Bioavailability, gastrointestinal transit, solubilization and faecal excretion of ursodeoxycholic acid in man

The bioavailability of ursodeoxycholic acid (UDCA), a cholesterolic gallstone dissolving agent, has been analysed in seven healthy human volunteers. After absorption of a capsule containing a 500 mg dose, the time course of plasma concentrations of the drug presented a double peak profile over a 240 min period. In order to explain this result, a second group of five subjects bearing a four-way jejunal catheter fitted with an occluding balloon, received an oral dose of 250, 500 or 750 mg of the drug. Simultaneous analyses of plasma UDCA concentrations and jejunal UDCA contents were carried out. UDCA is poorly soluble in the gastro-duodeno-jejunal contents of fasted subjects since 21-50% of the ingested doses were recovered in solid form. The profile of plasma concentration paralleled the amount of soluble UDCA present in intestinal lumen. When jejunal contents were infused below the balloon a second plasma peak appeared in cases corresponding to ingestion of higher doses of UDCA. In conclusion, pharmacological doses of UDCA are not readily soluble in the stomach and intestine of a healthy fasting man. In consequence, the bioavailability of the drug varies with its progressive solubilization in the gastrointestinal tract. The present results suggest that repeated daily doses of UDCA should improve its bioavailability in treated gallstone patients.

Metabolism of beta-muricholic acid in man

Labeled beta-muricholic acid was obtained from germfree rats given [24-14C]-chenodeoxycholic acid. It was crystallized with the same unlabeled bile acid extracted from germfree rat pooled biles. Five patients fitted with a T-tube after cholecystectomy were given orally 100 mg of the bile acid. Metabolites of beta-muricholic acid in bile, urine and feces were studied. Glyco- and tauro-beta-muricholic acid were the only metabolites detected in bile. The urinary bile acid pattern was complex and included free, glyco- and sulfoconjugated beta-muricholic acid, but no glucuronide was observed. Analysis of fecal bile acid showed very few metabolites: the 3 beta-epimer was identified; the 6 beta- and 7 beta-hydroxyls were apparently not transformed by human intestinal microflora.

Isolation and characterization of rat intestinal polyribosomes and RNA during absorption of fat. Increased translation in vitro of apo-AIV

Polyribosomes were isolated from the intestinal mucosa of fasted and fat-fed rats in the presence of ribonuclease inhibitors. Polyribosomes from fat-fed rats were larger and more efficient in incorporating radioactive aminoacids into proteins than those from fasted rats. Total RNA prepared by guanidine-HCl extraction, from the intestine of fasting and fat-fed rats were translated in vitro in a mRNA-dependent rabbit reticulocyte lysate system in the presence of 35S-methionine. Sodium dodecyl sulfate polyacrylamide gel electrophoresis and autoradiography of the synthesized peptides showed a relative increase in the radioactivity of some peptides of RNA from fat-fed animals and particularly a two fold increase in preapo-AIV indicating that the intestinal apo-AIV synthesis is under transcriptional regulation by the metabolic processes involved in fat transport, that is, triglyceride-rich lipoprotein production.

High-yield preparation of porcine hepatocytes for long survival after transplantation in the spleen

The creation of an auxiliary liver by autotransplantation of liver parenchymal cells into the spleen has mainly been studied in rats for the treatment of acute liver failure. In order to apply this procedure to humans with chronic liver insufficiency the aim of this work was: To demonstrate that hepatocytes can survive for long periods after autotransplantation into the spleen; to increase the yield of the isolation of hepatocytes obtained from pig livers since this animal has a more fibrous liver than rats or normal humans and consequently one which is more difficult to dissociate. In 21 pigs isolated hepatocytes were obtained with in collagenase dissociation technique, the yield being 1-3 X 10(7) cells per gram of liver and the viability 70-95%. The hepatocytes survived and maintained normal morphological and histochemical characteristics up to 7 months after transplantation, the date of sacrifice of the last animal.

Isolation, culture and characterization of adult human hepatocytes from surgical liver biopsies

A technique is described for isolation and culture of adult human hepatocytes from surgical liver biopsies. The mean cell yield was 1.75 X 10(7) cells per gm liver and viability averaged 80%. Hepatocytes were maintained in primary culture for about 10 days. Cell morphology and histochemical characteristics were similar to hepatocytes in vivo. Bile canaliculi were observed by electron microscopy. Intracellular albumin was demonstrated up to the 7th day of culture; albumin secretion rate was maximal (0.6 +/- 0.33 micrograms per hr per 10(6) cells) 5 days after plating. These studies demonstrate that adult human hepatocytes can be isolated from surgical biopsies with high yield, and differentiated function can be maintained for several days.

1-Acyl-lysolecithin acyltransferase and synthesis of biliary lecithins in rat liver

The role of lysolecithin acyltransferase activities in biliary lecithin formation was investigated, using livers perfused in the presence of labeled palmitoyl-lysolecithin and albumin, overloaded or not with linoleic acid. At the end of liver perfusion, the lecithins extracted from microsomes, mitochondria and plasma membranes displayed the same specific activity. Double-labeled lysolecithin was used to prove that labeled lecithins were synthesized by lysolecithin acylation. In the absence or presence of a linoleic acid overload, the level of lysolecithin incorporation into linoleyl and arachidonyl containing lecithin was identical. Hence fatty acids did not influence phosphatidylcholine synthesis by the acylation pathway. In vitro the rate of linoleyl lecithin synthesis was the same in plasma membranes, mitochondria and microsomes provided the linoleyl-CoA concentration was lower than 30 microM. Taurocholate was essential to the excretion of lecithin synthesized from lysolecithin and stimulated its synthesis. The specific activities of the two lecithin molecular species excreted in bile (linoleyl and arachidonyl) were not significantly different. These results enabled us to evaluate the contribution of the lysolecithin pathway to the synthesis of lecithin in liver and bile: this contribution in bile was less than 2% under the perfusion conditions used.

Intestinal absorption, excretion, and biotransformation of hyodeoxycholic acid in man

Five patients fitted with a biliary T-tube after cholecystectomy were given orally a tracer dose of [14C]hyodeoxycholic acid and 500 mg of the same unlabeled acid. Intestinal absorption and biotransformation, liver metabolism, bile secretion, fecal and urinary excretions of this acid or of its metabolites were studied. Hyodeoxycholic acid was well absorbed by the human intestine. It was not subjected to intestinal transformations and, particularly, did not produce a significant amount of lithocholic acid, which does not support the existence of intestinal bacterial 6 alpha-dehydroxylases. The percentage of hyodeoxycholic acid and of its metabolites recovered in bile varied from 11.5 to 31%. Two major metabolites were isolated from bile: glycohyodeoxycholic acid and hyodeoxycholic acid glucuronide. Analysis of urinary bile acids showed that a large proportion (30-84%) of the administered hyodeoxycholic acid was excreted by the kidney as a glucuronide. The large extent of both glucuronidation and urinary excretion of hyodeoxycholic acid is a unique example of bile acid metabolism and excretion in man.

Impaired glycosylation in liver microsomes of orotic-acid-fed rats

In rats fed orotic acid, the incorporation in liver subcellular fractions of sugars injected intraperitonealy is altered only for mannose, but not for fucose or galactose. Direct determinations of several glycosyltransferases are done in smooth and rough microsomes: fucosyl-, glactosyl-, N-acetylglucosaminyltransferase activities are at quite similar levels in normal and fatty livers. By contrast, sialyltransferase activity is increased (+50%) in smooth microsomes of fatty livers, while mannosyltransferase activity is inhibited by 30%. These alterations are not caused by interfering reactions (pyrophosphatases or proteases). For the mannosyltransferase activity, the inhibition is found in the dolichylphosphorylmannose intermediates. Kinetic studies suggest that there is deficiency of both enzyme and endogenous dolichyl phosphate.

Noninvasive measurement of nutrient portal blood shunting: an experimental study with [14C]ursodeoxycholic acid

All of the methods proposed for measuring portal blood flow are either invasive, estimate total rather than nutrient flow, and none has proved reliable in cirrhotic patients. A method has been derived from pharmacokinetic principles used for the calculation of bioavailability of drugs according to the route of administration (i.v. or p.o.) and tested experimentally in 20 pigs. A tracer dose of [14C]ursodeoxycholic acid, a biliary acid with a high-liver first-pass effect, is administered in the duodenum, and serial peripheral blood samples are taken. Later, the same dose of the same drug is administered i.v. The shunt fraction of portal blood F is obtained by the ratio of the areas under the plasma level vs. time curves ("AUC") after p.o. and i.v. administrations: (see formula in text). The pigs were divided into three experimental groups. (i) Group I: undisturbed portal flow; (ii) Group II: total diversion of portal blood with an end-to-side portacaval shunt, and (iii) Group III: partial diversion of portal blood through a side-to-side portacaval shunt. Portal flow was measured during surgery with an electromagnetic flowmeter above and below the shunt and the degree of shunting calculated. Results show that the shunt fraction measured with ursodeoxycholic acid is well-correlated with hemodynamic data. No overlap between Groups I and III is observed. It is concluded that the shunt fraction of nutrient portal blood can be measured with this noninvasive method. Minute amounts of ursodeoxycholic acid were used in order to be completely metabolized by the liver, even in spite of hepatocellular dysfunction. Therefore, this method should be valid in cirrhotic patients and be useful to decide the type of portasystemic shunt to propose for the decompression of gastroesophageal varices.

Lack of induction of VLDL apoprotein synthesis by medium chain fatty acids in the isolated rat liver

Long chain fatty acids (greater than C16) are known to induce the liver synthesis of very low density lipoprotein (VLDL) apoproteins. Since medium chain (less than C16) triglycerides are used as dietary fats and in parenteral nutrition, we have investigated the relative uptake, esterification and oxidation of 14C-labelled fatty acids of a chain length of C10-C14 by the perfused rat liver compared to palmitic acid at two different concentrations: tracer (control) and overload (200 or 600 mumol/200 ml perfusate). The effect on VLDL apoprotein synthesis was simultaneously estimated by 3H-leucine incorporation. The results show: (1) a rapid liver uptake of all fatty acids; (2) a substantial incorporation into liver lipids of C12-C14 and C16, and (3) a higher oxidation rate of medium chain compared to long chain fatty acids; a lack of induction of VLDL apolipoprotein by an overload of fatty acids shorter than palmitic acid in spite of their utilisation for liver and VLDL lipid synthesis. Possible explanations for these differences are discussed.

Metabolism of orotic acid: lack of orotate phosphoribosyltransferase in rat intestinal mucosa

The main enzymes involved in orotic acid metabolism, orotate phosphoribosyltransferase and orotidine 5'-phosphate decarboxylase, are associated as a multienzyme complex (complex U) which is present in the liver of most vertebrate species. Orotic-acid-enriched diets produce increased pyrimidine synthesis which competes with purine synthesis for 5-phosphoribosyl diphosphate, resulting in decreased adenylate levels in liver cells. Inhibition of secretion of very low density lipoproteins and hepatic steatosis is then observed. In contrast, lipoproteins secretion by the intestine is not impaired and fat does not accumulate in enterocytes. The aim of this work was to investigate whether orotate is differently metabolized in gut and in liver thus explaining the lack of effect on the intestinal lipoproteins secretion. Complex U was found in appreciable amounts in rat, mouse and rabbit livers; the intestinal mucosa of the two last species contains a much lower level of multienzyme complex whereas in rat intestine its activity cannot be detected. Indeed, radioactive aspartate and orotate were not incorporated into intestinal cells RNA. The absence of orotate metabolisation by lack of orotate phosphoribosyltransferase and orotidine 5'-phosphate decarboxylase activity in rat intestine would explain why this organ, in contrast to the liver, is protected against disturbances of nucleotide metabolism and lipoproteins secretion induced by orotic-acid-supplemented diets.

Pharmacokinetics and metabolism of [14C]ursodeoxycholic acid in the rat

The pharmacokinetics and metabolism of [14C]ursodeoxycholic acid have been studied after intravenous and intraduodenal administrations of tracer doses to bile fistula rats. Control animals were fed a standard diet. Two other groups received the same diet added with UDCA (5 mg/kg body wt. per day) (group A) or (20 mg/kg body wt. per day) (group B) for 3 weeks. The plasma clearance in control and treated animals after intravenous injection of [14C]UDCA (2 microCi. 20 microgram) followed an identical biphasic exponential curve with t 1/2 of 2 and 30 min, respectively. Biliary excretion kinetics were dependent on the treatment; after 10 min of injection, the recovery of radioactivity in bile was 29.8, 19 and 10.9% in group B, A and control, respectively. After intraduodenal administration, the same dose of [14C]UDCA, was rapidly absorbed and excreted in bile. The kinetic process was modified by treatment; at the 30-min peak of radioactivity, the fraction of the dose excreted in bile was: controls, 51.2%; group A, 32.8% and group B, 35.3%. Biotransformation of [14C]UDCA after intravenous and intraduodenal administrations was similar in the three groups. Radioactivity was mainly found as the tauroconjugate of UDCA and less than 5% 14C was recovered as beta-muricholic acid. Bile composition was modified by the UDCA diet as follows: biliary bile acids increased and cholesterol decreased proportionally to the dose; the relative ratio of biliary bile acids was changed, UDCA represented at most 7% in the group B.

[Probucol: mechanism of action (author's transl)]

The mechanism of action of probucol is discussed after human and animal studies. This drug seems to have a poor or no effect on the overall cholesterol synthesis in the body and does not produce any storage of cholesterol or its metabolic intermediates (i.e. desmosterol) in the tissues. The main effect of probucol could be a decreased synthesis and/or an acceleration of plasma lipoprotein turnover; disappearance of xanthomatous lesions under traitement suggest a facilitation of cholesterol transferred from tissues to the excretion or catabolic pathways; its relevance in atheroma lesions is to be proved. Probucol seems to be a non hepatotoxic drug and induces a decrease of lithogenic index of bile.

Effect of fasting on the lipid composition and enzyme activity of rat liver plasma membranes

After 24-h fasting, when the recovery of plasma membrane protein isolated from rat liver was unchanged, the enrichment in 5'-nucleotidase was decreased by 16%. Modifications of the lipid composition were also observed and resulted in a 27% decrease of the cholesterol/phospholipid molar ratio.

Effect of clofibrate on the enzyme activity of rat liver plasma membranes

The activity of 3 plasma membranes marker enzymes (5'-nucleotidase, Mg++-ATPase and alkaline phosphodiesterase-I) was determined in plasma membranes isolated from liver of control and of clofibrate-treated rats. A complete indentity of plasma membranes enzyme activity in the 2 groups of experimental animals was observed for the 3 enzymes studied.

Effect of acetone on VLDL secretion by the isolated rat liver

High acetone levels occur in uncontrolled diabetes and after isopropanol administration to rats. In both conditions, very low density lipoproteins (VLDL) secretion is depressed. Acetone, hoever, failed to affect the VLDL secretion rate by the isolated perfused rat liver, suggesting that this metabolite is not involved in impaired VLDL production in diabetes and after isopropanol administration.

Ultrastructural localization of apo-b and apo-c binding to very low density lipoproteins in rat liver

Hepatic synthesis of apo-B and apo-C and their binding to nascent very low density lipoproteins (VLDL) have been studied in fat-fed rats. Apolipoproteins were located in hepatocyte organelles by light and electron microscopy after immunoenzymatic staining using peroxidase-conjugated antibodies. Our results indicate that apo-B and apo-C are synthesized by membrane-bound ribosomes. Both apoproteins seem to be adsorbed simultaneously to the lipid core of VLDL in the lumen of the endoplasmic reticulum channels, at the junction zone between rough and smooth endoplasmic reticulum. Some additional protein presumably binds nascent VLDL in the Golgi apparatus as judged by the strong positive reaction of lipoprotein particles with peroxidase-labeled antibodies. Finally our data show that significant amounts of apo-B and apo-C are bound to the sinusoidal plasma membrane in fed rat livers which probably represent remnants of lipoprotein of intestinal origin since membrane-bound apolipoproteins virtually disappeared 24 h after lymphatic duct cannulation. It is suggested that nascent VLDL (apo-C poor) could be enriched in apo-C from lipoprotein remnants at the space of Disse.

Ribonucleic acid synthesis in rat liver during fatty acid stimulated secretion of very low density lipoproteins

Production of very low density lipoproteins by the liver depends on the cellular availability of fatty acids. It is stimulated by the uptake of free fatty acids from the plasma and by increased lipogenesis and is inhibited by actinomycin D, suggesting that RNA synthesis is involved in the regulation of apolipoprotein synthesis. This hypothesis has been investigated in rats in vivo and in isolated perfused livers with and without stimulation by fatty acid overload: [14C] orotate incorporation in liver polyribosomal RNA is 60 per cent greater in stimulated livers as compared to controls. This increase is primarily due to a higher incorporation in bound polysomes and in those containing at least six ribosomes and does not result from the inhibition of ribonuclease. RNase digestion of polysomal RNA (4.10(-10) M enzyme, 0 degrees C, 3 h) shows that there is twice as much radioactivity in the hydrolyzed RNA of stimulated livers as compared to controls. After partial purification of poly A-rich RNA by affinity chromatography, the mass yield and radioactivity are increased by 100 per cent in stimulated livers as compared to controls. In conclusion, de novo RNA synthesis seems to be necessary for fatty acid stimulation of VLDL production.

[The effects of Triton WR 1339 on the secretion of bile lipids in the rat]

Rats bearing a biliary fistula received i.v. a solution of Triton WR 1339. Bile and plasma lipid composition was studied every 2 hrs and compared to that of control rats injected with saline. Two hours after Triton injection a sharp decrease in the bile secretion of lecithins (-- 93%), cholesterol (-- 50%) and bile salts (-- 50%) was observed together with a fall in bile flow (-- 20%). Eight hours after Triton administration the biliary output of lecithins, cholesterol and bile salts was lowered to --73%, --50% and --34% respectively compared to control animals. At that time an accumulation of L.C.A.T. substrates (lecithins and cholesterol) was observed in the plasma of Triton group. These modifications of biliary lipids after inhibition of L.C.A.T. activity by Triton W 1339 could be the result of a decreased production of plasma lysolecithins and cholesterol esters suggesting that both lipids could be important precursors for the synthesis of bile constituents. Furthermore this support the view that the production by the liver of plasma and bile lipids follows two distinct pathways.

Effects of glycerol on VLDL secretion by the isolated rat liver

Stimulation of VLDL production by increasing fatty acid availability is now well established. However, a possible regulatory role of glycerol, another lipid precursor, in VLDL synthesis by the liver has not yet been substaniated. The present experiments investigate this problem using the isolated perfused rat liver. [14C] Glycerol uptake and metabolism were studied at two different glycerol concentrations: 1 mumol/perfusate (control) or 1.6 mmol/perfusate. VLDL production and lipid synthesis were investigated using [14C]leucine and several labelled fatty acids as precursors in control and glycerol-overloaded livers. Neoglycogenesis and lipogenesis from glycerol carbons are negligible in our conditions. The absolute amount of glycerol, but not the precentage, taken up by the liver, increased after raising its concentration in the perfusate. A major part of exogenous (plasmatic) glycerol was esterified with endogenous (non plasmatic) fatty acids. Incorporation of radioactive fatty acids into liver or plasma lipids was lower than in the the control group. Significant differences were observed between saturated and unsaturated fatty acids used as lipid precursors. Production of VLDL as assessed by radioactive leucine and fatty acid incorporation in the VLDL of the perfusate was depressed by glycerol. Glycerol partly inhibits the normal stimulation of VLDL production by plasmatic fatty acid overload.

[An immunoelectrophoretic study of plasma lipoproteins in a case of familial deficiency of lecithin-cholesterol acyltransferase (author's transl)]

The lipid composition and apopeptide patterns of plasma LDL and VLDL have been studied in a patient with LCAT deficiency, using two-dimensional immunoelectrophoresis. After absorption of a fat meal an elevation of VLDL apopeptides C was observed. Apo-LDL also showed an abnormal relative increase of apo-C. After heparin-induced lipolysis, a shift of apo-C from chylomicra and VLDL to LDL was noticed by the striking increase of apo-C in this last lipoprotein class. In view of the present and previously published results the possible mechanisms involved in lipoprotein catabolism in LCAT deficiency are discussed.

Mechanism of ethacrynic acid-induced choleresis in the rat

Ethacrynic acid (EA) was injected to rats with functional nephrectomy after a control period of steady-state bile flow sustained by taurocholate infusion. Biliary clearance of [14C]mannitol was measured in order to estimate canalicular bile flow and bile salt-independent fraction (BSIF). After EA infusion, bile flow increased by 56%; bile salt excretion rate decreased by 10%; electrolyte excretion rates all increased, principally Na+ and K+. Mannitol clearance increased in parallel with bile flow. The BSIF increased. EA was excreted into bile as a metabolite identified as the cysteine adduct of EA; its excretion rate was linearly correlated with the increment in bile flow. The results are consistent with the hypothesis that the biliary excretion of an EA derivative results in an osmotic water flow increasing the canalicular BSIF. Since EA ia a Na+-K+-ATPase inhibitor, it is necessary to reconsider the relationship between secretion of canalicular BSIF and active Na+ transport mediated by the Na+-K+-ATPase system.

Demonstration of two pools of albumin-bound fatty acids

The uptakes of albumin-bound nonesterified fatty acids and of [1-14C]palmitic acid complexed to albumin by the isolated perfused rat liver were compared. During perfusion, the rate of uptake of nonesterified fatty acids decreased and became zero when the fatty acid:albumin molar ratio reached 0.3, but the rate of uptake of radioactive palmitic acid remained constant. This finding suggests the existence of two pools of fatty acids bound to albumin with different fractional turnover rates. This view was supported by the fact that when delipidated albumin complexed in vitro to radioactive and nonradioactive fatty acids was used no difference was observed between the uptakes of nonesterified fatty acids and radioactive fatty acids by perfused liver. Similar results were found with albumin-bound radioactive fatty acid in vivo (obtained from rats fed radioactive palmitic acid), showing a homogeneous distribution of the label in both pools. The existence of two nonesterified fatty acid pools in plasma would arise from the differences in the nature of bonds between fatty acid and albumin molecules, which could determine the rate of exchange of fatty acids between the albumin-bound and soluble forms preceding their uptake by the cells.