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

Abstract

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), inulin (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 12 min and then slowly approached maximal values, whereas sucrose exhibited kinetically intermediate behavior. A similar pattern was observed after removal of >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 fluoresceindextran, 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 (>95%) of the blood-to-bile

Effects of methylenedianiline on tight junction permeability of biliary epithelial cells in vivo and in vitro

“…The manner in which inulin could modulate either bile flow or bilirubin excretion following DAPM administration is presently unknown. However, we do not consider that the constant infusion and excretion of inulin appreciably altered pathways of bile formation because B:P ratios of control rats in our study were comparable to those observed in isolated perfused rat livers with recirculating perfusate (Lake et al, 1985;Krell et al, 1987) as well as in anesthetized rats with ligated renal pedicles and bolus injections of inulin (Shanker and Hogben, 1961;Lorenzini et al, 1986;Smith and Boyer, 1982). Furthermore, we consider the minor alterations in biliary parameters caused by constant inulin infusion to be insufficient to invalidate the DAPMinduced increases in inulin B:P ratios which occurred hours later (Fig.…”

“…Studies using isolated segments of the rat bile duct have demonstrated that inulin can enter bile by diffusion through bile ducts/ ductules (Smith and Boyer, 1982). Perfusate to plasma ratios for solutes determined in the rat bile duct were comparable to those determined in isolated perfused livers (Lake et al, 1985;Lorenzini et al, 1986) or livers of anesthetized rats (Shanker and Hogben, 1961), which suggests that bile duct/ ductular TJs have similar permeability characteristics for inert solutes as canalicular TJs. P i and glucose are believed to enter bile via TJs (Handler et al, 1994;Krell et al, 1982), but solute movement through the paracellular route is restricted by both size and charge (Kan and Coleman, 1986;Smith and Boyer, 1982).…”

High density lipoproteins, but not other lipoproteins, provide a vehicle for sterol transport to bile.

“…Furthermore, with HDL, the increase in biliary plant sterol secretion was detected within 5-10 min after HDL was added to perfusions. This relatively rapid rate of plant sterol transport from HDL is consistent with the time of appearance in bile of radiolabeled UC after the intravenous injection of lipoproteins with radiolabeled UC in the intact rat (16) and may approximate the rate of transport of small water-soluble molecules from the blood into bile by a paracellular route (17). The mechanism of delivery and route of transit of cholesterol from the blood through the liver into bile has not been defined with any certainty.…”

Uptake ability of hepatic sinusoidal endothelial cells and enhancement by lipopolysaccharide

“…The accumulation of dextran around bile capillaries indicates the release of dextran into the capillary lumen. Dextran is known to be incorporated by hepatocytes via pinocytosis and to be secreted into the bile ducts (18). the monocyte-macrophage lineage was believed to be the major cell type functioning in the elimination of exogenous substances.…”