Backgrounds & Aims: We report a novel approach to study biliary water, bile acid, and HCO3 - transport: the microperfusion of intrahepatic bile duct units (IBDUs) isolated from normal rat liver. Methods: To study water transport, IBDUs were perfused in vitro with a membrane-impermeant fluorescent volume marker, fluorescein sulfonate; net water movement (Jv) and osmotic water permeability (Pf) were then calculated. To study solute transport, IBDUs were perfused with taurocholic acid (TCA) and bile acid uptake was calculated from the concentrations of TCA in the perfused and collected solutions. To study ion transport, IBDUs were perfused with the cell-impermeant pH-sensitive dye BCECF dextran; luminal pH was determined from fluorescence excitation ratios. Results: When inward (secretory) or outward (absorptive) osmotic gradients were established across IBDUs, water movement was observed from bath to lumen (i.e., secretion) and from lumen to bath (i.e., absorption). The perfused IBDUs absorbed TCA in a saturable, sodium-dependent manner; in addition, TCA absorption was blocked in a dose-dependent fashion by S0960, a specific inhibitor of the Na+/bile acid cotransporter. Addition of forskolin to HCO3 --containing (but not HCO3 --free) bath buffer resulted in lumen alkalinization reflecting HCO3 - transport into the lumen of perfused IBDUs. Conclusions: The results provide direct functional evidence that intrahepatic bile ducts both secrete and absorb water in response to osmotic gradients, actively absorb bile acid, and transport HCO3 -.
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