['During bile duct ligation (BDL), the growth of large cholangiocytes is regulated by the cyclic adenosine monophosphate (cAMP)/extracellular signal-regulated kinase 1/2 (ERK1/2) pathway and is closely associated with increased secretin receptor (SR) expression. Although it has been suggested that SR modulates cholangiocyte growth, direct evidence for secretin-dependent proliferation is lacking. SR wild-type (WT) (SR(+/+)) or SR knockout (SR(-/-)) mice underwent sham surgery or BDL for 3 or 7 days. We evaluated SR expression, cholangiocyte proliferation, and apoptosis in liver sections and proliferating cell nuclear antigen (PCNA) protein expression and ERK1/2 phosphorylation in purified large cholangiocytes from WT and SR(-/-) BDL mice. Normal WT mice were treated with secretin (2.5 nmoles/kg/day by way of osmotic minipumps for 1 week), and biliary mass was evaluated. Small and large cholangiocytes were used to evaluate the in vitro effect of secretin (100 nM) on proliferation, protein kinase A (PKA) activity, and ERK1/2 phosphorylation. SR expression was also stably knocked down by short hairpin RNA, and basal and secretin-stimulated cAMP levels (a functional index of biliary growth) and proliferation were determined. SR was expressed by large cholangiocytes. Knockout of SR significantly decreased large cholangiocyte growth induced by BDL, which was associated with enhanced apoptosis. PCNA expression and ERK1/2 phosphorylation were decreased in large cholangiocytes from SR(-/-) BDL compared with WT BDL mice. In vivo administration of secretin to normal WT mice increased ductal mass. In vitro, secretin increased proliferation, PKA activity, and ERK1/2 phosphorylation of large cholangiocytes that was blocked by PKA and mitogen-activated protein kinase kinase inhibitors. Stable knockdown of SR expression reduced basal cholangiocyte proliferation. SR is an important trophic regulator sustaining biliary growth.'] Does knockout of secretin receptor reduce large cholangiocyte hyperplasia in mice with extrahepatic cholestasis induced by bile duct ligation?