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

Ntcp-mediated uptake of bile salts at the basolateral membrane of hepatocytes is required for maintenance of their enterohepatic circulation. Expression of Ntcp is reduced in various experimental models of cholestasis 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 Ntcp in the elevated bile salt levels in mdr2 P-glycoprotein-deficient (-/-) mice. Plasma membranes were isolated from male wild-type (+/+) and mdr2 (-/-) mice for measurement of Na+-dependent taurocholate transport and assessment of Ntcp protein levels by Western blotting. Northern blot analysis and competitive reverse transcription-polymerase chain reaction were used to determine hepatic Ntcp mRNA levels. Kinetic analysis showed a 2-fold decrease in the Vmax of Na+-dependent taurocholate transport, with an unaffected Km in (-/-) mice compared with (+/+) controls. Ntcp protein levels were 4-6-fold reduced in plasma membranes of (-/-) mice relative to sex-matched controls. Surprisingly, hepatic Ntcp mRNA levels were not significantly affected in the (-/-) mice. Elevated plasma bile salt levels in mdr2 P-glycoprotein-deficient mice in the absence of overt cholestasis are associated with reduced Ntcp expression and transport activity. This is due to posttranscriptional down-regulation of Ntcp.