Perfused rat intrahepatic bile ducts secrete and absorb water, solute, andlons

Backgrounds & Aims: We report a novel approach to study biliary water, bile acid, and HCO3- transport: the microperfusion of intrahepatic bile duct un its (IBDUs) isolated from normal rat liver. Methods: To study water transpo rt, IBDUs were perfused in vitro with a membrane-impermeant fluorescent vol ume marker, fluorescein sulfonate; net water movement (J(v)) and osmotic wa ter permeability (P-f) were then calculated. To study solute transport, IBD Us were perfused with taurocholic acid (TCA) and bile acid uptake was calcu lated from the concentrations of ICA in the perfused and collected solution s. To study ion transport, IBDUs were perfused with the cell-impermeant pH- sensitive dye BCECF dextran; luminal pH was determined from fluorescence ex citation ratios. Results: When inward (secretory) or out ward (absorptive) osmotic gradients were established across IBDUs, water movement was observe d from bath to lumen (i.e., secretion) and from lumen to bath (i.e., absorp tion). The perfused IBDUs absorbed TCA in a saturable, sodium-dependent man ner; in addition, TCA absorption was blocked in a dose-dependent fashion by S0960, a specific inhibitor of the Na+/bile acid cotransporter. Addition o f forskolin to HCO3--containing (but not HCO3--free) bath buffer resulted i n lumen alkalinization reflecting HCO3- transport into the lumen of perfuse d IBDUs. Conclusions: The results provide direct functional evidence that i ntrahepatic bile ducts both secrete and absorb water in response to osmotic gradients, actively absorb bile acid, and transport HCO3-.