Taurocholate, but not taurodehydrocholate, increases biliary permeability to sucrose

Disruption of function and localization of tight junctional structures and Mrp2 in sustained estradiol-17beta-D-glucuronide-induced cholestasis

Estradiol-17beta-D-glucuronide (E(2)17G) induces immediate and profound but transient cholestasis in rats when administered as a single bolus dose. Here, we examined the consequence of sustained E(2)17G cholestasis and assessed the function and localization of the tight junctional proteins zonula occludens-1 (ZO-1) and occludin and of the canalicular transporter multidrug resistance-associated protein-2 (Mrp2). An initial dose of E(2)17G (15 mumol/kg iv) followed by five subsequent doses of 7.5 mumol/kg from 60 to 240 min induced a sustained 40-70% decrease in bile flow. Following their biliary retrograde administration, cholera toxin B subunit-FITC or horseradish peroxidase were detected at the sinusoidal domain, indicating opening of the paracellular route; this occurred as early as 15 min after the first dose as well as 15 min after the last dose of E(2)17G, but not following the administration of vehicle in controls. Localization of ZO-1 and occludin was only slightly affected under acute cholestatic conditions but was severely disrupted under sustained cholestasis, with their appearance suggesting a fragmented structure. Endocytic internalization of Mrp2 to the pericanalicular region was apparent 20 min after a single E(2)17G administration; however, Mrp2 was found more deeply internalized and partially redistributed to the basolateral membrane under sustained cholestasis. In conclusion, acute E(2)17G-induced cholestasis increased permeability of the tight junction, while sustained cholestasis provoked a significant redistribution of ZO-1, occludin, and Mrp2 in addition to increased permeability of the tight junction. Altered tight junction integrity likely contributes to impaired bile secretion and may be causally related to changes in Mrp2 localization.

Role of the hepatic artery in canalicular bile formation by the perfused rat liver. A multiple indicator dilution study

The role of the hepatic artery in tracer water exchange and regulation of permeation of small solutes during canalicular bile formation was studied in the rat using a system that permitted perfusion of both hepatic artery and portal vein. Hepatic vein and biliary multiple indicator dilution curves were obtained after injection of indicators into either vessel. The main difference in hepatic venous dilution curves was a 3.1-fold longer t0 (time spent in nonexchanging vessels) and a 5% larger equivalent water space after injection into the hepatic artery. Biliary tracer recovery of water was markedly higher after arterial injection than after portal vein injection. Both taurocholate and taurodehydrocholate stimulated bile flow and increased biliary tracer recovery after injection into either vessel. The biliary recovery of sucrose relative to that of water, which is a measure of biliary sucrose permeation, was much lower when given into the hepatic artery than when given into the portal vein. During taurocholate infusion, it decreased by 33% in the hepatic artery but increased 36% in the portal vein. Taurodehydrocholate, by contrast, did not affect permeation of sucrose given into the portal vein. Our studies demonstrate marked exchange of tracer water in the biliary epithelium. Taurocholate, but not taurodehydrocholate, increases permeation of sucrose into bile in the portal vein bed while both bile salts decrease it in the arterial bed.

Verapamil favorably influences hepatic microvascular exchange and function in rats with cirrhosis of the liver

The effect of the calcium channel blocking agent, verapamil, on microcirculatory patterns and hepatic function was investigated in the perfused liver of cirrhotic rats. Compared with controls, cirrhotic livers had higher vascular resistance, increased intrahepatic shunting, and smaller extravascular albumin space and larger extravascular sucrose space, as determined by a multiple-indicator dilution technique. Hepatic function, estimated by determining propranolol and antipyrine extraction, was markedly reduced in cirrhotic livers. Portal pressure was then reduced 25% either pharmacologically by verapamil or hydrodynamically by lowering inflow. Verapamil decreased vascular resistance by 22%. This was associated with a 38% reduction in intrahepatic shunting and a 62% increase in extravascular albumin space. Hydrodynamically lowering pressure had no or adverse effects. The verapamil-induced improvement in microcirculatory characteristics was associated with a significant improvement in oxygen consumption (+21%) and antipyrine clearance (+20%). We conclude that the microvascular distortions of liver cirrhosis in the rat are partially reversible by vasodilators like verapamil.

Control of biliary phospholipid secretion. Effect of continuous and discontinuous infusion of taurocholate on biliary phospholipid secretion

A major determinant of biliary lipid secretion is bile-salt secretion. Taurocholate (TC), a micelle-forming bile salt, was infused continuously at different rates in both isolated perfused livers and biliary-fistula rats. In both of these systems, infusion of TC brought about an elevated secretion of phosphatidylcholine for the duration of the TC infusion period. Initial phospholipid/bile-salt ratios in the bile were higher in the whole-animal model than in isolated livers, but at the higher infusion rates both secreted approx. 6 mol of phospholipid for every 100 mol of bile salt. The secretion of phospholipid, which was maintained even at high rates of bile-salt infusion, suggest a continuous and regulated phospholipid supply and secretion mechanism. In contrast, however, multiple short pulses of TC to the perfused liver, which brought about relatively equal biliary bile-salt output pulses, did not bring about equal phospholipid outputs, since the phospholipid peak size declined with each bile-salt pulse. These experiments taken together suggest either that a threshold (intracellular) bile-salt concentration may be required to 'switch-on' the phospholipid supply and that it may need to be maintained for continuous biliary phospholipid supply to the canalicular membrane.

Biliary secretion of fluid-phase markers by the isolated perfused rat liver. Role of transcellular vesicular transport

In these studies, we have used several approaches to systematically explore the contribution of transcellular vesicular transport (transcytosis) to the blood-to-bile movement of inert fluid-phase markers of widely varying molecular weight. First, under steady-state conditions, the perfused rat liver secreted even large markers in appreciable amounts. The bile-to-plasma (B/P) ratio of these different markers, including microperoxidase (B/P ratio = 0.06; mol wt = 1,879), insulin (B/P ratio = 0.09, mol wt = 5,000), horseradish peroxidase (B/P ratio = 0.04, mol wt = 40,000), and dextran (B/P ratio = 0.09, mol wt = 70,000), exhibited no clear ordering based on size alone, and when dextrans of two different sizes (40,000 and 70,000 mol wt) were studied simultaneously, the relative amounts of the two dextran species in bile were the same as in perfusate. Taurocholate administration produced a 71% increase in bile flow but little or no (0-20%) increase in the output of horseradish peroxidase, microperoxidase, inulin, and dextran. Second, under nonsteady-state conditions in which the appearance in or disappearance from bile of selected markers was studied after their abrupt addition to or removal from perfusate, erythritol reached a B/P ratio of 1 within 2 min. Microperoxidase and dextran appeared in bile only after a lag period of approximately 12 min and then slowly approached maximal values, whereas sucrose exhibited kinetically intermediate behavior. A similar pattern was observed after removal of greater than 95% of the marker from the perfusate. Erythritol rapidly reapproached a B/P ratio of 1, whereas the B/P ratio for sucrose, dextran, and microperoxidase fell much more slowly and exceeded 1 for a full 30 min after perfusate washout. Finally, electron microscopy and fluorescence microscopy of cultured hepatocytes demonstrated the presence of horseradish peroxidase and fluorescein-dextran, respectively, in intracellular vesicles, and fractionation of perfused liver homogenates revealed that at least 35-50% of sucrose, inulin, and dextran was associated with subcellular organelles. Collectively, these observations are most compatible with a transcytosis pathway that contributes minimally to the secretion of erythritol, but accounts for a substantial fraction of sucrose secretion and virtually all (greater than 95%) of the blood-to-bile transport of microperoxidase and larger markers. These findings have important implications with respect to current concepts of canalicular bile formation as well as with respect to the conventional use of solutes such as sucrose as markers of canalicular or paracellular pathway permeability.