TAUROURSODEOXYCHOLIC ACID STIMULATES HEPATOCELLULAR EXOCYTOSIS AND MOBILIZES EXTRACELLULAR CA++ MECHANISMS DEFECTIVE IN CHOLESTASIS

To assess the effects of tauroursodeoxycholic acid (TUDCA) on bile exc retory function, we examined whether TUDCA modulates vesicular exocyto sis in the isolated perfused liver of normal rats in the presence of h igh (1.9 mM) or low (0.19 mM) extracellular Ca++ and in cholestatic ra ts 24 h after bile duct ligation. In addition, the effects of TUDCA on Ca++ homeostasis were compared in normal and in cholestatic hepatocyt es. In the isolated perfused rat liver, TUDCA (25 muM) stimulated a su stained 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 st imulated 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 diffe rent between normal and cholestatic cells. However, in cholestatic cel ls [Ca++]i increases induced by TUDCA ( 10 muM) and its 7alpha-OH epim er taurochenodeoxycholic acid (50 muM) were reduced to 22% and 26%, re spectively, compared to normal cells. The impairment of TUDCA-induced [Ca++]i increase in cholestatic cells could be mimicked by exposing no rmal cells to low extracellular Ca++ (21%) or to the Ca++ channel bloc ker NiCl2 (23%). These data indicate that (a) dihydroxy bile acid-indu ced Ca++ entry may be of functional importance in the regulation of he patocellular vesicular exocytosis, and (b) this Ca++ entry mechanism a cross the plasma membrane is impaired in cholestatic hepatocytes. We s peculate that the beneficial effect of ursodeoxycholic acid in cholest atic liver diseases may be related to the Ca++-dependent stimulation o f vesicular exocytosis by its conjugate.