Decreased Na+-dependent taurocholate uptake and low expression of the sinusoidal Na+-taurocholate cotransporting protein (Ntcp) in livers of mdr2 P-glycoprotein-deficient mice

Background/Aims: Ntcp-mediated uptake of bile salts at the basolateral memb rane of hepatocytes is required for maintenance of their enterohepatic circ ulation. Expression of Ntcp is reduced in various experimental models of ch olestasis associated with increased plasma bile salt concentrations. Mdr2 P -glycoprotein-deficient mice lack biliary phospholipids and cholesterol but show unchanged biliary bile salt secretion and increased bile flow. These mice are evidently not cholestatic, but plasma bile salt concentrations are markedly increased. The aim of this study was to investigate the role of N tcp in the elevated bile salt levels in mdr2 P-glycoprotein-deficient (-/-) mice. Methods: Plasma membranes were isolated from male wild-type (+/+) and mdr2 (-/-) mice for measurement of Na+-dependent taurocholate transport and asse ssment of Ntcp protein levels by Western blotting, Northern blot analysis a nd competitive reverse transcription-polymerase chain reaction were used to determine hepatic Ntcp mRNA levels. Results: Kinetic analysis showed a 2-fold decrease in the V-max of Na+-depe ndent taurocholate transport, with an unaffected K-m in (-/-) mice compared with (+/+) controls. Ntcp protein levels were 4-6-fold reduced in plasma m embranes of (-/-) mice relative to sex-matched controls. Surprisingly, hepa tic Ntcp mRNA levels were not significantly affected in the (-/-) mice. Conclusions: Elevated plasma bile salt levels in mdr2 P-glycoprotein-defici ent mice in the absence of overt cholestasis are associated with reduced Nt cp expression and transport activity This is due to posttranscriptional dow n-regulation of Ntcp.