The physicochemical basis of cholesterol gallstone formation in man

Cholesterol and bile salt studies on the bile of patients with cholesterol gallstones

Gallbladder bile from patients with cholesterol stones and control patients without cholesterol stones was analysed for the relative concentrations of bile salts, phospholipids, and cholesterol. It was not possible to distinguish stone-forming from control biles when these were plotted on triangular coordinates. There was a significant increase in the dihydroxy: trihydroxy bile salt ratio in the patients with cholesterol gallstones. The maximum cholesterol holding capacity for the bile salts was determined and no difference could be detected in the ability of the bile salts from either cholesterol stone-containing and control bile samples to dissolve cholesterol.

Primate biliary physiology. 8. The effect of phenobarbital upon bile salt synthesis and pool size, biliary lipid secretion, and bile composition

Phenobarbital, by inducing liver microsomal enzymes, may affect bile acid synthesis from cholesterol and thus alter the secretion of biliary lipids and the composition of bile. We, therefore, determined the effects of phenobarbital on bile flow, biliary lipid secretion, bile acid synthesis, and bile-acid pool size. Using an experimental preparation that allows controlled interruption of the enterohepatic circulation (1), we administered 5 mg/kg per day of phenobarbital to healthy Rhesus monkeys for 1-2 wk to achieve steady-state conditions. Three animals were studied with an intact enterohepatic circulation and three with a total bile fistula, each animal served as its own control. Total bile flow and secretion of bile salt, phospholipid, and cholesterol were measured every 24 h during steady-state conditions. Further, under conditions of an intact enterohepatic circulation bile-acid synthetic rate was measured in three animals and pool size estimated in two animals during both control and drug treatment periods. Phenobarbital at doses of 5 mg/kg per day increased bile flow 30-50% in all animals (P < 0.001). The increased bile flow resulted from both an increased "bile-salt independent fraction" and an increased bile-salt secretion rate. Phenobarbital significantly increased bile salt (P < 0.01) and phospholipid secretion (P < 0.05) by about 30% but cholesterol secretion was not significantly changed. Consequently, the concentration of cholesterol relative to bile salt and phospholipid was decreased (P < 0.001). Phenobarbital significantly enhanced the maximal rate of bile acid synthesis 25-30% in all three monkeys with total bile fistulas (P < 0.05) and also augmented bile acid synthesis and pool size in animals with intact enterohepatic circulations despite the fact that the rates of bile salt returning to the liver in these animals would have inhibited bile acid synthesis in control animals. Thus, phenobarbital not only increases the maximal rate of bile acid synthesis but also alters the normal control mechanisms by which bile salts returning to the liver inhibit bile salt synthesis. The fact that phenobarbital treatment results in increased synthesis of bile salt and unchanged secretion of cholesterol is consistant with the view that the drug augments conversion of hepatic cholesterol to bile salt. The resulting decrease in relative cholesterol content in bile may have therapeutic implications for cholesterol gallstone therapy.

Mechanisms of lithogenic bile formation in American Indian women with cholesterol gallstones

Hepatic secretions of biliary lipids were estimated in 43 patients with and without cholesterol gallstones. Studies were carried out by a marker dilution technique employing duodenal intubation with a three-lumen tube. Hourly secretion rates of cholesterol, bile acids, and phospholipids were determined during constant infusion with liquid formula. In 17 American Indian women with gallstones, hourly outputs of biliary bile acids were significantly less than those in 7 Indian men and 12 Caucasian women without gallstones. These findings suggest that a decreased hepatic secretion of bile acids contributes significantly to the production of a lithogenic bile in Indian women. However, in Indian women with gallstones, secretion of biliary cholesterol was also significantly increased, as compared with Caucasian women without stones. Therefore, lithogenic bile in Indian women was, in most cases, due to a combined decrease in bile acid output and increase in cholesterol secretion. In an attempt to determine the mechanisms for these abnormalities, cholesterol balance studies were done in Indian women with gallstones and normal Indian men. Balance data were compared with results reported previously in non-Indian patients studied by the same techniques, and in general, Indian women showed a slight increase in fecal excretion of bile acids. Since bile acids in the enterohepatic circulation were relatively depleted in Indian women, these patients had a reduced fractional reabsorption. However, previous studies have shown that Caucasians can rapidly replenish bile acid pools in the presence of much greater intestinal losses, and it is suggested that among Indian women with gallstones, reduced secretion rates of bile acids are primarily the result of defective homeostatic regulation of bile acid synthesis. In Indian women with gallstones, at least two factors may have contributed to an increased availability of cholesterol in the liver for secretion into bile. First, cholesterol was inadequately converted into bile acids, and secondly, an increased amount of cholesterol was synthesized, as shown by the balance technique. This enhanced production of cholesterol can partially be explained by obesity, but other factors may also play a role.

Intermittent secretion of abnormal bile in patients with cholesterol gall stones

Gall bladder and hepatic bile was sampled from 66 patients undergoing elective operations on the biliary tract. Fifty-one patients had cholesterol gall stones but only 59% of these were found to have bile which was supersaturated with cholesterol. Repeated sampling of hepatic bile from patients with T-tubes showed that the secretion of supersaturated bile was intermittent.These results indicate that it is impossible to separate patients with cholesterol stones from controls simply by examination of the lipid composition of their bile, since an appreciable number of bile samples from patients with cholesterol stones were unsaturated.The fact that cholesterol gall stones form when the bile is supersaturated with cholesterol only intermittently suggests that the gall bladder may also have a part in their formation.

Effect of cholestyramine on bile acid metabolism in normal man

The effect of cholestyramine administration on the enterohepatic circulation of bile acids was studied in eight normal volunteers. In six subjects the metabolism of sodium taurocholate-(14)C was determined after its intravenous injection before and during the 6th wk of cholestyramine administration, 16 g/day. In two subjects, the metabolism of cholic acid-(14)C was observed before and during the 2nd wk of cholestyramine, 16 g/day. Bile acid sequestration resulted in a more rapid disappearance of the injected primary bile acid and its metabolic products. The composition of fasting bile acids was promptly altered by cholestyramine to predominantly glycine-conjugated trihydroxy bile acid. In four subjects, unconjugated bile acid-(14)C was administered during cholestyramine administration; the relative proportion of glycine-conjugated bile acid-(14)C before enterohepatic circulation was similar to the relative proportion of unlabeled glycine-conjugated bile acid present in duodenal contents after an overnight fast, indicating that a hepatic mechanism was responsible for the elevated ratios of glycine- to taurine-conjugated bile acid (G: T ratios) observed. The relative proportions of both dihydroxy bile acids, chenodeoxycholic and deoxycholic, were significantly reduced. Steatorrhea did not occur, and the total bile acid pool size determined after an overnight fast was unaltered by cholestyramine. These findings suggest that in normal man bile acid sequestered from the enterohepatic circulation by cholestyramine is replaced by an increase in hepatic synthesis primarily via the pathway leading to production of glycocholic acid.

The composition of hepatic and gallbladder bile in patients with gallstones

Bile specimens were obtained from 17 patients with gallstones and 21 patients with duodenal ulcer. The specimens were obtained from the former by needle aspiration of the gallbladder and common bile duct at operation and from the latter by duodenal intubation. The concentrations of bile salt, phospholipid, and cholesterol were measured. Gallbladder bile from gallstone patients contained significantly more cholesterol than did ;duodenal' bile from duodenal ulcer patients. Hepatic bile from gallstone patients contained significantly more cholesterol than did gallbladder bile from the same patients. When the data were plotted on triangular coordinates the relative composition lay within the zone of cholesterol solubility in all 21 ulcer patients. The relative composition of hepatic bile lay outside the zone of cholesterol solubility in five gallstone patients, at the limits of cholesterol solubility in a further three, and within the micellar zone in the remaining nine patients. This suggests that supersaturation of hepatic bile with cholesterol is not the sine qua non for the production of cholesterol gallstones.

Management of choledocholithiasis

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Cholesterol solubility in lecithin-bile salt systems

The method of sample preparation can markedly influence the rate of dissolution and attainment of supersaturated states of cholesterol. The equilibrium solubility of cholesterol, studied as a function of its physical state in a model bile system, is almost half that of previously accepted values. Slow attainment of the equilibrium state may have acted to bias previous studies. Extrapolation of our data to the clinical situation reveals that many persons considered normal by present standards actually possess bile that is supersaturated with respect to cholesterol and are thus potential gallstone formers.

The formation of abnormal bile and cholesterol gallstones from dietary cholesterol in the prairie dog

To study the pathogenesis of cholesterol gallstones, we fed 24 adult male prairie dogs a high cholesterol, egg yolk diet. 13 control animals received a cholesterol-free diet. All animals fed the egg yolk diet formed multiple gallstones in 2-6 months' time. These stones contained cholesterol, 77+/-14% by dry weight. No stones ocurred in the control group. The egg yolk-fed animals developed bile of altered chemical composition. The cholesterol concentration of hepatic and gallbladder bile increased significantly. The molar ratios of bile acid/cholesterol and phospholipid/cholesterol decreased in hepatic and gallbladder bile. The predominant bile acid shifted from cholic acid, 78% of the total bile acids, to chenodeoxycholic acid, 60% of the total. In common bile duct cannulated animals the high cholesterol diet produced increased secretion of cholesterol by the liver and increased bile flow. In animals fed the egg yolk diet for 2 months, cholesterol-4-(14)C was included in the daily diet for the next 4 months to establish an isotopic steady state. At autopsy the mean specific activity of cholesterol was similar in serum, liver, hepatic bile, gallbladder bile, and gallstones. Thus the cholesterol of gallstones apparently equilibrated constantly throughout the study and was not sequestrated as a static pool. The high cholesterol, egg yolk diet caused the secretion of an "abnormal bile" which led to precipitation of cholesterol from micellar solution. The increased bile cholesterol relative to bile acid and phospholipid favored stone formation. This dietary induction of cholesterol gallstones provided a unique animal model, in part but not completely analogous to human cholelithiasis.

Lithogenic bile in patients with ileal dysfunction

Ileal disease or resection causes bile salt malabsorption and a reduction in the bile salt content of bile. Since cholesterol solubility requires adequate bile salt concentrations, depletion of the bile salt content of bile might, therefore, jeopardize cholesterol solubility and predispose to cholesterol gallstone formation. To study this, we examined biliary lipid composition in 10 patients with ileal dysfunction and in 25 healthy controls. Biliary lipid composition, as analysed in cholecystokinin-stimulated, bile-rich duodenal fluid, was shown to be representative of gallbladder bile and reproducible on repeat duodenal intubation. Nine of the 10 patients with ileal dysfunction had an abnormal, supersaturated bile in which the limits of cholesterol solubility were exceeded, and while nine of 25 control subjects also had an unstable bile, the mean bile composition in the ileal dysfunction group was significantly different from the control population. These studies provide a physicochemical explanation for the clinical observation that patients with ileal dysfunction have an increased incidence of gallstones.

Biliary excretion of lecithin and cholesterol in the dog

The biliary excretion rates of bile acid, lecithin, and cholesterol were measured in unanesthetized dogs after interruption of enterohepatic circulation and during infusions of sodium taurocholate, sodium glycocholate, sodium dehydrocholate, SC2644 (a bicyclic organic acid with high choleretic potency), and secretin. Both lecithin output and cholesterol output were directly related to bile acid excretion rate. The curves describing these relationships were concave downward. Molar concentration ratios of lecithin-to-bile acid declined gradually from approximately 0.4 to 0.2 as bile acid output increased from approximately 1 to 70 mumoles/min. Cholesterol-to-lecithin molar ratios were highest (0.05-0.15) at very low rates of bile acid excretion, but descended rapidly to a plateau (0.03-0.04) which was constant over the entire range of bile acid excretion rates from 10 to 70 mumoles/min. Similar lipid excretion patterns were observed during glycocholate infusion, but secretin-induced choleresis and dehydrocholate-induced choleresis were unaccompanied by any increments in lecithin or cholesterol excretion and SC2644 (which caused a marked increase in canalicular bile production as measured by erythritol clearance) caused a depression of lipid excretion. The data are consistent with the view that lecithin moves passively from cell membranes to intracanalicular micelles, that transport of cholesterol is coupled to lecithin transport, and that there is also a small amount of independent passive transport of cholesterol from membranes to micelles. A model developed on these assumptions has been shown to behave in a fashion consistent with the entire range of these observations.

Fetal bile salt metabolism. II. Hepatic excretion of endogenous bile salt and of a taurocholate load

Bile salt metabolism was studied in fetal dogs 1 wk before term. The size and distribution of the fetal bile salt pool were measured, and individual bile salts were identified. The hepatic excretion of endogenous bile salts was studied in bile fistula fetuses, and the capacity of this excretory mechanism was investigated by the i.v. infusion of a load of sodium taurocholate-(14)C up to 20 times the endogenous pool size. The total fetal bile salt pool was 30.9+/-2.7 mumoles, of which two-thirds was in the fetal gallbladder. Expressed on a body weight basis, this was equal to approximately one-half the estimated pool size in the adult dog (119.2+/-11.3 vs. 247.5+/-33.1 mumoles/kg body wt). Measurable quantities of bile salt were found in small bowel (6.0+/-1.8 mumoles), large bowel (1.1+/-0.3 mumoles), liver (1.2+/-0.5 mumoles), and plasma (0.1+/-0.03 mumoles). Plasma bile salt levels were significantly greater in fetal than in maternal plasma (1.01+/-0.24 mug/ml vs. 0.36+/-0.06 mug/ml; P < 0.05). Fetal hepatic bile salt excretion showed a fall over the period of study from 2.04+/-0.34 to 0.30+/-0.07 mumoles/hr. The maximal endogenous bile salt concentration in fetal hepatic bile was 18.7+/-1.5 mumoles/ml. The concentration in fetal gallbladder bile was 73.9+/-8.6 mumoles/ml; and, in those studies in which hepatic and gallbladder bile could be compared directly, the gallbladder appeared to concentrate bile four- to fivefold.Taurocholate, taurochenodeoxycholate, and taurodeoxycholate were present in fetal bile, but no free bile salts were identified. The presence of deoxycholate was confirmed by thin-layer chromatography and gas liquid chromatography, and the absence of microorganisms in fetal gut suggests that it was probably transferred from the maternal circulation. After infusion of a taurocholate load, fetal hepatic bile salt excretion increased 30-fold, so that 85-95% of the dose was excreted by the fetal liver during the period of observation. Placental transfer accounted for less than 5% of the dose. Fetal bile volume increased 15-fold on average, while bile salt concentrations increased two- to threefold. It is concluded that bile salt is taken up, conjugated, and excreted by the fetal liver with remarkable efficiency. The excreted material is either stored and concentrated in the fetal gallbladder or released into the intestine and reabsorbed to be reexcreted in bile.

Intraluminal fat digestion in the chronic alcoholic

Intraluminal fat digestion and absorption were evaluated in 15 chronic alcoholic and nine healthy volunteer males by aspiration of a standard meal from the jejunum. Intraluminal bile salts were present in normal concentrations and micellar solubilization of fatty acids and monoglycerides was within normal limits. Pancreatic lipase concentrations, however, were significantly lower in the alcoholic patients. As a consequence, hydrolysis of triglycerides was slower resulting in abnormally low concentrations of fatty acids and micellar lipids. Recovery of lipase production toward normal was demonstrated after four to six weeks by repeated intubation studies. Thus, in the chronic alcoholic insufficient production of pancreatic lipase seems to be responsible for the reversible abnormality in the intraluminal digestion of fat.

Biliary lipid secretion and bile composition after acute and chronic interruption of the enterohepatic circulation in the Rhesus monkey. IV. Primate biliary physiology

Bile salts and phospholipids are both required to solubilize biliary cholesterol. Since interruption of the enterohepatic circulation (EHC) depletes bile of bile salts, we have examined in the rhesus monkey the effects of controlled interruption of the EHC on biliary secretion of bile salt, phospholipid, and cholesterol and on the relative proportions of these components in bile. Immediately after complete interruption of the EHC, bile secretion and bile composition remained normal for 2-3 hr. During the next 3 hr, however, secretion of all components decreased. Bile salt decreased to a greater extent than phospholipid and cholesterol, and the bile was now supersaturated with cholesterol. 12-24 hr after interruption of the EHC, a new steady state was reached in which there was a relative deficiency of bile salt and a relative increase in phospholipid and cholesterol. The resulting bile, although somewhat more saturated with cholesterol, was not supersaturated with cholesterol but was stable with respect to cholesterol solubility. Thus, bile instability conducive to gallstone formation occurs transiently within hours after interruption of the EHC. Prolonged large interruptions in the steady state animal also produce a relative bile salt deficiency, but in this situation cholesterol remains soluble in the bile of these animals because there occurs a concomitant relative increase in phospholipid. When the EHC was only partially interrupted, secretion rates and the relative concentration of bile salt, phospholipid, and cholesterol did not change significantly from control values until more than 20% of the bile was diverted. Modest changes in the relative composition of bile occurred when 33 and 66% of the bile was diverted, and these changes were very similar to those produced by resection of the distal small bowel.

Composition of basal and stimulated hepatic bile in baboons, and the formation of cholesterol gallstones

The baboon, Papio, has been found to be a model for the study of the pathogenesis of cholesterol cholelithiasis in man. Studies of the physiologic variations in hepatic bile composition indicate a cyclic pattern to the proportions of cholesterol, lecithin, and bile salt in hepatic bile. During reabsorption of the bile salt pool from the intestines (stimulated flow), hepatic bile is characteristically undersaturated with cholesterol. After reabsorption of the bile salt pool (basal flow), hepatic bile is characteristically supersaturated with cholesterol. This typical pattern of basal and stimulated hepatic bile occurs irrespective of the presence of cholesterol stones in the baboon. Recognition of these two types of hepatic bile and their interrelationship during admixture in the gallbladder provides new insight into the pathogenesis of gallstone formation.

Fetal bile salt metabolism. I. The metabolism of sodium cholate-14C in the fetal dog

Cholate metabolism was studied in fetal dogs 1 wk before term and was compared with cholate metabolism in adult dogs. Tracer amounts of sodium cholate-(14)C were administered to the fetus in utero by intravenous infusion over 6 hr. Fetal plasma disappearance, biliary excretion, tissue distribution, and placental transfer of cholate were measured over 10 hr. Infused cholate-(14)C was cleared rapidly from fetal plasma principally by the fetal liver and to a minor extent by placental transfer to the mother. The taurine conjugate was formed in the fetal liver and was excreted into the proximal small intestine via the biliary tree. Indirect evidence for the functioning enterohepatic circulation of bile salt in the fetus was obtained. Comparison with the results of similar experiments in adult dogs showed that the fetal liver was almost as efficient as the adult liver in the uptake, conjugation, and excretion of tracer amounts of cholate-(14)C. The maximal rate of excretion of radiolabel attained by the fetus was somewhat slower than in the adult (82.8 +/-1.4% and 96.1 +/-4.0% [mean +/-SE] of the infusion rate, respectively), and the proportion of the total dose excreted by the fetal liver during 10 hr was smaller (81.4 +/-1.3% vs. 96.6 +/-4.4%). This difference could be only partly accounted for by placental transfer (2.8 +/-0.6% of the fetal dose). Labeled cholate and taurocholate were excreted by the fetus at similar rates, which suggests that, under the conditions of study, conjugation had little influence on the rate of transfer of cholate across the liver cell. It is concluded that the fetal dog, 1 wk before birth, has a remarkably mature and efficient mechanism for the uptake and excretion of cholate.