No abstract

In vitro, addition of taurine to liver homogenates increases the proportion of cholic acid conjugated with taurine. In the present study, the relation between hepatic taurine concentration and the proportion of infused sodium cholate conjugated with taurine was studied in the whole organ. The isolated perfused liver was studied to eliminate possible transfer of taurine to or from the large extrahepatic poosl present in vivo. During cholate infusion, the proportion of taurocholate excreted in bile decreased, and the proportion of glycocholate increased in a complementary fashion. Infusion of taurine with cholate prevented these changes. Hepatic taurine concentration, calculated from measured hepatic taurine concentrations before and at the end of cholate infusion, fell. Fall in proportion of total bile acid excreted as taurocholate was most rapid at low hepatic taurine concentrations between about 1.4 and 0.65 mumol/g liver. Hepatic taurine concentrations is a major determinant of the proportion of bile acid conjugated with taurine.

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We have investigated the effects of hormones on the permeability of the hepatocyte tight junction to two probes, [14C]sucrose and horseradish peroxidase, using one-pass perfused rat livers. Using a single injection of horseradish peroxidase we have demonstrated that this probe can enter bile by two pathways that are kinetically distinct, a fast pathway, which corresponds to the passage of the probe through the hepatocyte tight junctions, and a slow pathway, which corresponds to the transcytotic entry into bile. The passage of horseradish peroxidase through the hepatocyte tight junctions was confirmed by electron microscopic histochemistry. Vasopressin, epinephrine, and angiotensin II, hormones that act in the hepatocyte through the intracellular mediators calcium, the inositol polyphosphates, and diacylglycerol, increased the bile-to-perfusion fluid ratio of [14C]sucrose and the rapid entry of horseradish peroxidase into bile, indicating that the permeability of the tight junctions to these probes was increased. The effect of these hormones was dose dependent and in the cases of angiotensin II and epinephrine was inhibited by the specific inhibitors [Sar1, Thr8]angiotensin II and prazosin, respectively. Dibutyryl adenosine 3',5'-cyclic monophosphate did not affect the [14C]sucrose bile-to-perfusion fluid ratio or the fast entry of horseradish peroxidase into bile. These results suggest that the hepatocyte tight junction can no longer be considered a static system of pores separating blood from bile. It is rather a dynamic barrier potentially capable of influencing the composition of the bile.

The composition of hepatic endosome subfractions was compared directly with that of plasma membranes and Golgi-apparatus fractions. The neutral lipid and phospholipid composition of two endosome subfractions separated on Nycodenz gradients from a parent endosome fraction was similar to that of plasma membranes. The phospholipid/cholesterol ratios and the sphingomyelin contents were high, as in plasma membranes. However, the phosphatidylserine content was low. Endosomal subfractions contained a simpler polypeptide profile than did plasma membranes. However, a large number of glycoproteins were common to both fractions. Two endosome-specific glycoproteins, of Mr 59 000 and 38 000, were identified. Sialic acid was present at concentrations higher than in plasma membranes. The results indicate that endosomal membranes have a similar composition to plasma membranes that probably reflects their functional interaction during endocytosis and receptor recycling.