A. Perea

Effect of Taurine on Total Parenteral Nutrition-Associated Cholestasis

A decrease in the formation/secretion of bile has been well documented in animals on total parenteral nutrition (TPN). Either an excess or an imbalance of amino acids (AA) has been most often implicated. In view of recent work showing that taurine promotes bile flow, bile acid secretion, and protects against hepatotoxic bile acids, the effect of adding taurine (15 mg/dL) to an AA solution was examined in guinea pigs on TPN for 3 days. The TPN-taurine group had a larger bile flow than the group without taurine and had bile acid secretory rates (BASR) similar to those of controls who were on saline by central catheter and had free access to food. Bile composition showed an increase in the secondary bile acid, 7-ketolithocholate and a concomitant decrease in chenodeoxycholate (CDC) in both experimental groups. Taurine led to a reversal of the usual predominance of glycine over taurine conjugated bile acids as well as to increases in HCO3 in cholesterol secretion. In response to a challenge with a large load of CDC, the TPN-taurine animals increased their BASR beyond those observed in the two other groups. These observations suggest that the addition of taurine to TPN solutions could play a role in the prevention of altered biliary function associated with AA solutions.

Short-term feeding of a diet enriched in phospholipids increases bile formation and the bile acid transport maximum in rats

Earlier studies suggested that the secretory rate maximum (SRm) of bile acid and the cholestasis which occurs after the SRm is reached may be determined by the hepatic or extrahepatic biliary phospholipid pool. We therefore investigated whether bile formation and the bile acid SRm could be influenced by feeding a diet enriched in phospholipids. Male rats were fed phospholipid (PLD) or triacylglycerol (TgD)-enriched diet for 3 days, and bile formation as well as biliary lipid output were measured on the 4th day. In other similarly fed groups, cholic acid was infused in stepwise increasing doses to determine the effect of PLD on the SRm of cholic acid. The plasma lipid levels were significantly lower in PLD and TgD diets compared to basal diet. But, while the levels of total cholesterol (CH), HDL-CH, and phospholipid (PH) were not significantly altered by PLD compared to TgD, the triacylglycerol levels were markedly increased by PLD. In the liver of PLD fed rats, triacylglycerol and CH ester contents decreased by 39 and 62%, respectively, while free CH and PH contents were not significantly changed. The PLD significantly augmented spontaneous bile flow, bile acid, PH and CH secretion rates compared to TgD diet (65, 124, 164 and 654%, respectively). The enhanced biliary secretory function was associated with an increase in pericanalicular vacuoles and diverticuli in centrilobular hepatocytes. Compared to TgD fed rats, PLD rats showed a 2-fold decrease in the ratio of cholic acid/chenodeoxycholic acid in bile and a significant decrease in the % contribution of taurine conjugated BA. The PH fatty acids in bile were similar in both groups except that in PLD group the % contribution of C18:2 was higher than in TgD group. No differences were found in plasma membrane CH/PH content or total fatty acid composition. During bile acid infusion, the SRm and the total cholic acid secreted were significantly higher in the PLD than in the TgD rats. Moreover, the cholestatic response observed after high bile acid dose was markedly reduced by PLD. The results show that short-term feeding of PLD induces changes in CH and bile acid metabolism which result in enhanced biliary output of CH and PH. The enhanced pool of biliary lipid may protect plasma membranes from the deleterious effects of high bile acid concentrations.

The role of dietary choline in the beneficial effects of lecithin on the secretion of biliary lipids in rats

Earlier studies showed that dietary soybean lecithin increases biliary lipid secretion, which mainly comes from the contribution of high density lipoprotein (HDL) and hepatic microsomal pools of phosphatidylcholine and cholesterol. In addition, a lecithin diet enhances bile secretion and prevents bile acid-induced cholestasis. This study evaluated the contribution of choline, a component of lecithin, to the observed effect of lecithin on biliary secretory function. Rats were fed either a control diet (CD), a choline diet (ChD) or a lecithin-enriched diet (LD) for 2 weeks. Results showed that like LD, ChD induced an increase in bile flow and bile acid secretion rate when compared with the control diet. However, unlike LD, ChD did not significantly increase biliary phospholipids and cholesterol output. An increase of hydrophilic bile acids (i.e. ursodeoxycholic and muricholic acids) in bile of rats fed choline could explain why the biliary phospholipid and cholesterol secretion was not increased. During taurocholic acid infusion, both experimental diets increased bile flow and the bile acid secretion rate maximum (BASRm). The cholestasis usually observed after the BASRm is reached was inhibited by ChD and LD. Both diets induced a decrease in plasma cholesterol (total and HDL), however, only LD induced statistically significant changes. Analysis of total cholesterol and phospholipid content of microsomes and canalicular membranes indicated no statistically significant difference between control and experimental groups either under basal conditions or after bile acid infusion. Similarly, the phospholipid classes and fatty acid composition of biliary phosphatidylcholine were not altered by feeding ChD and LD. We conclude that choline contributes to the beneficial effect of a lecithin diet on bile secretion. It is postulated that this effect may be attributed to modulation of HDL and an enhancement of the cholesterol and phospholipid pools destined for biliary secretion.

Nutrition and bile formation

Abstract This review summarizes current knowledge on mechanisms involved in hepatic bile formation and the role of diet as a modulator of this important liver function. It also includes cholestasis and nutritional interventions known to exert a beneficial effect in this pathology. Two components of the bile flow have been described: bile acid dependent (BADF) and bile acid independent (BAIF) flows and, several cellular structures are known to be involved in their generation. The membranes enzyme activities, transporters and pumps play a particularly important role in bile secretion. Of the macronutrients, dietary protein has been shown to markedly affect bile flow. Protein deficient diet results in a decrease of both BADF and BAIDF, and in increased susceptibility to bile acid (BA)-induced cholestasis. Amino acid mixtures included in TPN solutions as well as certain individual amino acids can induce cholestasis mainly through alterations of plasma membrane composition and function. Supplementation with taurine and S-adenosyl methionine prevents these forms of cholestasis by maintaining membrane integrity and function. The quantity and quality of dietary lipid influences bile secretion. Enhanced bile flow was observed with high polyunsaturated fat intake and was attributed to both higher BADF and BAIDF. Diets enriched in fish oil were found to result in the generation of greater bile flow when compared to diets enriched in corn oil. Dietary phospholipid (soybean lecithin) supplementation increases bile secretion and exerts a beneficial effect against BA-induced cholestasis probably by maintenance of membrane integrity. Although there is much information on the role of dietary carbohydrates, fibers, minerals and vitamins on cholesterol and BA metabolism, relatively little is known about their implication in bile formation. Finally certain dietary strategies such as energy restriction and starve-refeed regimen can enhance bile secretion by their effects on BADF and BAIDF through maintenance of membrane function. In conclusion, diet is an important modulator of bile formation and secretion by affecting BA synthesis and metabolism as well as membrane structure and function.

Effect of aging and dietary restriction on bile acid metabolism in rats

The aim of the present study was to determine whether increased output of phospholipid in bile during aging may be due to alteration of bile acid composition and stimulated hydrophobic bile acid formation. In female Sprague-Dawley rats we examined the influence of aging and life long dietary restriction (60% of thead libitum intake) on bile flow, total bile acid secretion, bile acid composition and conjugation pattern, as well as phospholipid output. Rats were cannulated at 3.5, 8–12 and 24–27 months of age and bile collected for analysis. With age, there was a significant reduction in bile flow and total bile acid secretion, however, phospholipid output increased. Restriction of dietary intake exerted a beneficial effect on the age-related decline in bile formation. Studies of bile composition indicated that 12α-hydroxylated bile acids (cholic acid and deoxycholic acid) secretion decreased in aged rats compared to 3.5-month-old rats. This was associated with a corresponding increase in secretion of chenodeoxycholic acid and hyodeoxycholic-ursodeoxycholic acid. However, the magnitude of the change in secretion of these bile acids could not account for the increased output of phospholipid in bile.

Characterization of liver lysosomal enzyme activity in hepatocytes, Kupffer and endothelial cells during aging: effect of dietary restriction.

The specific activity of 4 lysosomal enzymes was studied in homogenate, hepatocytes, Kupffer and endothelial cells isolated from the livers of female Sprague-Dawley rats aged 3.5, 12 and 24 months. Cells were obtained by enzymatic digestion and centrifugal elutriation. Cell viability was not affected by age or diet. In hepatocytes, the activities of all enzymes (acid phosphatase, beta-galactosidase, arylsulfatase B and cathepsin D) increased with age in rats fed ad libitum (A) but were not altered significantly by dietary restriction. The activities of all enzymes except acid phosphatase were systematically higher at 3.5 months of age in Kupffer and endothelial cells than in hepatocytes. Acid phosphatase, arylsulfatase B and cathepsin D activities increased with age in both Kupffer and endothelial cells. Beta galactosidase was decreased significantly with age in Kupffer cells but was elevated in endothelial cells. Rats exposed to dietary restriction (R) showed higher activities of beta-galactosidase, arylsulfatase B and cathepsin D when compared to corresponding A animals with the exception of the younger age group. No clear cut pattern was observed in acid phosphatase activity. Thus, the activities of liver lysosomal enzymes increase with age but the pattern of change differs with respect to enzyme and cell populations. The heightened enzyme activity in Kupffer and endothelial cells from R rats may reflect a more efficient phagocytic capacity in these animals.

Dietary restriction influences bile formation in aging rats

Food restriction is one of the most effective interventions which increases the survival of rodents and influences a variety of physiologic and pathologic processes. Thus, we examined whether life-long caloric restriction would influence bile formation, one of the important hepatic functions. Female Sprague Dawley rats were subjected soon after weaning to a restricted diet (60% of the diet consumed by the rats fed ad libitum) and bile formation determined at 3.5, 12 and 24 months of age. Rats had their bile ducts cannulated under nembutal anesthesia and bile collected at 10 min. intervals. Bile flow rate decreased 35% between 3.5 and 24 months of age. This decrease was associated with a reduction of the bile acid dependent fraction of bile flow (BADF) up to 12 months of age, thereafter the bile acid independent fraction (BAIF) also decreased. Phospholipid and cholesterol secretion rates increased with age, but did not correlate with bile acid secretion. In rats fed the restricted diet, bile flow was about 20% higher at 3-5 months of age when compared with the ad libitum fed group. This bile flow rate remained constant until 24 months of age. The increased bile flow was attributed to higher BADF and BAIF. The phospholipid and cholesterol secretion followed that of bile acids. It thus appears that dietary restriction exerts a beneficial effect on the age related decline in bile formation.

Studies on the effect of synthetic amino acid mixtures on bile secretion in the isolated perfused rat liver

Abstract Intrahepatic cholestasis may occur in patients receiving parenteral nutrition. Recent work has implicated amino acids in the perfusates in the pathogenesis of this cholestasis. Thus, we examined the effect of currently used amino acid solutions on liver structure and function using the perfused rat liver. Livers from male rats were perfused with Krebs-Ringer bicarbonate buffer for periods up to 90 min. Two different solutions (Travasol R , Vamin R ) were each added to the perfusion medium at amino acid concentrations of 0.05%, to 1.5%. The perfusate was sampled every 15 min for GTP, K + and Na + . Bile flow and bile acid secretion rate were monitored every 15 min. Livers were examined by light and electron microscopy. Infusion of amino acids did not change Na + and K + concentrations in the perfusate but decreased transaminases at all times studied. Bile flow decreased by 20% to 98% in a dose-dependent manner with Vamin R being slightly less cholestatic than Travasol R . Bile acid independent flow (BAIF) of the bile secreted was also reduced. The hepatic ultrastructure following amino acid perfusion was similar to that of livers after buffer alone and indicated absence of cellular damage. This study suggests that the amino acid solutios may induce cholestasis by interfering with the BAIF.

Moderate Long-Term Physical Activity Improves the Age-Related Decline in Bile Formation and Bile Salt Secretion in Rats

Abstract This study examined the effect of moderate long-term exercise, begun soon after weaning, on the age-related decline in bile formation and the secretory rate maximum (SRm) of taurocholate (TC), the major bile salt (BS) in rats. Eight-month-old sedentary (S) female Sprague-Dawley rats showed a significant decrease in basal and TC-stimulated bile formation when compared with 2.5-month-old S controls. As in younger rats, decreased biliary phospholipid (PL) output was associated with the TC SRm, but this change appeared much more rapidly in S animals, which also exhibited significant increased plasma lipids and higher TC concentrations in plasma and liver at the end of TC infusion. Exercise significantly improved bile flow (BF), including the bile salt-dependent and -independent fractions under basal and TC-stimulated conditions in 8 month-old rats. Although all the biliary parameters evaluated were improved, maximal BF and PL secretion values were affected the most and were virtually not different from those obtained in younger S animals. Exercise also significantly lowered the age-related elevation of plasma PL. Thus, moderate long-term exercise exerts a beneficial effect on hepatobiliary function and the BS SRm, an effect that may be attributed in part to increased availability of a biliary PL pool previously implicated in regulation of the BS SRm.

Lithocholic acid-induced cholestasis in newborn rats

Susceptibility to lithocholic acid-induced cholestasis was examined in developing rats aged 14, 21 and 70 days. After bile duct cannulation, bile was collected for 30 min prior to, and 120 min following an i.v. injection of [14C]lithocholic acid (LCA). The liver weight/body weight ratio of newborns was significantly smaller than that of adults (3.13 +/- 0.09 at 14 days vs. 4.76 +/- 0.20 at 70 days). Therefore, when the LCA dosage was calculated per g body weight, the livers of newborns were exposed to larger amounts of LCA. The dosage was then given per liver/body weight ratio for an accurate comparison with adults. Bile flow was reduced by 7-18% in newborns, while a maximum decrease of 90% was recorded in adults by 2 h post injection, the latter drop in bile flow being proportional to LCA retention in the liver. In contrast, LCA retention was minimal in young rats with 91-95% of it being excreted via the bile. The decreased susceptibility of newborns to cholestasis appears to be related to their capacity to excrete LCA.