Phospholipid degradation in, and protein content of, rat fistula bile. Contamination of bile with pancreatic juice

Effect of etofibrate on bile production in the normolipidemic rat

1. The effect of etofibrate, the ethandiol-1,2 diester of nicotinic and clofibric acids on bile production was studied in male rats that received a daily dose of 300 mg of etofibrate/kg body weight by stomach tube for 10 days and were compared with control rats receiving the medium. 2. The bile duct was cannulated, animals were intravenously given 1 microCi (4-14C)-cholesterol/100 b.w. and bile was collected at different intervals for a total of 4 hr. 3. Etofibrate treatment decreased plasma cholesterol and triglyceride concentrations and increased the bile flow. The cummulative amount of both bile volume and total bile radioactivity secreted increased linearly in all the animals; the respective slopes being higher in etofibrate treated rats than in controls. 4. The main labelled component found in the bile was always bile acids rather than cholesterol and the proportion of each of these compounds was similar in both groups. Neither was any difference between the groups found in the concentration of bile acids, cholesterol and phospholipids nor in the cholesterol/(bile+phospholipid) ratio. 5. Besides other factors, the present results indicate that an increase in bile flow and biliary cholesterol excretion in its free form and after its conversion into bile acids should contribute to the hypocholesterolemic effect of etofibrate.

Quantitative modifications induced by angiotensin II on rat bile secretion

Angiotensin II (ANG II) effects on bile secretion were studied in the rat. ANG II (0.1 microgram/kg) was injected into the jugular vein every 30 min and bile samples of 30 min were collected for 120 min. Blood arterial and portal venous pressures were simultaneously recorded before and after the administration of ANG II. Results showed that ANG II decreased bile flow and the excretion of sodium, potassium, chloride and bile acids whereas it increased pH, bile osmolality and the excretion rate of bicarbonate and calcium. ANG II also led to a rapid increase in mean arterial pressure as well as portal venous pressure which reverted to control values within 1 min. The present results suggest that ANG II may modulate bile flow and the excretion rate of the different biliary constituents. We have previously investigated atrial natriuretic factor effects on bile secretion and although the atrial factor antagonizes most of ANG II biological actions, unexpectedly, ANG II effects on bile secretion were not opposite to those of atrial natriuretic peptide. The modifications induced by ANG II on bile secretion may play an important role in pathophysiological conditions such as hypertensive states with increased ANG II circulating levels.

Atrial natriuretic factor modifies bile flow and composition in the rat

The effects of atrial natriuretic factor (ANF) on bile secretion were studied in the rat. ANF was injected 'in bolus' (5.0 micrograms/kg) every 30 min into the jugular vein. The common bile duct was cannulated and bile samples were collected every 30 min for 120 min. Systemic blood arterial pressure was registered before and after the administration of ANF. Results showed that ANF decreased bile flow and the excretion rate of sodium, potassium, chloride, bile acids, cholesterol and proteins. On the other hand, it increased pH and the excretion of bicarbonate and calcium. As ANF slightly reduces mean arterial pressure, its vascular effect may not be considered the major event. In addition, increased excretion of bicarbonate and calcium, and the fact that ANF vascular effect is short in time suggest that the peptide may have a non-vascular effect on the processes of bile formation and its modifications along the bile ducts. This extravascular effect may be exerted on the hepatocyte ions exchangers and/or at the ductal level on the processes of excretion and reabsorption of electrolytes and water.

Effects of niacin on biliary lipid output in the rat

The mechanisms for the hypocholesterolaemic action of niacin (nicotinic acid) were examined in rats administered niacin at a dose of 400 mg/kg body wt/day for either 2 or 4 weeks. Another group of rats were administered diosgenin, an inhibitor of acyl-CoA:cholesterol acyltransferase, as a 1% (w/w) supplement in the diet for 7 days. Both agents produced small increases in bile flow rates (up to 40%) and mild hepatotoxicity evidenced by small increases in serum transaminase activities. Niacin treatment for 2 or 4 weeks lowered serum cholesterol concentrations by 13% or 29%, respectively, with the greatest decrease occurring in the low density lipoprotein fraction. This was accompanied by relatively large increases in biliary cholesterol output (114% and 130% after 2 and 4 weeks treatment, respectively) with smaller increases in the biliary output of phospholipid (18% and 45%) and bile acid (26% and 14%). Diosgenin treatment increased serum cholesterol by 29% and increased the biliary output of cholesterol, phospholipid and bile acid by 800%, 10% and 45%, respectively. Thus, both agents increased the cholesterol saturation of bile (100% by niacin, 500% by diosgenin). Cholesterol and phospholipid in fistula bile from control rats were present in lamellar and micellar forms. Niacin treatment did not alter the physical form of biliary lipids whilst diosgenin caused the appearance of vesicular lipid in fistula bile. Thus, increased biliary secretion of cholesterol explains, at least in part, the hypocholesterolaemic action of niacin. In addition, since aggregation of biliary vesicles is involved in cholesterol gallstone formation in humans, the non-appearance of vesicular material in fistula bile from niacin-treated rats may be of some importance.

Biliary excretion of polyethylene glycol molecular weight 900. Evidence for a bile salt-stimulated vesicular transport mechanism

Polyethylene glycol molecular weight 900 (PEG-900) has been used as a marker of vectorial water transport into bile canaliculus. However, the mechanisms by which this compound is excreted have not been clarified. To gain more information on this process, we studied the biliary excretion of [3H]PEG-900 in rats during choleresis induced by canalicular choleretics. In addition, the effects of the microtubule inhibitors colchicine and vinblastine, and of the acidotropic agent chloroquine, on PEG-900 excretion were studied to determine whether a vesicular pathway is involved. Continuous i.v. infusion of either dehydrocholate (DHC, a non-micelle forming bile salt choleretic) or 4-methylumbelliferone (4-MU, a non-bile salt canalicular choleretic) at stepwise-increasing rates [0.7, 1.0 and 1.2 mumol.min-1.(100 g body wt)-1] induced a gradual increment in bile flow, whereas a transient increment of [3H]PEG-900 excretion was observed only during DHC-induced choleresis. Furthermore, studies in which two consecutive i.v. injections of DHC (10 mumol/100 g body wt) were administered showed that [3H]PEG-900 excretion induced by a second administration of DHC was 54% lower than that induced by the first one, despite a similar excretion in bile flow. Finally, colchicine (0.5 mumol/100 g body wt), vinblastine (0.5 mumol/100 g body wt) and chloroquine (50 mg/kg body wt) pretreatments inhibited the DHC-induced increment in biliary [3H]PEG-900 output, while DHC-induced choleresis was almost unaffected. Conversely, excretion of [14C]sucrose, when coadministered with [3H]PEG-900, was not impaired by the treatments. These results suggest that, unlike sucrose, PEG-900 excretion is not associated with canalicular water movements. Instead, it may be related to a vesicular transport process followed by a bile acid-stimulated discharge of secretory vesicles into bile through the lysosomal compartment.

Studies on the mechanism of bile salt-independent bile flow impairment in streptozotocin-induced hepatotoxicity

The main factors involved in the impairment of formation of the bile salt-independent bile flow (BSIF) in streptozotocin (SZ)-treated rats were examined. Twenty-four hours after SZ injection (50 mg/kg body wt, i.v.) bile flow, bile salt output and biliary excretion of the major inorganic electrolytes (sodium, chloride and bicarbonate) were significantly diminished. The relationship between bile flow and bile salt output obtained during the administration of sodium taurocholate at stepwise-increasing rates indicated that bile salt-independent bile flow (y-intercept) was diminished by 37% in SZ-treated rats. The relationship between electrolyte output and bile salt output showed that the fractions of sodium, chloride and bicarbonate excreted independently of bile salt (y-intercept) decreased to 59%, 47% and 67% of the control values respectively, while the amount of electrolyte secreted per unit of bile salt secreted was unaffected in SZ-treated rats. The hepatic activity of Na+,K(+)-adenosine triphosphatase (Na+,K(+)-ATPase) was decreased by 59% (P less than 0.05) in SZ-treated rats. Nicotinamide administered prior to SZ prevented the hyperglycemia indicative of SZ-induced diabetes, but had no effect on the decrease in Na+,K(+)-ATPase activity caused by the drug. These results suggest that SZ itself, and not its diabetogenic effect, decreases the BSIF by a mechanism that involves impairment of the biliary electrolyte excretion, which could be the result of the inhibition of the hepatic Na+,K(+)-ATPase activity.

Biliary lipid secretion in the rat. The uncoupling of biliary cholesterol and phospholipid secretion from bile acid secretion by sulfated glycolithocholic acid

Glycolithocholic acid and its sulfated derivative are major metabolites of the secondary bile acid lithocholic acid in man. Both compounds are known to induce cholestasis in experimental animals. We compared the effects of these endogenous hepatotoxins on bile production and biliary lipid composition in rats with chronic biliary drainage. The compounds were administered enterally at relatively low rates (5-50% of the rats' endogenous bile acid secretion in these experiments) to simulate enterohepatic circulation. Both compounds were substantially secreted into bile (more than 90% of dose); sulfated glycolithocholic acid unchanged and glycolithocholic acid after hepatic hydroxylation predominantly in the form of glyco-beta-muricholic acid (cf. Kuipers et al. (1986) Am. J. Physiol. 251, G189-G194). Neither glycolithocholic acid nor its sulfated derivative affected the biliary excretion of endogenous bile acids or bile flow in these experiments. In spite of this, phospholipid and cholesterol secretion were significantly reduced by sulfated glycolithocholic acid but were not altered by glycolithocholic acid. Phospholipid and cholesterol secretion rapidly decreased to 25 and 50% of their initial values, respectively, at biliary output rates of sulfated glycolithocholic acid up to 2 mumol/h, and did not further decrease when this output was increased to 6 mumol/h. Small unilamellar liposomes consisting of cholesterol, [Me-14C]choline-labeled phosphatidylcholine, phosphatidylserine and [3H]cholesteryl oleate in a 5:4:1:0.1 molar ratio were employed to label intrahepatic lipid pools. Administration of sulfated glycolithocholic acid slightly reduced bile acid synthesis from [3H]cholesteryl oleate, but significantly reduced the biliary secretion of [14C]phospholipid. Glycolithocholic acid did not affect the hepatic processing of liposomal lipids. It is concluded that sulfated glycolithocholic acid at low doses causes the uncoupling of biliary lipid secretion from that of bile acids, which might represent in initiating event in sulfated glycolithocholic acid hepatotoxicity.