Morphometric ultrastructural analysis of horseradish peroxidase–containing structures has been performed in vivo, in rat liver and, in vitro, in isolated bile ducts to determine whether a transcytotic vesicle pathway exists in biliary epithelial cells. In vivo, horseradish peroxidase (100 mg/kg body wt) was given by intraportal injection in normal rats (n = 15) or 1 hr after administration of 600 mg/kg valproic acid (n = 15). Ultrastructural morphometric analysis was conducted on livers between 1 and 40 min after horse-radish peroxidase injection. In vitro, bile ducts were isolated on collagenase digestion, incubated in horseradish peroxidase for 3 min and prepared for electron microscopy immediately or after incubation for another 5, 10, 15 or 20 min in horseradish peroxidase–free medium at 37° C. In four experiments, colchicine (10−5 mol/L) or β-Iumicolchicine (10−5 mol/L) was added to the culture medium 2 hr before horseradish peroxidase. In a separate series of experiments, 50 μmol/L taurocholic acid or 500 μmol/L ursodeoxycholic acid was added to the culture medium 12 min before horseradish peroxidase. The volume density (percent area) of horseradish peroxidase–containing structures was analyzed in the 1-μm-wide area of basolateral or apical cytoplasm. In vivo, horseradish peroxidase–containing structures maximally increased from the basolateral to the periluminal region over a 20-min interval (percent area increased from 0.09 ± 0.12 to 2.02 ± 0.33; p < 0.001) and over a 10-min interval in valproic acid–treated animals (from 0.17 ± 0.11 to 2.05 ± 0.36; p < 0.001). In vitro, horseradish peroxidase immediately labeled vesicles in the basolateral cytoplasm. Within 15 min, the vesicles were labeled in the periluminal region (percent area increased from 0.36 ± 0.08 to 1.90 ± 0.17; p < 0.001). Colchicine but not β-lumicolchicine decreased the volume density of labeled structures in the apical cytoplasm (percent area at 15 min, 1.94 ± 0.24 after β-lumicolchicine and 1.04 ± 0.29 after colchicine; p < 0.01). Taurocholic or ursodeoxycholic acid did not change the migration pattern of labeled vesicles, but peroxidase tended to appear earlier in the apical cytoplasm, especially after taurocholic acid. In addition, taurocholic acid increased the percentage of labeled tubules in the apical cytoplasm. These studies show that a polarized tubulovesicular transcytotic pathway exists in rat biliary epithelium and is microtubule dependent. These tubulovesicular structures are labeled with horseradish peroxidase, which is rapidly transported from the cell periphery to the luminal area. This process appears to be stimulated by choleretic drugs such as valproic acid and taurocholic or ursodeoxycholic bile salts. (HEPATOLOGY 1993;18:422–432).
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