Bile formation in the rat: the role of the paracellular shunt pathway

Transepithelial movement of water and solute occurs both through the cell membrane as well as across the intercellular junctional complex (paracellular shunt pathways). Permeability of paracellular shunt pathways is increase by transmucosal osmotic gradients, and in certain epithelia these changes are associated with bullous-like deformations (blisters) of the zonula occludens and localization of lanthanum within junctional complexes. Although bile acids increase biliary secretion by osmotic forces, the source of this water movement into bile is not known. In the present studies we examined whether a choleretic infusion of sodium dehydrocholic acid (DHC) or its taurine conjugate, taurodehydrocholate, altered the solute permeability characteristics and morphologic appearance of the junctional complexes of rat hepatocytes. Animals were continuously infused for 1 hr with 1% albumin--0.9% NaCl alone or 120 mumol of DHC and bile flow and biliary clearance of [14C]sucrose, an indirect marker of biliary permeability were measured. The number of intercellular blisters adjacent to the bile canaliculus were counted in an unbiased manner from photographs obtained with scanning electron microscopy. Bile flow and the biliary sucrose clearance remained unchanged in control animals whereas DHC infusions resulted in a progressive increase in the biliary clearance of [14C]sucrose during the 60 min of infusion even though the choleretic response to DHC was stable during the final 30 min of infusion. DHC infusions produced surface invaginations, or blisters, (0.1--0.7 micrometer in diameter) which were located immediately adjacent to the hemi-bile canaliculus and occurred with a frequency of 1.62 +/- 0.08 per hepatocyte surface, which was fivefold greater than observed in controls. In separate groups of animals 5 mM ionic lanthanum chloride was perfused intraportally after taurodehydrocholate infusions, and the number of junctional complexes that contained the electron dense marker were quantitated by transmission electron microscopy. Localization of lanthanum in the junctional complexes of fasted control animals was not observed, whereas approximately equal to 50% of the zonula occludens in DHC-infused animals contained lanthanum which was also occasionally identified within the lumen of the bile canaliculus. These results indicate that infusions of DHC cause blisters adjacent to the junctional complex of rat hepatocytes in association with changes in solute conductivity of the zonula occludens to cations such as ionic lanthanum chloride, and presumably to larger solutes such as sucrose. Qualitatively similar morphologic findings were also observed during the infusion of sodium taurocholate at physiologic rate (40 mumol/h). These studies suggest that the paracellular shunt pathway in the liver is an important site for bile acid-induced water and solute movement into bile.