; D050608:Sodium-Phosphate Cotransporter Proteins; D050611:Sodium-Phosphate Cotransporter Proteins, Type III; D050614:Sodium-Phosphate Cotransporter Proteins, Type IIb; D027981:Symporters / Q000235:genetics / Q000378:metabolism*; D014018:Tissue Distribution
chemical_list: D010710:Phosphates; D012333:RNA, Messenger; C493588:Slc20a1 protein, rat; C493559:Slc34a2 protein, rat; D050608:Sodium-Phosphate Cotransporter Proteins; D050611:Sodium-Phosphate Cotransporter Proteins, Type III; D050614:Sodium-Phosphate Cotransporter Proteins, Type IIb; D027981:Symporters