Tauroursodeoxycholic acid stimulates hepatocellular exocytosis and mobilizes extracellular Ca⁺⁺ mechanisms defective in cholestasis

To assess the effects of tauroursodeoxycholic acid (TUDCA) on bile excretory function, we examined whether TUDCA modulates vesicular exocytosis in the isolated perfused liver of normal rats in the presence of high (1.9 mM) or low (0.19 mM) extracellular Ca⁺⁺ and in cholestatic rats 24 h after bile duct ligation. In addition, the effects of TUDCA on Ca⁺⁺ homeostasis were compared in normal and in cholestatic hepatocytes. In the isolated perfused rat liver, TUDCA (25 μM) stimulated a sustained increase in the biliary excretion of horseradish peroxidase, a marker of the vesicular pathway, in the presence of high, but not low extracellular Ca⁺⁺ or in the cholestatic liver. In contrast, TUDCA stimulated bile flow to the same extent regardless of the concentration of extracellular Ca⁺⁺ or the presence of cholestasis. In indo-1-loaded hepatocytes, basal cytosolic free Ca⁺⁺ ([Ca⁺⁺](i)) levels were not different between normal and cholestatic cells. However, in cholestatic cells [Ca⁺⁺](i) increases induced by TUDCA (10 μM) and its 7α-OH epimer taurochenodeoxycholic acid (50 μM) were reduced to 22% and 26%, respectively, compared to normal cells. The impairment of TUDCA- induced [Ca⁺⁺](i) increase in cholestatic cells could be mimicked by exposing normal cells to low extracellular Ca⁺⁺ (21%) or to the Ca⁺⁺ channel blocker NiCl₂ (23%). These data indicate that (a) dihydroxy bile acid-induced Ca⁺⁺ entry may be of functional importance in the regulation of hepatocellular vesicular exocytosis, and (b) this Ca⁺⁺ entry mechanism across the plasma membrane is impaired in cholestatic hepatocytes. We speculate that the beneficial effect of ursodeoxycholic acid in cholestatic liver diseases may be related to the Ca⁺⁺-dependent stimulation of vesicular exocytosis by its conjugate.