Accumulation Of Bile Salts Research Articles

Hepatobiliary transport

The alterations of hepatobiliary transport that occur in cholestasis can be divided into primary defects, such as mutations of transporter genes or acquired dysfunctions of transport systems that cause defective canalicular or cholangiocellular secretion, and secondary defects, which result from biliary obstruction. The dysfunction of distinct biliary transport systems as a primary cause of cholestasis is exemplified by the genetic defects in progressive familial intrahepatic cholestasis or by the direct inhibition of transporter gene expression by cytokines. In both, the hepatocellular accumulation of toxic cholephilic compounds causes multiple alterations of hepatocellular transporter expression. In addition, lack of specific components of bile caused by a defective transporter, as in the case of mdr2/MDR3 deficiency, unmasks the toxic potential of other components. The production of bile is critically dependent upon the coordinated regulation and function of sinusoidal and canalicular transporters, for instance of Na +-taurocholate cotransporting polypeptide (NTCP) and bile salt export pump (BSEP). Whereas the downregulation of the unidirectional sinusoidal uptake system NTCP protects the hepatocyte from further intracellular accumulation of bile salts, the relative preservation of canalicular BSEP expression serves to uphold bile salt secretion, even in complete biliary obstruction. Conversely, the strong downregulation of canalicular MRP2 (MRP, multidrug resistance protein) in cholestasis forces the hepatocyte to upregulate basolateral efflux systems such as MRP3 and MRP1, indicating an inverse regulation of basolateral and apical transporters. The regulation of hepatocellular transporters in cholestasis adheres to the law of parsimony, since many of the cellular mechanisms are pivotally governed by the effect of bile salts. The discovery that bile salts are the natural ligand of the farnesoid X receptor has shown us how the major bile component is able to regulate its own enterohepatic circulation by affecting transcription of the genes critically involved in transport and metabolism.

Hepatic sinusoidal fibrosis induced by cholesterol and stilbestrol in the rabbit: 1. Morphology and inhibition of fibrogenesis by dipyridamole

This study documents the hepatic morphology and the ultrastructure of a model of hepatic fibrosis in rabbits. Rabbits were given a cholesterol-supplemented diet (1%), a stilbestrol diet (10 mg subcutaneously twice a week), or both treatments simultaneously for 7 weeks. Rabbits given the combined treatment developed sinusoidal and portal fibrosis with only a mild disturbance of acinar vascular relationships. Ultrastructurally, there was marked widening of the spaces of Disse by collagen fibers, basement membrane material adjacent to endothelial cells and hepatocytes, blunted hepatocellular microvilli, activated stellate cells, lipid droplets in endothelial cells and hepatocytes, and degranulated platelets in sinusoids. The hepatic hydroxyproline content was markedly increased (12.0 +/- 5.2 vs. 4.8 +/- 1.5 mmol/g of liver dry weight; P < .001). Plasma bile acids were markedly increased (222 +/- 180 vs. 12 +/- 5 in controls; P < .001). Dipyridamole (25 mg every 12 hours) that was given in addition to cholesterol and stilbestrol decreased the hepatic collagen content (-49% and -48%, in two experiments; P < .05 in both) and splenomegaly. This model provides a reliable method for the production of extensive sinusoidal fibrosis with capillarization of sinusoids. Hepatocellular degeneration is only mild to moderate, and fibrosis occurs slowly without the sudden pathological changes that occur with other models of hepatic fibrosis, such as with the administration of CCl4 or galactosamine. The mechanism of injury may involve the accumulation of bile salts or the generation of free radicals from cholesterol oxidation products. The possibility that the sinusoidal release of platelet-derived factors may have a role in the activation of stellate cells (lipocytes) is supported by the suppression of fibrogenesis by dipyridamole.

Hepatic sinusoidal fibrosis induced by cholesterol and stilbestrol in the rabbit: 1. Morphology and inhibition of fibrogenesis by dipyridamole

This study documents the hepatic morphology and the ultrastructure of a model of hepatic fibrosis in rabbits. Rabbits were given a cholesterol- supplemented diet (1%), a stilbestrol diet (10 mg subcutaneously twice a week), or both treatments simultaneously for 7 weeks. Rabbits given the combined treatment developed sinusoidal and portal fibrosis with only a mild disturbance of acinar vascular relationships. Ultrastructurally, there was marked widening of the spaces of Disse by collagen fibers, basement membrane material adjacent to endothelial cells and hepatocytes, blunted hepatocellular microvilli, activated stellate cells, lipid droplets in endothelial cells and hepatocytes, and degranulated platelets in sinusoids. The hepatic hydroxyproline content was markedly increased (12.0 +/- 5.2 vs. 4.8 +/- 1.5 mmol/g of liver dry weight; P < .001). Plasma bile acids were markedly increased (222 +/- 180 vs. 12 +/- 5 in controls; P < .001). Dipyridamole (25 mg every 12 hours) that was given in addition to cholesterol and stilbestrol decreased the hepatic collagen content (-49% and -48%, in two experiments; P < .05 in both) and splenomegaly. This model provides a reliable method for the production of extensive sinusoidal fibrosis with capillarization of sinusoids. Hepatocellular degeneration is only mild to moderate, and fibrosis occurs slowly without the sudden pathological changes that occur with other models of hepatic fibrosis, such as with the administration of CCl4 or galactosamine. The mechanism of injury may involve the accumulation of bile salts or the generation of free radicals from cholesterol oxidation products. The possibility that the sinusoidal release of platelet-derived factors may have a role in the activation of stellate cells (lipocytes) is supported by the suppression of fibrogenesis by dipyridamole. (Hepatology 1996 Oct;24(4):855-64)

Heterogeniteit Van De Leverparenchymcellen

SummaryLiver parenchymal cells reveal an important heterogeneity in their functions, which can mainly be linked to their different localization in the simple liver acinus of Rappaport. Hepatocytes of zone 1 are situated close to the vascular axis consisting of a terminal hepatic arteriole and terminal portal venule. As such, these hepatocytes receive blood rich in oxygen, nutrients and bile salts coming from the gastrointestinal tract, and hormonal factors. Metabolically, this region is characterized by the oxydative breakdown of carbohydrates, amino acids and fatty acids. Gluconeogenesis, which is an endergonic process that must be driven by the oxydative catabolism, is also a function of zone 1 hepatocytes. This region of the acinus should always release glucose and this function is induced by high ratio of (glucagon + catecholamines)/insulin in this zone. Acinar zone 1 hepatocytes are the first to receive blood and substrates and contribute predominantly to transport of bile salts and bilirubin and to the secretion of the bile salt-dependent fraction of canalicular bile production.This corresponds to a greater diameter of bile canaliculi and a predominant localization of« Golgirich area » in this region. In patients with longstanding biliary obstruction, « cholate stasis » can be observed histologically, which may represent an accumulation of bile salts in periportal hepatocytes.Perivenous hepatocytes of zone 3 are the most distant from the supplying blood vessels and receive blood that has already exchanged oxygen, nutrients and hormones with cells in zones 1 and 2. Metabolically, this region is characterized by processes that can occur in anaerobic conditions such as glycoly sis and liponeogenesis. This perivenous zone should always take up glucose and this function is induced by a high ratio insulin/(glucagon + catechola mines) in this region.Hepatocytes of zone 3 also contribute predomi nantly to cytochrome P-450 dependent drug metabolism, which corresponds to the predominant lo calization of the smooth endoplasmic reticulum in zone 3. The peculiar susceptibility of perivenous he patocytes during drug-induced liver disease can also be explained by the predominant formation of toxic metabolites in zone 3 by the cytochrome P-450 system.The cells of zone 3 do not contribute significantly to bile salt and bilirubin transport in physiologic conditions but may be « recruted » as the concen tration of incoming bile salts or bilirubin is increa sed. These cells may also contribute to the generation of canalicular bile, more specifically to the formation of the so-called bile salt-non dependent fraction.

Mechanism of Cholestasis

To determine whether changes in the function of the hepatocellular microsomal biotransformation system play a role in the genesis of cholestasis or result from it, the biochemical alterations in the biotransformation system and the appearance of the endoplasmic reticulum were compared in rats during the 1st 4 days after bile duct ligation. The time course of the changes in the structure of the hepatocytes was also compared with the results of commonly used hepatic tests. Cholestasis is characterized by decreased metabolism of substrates bound to the lipoprotein of cytochrome P-450 despite an increase in the amount of smooth endoplasmic reticulum. This increase occurs after cholestasis is apparent and, therefore, results from it. The changes in the endoplasmic reticulum with hypertrophy of the smooth form may be adaptive in nature. The high but transient elevation of transaminase activity soon after bile duct ligation was not associated with morphological signs of injury. The later injurious effects of cholestasis are thought to be caused, at least in part, by the accumulation of bile salts, which by their detergent action decrease the function of the hypertrophic smooth endoplasmic reticulum.

BILE SALT ABSORPTION IN THE IRRADIATED RAT.

Measurements of radioactivity in bile secreted by bile duct-cannulated rats after intraluminal injection of the labeled bile salt indicated bile salt absorption rates in the duodenum, ileum, and large intestine in the ratio of 1:30:2. Three days after exposure of the abdomen to 1,000 or 1,500 R of X-irradiation, absorption from the ileum was decreased by 14 and 53%, respectively. Five days after exposure to 1,500 R, absorption from the ileum was decreased by 70%. The radiomimetic nitrogen mustard, HN2, also decreased absorption. This was shown to be a radiation-induced absorption defect which probably resulted in increased excretion of bile salts, rather than an impaired secretion of bile by the liver, by injecting labeled bile salts intravenously or parenterally. At 4 days after 1,500 R and 1 day after intravenous injection of C14-labeled cholic acid, the accumulation of labeled bile salt in the lower bowel was almost twice the unirradiated value. The level of radioactivity secreted in the bile at that time was one-half control value and that present in the small intestine only one-sixth the unirradiated level, suggesting a dilution of the labeled bile salt as a result of de novo synthesis and a substantial accumulation of bile salts in the lower small intestine and large bowel.