Expression of the rat liver Na+/taurocholate cotransporter is regulated in vivo by retention of biliary constituents but not their depletion.

Expression and function of the hepatic Na+/taurocholate cotransporter (ntcp) are down-regulated in several models of experimental cholestasis. To test whether retention and/or depletion of biliary constituents are involved in ntcp regulation, ntcp expression was quantified in several animal models with altered levels of these constituents. In choledochocaval fistula rats (CCF) (retention model), ntcp mRNA expression specifically declined after 1 and 3 days by 76 +/- 4% (P < .005) and 31 +/- 9% (P < .05), respectively, returning to control levels by 7 days. However, protein expression as assessed by Western blotting remained unchanged for up to 7 days of CCF. In rats with bile fistulas (depletion model) for 0.5, 1, 2, 4, and 7 days, both ntcp protein and mRNA expression remained unaltered. Infusion of either taurocholate or taurochenodeoxycholate for 12 hours also did not effect ntcp mRNA expression in intact animals, probably because of its inability to increase serum and intrahepatic bile acid levels. In rats with selective bile duct ligation (SBDL), ntcp mRNA levels were down-regulated by 40 +/- 10% (P < .05) only after 12 and 24 hours in ligated lobes, and mRNA levels returned to control values in these lobes after 2 and 4 days. ntcp mRNA expression remained unchanged in the nonobstructed lobes at any time. When data from CCF and SBDL rats were combined, serum bile acids correlated linearly with ntcp mRNA (r = .62, P < .0005) over a 0 to 110-micromol/L range. Our results indicate that ntcp is constitutively expressed and remains uneffected by either depletion or increased flux of biliary constituents. However, retention of biliary constituents results in rapid down-regulation of ntcp mRNA, consistent with the concept that hepatocytes may be protected from bile acid toxicity during cholestasis by this mechanism.