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©2013 UpToDate ® Print Email Hepatocellular and biliary epithelial transport proteins Conjugated bilirubin and certain other organic anions (OA-) are actively transported from the hepatocyte into the bile canaliculus by the multidrug resistance related protein, MRP 2 (cMOAT). The Na+/K+-ATPase and the K+ channel together maintain a negative intracellular potential of -35mV that contributes to the transport of bilirubin glucuronide into the bile channel. The sodium-taurocholate cotransporter (NTCP) mediates the cotransport of conjugated bile salts (BS-) and sodium (Na+) from portal blood. Organic anion transport polypeptide (OATP) undertakes sodium- independent transport of bile salts (BS-) and other organic anions (OA-) from the portal blood into the hepatocyte. The sister of P-glycoprotein (SPGP) actively transports bile salts (BS-) into the bile. The large scale transport of bile salts by this protein is the main stimulus for bile flow. The multidrug resistance-3-P-glycoprotein (MDR 3) is responsible for the ATP-dependent translocation of phosphatidylcholine (PL) from the inner to the outer leaflet of the membrane bilayer. Multidrug resistance-1-P- glycoprotein (MDR 1) actively transports organic cations (OC+), xenobiotics, and cytotoxins into the bile. FIC1 transports aminophospholipid from the outer to the inner leaflet of the membrane bilayer of the bile canaliculus. Mutations in the gene for this protein has been found in patients with PFIC I and BRIC. AE2 = Chloride-bicarbonate anion exchanger isoform 2.