Intrahepatic sources of biliary-like micelles

Effects of ligation and reperfusion of hepatic afferent vessels on the composition of liver cell membrane in the rat: 1H- and 31P-magnetic resonance spectroscopic analysis

Biochemical changes that occur within hepatic tissue of the rat during ischemia and subsequent reperfusion were investigated using magnetic resonance spectroscopy of liver extracts. Hepatic ischemia was produced in the rat by a continuous clamping of the left branches of the hepatic artery and portal vein. In the reperfusion experiments, the vascular clamps were released after 30 or 120 min of ischemic periods. At the end of the periods of ischemia and/or reperfusion, the left and middle hepatic lobes were dissected and processed for subsequent 1H-MRS and 31P-MRS analyses. Phosphatidylcholine, sphingomyelin, phosphatidic acid and phosphatidylethanolamine contents all showed reduction of about 30% after 120 min of ischemia. In contrast, the content of lysophosphatidylcholine showed relatively small changes following ischemia. Ten minutes after initiation of reperfusion, further decline of the total phospholipid content resulting in as much 42% reduction was observed. Then it recovered to nearly the control level when ischemia was for 30 min, but to only 65% of the control level when ischemia was for 120 min. The cholesterol/-N-(CH3)3 ratio, generally regarded as a parameter for membrane fluidity, showed about a 40% increase when ischemia was for 120 min, a change toward decreased membrane fluidity. These results appear to reflect ischemia/reperfusion-induced changes of membrane phospholipid metabolism.

Bile acid-dependent vesicular transport of lysosomal enzymes into bile in the rat

BACKGROUND

Bile acids may stimulate the movement of hepatocyte vesicles and enhance their fusion with the biliary canaliculus. The present study examined the effects of various bile acids on the exocytosis of the contents of hepatocyte lysosomes into the biliary canaliculus.

METHODS

The effects of various bile acids on hepatocyte lysosome movement and on exocytosis of the contents of hepatocyte lysosomes into the biliary canaliculus were determined from the distribution of fluorescein isothiocyanate-dextran--labeled lysosomes in hepatocyte couplets and by quantitating biliary lysosomal enzyme output in rats.

RESULTS

Hydrophobic as well as hydrophilic and nonmicellar bile acids were found to stimulate to a similar degree the output of lysosomal enzymes into bile, indicating that bile acid-induced change of canalicular or lysosomal membrane fluidity is not responsible for enhanced exocytosis. The taurocholate-dependent increase in lysosomal enzyme excretion was completely blocked by either microtubule or microfilament inhibition, suggesting that these subcellular structures are involved in bile acid-dependent vesicular transport. Fluorescent microscopy studies showed that taurocholate causes a microtubule-dependent translocation of lysosomes towards the canaliculus in hepatocyte couplets, which occurred at the same time as increased output of lysosomal enzymes into bile.

CONCLUSIONS

The results suggest that bile acids modulate vesicle traffic towards the canaliculus by a mechanism unrelated to bile acid interaction with the vesicle membrane.

Microtubule-independent choleresis and anti-cholestatic action of tauroursodeoxycholate in colchicine-treated rat liver

In order to cast light on the anti-cholestatic and cytoprotective properties of ursodeoxycholic acid (UDCA), intrahepatic transport and secretion of bile salts and biliary phospholipids were investigated by using isolated perfused livers from colchicine-pretreated rats. Administration of taurocholic acid (TCA) after colchicine pretreatment induced marked cholestasis. Tauroursodeoxycholic acid (TUDCA) treatment, in contrast, was associated with maintenance of bile flow, with excretion rates of bile acids and phospholipids similar to those in control animals. Furthermore, TCA-induced cholestasis in colchicine-treated rat livers was clearly decreased by co-administration of TUDCA. Although simultaneous addition of UDCA also showed slight improvement, with or without taurine pre-treatment, biliary bile-salt analysis also showed that cholestasis was markedly remitted as the excretion of taurine-conjugated UDCA was increased. The results suggest that the cytoprotective and anti-cholestatic effects of TUDCA may be linked to action at the intrahepatocyte level, represented by mild detergent effects on organelle lipids and preservation of intracellular transport even under microtubule-dysfunctional conditions. In addition, it was indicated that cytoprotective effects of UDCA may also be exerted after its conjugation with taurine inside hepatocytes.

Cholestasis, metabolism and biliary lipid secretion during perfusion of rat liver with different bile salts

The biological effects of bile acids depend largely upon their molecular structure. When bile acid uptake exceeds the maximal biliary secretory rate (SRm) cholestasis occurs. In order to characterize the influence of bile acid structure on its cholestatic potency we systematically studied SRm, maximal bile flow, maximal and cumulative phospholipid and cholesterol secretion with different taurine-conjugated tri-, di- and keto bile acids (Table I) in the isolated perfused rat liver. Bile acids with a high critical micellar concentration (CMC) promoted the greatest bile flow; a positive non-linear correlation between CMC and maximal bile flow was found. 3 alpha-Hydroxylated bile acids with a hydroxyl group in 6 alpha and/or 7 beta position and lacking a 12 alpha hydroxy group had a high SRm. SRm was not related to CMC or maximal bile flow, respectively. Phospholipids and cholesterol were secreted in a nearly fixed ratio of 12:1; a strong linear relationship could be observed. Cumulative phospholipid secretion over 48 min was significantly lower for non and poor micelle forming bile acids (TDHC and TUC) than for those with comparatively low CMC values (TUDC, TC, THC, THDC, TCDC) (70-140 vs. 210-450 nmol/g liver). At SRm all bile acids with good micelle forming properties showed a similar cumulative biliary lipid output. However, when biliary lipid output was related to 1 mumol bile acid secreted bile acids with a low SRm induced the highest lipid secretion (TCDC, TC). These data (1) demonstrate that a 6 alpha and/or a 7 beta hydroxy group on the steroid nucleus reduce cholestatic potency if the 12 alpha hydroxy group is absent, (2) suggest that in the case of micelle forming bile acids the total amount of phospholipids secreted in bile (depletion of cellular phospholipids) is associated with the occurrence of cholestasis whereby bile acids with a low SRm deplete the cellular phospholipid content at much lower bile acid concentrations than those with a higher SRm and (3) imply that bile acids with non and poor micelle forming properties (TDHC, TUC) presumably do not cause cholestasis (solely) by depletion of cellular phospholipids.

Taurocholate stimulates transcytotic vesicular pathways labeled by horseradish peroxidase in the isolated perfused rat liver

The effect of taurocholate on transcytotic vesicular pathways labeled with horseradish peroxidase was assessed in isolated perfused rat liver preparations. Forty-five minutes after a horseradish peroxidase load in a recirculating system, continuous infusion of taurocholate but not taurodehydrocholate significantly increased horseradish peroxidase excretion in bile by 50% compared with controls. When horseradish peroxidase (25 mg) was pulse loaded for 1 minute in control perfusions, it appeared in bile in early (4-6 minutes) and late (20-25 minutes) peaks, the latter accounting for 90% of total horseradish peroxidase output. Taurocholate infusion significantly increased horseradish peroxidase output in both early and late peaks, whereas only a small increase in the early peak was observed with taurodehydrocholate. Colchicine pretreatment increased the early peak in bile but abolished the second peak. Electron micrographs from control livers revealed the accumulation of horseradish peroxidase-containing vesicles in pericanalicular regions at early (2 minutes) as well as late (18 minutes) periods. When a morphometric analysis of electron micrographs was performed from pericanalicular regions 2 minutes after a 1-minute pulse of horseradish peroxidase (500 mg), taurocholate but not taurodehydrocholate increased both the density and percent area of horseradish peroxidase-containing vesicles compared with controls. In contrast, colchicine pretreatment had no effect on the density of the early-appearing vesicles, although their individual sizes were reduced. Taurocholate but not taurodehydrocholate also increased the percent of tubular structures in the pericanalicular region. These findings indicate that taurocholate stimulates both early and late transcytotic vesicle pathways and therefore probably microtubule-independent vesicle pathway is present in hepatocytes that must be distinguished from paracellular routes.

Tubulovesicular transcytotic pathway in isolated rat hepatocyte couplets in culture. Effect of colchicine and taurocholate

Isolated rat hepatocyte couplets in short-term culture (6 h) were labeled for 3 min with horseradish peroxidase (HRP) to characterize the transcytotic vesicle transport pathway in this cell culture system that retained an "apical" canalicular membrane polarity. Microtubules were identified with monoclonal antibodies to beta-tubulin and fluorescein iso-thiocyanate-labeled goat-antimouse antibody and were concentrated in the apical domain, a structural polarity that was eliminated by pretreatment with colchicine. In control cells, HRP immediately labeled vesicles and tubules in the submembrane regions of the periphery of the cell. Within 10 min tubules and vesicles were prominently labeled in pericanalicular regions, a process blocked by colchicine but not by lumicolchine or taurocholate administration. A quantitative morphometric analysis utilizing a Zeiss Videoplan-2 image analyzer established that (a) HRP-containing structures increased in density, area, length, and diameter in the pericanalicular region by 10 min; (b) colchicine, but not lumicolchicine, pretreatment diminished their density, area, and length; and (c) taurocholate (50 microM), a choleretic and biliary lipid-stimulating bile acid, had no effect on HRP density or percentage of area in the pericanalicular region, but decreased the diameter of the pericanalicular HRP-containing structures and increased the percentage of tubules containing HRP from 29% to 40%. Tubules were particularly prominent in thick sections (400 nm) in both peripheral and pericanalicular regions and were viewed as continuous anastomosing linear arrays in stereo-paired micrographs. These studies established that isolated rat hepatocyte couplets maintain a highly polarized tubulovesicular transcytotic pathway in short-term culture that is micro-tubule-dependent. Taurocholate stimulates the transformation of tubules from vesicles in this isolated rat hepatocyte couplet system.