Prolonged administration of secretin to normal rats increases biliary proliferation and secretin-induced ductal secretory activity

Background and aim: Cholangiocyte proliferation is coordinately regulated by a number ofgastrointestinal hormones/peptides, some of which display stimulatory effects and some have inhibitoryactions on cholangiocyte proliferation. Enhanced biliary proliferation [for example after bile duct ligation(BDL) and partial hepatectomy] is associated with increased expression of secretin receptor (SR), cysticfibrosis transmembrane conductance regulator (CFTR) and Cl–/HCO3– anion exchanger 2 and secretinstimulatedductal secretion, whereas loss/damage of bile ducts [for example after acute carbon tetrachloride(CCl4) administration] is associated with reduced secretin-stimulated ductal secretory activity. There isgrowing information regarding the role of gastrointestinal hormones the regulation of biliary growth.For example, while gastrin, somatostatin and serotonin inhibit bile duct hyperplasia of cholestatic rats bydownregulation of cAMP signaling, secretin has been shown to stimulate the proliferation of normal mice byactivation of cyclic adenosine 3 ,5 -monophosphate (cAMP)-dependent signaling. However, no informationexists regarding the stimulatory effects of secretin on biliary proliferation of normal rats. Thus, we evaluatedthe in vivo and in vitro effect of secretin on biliary proliferation, the expression of markers key of ductalsecretion and secretin-stimulated ductal secretion.Methods: Normal male rats were treated with saline or secretin (2.5 nmoles/kg BW/day by osmotic minipumpsfor one week). We evaluated: (I) intrahepatic bile duct mass (IBDM) in liver sections and PCNA expression inpurified cholangiocytes; (II) SR and CFTR mRNA expression and secretin-stimulated cAMP levels in purifiedcholangiocytes; and (III) secretin-stimulated bile and bicarbonate secretion in bile fistula rats. In vitro, normal ratintrahepatic cholangiocyte lines (NRIC) were treated with BSA (basal) or secretin (100 nM) for 24 to 72 hours inthe absence/presence of a PKA or a MEK inhibitor before evaluating proliferation by MTS assays.Results: Prolonged administration of secretin to normal rats increased IBDM and PCNA expression inpurified cholangiocytes compared to saline-treated normal rats. Also, secretin increased the expression ofproteins (SR and CFTR) that are key in the regulating ductal secretion and enhanced secretin-stimulatedcAMP levels and bile and bicarbonate secretion. In vitro, secretin increased the proliferation of NRIC,increase that was prevented by PKA and MAPK inhibitors.Conclusions: We have demonstrated that secretin stimulates both in vivo and in vitro biliary proliferation andsecretin-stimulated ductal secretory activity in normal rats. We suggest that the stimulatory effect of secretin onbiliary proliferation and secretion may be important for preventing biliary dysfunction during ductopenic disorders.