Determinants of bile secretion: effect of bile salt structure on bile flow and biliary cation secretion

Absence of Intestinal Microbiota during Gestation and Lactation Does Not Alter the Metabolic Response to a Western-type Diet in Adulthood

SCOPE

Microbiota composition in early life is implied to affect the risk to develop obesity in adulthood. It is unclear whether this risk is due to long-lasting microbiome-induced changes in host metabolism. This study aims to identify whether the presence or total absence of early-life microbiota affects host metabolism in adulthood.

METHODS AND RESULTS

The effects of a germ-free (Former GF) versus conventional status during gestation and lactation on the metabolic status in adult offspring are compared. Upon conventionalization at weaning, all mice were metabolically challenged with a Western-type diet (WTD) at 10 weeks age. Between age 10 and 30 weeks, a former GF status does not notably affect overall body weight gain, cholesterol metabolism, glucose tolerance or insulin sensitivity at adult age. However, Former GF mice have lower bile flow and bile acid secretion in adulthood, but similar bile acid composition.

CONCLUSIONS

A germ-free status during gestation and lactation does not substantially affect key parameters of the metabolic status before 10 weeks of age on chow diet or in adulthood following a WTD challenge. These data imply that microbiota in early life does not critically affect adult metabolic plasticity.

The importance of membrane microdomains for bile salt-dependent biliary lipid secretion

Alternative models explaining the biliary lipid secretion at the canalicular membrane of hepatocytes exist: successive lipid extraction by preformed bile salt micelles, or budding of membrane fragments with formation of mixed micelles. To test the feasibility of the latter mechanism, we developed a mathematical model that describes the formation of lipid microdomains in the canalicular membrane. Bile salt monomers intercalate into the external hemileaflet of the canalicular membrane, to form a rim to liquid disordered domain patches that then pinch off to form nanometer-scale mixed micelles. Model simulations perfectly recapitulate the measured dependence of bile salt-dependent biliary lipid extraction rates upon modulation of the membrane cholesterol (lack or overexpression of the cholesterol transporter Abcg5-Abcg8) and phosphatidylcholine (lack of Mdr2, also known as Abcb4) content. The model reveals a strong dependence of the biliary secretion rate on the protein density of the membrane. Taken together, the proposed model is consistent with crucial experimental findings in the field and provides a consistent explanation of the central molecular processes involved in bile formation.

Biliary secretion of rosuvastatin and bile acids in humans during the absorption phase

AIM

The aim of this study was to investigate the biliary secretion of rosuvastatin in healthy volunteers using an intestinal perfusion method after administration of 10mg rosuvastatin dispersion in the intestine.

METHODS

The Loc-I-Gut tube was positioned in the distal duodenum/proximal jejunum and a semi-open segment was created by inflating the proximal balloon in ten volunteers. A dispersion of 10mg rosuvastatin was administered below the inflated balloon and bile was collected proximally of the inflated balloon. Bile and plasma samples were withdrawn every 20 min during a 4h period (absorption phase) and additional plasma samples were collected 24 and 48 h post-dose.

RESULTS

The study showed that there is a substantial and immediate transport of rosuvastatin into the human bile, with the maximum concentration appearing 42 min after dosing, 39,000+/-31,000 ng/ml. Approximately 11% of the administered intestinal dose was recovered in the bile after 240 min. At all time points the biliary concentration exceeded the plasma concentration, and the average bile to plasma ratio was 5200+/-9200 (range 89-33,900, median 2000). We were unable to identify any bile-specific metabolites of rosuvastatin in the present study.

CONCLUSION

Rosuvastatin is excreted via the biliary route in humans, and the transport and accumulation of rosuvastatin in bile compared to that in plasma is rapid and extensive. This intestinal perfusion technique offers a successful way to estimate the biliary secretion for drugs, metabolites and endogenous substances during the absorption phase in healthy volunteers.

Bile acids analysis: a tool to assess graft function in human liver transplantation

The expanding use of "sub-optimal" grafts due to donor organ shortage increases the importance of accurate graft assessment before liver transplantation. Bile secretion is an early sign of recovering hepatic function post-transplant. The role of bile acid analysis in assessing graft function before and immediately after liver transplantation has been investigated. Two hundred and sixteen samples of hepatic bile were collected from 35 donors and 13 recipients. Clinical data, bile flow, total bile acid concentration, apparent choleretic activity and bile acid composition were assessed. Sub-optimal donor livers showed a low apparent choleretic activity and a different bile acid composition when compared to normal grafts. In recipients, the pattern of recovery of bile secretion immediately after reperfusion was a useful predictor of graft function. This study characterises bile acid secretion of liver grafts and remarks the potential value of bile acid analysis to assess donor liver quality and early post-transplant graft function.

Bile acid conjugation in early stage cholestatic liver disease before and during treatment with ursodeoxycholic acid

The efficiency of bile acid conjugation before and during therapy with 600 mg/day of ursodeoxycholic acid was measured in seven adult patients with early chronic cholestatic liver disease (6 with primary biliary cirrhosis; 1 with primary sclerosing cholangitis). Duodenal bile samples were obtained by aspiration and the proportion of unconjugated bile acids was determined using lipophilic anion exchange chromatography to separate bile acid classes, followed by analysis of individual bile acids by gas chromatography-mass spectrometry. The proportion of conjugated bile acids was determined by high-performance liquid chromatography. Use of a (99m)Tc-HIDA recovery marker permitted the absolute mass of unconjugated bile acids in the gallbladder to be calculated. Unconjugated bile acids comprised 0.4% of total biliary bile acids before and 0.2% during ursodeoxycholic acid therapy, indicating highly efficient conjugation of bile acids. During therapy, percentage unconjugated ursodeoxycholic acid significantly increased from (mean +/- S.D.) 13 +/- 13% to 54 +/- 12%; P < 0.002. When the unconjugated and conjugated fractions of bile acids were compared, there was an enrichment in unconjugated fraction for cholic acid and ursodeoxycholic acid and a depletion for chenodeoxycholic acid both in basal condition and during ursodeoxycholic acid therapy, suggesting that hydrophilic bile acids were conjugated less efficiently. During therapy, the conjugation efficiency significantly increased for cholic acid and ursodeoxycholic acid. The pretreatment mass of total unconjugated bile acids in the gallbladder was (mean +/- S.D.) 4.4 +/- 3.2 mumol, and was not significantly changed by ursodeoxycholic acid therapy (6.2 +/- 3.5 mumol). However, ursodeoxycholic acid therapy caused a significant increase in the mass of unconjugated ursodeoxycholic acid. It is concluded that endogenous bile acids and exogenous ursodeoxycholic acid when given at the usual dose are efficiently conjugated in patients with early cholestatic liver disease. Despite showing increased biliary unconjugated ursodeoxycholic acid during its oral administration, our data do not lend support to the occurrence of hypercholeresis due to cholehepatic shunting of bile acids.

Effect of acute bile acid pool depletion on total and ionized calcium concentrations in human bile

Although calcium salts are important components of gallstones, there are few data on the total and ionized calcium content of human bile. Therefore, in 14 fasting T-tube patients studied 7-11 days after cholecystectomy, we measured bile flow, bile acid [BA], total [CaTOT] and free ionized [Ca++] calcium concentrations, in 20-30 min bile collections during acute BA pool depletion induced by 6-8 h of continuous bile drainage. During washout of the BA pool there were parallel falls in bile flow, BA output and total calcium output (correlation coefficients ranging from 0.59 to 0.99; P < 0.02-0.001). In 12 of the 14 patients, [CaTOT] also fell (from 1.84 +/- 0.29 to 1.32 +/- 0.34 mmol L-1) in parallel with [BA] (from 34.0 +/- 14.0 to 8.2 +/- 8.0 mmol L-1; r = 0.75-0.98; P < 0.005). In contrast, biliary [Ca++] remained virtually unchanged. These data suggest that the BAs are linked to the bound, rather than to the free, ionized, fraction of biliary calcium, which is consistent with in vivo calcium binding by BAs. A model is proposed in which BA-induced biliary calcium secretion results from (i) bile acid-induced water flow via solvent drag; and (ii) calcium binding in the bile canaliculus by bile acids, which induces paracellular diffusion of Ca++, thereby maintaining [Ca++] independent of [BA].

Changes in serum calcium levels influence biliary calcium levels in humans

BACKGROUND/AIMS

There are few data on the influence of serum calcium on biliary total and ionized calcium levels in humans. The aims of the study were to increase serum calcium levels short-term by intravenous calcium infusion and study the resultant changes in total and ionized calcium concentrations ([CaTOT] and [Ca2+]) in T-tube bile.

METHODS

Serum and biliary total and ionized calcium concentrations were measured over an 8-hour period in 7 postcholecystectomy patients with T tubes before, during, and after a 4-hour intravenous infusion of 10% calcium gluconate.

RESULTS

During the infusion, serum [CaTOT] increased from 2.08 +/- 0.14 mmol/L (mean +/- SD) to 3.18 +/- 0.33 mmol/L, and serum [Ca2+] increased from 1.13 +/- 0.13 mmol/L to 1.68 +/- 0.13 mmol/L. After a 20-40-minute time lag, there were corresponding increases in biliary [CaTOT] from 1.90 +/- 0.45 mmol/L to 2.80 +/- 0.52 mmol/L and in biliary [Ca2+] from 0.70 +/- 0.11 mmol/L to 1.19 +/- 0.16 mmol/L. When the data were pooled, serum [Ca2+] showed significant correlations with both biliary [CaTOT] (n = 128; r = 0.56; P < 0.001) and biliary [Ca2+] (n = 128; r = 0.64; P < 0.001).

CONCLUSIONS

These results support the hypothesis that the biliary tree is freely permeable to calcium ions and that serum calcium level is one determinant of biliary calcium concentration. Our data may also explain the observation that patients with hypercalcemia are reported to have a greater than normal prevalence of calcified gallstones.

Pharmacology of ursodeoxycholic acid, an enterohepatic drug

The pharmacokinetics, metabolism, as well as the pharmacodynamic actions of ursodeoxycholic acid are reviewed and related to its physicochemical properties. Ursodeoxycholic acid is absorbed incompletely because of its low aqueous solubility. After absorption, it is conjugated with glycine or taurine and circulates with the endogenous bile acids. At usual doses (8-10 mg/kg/day), the pool of ursodeoxycholyl conjugates constitutes 30-60% of circulating bile acids. Ursodeoxycholic acid is metabolized by intestinal bacteriae to lithocholic acid which does not accumulate in the circulating bile acids because of efficient hepatic sulfation. Administration of ursodeoxycholic acid causes decreased cholesterol absorption, increased bile acid biosynthesis, and decreased biliary cholesterol secretion. Ursodeoxycholic acid is a choleretic agent, as all bile acids, but differs from other dihydroxy-bile acids in being non-cytotoxic because it has less affinity for membranes, and when present at micellar concentrations does not solubilize membranes. Chronic administration of ursodeoxycholic acid appears to increase canalicular transport.

Subcellular and molecular mechanisms of bile secretion

One of the liver's principal functions is the formation of bile, which is requisite for digestion of fat and elimination of detoxified drugs and metabolites. Bile is a complex fluid made up of water, electrolytes, bile acids, pigments, proteins, lipids, and a multitude of chemical breakdown products. In this review, we have summarized the source of various biliary components, the route by which they end up in bile, including the underlying subcellular and molecular mechanisms, and their contribution to bile formation. One of the reasons why bile formation is so complex is that there are many mechanisms with overlapping substrate specificities, i.e., many biochemically unrelated biliary constituents share common transport mechanisms. Additionally, biliary constituents may reach bile by more than one pathway. Some biliary components are critical for bile formation; others are of minor significance for bile formation but play a major physiological role. The major driving force for bile formation is the uptake and transcellular transport of bile salts by hepatocytes. The energy for bile formation comes from the sodium gradient created by the basolateral Na+/K(+)-ATPase, to which bile salt transport is coupled. The secretory pathway for bile salts involves uptake at the basolateral surface of the hepatocyte, vectorial transcellular movement, and transport across the canalicular membrane into the canalicular lumen. Hydrophilic bile salts are taken up via a sodium-dependent, saturable, carrier-mediated process coupled to the Na+/K(+)-ATPase. This uptake mechanism is also shared by other substrates, such as electroneutral lipids, cyclic oligopeptides, and a wide variety of drugs. Hydrophobic bile acids are taken up by a sodium-independent facilitated carrier-mediated mechanism in common with other organic ions, including sulfated bile acids, sulfobromophthalein, bilirubin, glutathione, and glucuronides, or by nonsaturable passive diffusion. Two major carrier proteins have been identified on the hepatocyte basolateral membrane: a 48-kDa protein that appears to be involved with Na(+)-dependent bile salt uptake, and a 54-kDa protein, thought to be associated with Na(+)-independent bile salt uptake. The intracellular transport of bile salts may involve cytosolic carrier proteins, of which several have been identified. Some evidence suggests a vesicular transport mechanism for bile salts. Since bile acids clearly do not enter the cell by endocytosis, formation of transport vesicles must be a more distal event in the transcellular translocation process. Some bile salts appear to be transported within the same unilamellar vesicles that are involved in the secretion of cholesterol and phospholipid.(ABSTRACT TRUNCATED AT 400 WORDS)

Acute infusions of bile salts increase biliary excretion of iron in iron-loaded rats

The mechanisms of biliary excretion of iron are not well known. The aim of this study was to examine the effect of choleresis induced by several agents on biliary iron excretion in iron-loaded rats. Iron overload was obtained with a diet supplemented by 3% iron carbonyl during a 6-week period. Bile was collected with an external bile fistula. Biliary iron concentration was measured by atomic absorption spectrophotometry, and hepatic iron concentration was measured by a chemical method. Compared with controls, iron overload resulted in a 14-fold increase in hepatic iron concentration but only a 3.9-fold increase in biliary iron output. In iron-loaded rats, taurocholate infusion caused a 1.8-fold significant increase in biliary iron output. Dehydrocholate, given at the same dose, induced a significant but less pronounced (1.3-fold) increase in biliary iron output in spite of a higher bile flow. Taurochenodeoxycholate, tauroursodeoxycholate, and tauro-7-ketolithocholate induced an increase in biliary iron output similar to that observed with taurocholate. The canalicular bile salt-independent choleretic dihydroxydibutyl ether caused a significant but less pronounced increase in biliary iron output (1.4-fold). These results confirm that in iron-loaded rats biliary iron excretion is increased much less than hepatic iron concentration. They show that in iron loaded rats (a) bile salts can increase biliary iron secretion, and (b) this increase is related in part to choleresis and in part to bile salts themselves. This increase may be related to an interaction of iron with bile salt monomers and/or micelles.

Recovery of bile secretion following orthotopic liver transplantation

Recovery of hepatic function following orthotopic liver transplantation includes the ability to produce 'adequate' bile. What constitutes adequate bile flow, however, has not previously been defined. The present study was undertaken to characterize biliary water and electrolyte secretion following hepatic transplantation. Bile was sampled from nine liver transplant recipients for 15-25 consecutive days during chronic t-tube biliary drainage. Liver biopsies and t-tube cholangiograms were unremarkable in all patients. During the first post-operative day mean bile flow, bile salt concentration, [BS], and bile salt output (BSO) were 60.0 microliters/min, 6.8 mM and 0.41 mumol/min, respectively. [BS] increased over days 1-5 and then plateaued at 12.2 mM over days 6-25 post-transplant. BSO and bile flow increased over days 1-12 before achieving steady-state values of 4.52 mumol/min and 334.7 microliters/min, respectively. In each patient bile flow increased linearly with increasing BSO. Choleretic index (CI), varied from 36.9-77.1 microliters/mumol (mean: 50.7 +/- 8.8). The y-intercept for this relationship ranged from 52.4-156.9 microliters/min (mean: 95.9 +/- 81.8). Only primary bile salts (82% cholate and 17% chenodeoxycholate), were observed in the bile of each patient. Biliary electrolyte concentrations were similar to that observed in plasma. Each was relatively unaffected by changes in bile flow and BSO. Electrolyte outputs increased linearly with respect to both BSO and bile flow. We conclude that recovery of bile secretion following orthotopic liver transplantation occurs gradually over a 10-12 day period and is strongly dependent upon bile salt secretion.