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1 Division of Clinical Pharmacology and Toxicology, Department of Medicine, University Hospital Zurich, Zurich, Switzerland
2 Institute of Physiology, University of Zurich, Zurich, Switzerland
* To whom correspondence should be addressed. E-mail: bstieger{at}kpt.unizh.ch.
Hepatocytes and cholangiocytes release adenosine triphosphate into bile, where it is rapidly degraded into adenosine and inorganic phosphate (Pi). In rat, biliary Pi concentration (0.01 mM) is about 100fold and 200fold lower than in hepatocytes and plasma, respectively, indicating active reabsorption of biliary Pi. This study aimed to functionally characterize canalicular Pi reabsorption in rat liver and to identify the involved Pi transport system(s). Pi transport was determined in isolated rat canalicular liver plasma membrane (cLPM) vesicles using a rapid membrane filtration technique. Identification of putative Pi 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 cLPM vesicles demonstrated sodium-dependent Pi uptake. Initial Pi uptake rates were saturable with increasing Pi concentrations, exhibiting an apparent Km value of ~ 11 µM. Pi 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 Pi from primary hepatic bile and thus might play an important role in the regulation of biliary Pi concentration.
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