Two alternative mechanisms for the mechanism of phosphatidylcholine (PC) excretion into bile. (A) Bile salts are transported into the canalicular lumen by the canalicular bile salt transporter (cBST) (now termed BSEP), and PC accumulates on the luminal side of the canalicular membrane by the action of MDR3. Luminal bile salts then extract the phospholipid from the membrane into micelles. (B) MDR3 flops PC to the external domains of the canalicular membrane bilayer which extrude into the bile lumen and are destabilized by bile salts which pinch off the membrane. Reprinted, with permission, from Ref. (76) as modified from Oude Elferink RPJ, Tytgat GNJ, Groen AK. Faseb J, 11: 19, 1997(427).