CHOLESTASIS-INDUCED ALTERATIONS OF THE TRANS AND PARACELLULAR PATHWAYS IN RAT HEPATOCYTES

Bile secretion depends on the vectorial transport of solutes from bloo d to bile and involves three different pathways: transcellular pathway s mediated by transport proteins distributed asymmetrically in the bas olateral and canalicular plasma membrane and by transcytotic vesicles, and a paracellular pathway allowing selective diffusion through tight junctions. All three pathways are impaired differentially by extrahep atic (bile duct ligation) or intrahepatic (ethinyloestradiol) cholesta sis. Ethinyloestradiol treatment leads to tight junctional defects tha t are less severe than those induced by bile duct ligation. Junctional impairment is reflected functionally in increased permeability for ho rseradish peroxidase and structurally by decreased strand numbers and increased junctional length, but not by alterations at the level of th e individual strands. The parallelism of physiological and morphologic al perturbations indicates a structure-function relationship in hepato cellular tight junctions. In addition, impaired functional integrity o f tight junctions following bile duct ligation is reflected in a parti al loss of hepatocellular surface polarity owing to redistribution of some, but not all, domain-specific plasma membrane antigens, which mig ht mimic the behaviour of transport systems. After ethinyloestradiol t reatment no alterations of surface polarity were observed. Thus, immun ohistochemistry supports the view that ethinyloestradiol results in le ss severe impairment of the tight junctions than bile duct ligation. F inally, bile duct ligation, but not ethinyloestradiol, affects the tra nscytotic vesicular pathway; severe impairment of this is reflected in the absence of a late horseradish peroxidase peak in bile and also in the accumulation of pericanalicular vesicles that are immunopositive for canalicular membrane proteins and accessible for bulk phase endocy tic markers. This view is supported by the disappearance of the perica nalicular vesicles simultaneously with the resumption of transcytotic horseradish peroxidase transport following release of ligation, a find ing that indicates rapid restoration of this defect.