Class III P-glycoproteins mediate the formation of lipoprotein X in the mouse

Cholestasis is associated with hypercholesterolemia and appearance of the abnormal lipoprotein X (LpX) in plasma. Using mice with a disrupted Mdr2 gene, we tested the hypothesis that LpX originates as a biliary lipid vesicle. Mdr2-deficient mice lack Mdr2 P-glycoprotein, the canalicular translocator for phosphatidylcholine, and secrete virtually no phospholipid and cholesterol in bile. Bile duct ligation of Mdr2(+)/+ mice induced a dramatic increase in the plasma cholesterol and phospholipid concentration. Agarose electrophoresis, density gradient ultracentrifugation, gel permeation, and electron microscopy revealed that the majority of phospholipid and cholesterol was present as LpX, a 40-100 nm vesicle with an aqueous lumen. In contrast, the plasma cholesterol and phospholipid concentration in Mdr2(-)/- mice decreased upon bile duct ligation, and plasma fractionation revealed a complete absence of LpX. In mice with various expression levels of Mdr2 or MDR3, the human homolog of Mdr2, we observed that the plasma level of cholesterol and phospholipid during cholestasis correlated very closely with the expression level of these canalicular P-glycoproteins. These data demonstrate that during cholestasis there is a quantitative shift of lipid secretion from bile to the plasma compartment in the form of LpX. The concentration of this lipoprotein is determined by the activity of the canalicular phospholipid translocator.     

The clinical significance of type 1 fiber predominance

Having recently demonstrated that taurine supplementation prevents total parenteral nutrition (TPN)-induced cholestasis, we chose to use this model to examine plasma membrane composition in relation to bile formation. Male guinea pigs received daily a mixture of glucose and of the amino acid solution Travasol with or without added taurine (1.2 mM). After 3 days, bile was collected and liver plasma membrane fractions enriched in sinusoidal lateral membrane and bile canalicular membrane domains were isolated. In animals receiving TPN alone, bile flow and biliary secretory rate of bile acid and bicarbonate decreased significantly compared with controls. Although membrane ATPases (Na+K+ and Mg+) were unchanged, TPN induced an increase in the lipid to protein ratio and a decrease of polyunsaturated fatty acids, in conjunction with a higher content of diene conjugates in sinusoidal lateral membrane fractions. Taurine corrected these changes and, in addition, reduced significantly the cholesterol to phospholipid ratio in both membrane fractions. The data show that changes in liver cell membranes occur in TPN-induced cholestasis and suggest that free radical injury may play a role. As taurine prevented cholestasis as well as membrane changes, it is suggested that taurine should be added to amino acid solutions used for parenteral nutrition.     

Time-related changes of cold-stored rat liver in University of Wisconsin solution. Effect of ursodeoxycholate

Livers of Wistar rats were stored between 0 and 36 hrs. in the University of Wisconsin preservation liquid in order to determine time-related biochemical and morphological hepatic changes. Ursodeoxycholate (100 microM) was also added in the medium to test the hepatoprotective properties of the bile salt. Biochemical assays were performed on hepatic microsomes, plasma and biliary canalicular membranes. Protein and lipid composition of the microsomal and baso-lateral plasma membranes remained stable. Protein and cholesterol content of the biliary canalicular membranes decreased, phospholipid/cholesterol ratio increased between 0 and 36 hrs.; it resulted in a leak of 5'-nucleotidase and leucine amino peptidase activity of these biliary canalicular membranes, especially up to 12 hrs. Between 0 and 36 hrs., the lipid and protein content remained stable in the plasma membranes, as well as both tested enzymatic activities. Observations under electron microscopy showed alterations and underlined fragility of the bile canaliculi, particularly after 24 hrs. preservation. Ultrastructure of sinusoidal membranes showed damaged microvilli. Endoplasmic reticulum remained unchanged, in relation to the stability of the microsomal lipidic, proteic content and hydroxymethylglutaryl-coenzyme A reductase activity, except the decreased protein content after preservation for 36 hrs without ursodeoxycholate. Ursodeoxycholate by itself did not protect against the described disturbances.     

The lipid composition of plasma membrane subfractions originating from the three major functional domains of the rat hepatocyte cell surface

1. The neutral and phospholipid compositions of three rat liver plasma membrane subfractions originating predominantly from the three major functional domains of the hepatocyte viz the blood sinusoidal, contiguous and bile canalicular fractions, were determined. 2. The sinusoidal and canalicular plasma membrane subfractions, both of which were vesicular, contained a higher lipid to protein weight ratio than the contiguous plasma membrane subfraction that consisted of membrane strips, junctional complexes and some larger vesicles. The three plasma membrane subfractions contained a similar neutral lipid to phospholipid ratio. The highest unesterified cholesterol content was associated with the canalicular plasma membrane subfraction. 3. The phospholipid profiles of the three subfractions were generally similar. However, the canalicular plasma membrane subfraction contained a higher proportion of sphingomyelin than the other subfractions. 4. Correlations between the neutral and phospholipid composition of the subfractions and membrane integrity and function are discussed, especially with respect to a possible role of lipids in governing the resilience of the canalicular plasma membrane to the action of bile salts.     

A longitudinal study of circulating lymphocyte subsets in the peripheral blood during the acute stage of Guillain-Barré syndrome

We investigated sequential changes in bile flow, serum and biliary biochemical parameters in phalloidin-induced cholestasis in rats. Intrahepatic cholestasis was induced by administration with phalloidin (500 microg/kg) for 7 days, and then the animals were allowed to survive for 1, 2, 4, 7, 14 and 28 days after the last treatment. In phalloidin-treated rats, bile flow significantly decreased up to 4 days of recovery, compared with the control animals. In contrast, serum ALP activity, LAP activity, cholesterol concentration and phospholipid concentration exhibited a marked elevation throughout the recovery periods. For biliary parameters, bilirubin excretion rate was unchanged but, cholesterol excretion rate showed a marked decrease throughout the recovery periods. These results demonstrate that some parameters, particularly important indexes of cholestasis (serum ALP, cholesterol, bile flow and so on), continued significant changes at least 4 days after the last administration of phalloidin. These results demonstrate that successive treatment with phalloidin can cause damage in most of serum and biliary parameters at a chronic stage of cholestasis. Thus, our findings may provide useful information for diagnosis of drug-induced cholestasis and help to further elucidate the biochemical mechanisms of drug-induced cholestasis in humans.     

Effect of ethynylestradiol on biliary excretion of bile acids, phosphatidylcolines, and cholesterol in the bile fistula rat

The effects of ethynylestradiol on endogenous bile acids, their capacity to conjugate and excrete intravenously infused cholic acid, the concentrations of biliary cholesterol and lecithin, and the individual molecular species of phosphatidylcholine have been determined in male and female Sprague-Dawley rats. Endogenous biliary bile acids were analyzed by gas-liquid chromatography-mass spectrometry. Eleven bile acids were identified and several minor bile acids, primarily muricholates, could not be completely characterized. After 5 days of treatment with ethynylestradiol (1 mg/kg per day), the percentage of cholic acid decreased and the percentage of 6beta-hydroxylated bile acids, including several monounsaturated species, increased. Ethynylestradiol caused a decrease in bile acid-independent bile flow. Intravenous infusion of cholic acid at a high concentration caused cholestasis in control animals but, after ethynylestradiol treatment, cholestasis developed during the infusion of a much lower concentration of cholate, indicating a lowered threshhold for bile acid-induced cholestasis. In the treated rats, there was a slight increase in excretion of unconjugated endogenous bile acids, and a striking impairment of conjugation of intravenously administered cholic acid. One of the few sex-related differences observed was an increased concentration of biliary phospholipids in untreated male rats. Both phospholipid and cholesterol concentrations in the bile were higher in the treated animals. The molar percentage of cholesterol was always 1-2%, but it was slightly higher in treated animals, especially males. Ethynylestradiol treatment also affected biliary phospholipid by causing a marked increase of phosphatidylcholine species containing palmitic and oleic acid residues and a decrease of species containing stearic and linoleic acid residues. There was no increase in biliary excretion of long chain polyunsaturated species, which might have indicated damage to membranes, in response to ethynylestradiol either alone or with cholic acid infusion. Some of these ethynylestradiol-induced changes in biliary bile acid and lipid excretion are probably peculiar to the rat, but others, such as the increase in molar percentage of cholesterol and cholestasis, may be relevant to disorders in man, especially cholesterol gallstones and idiopathic cholestasis of pregnancy.     

The rat liver ecto-ATPase is also a canalicular bile acid transport protein

A approximately 110-kDa glycoprotein purified from canalicular vesicles by bile acid affinity chromatography has been identified as the canalicular bile acid transport protein. Internal amino acid sequence and chemical and immunochemical characteristics of this protein were found to be identical to a rat liver canalicular ecto-ATPase. In order to definitively determine whether these were two activities of a single polypeptide, we examined the possibility that transfection of cDNA for the ecto-ATPase would confer bile acid transport characteristics, as well as ecto-ATPase activity, on heterologous cells. The results show that transfection of the ecto-ATPase cDNA conferred on COS cells de novo synthesis of a approximately 110-kDa polypeptide, as immunoprecipitated by antibody to the purified canalicular bile acid transport protein and conferred on COS cells the capacity to pump out [3H]taurocholate with efflux characteristics comparable with those previously determined in canalicular membrane vesicles (Km = 100 microM; Vmax = 200 pmol/mg of protein/20 s). A truncated ecto-ATPase cDNA, missing the cytoplasmic tail, was targeted correctly to the cell surface but did not confer bile acid transport activity on COS cells. The results of this study also show that the canalicular ecto-ATPase/bile acid transport protein is phosphorylated on its cytoplasmic tail and that its phosphorylation is stimulated by activation of protein kinase C and inhibited by inhibitors of protein kinase C activation. Moreover, inhibition of protein kinase C activation by staurosporine completely abrogates bile acid transport but does not affect ATPase activity. This study, therefore, demonstrates that the rat liver canalicular ecto-ATPase is also a bile acid transport protein, that the capacity to pump out bile acid can be conferred on a heterologous cell by DNA-mediated gene transfer, and that phosphorylation within the cytoplasmic tail of the transporter is essential for bile acid efflux activity but not for ATPase activity.     

Increased hepatic Na,K-ATPase activity during hepatic regeneration is associated with induction of the beta1-subunit and expression on the bile canalicular domain

Cellular and molecular mechanisms regulating the activity of the sodium pump or Na,K-ATPase during proliferation of hepatocytes following 70% liver resection have not been defined. Na,K-ATPase may be regulated by synthesis of its alpha- and beta-subunits, by sorting to either the sinusoidal or apical plasma membrane domains, or by increasing membrane lipid fluidity. This study investigated the time course of changes during hepatic regeneration for Na, K-ATPase activity, lipid composition and fluidity, and protein content of liver plasma membrane subfractions. As early as 4 h after hepatic resection, Na,K-ATPase activity was increased selectively in the bile canalicular fraction. It reached a new steady state at 12 h and remained elevated for 2 days. Although hepatic regeneration was associated with a reduced cholesterol/phospholipid molar ratio and increased fluidity, measured with two different probes, these changes in lipid metabolism were in the sinusoidal membrane domain. The Na,K-ATPase beta1-subunit, but not the alpha1-subunit, was increased selectively at the bile canalicular surface as shown by immunoblotting of liver plasma membrane subfractions and the morphological demonstration at both the light and electron microscopic levels. Furthermore, cycloheximide blocked the rise in beta1-subunit mRNA levels. Since the time course for beta1-subunit accumulation was similar to that for activation of Na,K-ATPase activity, this change implicated the beta1-subunit in activating sodium pump activity.     

Alternative reproductive strategies in the ruff, Philomachus pugnax: a mixed ESS?

BACKGROUND/AIMS: Hepatic graft dysfunction is a major management problem in the early post-liver transplantation period. Our aims were to study how liver transplantation per se affects bile formation, and to investigate the role of cyclosporine in the pathogenesis of early graft dysfunction. METHODS: Syngeneic liver transplantation used male Lewis rats. Two weeks after transplantation, the rats were randomly assigned to receive either daily subcutaneous injections of cyclosporine 10 mg/kg for 1 week (n=8), or daily saline injections (Placebo, n=8). 24-h bile collections were performed 18 h after the last injection. Eight non-transplanted rats served as controls. RESULTS: Liver transplantation per se (Placebo) significantly increased basal bile flow (51%), particularly that portion which was bile salt-independent flow (81%), but did not impair bile salt kinetics or biliary lipid composition. Cyclosporine reduced basal bile flow and bile salt-independent flow by 41% and 30%, respectively. Bile salt synthesis was 52% suppressed, leading to a 22% decrease in the bile salt pool size. The recycling frequency of the bile salt pool was unaffected. The drug inhibited bile salt (37%) and phospholipid (23%) outputs; cholesterol secretion remained unaltered. This significantly elevated the cholesterol saturation of bile (25%). CONCLUSIONS: Liver transplantation per se is choleretic and does not impair bile formation or lipid composition in this inbred rat model. Parenteral administration of high-dose cyclosporine induces cholestasis by inhibiting bile salt secretion and BSIF. Bile salt synthesis is down-regulated and the bile salt pool size decreased. The drug adversely affects biliary lipid composition by differential inhibition of bile salt and phospholipid outputs relative to an unchanged cholesterol secretion.     

Attenuation of WAF1/Cip1 expression by an antisense adenovirus expression vector sensitizes glioblastoma cells to apoptosis induced by chemotherapeutic agents 1,3-bis(2-chloroethyl)-1-nitrosourea and cisplatin

Bile salts regulate the subselection of phosphatidylcholine species secreted into bile and thereby modulate bile metastability. The aim of this study was to determine whether bile salts alter phosphatidylcholine species of the canalicular membrane, and if they do, to clarify whether the cytoprotective action of hydrophilic bile salts is associated with modulation of phosphatidylcholine composition in cell membrane bilayers. Bile salt-pool-depleted rats were infused intravenously with sodium taurocholate at a constant rate (200 nmol/min/100 g body wt) for 2 hr, followed by infusion of either sodium tauroursodeoxycholate, sodium tauroalphamuricholate, or sodium taurobetamuricholate (200 nmol/min/100 g) for 2 hr. Biliary outputs of cholesterol and phosphatidylcholine and phosphatidylcholine hydrophobicity in bile and subcellular fractions were determined. The cytoprotective action of hydrophilic bile salts was determined by the release of canalicular membrane-localizing enzymes (alkaline phosphatase, leucine aminopeptidase) into bile. Tauroursodeoxycholate, taurobetamuricholate, and tauroalphamuricholate decreased the release of these enzymes when compared to values under taurocholate infusion. Bile phosphatidylcholine hydrophobicity was also decreased by the bile salts, whereas the cholesterol/phosphatidylcholine ratio was increased. In contrast, phosphatidylcholine hydrophobicity in the canalicular membrane was increased by these three bile salts. In conclusion, hydrophilic bile salts promote biliary secretion of relatively hydrophilic phosphatidylcholine secretion into bile, and consequently phosphatidylcholine hydrophobicity in canalicular membranes increased. Such an alteration in phosphatidylcholine species within canalicular membrane enhances its lateral packing density with less fluidity, and this may account, in part, for the cytoprotective action of hydrophilic bile salts against hydrophobic bile salts.     

Bile salt-membrane interactions and the physico-chemical mechanisms of bile salt toxicity

We present evidence that ursodeoxycholate prevents toxicity of more hydrophobic bile salts by inhibiting micellar solubilization of membrane lipids. Using both centrifugal ultrafiltration and gel filtration methods we studied leakage of inulin from vesicles composed of egg phosphatidylcholine and cholesterol. We observed that the addition of tauroursodeoxycholate to taurodeoxycholate reduced leakage of inulin from large unilamelar vesicles compared to that seen with taurodeoxycholate alone. This protective effect was observed only at high membrane cholesterol:phospholipid ratios (> or = 0.5). By gel filtration we found that fractional leakage of inulin from vesicles was identical to fractional phospholipid solubilization, indicating that release of inulin from vesicles results from membrane dissolution rather than from increased permeability of otherwise intact membranes. Addition of tauroursodeoxycholate to taurodeoxycholate was found to suppress the dissolution of phospholipid from cholesterol-rich vesicles. Bile salts were found to absorb to vesicles with an affinity proportional to their relative hydrophobicity, as estimated by reverse phase HPLC. Adsorption affinity decreased progressively with increasing membrane cholesterol content. Different bile salts displaced each other from membranes in proportion to their respective binding, affinities. Tauroursodeoxycholate, which absorbed to membranes with low affinity, displaced taurodeoxycholate from vesicles only weakly. Based on these findings we postulate that bile salts may damage the liver through solubilization of canalicular membrane lipids. Ursodeoxycholate may protect the liver by inhibiting dissolution of the cholesterol-rich canalicular membrane by more hydrophobic endogenous bile salts. Biliary secretion of vesicles rich in phosphatidylcholine may buffer the intermicellar concentration of bile acids at levels below those required to disrupt the cholesterol-rich canalicular membrane; thus biliary vesicle secretion may have evolved as a mechanism to protect the biliary epithelium from injury by luminal bile salts.     

Ethinylestradiol treatment induces multiple canalicular membrane transport alterations in rat liver

We investigated the effects of 17 alpha-ethinylestradiol treatment of rats on various transport functions in isolated basolateral and canalicular liver plasma membrane vesicles. Both membrane subfractions were purified to a similar degree from control and cholestatic livers. Although moderate membrane lipid alterations were predominantly observed in basolateral vesicles, no change in basolateral Na+/K(+)-ATPase activity was found. Furthermore, while Na(+)-dependent taurocholate uptake was decreased by approximately 40% in basolateral vesicles, the maximal velocity of ATP-dependent taurocholate transport was decreased by 63% in canalicular membranes. In contrast, only minimal changes or no changes at all were observed for electrogenic taurocholate transport in "cholestatic" canalicular membranes and total microsomes, respectively. However, canalicular vesicles from cholestatic livers also exhibited marked reductions in ATP-dependent transport of S-(2,4-dinitrophenyl)glutathione and in Na(+)-dependent uptake of adenosine, while in the same vesicles HCO3-/SO4- exchange and Na+/glycine cotransport activities were markedly stimulated. These data show that in addition to the previously demonstrated sinusoidal transport abnormalities ethinylestradiol-induced cholestasis is also associated with multiple canalicular membrane transport alterations in rat liver. Hence, functional transport alterations at both polar surface domains might ultimately be responsible for the inhibitory effects of estrogens on the organic anion excretory capacity and on bile formation in rat liver.     

Evidence for a luteinizing hormone surge center in the hypothalamus of the pig

Lipoprotein-X (Lp-X) is an abnormal low-density lipoprotein frequently found in liver disease. It is regarded as the most sensitive and specific biochemical parameter for the diagnosis of intra- and extrahepatic cholestasis. Moreover, Lp-X is supposed to contribute to the development of hypercholesterolemia in cholestatic liver disease, because it fails to inhibit de novo cholesterol synthesis. This investigation will focus on the relationship between the presence of Lp-X and serum lipid concentrations in cirrhosis. The significance of Lp-X in the diagnosis of cholestasis, compared with alkaline phosphatase (AP), gamma-glutamyl transferase (GGT), and bilirubin levels, will be assessed as well. The present cross-sectional study includes 212 patients with histopathologically proven cirrhosis. The detection of Lp-X and the quantification of -, beta-, and pre-beta-cholesterol was based on agar gel electrophoresis and polyanion precipitation. For the characterization of liver function, the concentrations of albumin and bilirubin, the activities of liver enzymes, and coagulation times were assessed. In a subgroup of 40 individuals, liver biopsies were re-evaluated to confirm or exclude intrahepatic cholestasis. As a result, there was no association between the appearance of Lp-X and total cholesterol concentrations. While all patients with Lp-X showed intrahepatic cholestasis (predictive value of the positive test = 1), only 16 of 28 patients with cholestasis formed Lp-X (sensitivity = 0.57). The activities of AP and of GGT, as well as the concentrations of bilirubin, were strongly elevated in most patients, with and without cholestasis. The predictive values of AP, GGT, and bilirubin were 0.77, 0.69, and 0.74 for the positive test and 0.5, 0, and 0.6 for the negative test, respectively. We conclude that Lp-X is not related to hypercholesterolemia in cirrhosis. The positive, but not the negative, Lp-X test has high predictive value for the diagnosis of cholestasis in cirrhosis. The biochemical parameters traditionally used for the assessment of extrahepatic cholestasis, AP, GGT, and bilirubin, do not support the diagnosis of intrahepatic cholestasis caused by cirrhosis.     

Lovastatin decreases de novo cholesterol synthesis and LDL Apo B-100 production rates in combined-hyperlipidemic males

The effect of lovastatin, an inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase activity, on the kinetics of de novo cholesterol synthesis and apolipoprotein (apo) B in very-low-density lipoprotein (VLDL), intermediate-density lipoprotein (IDL), and low-density lipoprotein (LDL) was investigated in five male patients with combined hyperlipidemia. Subjects were counseled to follow a Step 2 diet and were treated with lovastatin and placebo in randomly assigned order for 6-week periods. At the end of each experimental period, subjects were given deuterium oxide orally and de novo cholesterol synthesis was assessed from deuterium incorporation into cholesterol and expressed as fractional synthesis rate (C-FSR) and production rate (C-PR). Simultaneously, the kinetics of VLDL, IDL, and LDL apo B-100 were studied in the fed state using a primed-constant infusion of deuterated leucine to measure fractional catabolic rates (FCR) and production rates (PR). Drug treatment resulted in significant decreases in total cholesterol (-29%), VLDL cholesterol (-40%), LDL cholesterol (-27%), and apo B (-16%) levels and increases in HDL cholesterol (+13%) and apolipoprotein (apo) A-I (+11%) levels. Associated with these plasma lipoprotein responses was a significant reduction in both de novo C-FSR (-40%; P = .04) and C-PR (-42%; P = .03). Treatment with lovastain in these patients had no significant effect on the FCR of apoB-100 in VLDL, IDL, or LDL, but resulted in a significant decrease in the PR of apoB-100 in IDL and LDL. Comparing the kinetic data of these patients with those of 10 normolipidemic control subjects indicates that lovastatin treatment normalized apoB-100 IDL and LDL PR. The results of these studies suggest that the declines in plasma lipid levels observed after treatment of combined hyperlipidemic patients with lovastatin are attributable to reductions in the C-FSR and C-PR of de novo cholesterol synthesis and the PR of apoB-100 containing lipoproteins. The decline in de novo cholesterol synthesis, rather than an increase in direct uptake of VLDL and IDL, may have contributed to the decline in the PR observed.     

Hepatic concentrations of proenkephalin-derived opioids are increased in a rat model of cholestasis

The liver of adult rats with cholestasis secondary to bile duct resection has been shown to express the proenkephalin gene and, by immunohistochemical stains, to contain met-enkephalin. To further study hepatic opioids in cholestasis, concentrations of proenkephalin-derived endogenous opioids were measured in a rat model of cholestasis by the use of radioimmunoassays. The specificity of the immunoreactivity detected by the assays was confirmed by high performance liquid chromatography (HPLC). In adult male rats with cholestasis due to BDR, the concentrations of three proenkephalin-derived opioid peptides were increased. Specifically, the mean hepatic concentrations of met-enkephalin, Met-Enk-Arg6-Phe7 and leu-enkephalin were 2.5 (p < 0.005), 2.1 (p < 0.005) and 2.5 (p < 0.01) fold higher than the corresponding mean for controls. These findings provide further independent evidence that opioid peptides accumulate in the liver in a model of cholestasis and are consistent with de novo synthesis of opioid peptides occurring in the cholestatic liver. This phenomenon may have relevance to the altered function of the opioid system in cholestasis and to the role of the liver as a neuroendocrine organ.     

Cell adhesion force microscopy

The monohydroxy bile acid, taurolithocholate (TLC), causes cholestasis in vivo and in isolated perfused livers. It is also cholestatic in vitro and, in this study using isolated rat hepatocyte couplets, causes a reduction of the accumulation of (fluorescent) bile acid in the canalicular vacuoles (cVA) of this polarized cell preparation. The hepatoprotective bile acid, tauroursodeoxycholate (TUDCA), partially protects against the action of TLC when added at the same time. It also partially reverses the cholestatic effect if added after the cells have been exposed to TLC. A second hepatoprotective compound, S-adenosyl-L-methionine (SAMe) also not only partially protects against the action of TLC when added at the same time, but it too is able to partially reverse the cholestatic effect. Neither hepatoprotective agent is fully effective alone, but their effects are additive. In combination, a full restoration of cVA is observed in moderate cholestasis, but not in severe cholestasis. We discuss briefly some possible mechanisms involved in the additive mode of action of both hepatoprotective compounds. In summary, we show for the first time that SAMe and TUDCA can exert an additive effect in the amelioration of TLC-induced cholestasis in isolated rat hepatocyte couplets. This finding may be of possible clinical relevance.     

Mechanism of biliary lipid secretion in the rat: a role for bile acid-independent bile flow?

Bile acid-induced lipid secretion was compared in unanesthetized normal control and Groningen Yellow Wistar rats during variations in endogenous bile acid output. Groningen Yellow rats express a genetic defect in the biliary secretion of various organic anions. During a 5-hr period after interruption of the enterohepatic circulation, bile acid secretion decreased from 36.4 +/- 1.8 to 1.9 +/- 0.3 mumol per 30 min in normal control rats and from 37.1 +/- 2.8 to 1.8 +/- 0.2 mumol per 30 min in Groningen Yellow rats, respectively (mean +/- S.E.M., n = 5). The relationship between bile acid secretion and bile flow showed similar slopes (normal control, 8.74 +/- 0.44 microliter/mumol and Groningen Yellow rats, 7.71 +/- 0.42 microliter/mumol) but different y-intercepts (normal control, 243 +/- 8 and Groningen Yellow, 127 +/- 4 microliters per 30 min; p < 0.001), corresponding to a 47% reduction of the bile acid-independent fraction of bile flow in Groningen Yellow rats. During the course of the experiment, the ratio of lipids (phospholipids plus cholesterol) to bile acids increased in both strains more than threefold but was permanently higher in Groningen Yellow than in normal control rats (p = 0.035), implying that Groningen Yellow rats continuously secreted more lipid per bile acid. No differences in bile acid pool composition or in bile canalicular membrane composition and fluidity between the two strains were detected. The results indicate that apart from previously demonstrated factors (bile acid concentration, bile acid composition and hydrophilic organic anion concentration in bile), another parameter affects the efficacy of bile acids to induce biliary lipid secretion.(ABSTRACT TRUNCATED AT 250 WORDS)