Biochemistry of bile secretion

The influence of dietary saturated and unsaturated fat on hepatic cholesterol metabolism and the biliary excretion of chylomicron cholesterol in the rat

The biliary excretion of [3H] cholesterol carried in chylomicrons derived from palm oil (rich in long chain saturated fatty acids), olive oil (rich in monounsaturated fatty acids) or corn oil (rich in n-6 polyunsaturated fatty acids was studied in vivo in rats fed the corresponding oil in the diet for 21 days. The secretion of radioactivity into bile as both bile acids and unesterified cholesterol was significantly slower in the animals fed palm oil as compared to those given olive or corn oil, indicating that dietary saturated fat retards the excretion of cholesterol from the diet as compared to mono- or n-6 polyunsaturated fat. In order to investigate the mechanisms underlying these differences, the influence of the three high fat diets on cholesterol esterification, cholesteryl ester hydrolysis and bile acid synthesis in the liver and on biliary lipid output were also measured. The ratio of cholesterol esterification to cholesteryl ester hydrolysis was markedly raised in the olive and corn oil-fed as compared to palm oil-fed animals. Biliary cholesterol secretion was higher in corn oil-fed rats than in those fed olive or palm oil or a low fat diet, and this was associated with a markedly increased lithogenic index in these animals. The activity of cholesterol 7alpha hydroxylase was higher in the olive and corn oil-fed than in the palm oil-fed animals, although the expression of mRNA for the enzyme was increased only in the olive oil diet group. After 20 h biliary drainage, the rate of bile acid secretion into bile was increased in the rats fed olive and corn oil rather than to palm oil. These findings indicate that feeding rats mono- or n-6 polyunsaturated as compared to saturated fat in the diet promotes the storage of cholesteryl ester in the liver and leads to increased bile acid synthesis, resulting in the more rapid excretion of cholesterol originating from the diet via the bile.

Extracellular and intracellular regulation of biliary lecithin hydrophobicity

Bromosulfophthalein and papaverine have been demonstrated to inhibit biliary lipid secretion without affecting secretion of bile salts in normal rats, so-called uncoupling. Bromosulfophthalein inhibits the capacity of intracanalicular bile salt micelles to induce biliary lipid secretion, and papaverine inhibits vesicular transport within the hepatocyte. We compared the effects of bromosulfophthalein and papaverine on biliary lipid secretion in normal Sprague-Dawley rats and Eizai hyperbilirubinuria rats. The fatty acyl chain saturation in biliary lecithin increased during bromosulfophthalein infusion and decreased during papaverine infusion in Sprague-Dawley rats. Bromosulfophthalein had no effect on biliary lipid secretion in Eizai rats, while papaverine induced uncoupling. The degree of fatty acyl chain saturation in biliary lecithin was unchanged during bromosulfophthalein infusion, but decreased with papaverine in Eizai rats. We deduce that selection of biliary lecithin species occurs at various points in the lipid transport pathway at intracellular and intracanalicular sites.

Effects of nitric oxide on leukotriene D4 decreased bile secretion in the perfused rat liver

Nitric oxide (NO) and leukotrienes are potent vasoactive agents that are involved in the control of portal blood flow. The present study investigated the role of leukotriene D4 and NO in a non-recirculating constant pressure rat liver perfusion model to analyse their interchanges on portal flow and bile secretion. The addition of leukotriene D4 (20 nM) to the perfusate for 5 minutes resulted in a decrease in portal blood flow (-55.3%), in bile flow (-24.4%) as well as bile acid release (-35.2%). In parallel, leukotriene D4 increased glucose output. The administration of a lower dose of leukotriene D4 (5 nM) reduced the respective parameters to a lesser degree, indicating dose-dependence. The addition of NO via the infusion of sodium nitroprusside (0.05 mM, 1 mM) reduced the effect of leukotriene D4 on portal flow, bile flow and bile acid secretion whereas the leukotriene D4 effects on hepatic glucose output remained unaffected. Correlation coefficient between decrease in portal flow and reduction of bile flow by infusing leukotriene D4 was R = 0.91, while in the presence of sodium nitroprusside R = 0.85. These results suggest that the leukotriene D4-induced cholestasis is dependent on portal flow. In contrast, hepatic vasoconstriction does not contribute to glycogenolysis stimulated by leukotriene D4 in the perfused liver.

Effect of cholestasis induced by organic anion on the lipid composition of hepatic membrane subfractions and bile in rats

Several organic anions inhibit the secretion of cholesterol and phospholipid into bile without affecting total bile acid secretion (uncoupling). The uncoupling induced by sulphobromophthalein (BSP) alters the fatty acid composition of biliary lecithin. The purpose of this study was to investigate the relationship between the lipid composition of bile and of liver subcellular membrane fractions during BSP-induced uncoupling. After depletion of the bile salt pool, rats fitted with a bile duct cannulus were infused with sodium taurocholate given either alone or with BSP. Bile was collected and liver microsomes and canalicular membranes were isolated for analysis of lipid composition. In bile, uncoupling increased the cholesterol/phospholipid ratio (C/P ratio) and the saturated/unsaturated fatty acid ratio (S/U ratio) in phosphatidylcholine. The C/P ratio was increased in the canalicular membrane, but the membrane phosphatidylcholine S/U ratio was decreased during uncoupling. In microsomes, the S/U ratio of membrane phosphatidylcholine was slightly increased, but the C/P ratio was unaffected during uncoupling. These results support the hypothesis that an increased secretion of hydrophobic phosphatidylcholine species from the canalicular membrane into bile reduces the proportion of hydrophobic phosphatidylcholine species in the canalicular membrane during uncoupling. The decreased contribution of hydrophobic phosphatidylcholine species may ameliorate the decrease in membrane fluidity resulting from the accumulation of cholesterol in the canalicular membrane and stimulate the synthesis of hydrophobic phosphatidylcholine species in the microsomes.

Cholestatic effects of cyclosporine in the rat

BACKGROUND

Previous studies of cyclosporine-induced cholestasis were flawed by confounders encountered in human studies and discrepancies in acute animal experiments. Even the cyclosporine vehicle, polyoxyethylated castor oil (Cremophor EL), had been implicated in cholestasis. The purpose of this study was to investigate how cyclosporine affects bile salt kinetics and biliary lipid secretion in a rat model under steady state conditions.

METHODS

Three groups of male Lewis rats (n=10) were given daily subcutaneous injections of either cyclosporine (CsA; 10 mg/kg body weight), Cremophor, or NaCl (control) for 1 week. Twenty-four-hour bile collection was performed 18 hr after the last injection. The first hour's output measured bile flow and organic bile solute secretion rates. Bile salt pool size and basal synthesis were determined with the washout technique.

RESULTS

CsA significantly reduced basal bile flow and bile salt secretion by 25%. Bile salt synthesis was suppressed 45% (CsA: 3.50+/-0.8 micromol/g liver/24 hr vs. control: 6.31+/-1.17 micromol/g liver/24 hr; P<0.05), which resulted in a 28% reduction in the bile salt pool size (CsA: 16.9+/-1.9 micromol/g liver vs. control: 23.6+/-2.0 micromol/g liver; P<0.05). Bile salt-independent flow was significantly suppressed (29%), whereas bile salt-dependent flow was only modestly reduced. Biliary phospholipid output decreased 23% (CsA: 11.7+/-0.8 nmol/min/g liver vs. control 15.2+/-1.1 nmol/min/g liver; P<0.05), but cholesterol secretion was unaltered, resulting in a 29% increase in the cholesterol saturation index (CsA: 0.40+/-0.03 vs. control 0.31+/-0.02; P<0.05). Cremophor had no significant effects on bile secretion or bile salt kinetics.

CONCLUSIONS

CsA induces cholestasis by decreasing both bile flow and bile salt secretion. Its suppression of bile salt synthesis reduces the bile salt pool size. The drug inhibits bile salt and phospholipid secretion without a corresponding change in cholesterol secretion and thus elevates cholesterol saturation in bile, a potential risk for gallstone formation.

Taurolithocholate can inhibit the biliary discharge of lysosomes in the rat

The natural bile salt taurolithocholate (TLC) impairs the biliary excretion of lipids and proteins, which are known to reach the canaliculus via vesicles. In this study we examined whether these observations could be extended to the exocytic discharge of lysosomal contents into bile. The single intravenous injection of a cholestatic dose of TLC, 3 micromol/100 g body wt., markedly inhibited the biliary excretion of the lysosomal enzymes acid phosphatase and beta-glucuronidase, despite the excretion of bile salts being normalized after a transient diminution. Under such a condition, TLC did not affect the normal transport to and the processing in lysosomes of the exogenously administered [14C]sucrose-labeled horseradish peroxidase. However, the biliary excretion of the radioactive lysosomal metabolites of the protein was significantly reduced. The results indicate that TLC can inhibit the biliary discharge of lysosomes in the rat without altering the functional integrity of these organelles. Possible explanations for these findings are discussed.

Expression of a non-MDR2-coded liver phosphatidylcholine membrane transport protein in Xenopus laevis oocytes

Phosphatidylcholines (PC) are secreted into the bile via a membrane transport protein(s). Recently, evidence for ATP-dependent mdr2-encoded PC transport as well as for carrier-mediated PC transport had been reported. Therefore, we investigated whether mdr2 P-glycoprotein is involved in the transport of a water-soluble short chain phosphatidylcholine analogue L-alpha-dibutyroyl-PC (diC4PC) induced by expression of liver mRNA in Xenopus laevis oocytes. Expression of mouse and rat mdr2 cRNA did not result in diC4PC net uptake in Xenopus laevis oocytes. By contrast oocytes showed a similar carrier-mediated uptake activity for diC4PC after injection of mouse, rat and human liver total mRNA (Km 7.7, 9.6, and 11.6 mM). Antisense inhibition of mdr2 mRNA expression increased diC4PC uptake induced by total liver mRNA from mouse and rat. The present data prove the existence of a specific mRNA for a non-mdr2-coded cell membrane PC carrier in mouse, rat, and human liver which exhibits similar transport affinity for diC4PC as the PC carrier in rat liver canalicular membranes.

Biliary secretion of anionic polypeptide fraction is not coupled to that of phospholipids and cholesterol in rats

Anionic polypeptide fraction (APF) is a phospholipid- and calcium-binding apoprotein present in animal and human bile, predominantly associated with cholesterol-phospholipid vesicles. In bile, the protein may play a physiological role in preventing precipitation of calcium salts. APF has also been suggested to be of regulatory importance in the process of biliary lipid secretion. The aim of the present study was to investigate whether the secretion rates of APF and that of biliary lipids are coupled, which would support a physiological role of APF in biliary lipid secretion. Biliary secretion rates of bile acids, phospholipids, and cholesterol were experimentally modulated in three different rat models. Secretion rates of APF were compared with that of bile acids, lipids, and with that of two other biliary proteins, the lysosomal protein beta-glucuronidase and apolipoprotein A-I (apo A-I). Model 1: diurnal variation in bile formation during chronic bile diversion; model 2: specific inhibition of biliary phospholipid and cholesterol, but not of bile acid secretion by infusion of the organic anion, sulfated lithocholyltaurine; model 3: acute interruption of the enterohepatic circulation in unanesthetized rats. The diurnal variation in bile formation involved a parallel increase of the biliary secretion rates of bile acids (+56 +/- 7%, mean +/- SD), phospholipids (+53 +/- 29%), cholesterol (+73 +/- 54%), and APF (+72 +/- 86%) during the night phase of the cycle. Infusion of sulfated lithocholyltaurine inhibited biliary phospholipid and cholesterol secretion (-78 +/- 15%, and -54 +/- 25%, respectively), but did not affect biliary bile acid or APF secretion rate (-19 +/- 14%, and +12 +/- 107%, respectively). Within 4 hours after interruption of the enterohepatic circulation, bile secretion rates for bile acids (-92 +/- 3%), phospholipids (-74 +/- 13%), cholesterol (-64 +/- 8%), and APF (-58 +/- 24%) rapidly declined to a new steady-state level. Correlation analysis using the data from the three experimental models indicated that the biliary secretion rate of APF was independent from that of phospholipids, cholesterol, beta-glucuronidase, and, presumably, apolipoprotein A-I, and positively correlated to bile acid secretion rate and bile flow. The data from three experimental models indicate that the biliary secretion rates of APF and of phospholipids/cholesterol are not coupled and, therefore, do not support a direct physiological role of APF secretion in biliary lipid secretion. APF secretion into bile may, at least partially, be controlled by biliary bile acid secretion.

Interactions between organic anions, micelles and vesicles in model bile systems

Biliary lipid secretion probably involves both 'micellization' and 'vesiculization' of bile-canalicular membrane lipids. Several hydrophilic organic anions inhibit the secretion of lipids into the bile without altering bile salt secretion [Verkade, Vonk and Kuipers (1995) Hepatology 21, 1174-1189]. Hydrophobic organic anions do not interfere with biliary lipid secretion. We investigated whether the organic-anion-induced inhibition of biliary lipid secretion in vivo could be attributed to inhibition of micellization, by the application of in vitro models of micellization. Carboxyfluorescein was entrapped in a self-quenching concentration in small unilamellar vesicles (SUV) composed of cholesterol/egg phosphatidylcholine (molar ratios 0, 0.2 and 0.5). Certain organic anions clearly affected the bile-salt-induced release of fluorescence from these SUV, reflecting interference with micellization. However, the effects of hydrophilic and hydrophobic organic anions did not correspond with their effects on biliary lipid secretion in vivo, irrespective of the bile salt species used (taurocholate, taurodeoxycholate or tauroursodeoxycholate) and of the lipid composition of the SUV. Ultracentrifugation and dynamic light-scattering studies indicated that organic anions do interact with bile salt/ phosphatidylcholine/cholesterol mixed micelles, but that they do not inhibit micellization, for example by competing with phosphatidylcholine and/or cholesterol for incorporation into mixed micelles. In conclusion, the present in vitro data indicate that the in vivo mechanism of organic-anion-induced inhibition of biliary lipid secretion is not mediated by inhibition of micellization.

Hepatic inactivation of Leu-enkephalin

Leu-enkephalin radiolabelled at the N-terminal tyrosine by two different methods was presented to isolated perfused rat livers. Approximately 10% of a pulse of tritiated Leu-enkephalin was taken up first-pass; this was increased to 62% when the peptide was iodinated with Bolton and Hunter reagent. Uptake of both forms of radiolabelled Leu-enkephalin was inhibited by taurocholate in a concentration-dependent manner. The proportion of internalised radioactivity secreted into bile also differed but in both cases showed a very rapid time-course similar to that of [24-(14)C]taurocholate and suggestive of non-endocytic transfer via membrane transport proteins. Pre-perfusion with the aminopeptidase inhibitor bestatin increased uptake of 3H-labelled Leu-enkephalin from 10% to 23%; no further increase occurred when the endopeptidase 24.11 inhibitor thiorphan was also present. On infusion of the native peptide into rat livers, 80% of Leu-enkephalin immunoreactivity was lost between the pre- and post-hepatic perfusate; this was reduced to 65% in the presence of 10(-5) M bestatin. The almost total release of the N-terminal tyrosine from 3H-labelled Leu-enkephalin which escaped first-pass uptake confirmed that substantial sinusoidal metabolism had occurred. Low levels of aminopeptidase N were visualised in the sinusoidal membrane using a specific monoclonal antibody coupled to peroxidase staining. Thus, hepatic inactivation of Leu-enkephalin is primarily via hydrolysis mediated by cell surface peptidase (including aminopeptidases) whilst uptake of the intact peptide, probably by a bile salt transport protein, is quantitatively minor unless the N-terminus is blocked by Bolton and Hunter reagent or peptidase inhibitors are present.

Post-natal changes in biliary lipids in suckling goat kids

The post-natal composition of biliary lipids in the bile of suckling goat kids has been studied during the first month of life. Samples of hepatic and vesicular bile were obtained from animals fed with milk. Bile flow increased with age and the concentration of bile acids also rose up to 14 days of age. Cholesterol reached maximum values immediately after birth and then decreased. Bile acids conjugated with taurine predominated and this pattern of preferential tauroconjugation persisted at all ages, as has also been found in carnivores and sheep. Comparison of vesicular and hepatic bile composition revealed a poor concentration capacity of the gallbladder in pre-ruminant kids.

Prevention of cholesterol cholelithiasis by dietary unsaturated fats in hormone-treated female hamsters

We examined the effect of diet on gallstone incidence and the composition of biliary phosphatidylcholines in methyltestosterone-treated female hamsters. These hamsters were fed a nutritionally adequate purified lithogenic diet containing 2% corn oil, 4% butterfat, 0.3% cholesterol, and 0.05% methyltestosterone, resulting in a cholesterol gallstone incidence of 86%. This incidence was lowered when mono- and polyunsaturated fats or fatty acids were added to the diet: 2.5% oleic acid resulted in total prevention of cholesterol cholelithiasis, 2.5% linoleic acid, and 4% safflower oil (78% linoleic acid content) reduced gallstone incidence to 26 and 8%, respectively. An additional 4% butterfat (29% oleic acid content) produced gallstones in 50% of the animals. At the end of the 6-wk feeding period, the bile of all hamsters was supersaturated with cholesterol. The major biliary phosphatidylcholine species in all groups were (sn-1-sn-2): 16:0-18:2, 16:0-18:1, 18:0-18:2, 16:0-20:4, and 18:2-18:2. The safflower oil- and linoleic acid-fed hamsters exhibited an enrichment of 16:0-18:2 (16-18%); added butterfat or oleic acid increased the proportion of 16:0-18:1 (9 and 25%, respectively). We conclude that the phosphatidylcholine molecular species in female hamster bile can be altered by dietary fats/fatty acids and that mono- and polyunsaturated fatty acids play a role in suppressing the induced cholelithiasis.

Effects of feed antibiotic avoparcine on organ morphology in broiler chickens

Groups of 90 male broilers each were administered the antibiotic avoparcine mixed into feed in concentrations of 7.5, 10, and 15 ppm and achieved a higher mean body weight than the controls fed without this admixture. At the end of the 70-d fattening period, histological examination was carried out on selected individuals. The small intestine, liver, bursa of Fabricius, thymus, thyroid gland, pancreas, kidneys, heart, and skeletal muscle were observed on paraffin sections stained with hematoxylin and eosin. Cell proliferation was assessed in the liver and small intestine by means of bromodeoxyuridine labeling. The exposure to avoparcine resulted in a decreased cell proliferation in both tissues when compared to controls. In addition, hypertrophy of the hepatocytes and development of reactive lymphoid tissue in the bursa of Fabricius, which occurred in the controls, were absent in the treated animals. These observations indicate that the growth-promoting effect of avoparcine is related to a restriction in the host animals of responses to intestinal bacteria. No adverse pathological changes were observed in the examined tissues, indicating that avoparcine was well tolerated.

Current concepts of hepatic uptake, intracellular transport and biliary secretion of bile acids: physiological basis and pathophysiological changes in cholestatic liver dysfunction

Hepatic sinusoidal uptake of bile acids is mediated by defined carrier proteins against unfavourable concentration and electrical gradients. Putative carrier proteins have been identified using bile acid photoaffinity labels and more recently using immunological probes, such as monoclonal antibodies. At the sinusoidal domain, proteins with molecular weights of 49 and 54 kDa have been shown to be carriers for bile acid transport. The 49 kDa protein has been associated with the Na(+)-dependent uptake of conjugated bile acids, while the 54 kDa carrier has been involved in the Na(+)-independent bile acid uptake process. Within the hepatocyte, cytosolic proteins, such as the glutathione S-transferase (also designated the Y protein), the Y binders and the fatty acid binding proteins, are able to bind bile acids and possibly facilitate their movement to the canalicular domain. At the canalicular domain a 100 kDa carrier protein has been isolated and it has been shown by several laboratories that this particular protein is concerned with canalicular bile acid transport. The system is ATP-dependent and follows Michaelis-Menten kinetics. Interference with bile acid transport has been demonstrated by several chemicals. The mechanisms by which these chemicals inhibit bile acid transport may explain the apparent cholestatic properties observed in patients and experimental animals treated with these agents. Several studies have shown that Na+/K(+)-ATPase activity is markedly decreased in cholestasis induced by ethinyloestradiol, taurolithocholate and chlorpromazine. However, other types of interference have been described and the cholestatic effects may be the result of several mechanisms. Cholestasis is associated with several adaptive changes that may be responsible for the accumulation of bile acids and other cholephilic compounds in the blood of these patients. It may be speculated that the nature of these changes is to protect liver parenchymal cells from an accumulation of bile acids to toxic levels. However, more detailed quantitative experiments are necessary to answer questions with regard to the significance of these changes and the effect of various hepatobiliary disorders in modifying these mechanisms. It is expected that the mechanisms by which bile acid transport is regulated and efforts to understand the molecular basis for these processes will be among the areas of future research.

Rôle of bile in the elimination of dipropyl sulphone from the male rat

Following oral administration of [35S]-dipropyl sulphone to male Wistar rats (4.24 mmol/kg body wt), the only radioactive component subsequently found in the blood and bile was the sulphone. Biliary excretion played an important rôle in the elimination of this compound, with 16% of the dose excreted during the first twenty-four hours and 33% passing through the bile duct over a two day period. Bile/plasma concentration ratios remained constant during the first day (c. 46-fold), suggesting that a concentration process was taking place and that active transport of this low molecular weight compound (150 Da) into the bile was occurring.

Fate of dipropyl sulphone in rat

1. Dipropyl [35S]-sulphone was administered by gavage (4.24 mmol/4 ml/kg body weight) to the adult male Wistar rat following an overnight fast. 2. Urine was the major route of excretion (83%) with more radioactivity appearing during the second day (47%) than the first (28%). Only small amounts were found in the faeces (10%). Biliary excretion played an important role with substantial amounts of the dose (33%) passing through the bile duct during 0-48 h. A near total recovery was achieved suggesting that only small amounts (2%) may have been lost as volatile components. 3. Metabolism was limited, the majority (> or = 98%) of the sulphone being recovered unchanged. Oxidation of the sulphur with the formation of inorganic sulphate was the only pathway observed.

Hepatobiliary function and toxicity in vitro using isolated hepatocyte couplets

1. Hepatocyte couplets can be routinely prepared from rat liver to produce a suitable in vitro model for polarized primary cells. 2. Centrifugal elutriation provides a means of producing enriched subpopulations of periportal and perivenous couplets from the same liver, thus providing a means of studying the influence of zonal heterogeneity on hepatobiliary function. 3. The maintenance of structural and secretory polarity demonstrated by hepatocyte couplets provides a convenient in vitro system for mechanistic studies of factors both regulatory and adversely affecting hepatobiliary functions. 4. Couplets are also uniquely appropriate for specific studies of regulation at the biliary pole, on the performance of junctions and on the maintenance and rate of transcytotic movement. 5. The possibility also exists that effects of an in vivo pre-exposure to agents causing hepatobiliary dysfunction can be assessed in couplets ex vivo.

Enhancement of Mdr2-mediated phosphatidylcholine translocation by the bile salt taurocholate. Implications for hepatic bile formation

Expression of the Mdr2-protein in secretory vesicules (SVs) from the yeast mutant sec6-4 causes a time- and temperature-dependent enhancement of phosphatidylcholine (PC) translocation from the outer to the inner leaflet of the SV lipid bilayer. We show that this activity is independent of changes either in the membrane potential or the pH gradient (inside positive) generated in these SVs by the yeast proton-translocating PMA1 ATPase. However, loading of the SVs with the primary bile salt taurocholate results in an apparent enhancement of Mdr2-mediated PC translocation activity. Reducing the intravesicular taurocholate (TC) concentration by dissipating the electrochemical potential across the SV membranes eliminates the enhancing effect of TC. Three lines of evidence suggest that the enhanced Mdr2-mediated PC translocation activity is not caused by a regulatory effect of TC on Mdr2 but rather reflected the formation of TC/PC aggregates or micelles in the lumen of SVs. First, significantly higher detergent concentrations are required to reveal the fluorescence of (7-nitro-2-1,3-benzoxadiazol-4-yl)amino-PC molecules translocated in Mdr2-SV under conditions of TC stimulation than under control conditions; second, the nonmicelle-forming bile salt taurodehydrocholate does not cause enhancement of PC translocation in Mdr2-SVs; third, enzyme marker studies indicate that TC behaves as a potent lipid solubilizer directly extracting PC molecules out of the bilayer without causing leakage. This results in the formation of intravesicular aggregates or mixed micelles, and provokes the apparent stimulation of Mdr2 activity. These data demonstrate a unique relationship between Mdr2, PC, and TC in the process of bile formation and secretion.

Absence of correlation between hepatic function and characteristics of migrating motor complexes in the gastrointestinal tract

BACKGROUND

Cyclic changes in gallbladder filling and emptying during the migrating motor complex (MMC) cycle have been demonstrated by scintigraphy. However, a possible cyclic change in the hepatic function and handling of the pharmacologic agents used for scintigraphy during the MMC cycle could have an influence on these results. The aim of the present study was to investigate the hepatic handling of cholic acid and mebrofenin in relation to the MMCs of the gastrointestinal tract.

METHODS

The plasma disappearance rate of 14C-cholic acid and the hepatic uptake and excretion of 99mTc-mebrofenin were examined during phase I and phase II of the MMC in six healthy male volunteers.

RESULTS

The plasma disappearance rate of 14C-cholic acid showed a biexponential course with an initial rapid and late slow phase after a bolus injection. There were no significant differences between the initial or late plasma disappearance rate of 14C-cholic acid during phase I as compared with phase II. The results of the time-activity curves from the 99mTc-mebrofenin scintigraphy showed an exponential rapid increase in radioactivity followed by an almost linear slow decrease after a bolus injection. There was no significant difference between phase I and phase II in any of the variables studied.

CONCLUSION

The lack of a relationship between hepatic handling of cholic acid and mebrofenin and MMC excludes this as a possible source of error in the investigations of the dynamic function of the enterohepatic circulation and especially gallbladder motility by the use of either cholic acids or iminodiacetic acid derivatives as investigative agents.

Fractionation of livers following diosgenin treatment to elevate biliary cholesterol

The plant saponin, diosgenin, is known to induce a marked increase in biliary cholesterol/phospholipid ratio. We reasoned that putative biliary lipid supply vesicles might be similarly enriched with cholesterol. Seven-day diosgenin feeding to rats resulted in significantly increased biliary cholesterol and cholesterol/phospholipid ratio, but had no effect on total cholesterol or phospholipid content of the liver. Subcellular fractionation of livers showed no selective increase in any fraction (nuclear, mitochondrial, lysosomal, microsomal) of the homogenate. Further subfractionation of microsomal or nuclear (plasma membrane) fractions also showed no difference between control and diosgenin groups. Thus, no intracellular vesicle fraction has been identified with the provision of the enhanced biliary cholesterol and the results are discussed in terms of the possible involvement of cytosolic lipid-binding proteins as putative lipid carriers to the canalicular membrane as an alternative to the presence of the lipid in lipid supply vesicles.

Comparative studies on bile flow and biliary lipid excretion after bile-acid loading in normal and partially hepatectomized rats

This study was performed to investigate sequential changes in bile secretion and biliary lipids after taurocholic acid (TCA) loading of regenerating rat liver. TCA was administered intravenously at stepwise-increasing doses to groups of non-operated control and partially hepatectomized rats, 24, 72 and 168 h after surgery. Bile flow, bile-acid output (BAO) and phospholipid output (PLO) (expressed per gram of liver) in partially hepatectomized rats increased more than in the controls. Using an isolated perfusion rat-liver system, TCA infusion was also carried out on groups of non-operated control and hepatectomized rats 72 h after operation. Again bile flow, BAO and PLO (expressed per gram of liver) were significantly higher in the partial hepatectomy case, mirroring the results obtained in vivo. When horseradish peroxidase (HRP) was pulse-loaded in isolated perfusion preparations, the second peak of biliary HRP secretion in hepatectomized rats was significantly higher than in controls. We conclude that increased bile-acid flow in partially hepatectomized rats is dependent upon acceleration of vesicular transport accompanying or following proliferation in regenerating livers.

Regulation of biliary lipid secretion by mdr2 P-glycoprotein in the mouse

Disruption of the mdr2 gene in mice leads to a complete absence of phospholipid from bile (Smit, J. J. M., et al. 1993. Cell. 75:451-462). We have investigated the control of both mdr2 P-glycoprotein (Pgp) expression and bile salt secretion on biliary lipid secretion in the mouse. Lipid secretion was monitored at various bile salt output rates in wild-type mice (+/+), heterozygotes (+/-), and homozygotes (-/-) for mdr2 gene disruption. In (-/-) mice, phospholipid secretion was negligible at all bile salt output rates. In (+/-) mice, a curvilinear relation between bile salt and phospholipid secretion was observed similar to that in (+/+) mice; however, at all bile salt secretion rates phospholipid secretion was reduced compared to (+/+) mice, indicating that mdr2 Pgp exerts a strong control over secretion. Infusion of increasing amounts of taurocholate up to maximal secretory rate led to a decline in the phospholipid and cholesterol secretion in both (+/+) and (+/-) mice in accordance to what has been observed in other species. In contrast, in (-/-) mice cholesterol secretion increased under these conditions while phospholipid output remained extremely low. The increased cholesterol secretion may represent extraction of cholesterol from the canalicular plasma membrane by taurocholate micelles as opposed to the concomitant secretion of both phospholipid and cholesterol in the presence of a functional mdr2 Pgp. Increased bile flow in (-/-) mice could be attributed completely to an increase in the bile salt-independent fraction and may therefore be caused by the bile duct proliferation in these mice.

Isolation of rat pgp3 cDNA: evidence for gender and zonal regulation of expression in the liver

Distinct differences exist in the function and regulation of the individual p-glycoprotein (pgp) members in many species. In order to study regulation of individual pgp mRNA isoforms it is, therefore, necessary to have probes that can distinguish between the various pgp isoforms. However, to date few studies examining hepatic gene expression in rat liver have used pgp gene specific probes. Towards this end we screened a cDNA library constructed from a normal rat liver with a human pgp1 cDNA and isolated a partial cDNA for class III pgp, rat pgp3. By comparison of the sequence of this new rat pgp3 cDNA with genomic and cDNA sequences for rat pgp1 and rat pgp2 we selected oligonucleotide probe sequences that would allow us to differentiate between the highly homologous rat pgp2 and pgp3 genes on Northern blots and by polymerase chain reaction (PCR). We found that pgp3, for both male and female rats, was the predominant form of pgp expressed in normal rat liver with males consistently expressing several-fold lower levels of pgp3 than females. Because many genes are zonally expressed in the hepatic acinus we examined the possibility that pgp3 might show heterogeneous distribution as well. We found, by in situ hybridization of paraformaldehyde-fixed rat liver sections that pgp3 was distributed non-uniformly across the hepatic acinus with a gradient that showed the highest expression toward the terminal hepatic venule. We confirmed this finding by selectively isolating hepatocytes from either the terminal hepatic venular or periportal zones using a digitonin/collagenase perfusion procedure. Application of specific pgp3 PCR primers to RNA isolated from hepatocytes from these areas confirmed that pgp3 mRNA was the predominant form in the hepatocytes surrounding the terminal hepatic venule. Finally, we examined pgp3 expression in a variety of tissues by Northern blot analysis and found that pgp3 was most highly expressed in the liver and gastrointestinal tract (with a gradient of expression from small to large intestine), while low levels were found in the kidney, heart and brain. Pgp3 mRNA was undetectable in the adrenal gland and in skeletal muscle. In summary, using rat pgp gene specific oligonucleotide probes we found that pgp3 gene expression is regulated by anatomic location with the highest mRNA expression in organs that are involved in drug detoxification. Our results also demonstrate heterogeneity of hepatic rat pgp3 gene expression, which is influenced by both gender and by acinar location.

The mutant Eisai hyperbilirubinemic rat is resistant to bile acid-induced cholestasis and cytotoxicity

We investigated bile flow and biliary excretion of bile acids in the Eisai hyperbilirubinemic rat, a Sprague-Dawley mutant rat with conjugated hyperbilirubinemia, using both in vivo and in vitro models. In vivo bile flow was lower in Eisai hyperbilirubinemic rats than in the control rats before and after taurocholate was infused. After taurocholate was infused, bile acid output was similar in the Eisai hyperbilirubinemic rats and control rats. In the isolated perfused rat liver, biliary excretion of bile acids was higher in the Eisai hyperbilirubinemic rats than in the control rats after a high-dose infusion of taurocholate (0.33 mumol/min/gm liver). Infusion of taurochenodeoxycholate (0.22 mumol/min/gm liver) did not produce cholestasis and did not reduce the biliary excretion of bile acids in the Eisai hyperbilirubinemic rats. Taurochenodeoxycholate significantly increased the phospholipid/bile acid molar ratio and slightly reduced bile acid-induced alkaline phosphatase output into bile. The release of lactate dehydrogenase from the perfused liver 30 min after the start of the taurochenodeoxycholate infusion was 10 times lower in the Eisai hyperbilirubinemic rats than in the control rats (2.0 +/- 0.8 vs. 28.7 +/- 6.8 mU/min/gm liver). When the isolated perfused rat liver was infused with a 1-min pulse of horseradish peroxidase (25 mg), we observed an early and late peak of biliary excretion of horseradish peroxidase. The Eisai hyperbilirubinemic rats showed a significant increase in the late peak.(ABSTRACT TRUNCATED AT 250 WORDS)

Ultrastructure of the intracellular membranous system of rat hepatocytes in intrahepatic cholestasis induced by phalloidin

To investigate the effect of a thickened pericanalicular ectoplasm in tubulovesicular transport and biliary excretion, we examined the ultrastructure of the intracellular membranous system in rat hepatocytes with and without phalloidin treatment, by transmission electron microscopy and scanning electron microscopy combined with the Aldehyde prefix Osmium-Dimethyl Sulfoxide-Osmium method. Hepatocytes possessed elaborate networks of tubules around bile canaliculi, and some of them extended to the bile canaliculi in control rats. Vesicles were also present around the bile canaliculus. Treatment of rats with phalloidin produced a thick pericanalicular ectoplasm around the bile canaliculus visualized by transmission electron microscopy, and the density of vesicles (p < 0.001) and tubules (p < 0.001) within 0.5 microns around the bile canaliculus significantly decreased in phalloidin-treated rats. The number of lysosomes in hepatocytes apparently increased in phalloidin-treated rats; however, they were rarely observed around the bile canaliculus. The Aldehyde prefix Osmium-Dimethyl Sulfoxide-Osmium method produced an organelle-free space around the bile canaliculus by removing the thick pericanalicular ectoplasm in scanning electron microscopic examination, and the thickened pericanalicular ectoplasm inhibited the approach of intracellular membranes to the canalicular membrane in the transmission electron microscopic examination. In some pathological cholestatic conditions, the thickened pericanalicular ectoplasm may inhibit not only bile canalicular contraction but also biliary excretion of substances, which is mediated by the tubulovesicular transport system.

Tubulovesicular transport of horseradish peroxidase in isolated rat hepatocyte couplets: effects of low temperature, cytochalasin B and bile acids

The transcytotic vesicular pathway in isolated rat hepatocyte couplets was investigated using horseradish peroxidase. Ten to 20 min after horseradish peroxidase labeling, vesicles and tubules containing horseradish peroxidase were observed to be predominantly around the bile canaliculi. In hepatocytes incubated in a 4 degrees C medium for 10 min after horseradish peroxidase labeling, few horseradish peroxidase-containing structures were observed around the bile canaliculi, and the fine reticular immunofluorescence of microtubules was reduced. Cells treated with cytochalasin B (a microfilament inhibitor) showed a fair number of horseradish peroxidase-containing structures around the markedly dilated bile canaliculi and the distribution of microtubules was preserved. Cells labeled by horseradish peroxidase and then incubated for 10 min in a horseradish peroxidase-free medium containing 50 mumol/L of taurocholic acid, ursodeoxycholic acid or tauroursodeoxycholic acid had more tubular structures containing horseradish peroxidase around the bile canaliculi than control cells, whereas 50 mumol/L of taurochenodeoxycholic acid, taurodeoxycholic acid, dehydrocholic acid and taurodehydrocholic acid each failed to increase the number of tubular structures. These findings show that horseradish peroxidase was transported in hepatocyte couplets from the cell periphery to the bile canalicular front through the tubulovesicular pathway, depending on cytoplasmic microtubules. Cytoplasmic microfilaments appeared to play a minor role in this transport. Several specific bile acids such as taurocholic acid, ursodeoxycholic acid and tauroursodeoxycholic acid each promoted the tubular transformation.

The synergistic action (cross-talk) of glucagon and vasopressin induces early bile flow and plasma-membrane calcium fluxes in the perfused rat liver

A study was made of the initial responses of perfusate Ca2+ fluxes and bile flow to Ca(2+)-mobilizing agonists, following refinements to the methods for analysing these parameters in the perfused rat liver. Net Ca2+ efflux induced by vasopressin commences at 15 s, reaches a maximal rate at 35 s and declines to zero by 55 s, when Ca2+ influx commences. Vasopressin-induced increases in bile flow commence by 20 s, attain a maximal rate by 35 s and begin to decline at 50 s, to reach basal values by 90 s. Concomitant administration of glucagon modifies each of these actions of vasopressin in the following ways: it decreases by 5 s the time of onset of net Ca2+ efflux, and the time and magnitude of such efflux, and the time of onset of bile flow is decreased to 15 s, and the flow reaches maximal rates by 30 s. When the alpha 1-adrenergic agonist phenylephrine is used in place of vasopressin, Ca2+ efflux commences at 17-18 s and is greater in magnitude; little bile flow is induced by this agonist. Glucagon modifies the action of phenylephrine in the following ways: the onset of Ca2+ efflux is brought forward by 2-3 s, it is of lower magnitude and Ca2+ influx begins by 45 s; bile flow commences by 15-20 s, and reaches a maximum at 30 s, where the rate is much greater than in the absence of glucagon; this rate gradually declines to be near basal by 80 s. The onset of agonist-induced oxygen uptake was also brought forward by the co-administration of glucagon. Comparison of agonist-induced plasma-membrane Ca2+ fluxes and bile flow (with or without glucagon administration) suggests that correlations can be made between net Ca2+ fluxes and the transient increases seen in bile flow.

S-adenosylmethionine prevents total parenteral nutrition-induced cholestasis in the rat

Both an excess and an imbalance of amino acids have been associated with total parenteral nutrition-induced cholestasis. The present study was undertaken to further our understanding of this condition in light of observations that methyl donor amino acids may be protective. Rats were maintained on Travasol (3.4 g amino acids/24 h) and dextrose (10.2 g/24 h) with and without the "active methyl" S-adenosylmethionine at a dose of 75 mg/kg/24 h for 5 days, and compared to control rats on dextrose alone (10.2 g/24 h) with free access to rat chow. Bile flow (microliters/min) was lower (p < 0.025) in the Travasol (8.65 +/- 0.78) than in the control group (12.30 +/- 0.52) and was restored in the Travasol+S-adenosylmethionine animals (11.42 +/- 10). Furthermore, the bile acid secretory rate (mumol/h) was higher (p < 0.05) with S-adenosylmethionine (23.34 +/- 3.71) than without S-adenosylmethionine (14.16 +/- 2.19). As expected, the molar ratio of biliary cholesterol was lower (p < 0.005) in both total parenteral nutrition groups. However, in the total parenteral nutrition group without S-adenosylmethionine, there was also a decrease in the molar ratio of phospholipids which correlated well with the bile acid secretory rate. Analysis of liver plasma membranes showed that a lower activity of Na+K(+)-ATPase (mumol Pi/mg protein/h) (p < 0.005) in the Travasol animals (6.26 +/- 0.53) was restored to control values (15.20 +/- 1.43) by the addition of S-adenosylmethionine (17.07 +/- 2.87). In the three groups, a close correlation was observed between Na+K(+)-ATPase activity and bile flow.(ABSTRACT TRUNCATED AT 250 WORDS)

On the transfer of serum proteins to the rat intestinal juice

The in vivo pattern of serum proteins in the rat small-intestinal juice was characterized by crossed immunoelectrophoresis. Immunoglobulins and albumin, alpha-1-antitrypsin, transferrin, and orosomucoid were present. Larger serum proteins were absent (ceruloplasmin, haptoglobin, alpha-1-macroglobulin, alpha and beta lipoproteins). Thus, apart from immunoglobulins, only serum proteins with a molecular mass less than approximately 100 kDa were demonstrated. The origin and epithelial transfer were further characterized, using albumin as a model. No sign of local synthesis of albumin by the enterocytes was found by Northern blotting, and no albumin was found in the Golgi complex by immunogold electron microscopy. By immunogold electron microscopy a heavy labelling of albumin was observed in the interstitial spaces between the villus enterocytes. Where the enterocytes disintegrated, albumin was seen to leak out into the intestinal lumen from the opened interstitial spaces. A weak labelling was also found in the lysosomal/endosomal-like structures, especially in the crypt enterocytes, indicating pinocytosis of albumin. We conclude that the main reason for the occurrence of certain serum proteins in the intestinal juice is a selective passage through the capillary wall followed by passive intercellular transport via delivery of the serum in the interstitial space during disintegration of the enterocytes.

Fate of dipropyl sulphide and dipropyl sulphoxide in rat

1. Dipropyl [35S]-sulphide and dipropyl [35S]-sulphoxide were administered by gavage (4.24 mM/4 ml/kg body wt) to adult male Wistar rats following an overnight fast. 2. Urine was the major route of excretion for both compounds, with more radioactivity appearing during the second day (c. 43%) than the first (c. 26%). Only small amounts were found in the faeces (c. 5%). Biliary excretion played an important role with substantial amounts of the dose (c. 25%) passing through the bile duct during 0-48 h. Following ingestion of the sulphide large quantities of radioactivity (18%) were detected in exhaled air. Near total recoveries were achieved for both compounds, although 13% of the radioactivity remained within the carcass 3 days after administration of the sulphoxide. 3. Absorption and elimination half-lives were in the region of 5 and 8 h, respectively, for both compounds, with the sulphoxide plasma profile showing a prolonged plateau region. 4. Metabolism was limited to oxidation of the sulphur with the formation of the sulphoxide and sulphone, and trace amounts of inorganic sulphate.

Crosstalk between calcium- and cyclic AMP-mediated signalling systems and the short-term modulation of bile flow in normal and cholestatic rat liver

The flow of bile is subject to short-term modulation by glucagon and calcium-mobilizing hormones. Of potential relevance is the crosstalk between the second messenger-mediated signal transducing systems of these agonists. This latter point has revealed an area of investigation that should enable further insights to be made into a physiological network that interrelates bile flow, hepatocellular calcium movements and hormone action. This information in turn may provide insights into the etiology and treatment of human and animal diseases in which cholestasis is an underlying feature.

Homozygous disruption of the murine mdr2 P-glycoprotein gene leads to a complete absence of phospholipid from bile and to liver disease

Two types of P-glycoprotein have been found in mammals: the drug-transporting P-glycoproteins and a second type, unable to transport hydrophobic anticancer drugs. The latter is encoded by the human MDR3 (also called MDR2) and the mouse mdr2 genes, and its tissue distribution (bile canalicular membrane of hepatocytes, B cells, heart, and muscle) suggests a specialized metabolic function. We have generated mice homozygous for a disruption of the mdr2 gene. These mice develop a liver disease that appears to be caused by the complete inability of the liver to secrete phospholipid into the bile. Mice heterozygous for the disrupted allele had no detectable liver pathology, but half the level of phospholipid in bile. We conclude that the mdr2 P-glycoprotein has an essential role in the secretion of phosphatidylcholine into bile and hypothesize that it may be a phospholipid transport protein or phospholipid flippase.

Characterization of biliary proteins and composition of bile from sheep and goats

Higher lipid content in gall bladder bile of sheep and goats as compared with monogastric species was due to higher proportion of glycerides. The volume of bile per gall bladder was higher in sheep than in goats. Bile from sheep and goats has a higher content of proteins and lower contents of dry matter, cholesterol and phospholipids compared with monogastric species. Biliary proteins partially purified by the ammonium sulfate precipitation method were characterized and quantified by polyacrylamide gel electrophoresis (PAGE), fast protein liquid chromatography (FPLC) and immunodiffusion. FPLC and PAGE profiles indicate that biliary proteins of sheep and goats are mainly of low mol. wt. of upto 65 KDa. Proportions of immunoglobulin (Ig) like molecules in biliary proteins precipitated by ammonium sulfate of different degrees of saturation, were 10.4 to 26.7 and 21.9 to 30.7 per cent of total proteins in bile of goats and sheep. IgG and IgA like molecules were the predominant Ig and lacked carbohydrates. On immunodiffusion such Ig like molecules did not show cross reactivity with humoral Ig. It was concluded that sheep and goats appear to secrete Ig into bile after deglycosylation and partial degradation. The proteins of smaller molecular weight may be secretory fragments or degradation products of Ig.

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.

Cholesterol crystallization-promoting activity of aminopeptidase-N isolated from the vesicular carrier of biliary lipids

Different hydrophobic glycoproteins are associated to native biliary vesicles, which are the major carrier of biliary cholesterol. Some of these proteins promote cholesterol crystallization, a key step in cholesterol gallstone formation. This study was specifically conducted to identify the 130 kDa biliary vesicle-associated glycoprotein and to determine its in vitro effect on the cholesterol crystal formation time. The 130 kDa vesicular glycoprotein was identified as aminopeptidase-N by amino acid sequencing and specific enzymatic assay. Polyclonal antibodies raised against aminopeptidase-N allowed us to determine its concentration in human hepatic bile, which varied from 17.3 to 57.6 micrograms/ml. Aminopeptidase-N showed a concentration-dependent cholesterol crystallization activity when it was added to supersaturated model bile at a concentration range usually found in native bile. Because of this promoting effect on in vitro cholesterol crystal formation, we suggest that biliary aminopeptidase-N may play a critical role in the pathogenesis of cholesterol gallstone disease.

Bile secretion and liver regeneration in partially hepatectomized rats

OBJECTIVE

The purpose of this research was to study the correlation between bile secretion and the liver regeneration in the partially hepatectomized rat.

SUMMARY BACKGROUND DATA

Significant alteration in bile formation and secretion is expected in the liver after hepatectomy. There is scant literature, however, about the effects of liver regeneration in bile secretion.

METHODS

The work was done in rats with 50% hepatectomy, 75% hepatectomy, and sham operation as the control. A chronic common bile duct fistula and a duodenal cannula were established for bile collection and the sample analysis on days 1, 3, 5, 7, and 9.

RESULTS

With size reduced in the liver after 50% and 75% hepatectomy, the total bile volume decreased 45.9% and 51.5%, bile salt independent flow decreased 59.3% and 64.9%, bile salt secretion rate decreased 36.1% and 43.4%, bile salt basal synthesis rate decreased 52.3% and 56.4%, phospholipid secretion rate decreased 52.6% and 68.0%, and cholesterol secretion rate decreased 54.3% and 72.4% from control on day 1, respectively. All changes returned to the control level in 3 to 9 days with accompanying increasing liver size during regeneration.

CONCLUSION

Alterations of total bile flow, bile salt independent flow, bile salt secretion rate, bile salt basal synthesis rate, and biliary lipid secretion after partial hepatectomy correlate with the liver regeneration rate in rats. Partial hepatectomy reduces the bile salt independent fraction calculated as per 100 g body weight rather than the dependent fraction. The study of bile salt and biliary lipid secretion is a useful method for monitoring synthetic function in liver regeneration in vivo.

Effect of diosgenin on biliary cholesterol transport in the rat

Biliary cholesterol output in rats was stimulated over 3-fold by feeding diosgenin for 5 days, whereas biliary outputs of phospholipid and bile salts were not changed by diosgenin feeding. Isolating and perfusing the liver without bile salts resulted in a rapid and substantial decrease in biliary bile salt output; bile salt depletion abolished the diosgenin-induced increment in biliary cholesterol output, showing that the diosgenin-elevated biliary cholesterol output was bile-salt-dependent. Diosgenin treatment also produced a significant decrease in biliary alkaline phosphodiesterase I. Fresh bile obtained from control and diosgenin-fed rats was subjected to gel-permeation chromatography in order to separate different-sized biliary cholesterol carriers. Two major peaks of cholesterol were eluted, with cholesterol also being eluted between the peaks. The cholesterol peak eluted at the lower molecular mass (20-30 kDa) was observed in all bile samples. The higher-molecular-mass peak, which was eluted at the void volume, was not observed in all biles; control biles contained very little high-molecular-mass form of cholesterol, whereas biles from the diosgenin group contained up to 47% of cholesterol in the high-molecular-mass fraction. Diosgenin treatment produced a range of elevated biliary cholesterol values which positively correlated with the proportion of cholesterol contained in the high-molecular-mass fraction (r = 0.98). The results show that diosgenin induced a marked bile-salt-dependent increase in biliary cholesterol output and a shift in biliary cholesterol transport to higher-molecular-mass structures.

Effects of a hypercholesterolaemia-inducing diet on biliary electrolytes and lipid secretion in the rat

The effects of a high cholesterol/cholate diet on the biliary secretion of bile acids, cholesterol, phospholipid and inorganic electrolytes in the rat were examined. Wistar rats were fed a hypercholesterolaemia-inducing diet (HID) for 20 days prior to the biliary experiments. Rats fed a standard laboratory diet were used as controls. The HID diet increased plasma cholesterol concentrations and the hepatic content of total, free and esterified cholesterol, without changes in ALP*, ALT and AST plasma activities. Bile flow and biliary secretion of bile acids and inorganic electrolytes were markedly increased in the hypercholesterolaemic animals. The stimulated biliary secretion was due to an increase in both the bile acid-dependent and bile acid-independent fractions of bile flow. An increase in the bile acid pool induced by the administration of exogenous cholic acid/cholesterol would account for these changes in bile flow and inorganic electrolyte secretion. Nevertheless, the increase in the bile acid-independent bile flow could be also related, at least to some extent, to a higher efficiency of bicarbonate transport into bile. The HID diet also increased both cholesterol and phospholipid biliary outputs, whereas it did not modify the relationship between lipid and bile acid secretion. The lithogenic index of bile was reduced in the rats after the HID regimen due to a relatively higher increase in the biliary outputs of phospholipids and bile acids than of cholesterol.

The mode of action of primary bile salts on human platelets

Cholate and its conjugated amide derivatives glycocholate and taurocholate solubilized human platelets differently as studied by the observations on: (1) the change in optical absorbance of platelet suspension, (2) marker leakiness and (3) component solubility. Cholate ruptures the membrane in an all-or-none process, while both conjugated derivatives shed off both proteins and lipids. The shed lipids formed vesicles and could be separated from the proteins. The conjugated salts gradually chop off the cell membrane into pieces causing the cells to become small spheres (1.5 microns in diameter) as revealed by scanning electron microscopy, which also revealed that morphological change of platelet in these bile salts depended on both concentration and incubation period. Platelets at the prelytic-stage concentration of these three salts deformed initially to spiculate disc and finally to a stretched-out flat form. Also, in the prelytic stage of these bile salts, platelets showed inhibited responses to thrombin which did not happen to platelets in deoxycholate (Shiao et al. (1989) Biochim. Biophys. Acta 980, 56-68.).