Inhibitory effect of unconjugated bile acids on the enterohepatic circulation of methotrexate

The effect of unconjugated cholic and deoxycholic acids on intestinal and hepatic transport and bile secretion of methotrexate was studied using everted sacs of rat proximal jejunum and isolated perfused rat liver. Cholic and deoxycholic acids competitively inhibit the mucosal-to-serosal transport of methotrexate (Ki, 0.08 and 0.06 mM, respectively). Cholic and deoxycholic acids also decrease intestinal tissue content of methotrexate in a concentration-dependent manner. Structural and functional damage to the intestinal mucosa does not occur in tissue treated with 0.1 mM and lower concentration of deoxycholic acid as assessed by histological studies, transmural potential difference measurements and the release of the cytoplasmic marker enzyme, lactate dehydrogenase. In the isolated liver, cholic and deoxycholic acids inhibit the uptake, retention and biliary secretion of methotrexate. At 1 mM cholic and deoxycholic acids, 72 and 80% inhibition in liver uptake and 93 and 99% inhibition in bile secretion of 1 microM methotrexate are observed, respectively. These studies demonstrate that unconjugated bile acids inhibit the enterohepatic circulation of methotrexate by impairing its intestinal transport and hepatic uptake and retention and biliary secretion.     

The enterohepatic nuclear receptors are major regulators of the enterohepatic circulation of bile salts

Recent studies have established that bile salts are signaling molecules, besides their classic function in dietary lipid absorption and cholesterol metabolism. Bile salts signal by activating mitogen-activated protein kinase (MAPK) pathways and nuclear receptors like farnesoid X receptor-alpha (FXRalpha). FXRalpha activation increases the expression of direct FXRalpha target genes involved in bile salt transport and detoxification, and decreases expression of indirect FXRalpha target genes involved in bile salt biosynthesis and uptake. These actions prevent toxic accumulation of bile salts in the enterohepatic organs. A network of interactions with other nuclear receptors and MAPK pathways may protect the liver against pathological elevation of bile salts and cholestasis. Therefore treatment of cholestasis might benefit from the development of FXRalpha agonists.     

The enterohepatic nuclear receptors are major regulators of the enterohepatic circulation of bile salts

Recent studies have established that bile salts are signaling molecules, besides their classic function in dietary lipid absorption and cholesterol metabolism. Bile salts signal by activating mitogen-activated protein kinase (MAPK) pathways and nuclear receptors like farnesoid X receptor-α (FXRα). FXRα activation increases the expression of direct FXRα target genes involved in bile salt transport and detoxification, and decreases expression of indirect FXRα target genes involved in bile salt biosynthesis and uptake. These actions prevent toxic accumulation of bile salts in the enterohepatic organs. A network of interactions with other nuclear receptors and MAPK pathways may protect the liver against pathological elevation of bile salts and cholestasis. Therefore treatment of cholestasis might benefit from the development of FXRα agonists.     

Regulation of pancreatic and gallbladder functions by intraluminal fatty acids and bile acids in man.

The effects of intraduodenal glycerol, fatty acid (FA) chain length and FA loads, and bile acid (BA) concentrations on pancreatic and gallbladder function were investigated in 31 healthy volunteers by a perfusion method. FA absorption rates in the duodenum and proximal jejunum were measured simultaneously. Pancreatic and gallbladder responses were augmented by increasing FA chain length and FA loads until the "maximal" secretory capacity of the pancreas and gallbladder emptying was attained. Glycerol had no effect. Raising BA concentrations above the critical micellar concentration accelerated FA absorption rates but decreased the magnitude of pancreatic and gallbladder responses to FA. Higher BA concentrations exerted an opposite effect, slowing FA absorption and increasing pancreatic and gallbladder responses. Indeed, a significant, inverse correlation was found between FA absorption and pancreatic and gallbladder responses to FA, suggesting a relationship between the length of intestine exposed to FA and the amount of cholecystokinin (and/or other neurohormonal factors) released, which stimulates pancreatic secretion and gallbladder contraction.     

Total fasting serum bile acids and beta-hexosaminidase in alcoholic liver disease

Total serum bile acid levels and beta-hexosaminidase activity were studied in 22 normal subjects, 35 non-cirrhotic patients with acute alcohol intoxication, 45 patients with alcoholic liver cirrhosis and 11 patients with alcoholic liver cirrhosis and surgical portal-systemic shunts. Comparison was made with traditional liver function tests. beta-Hexosaminidase was most frequently elevated in acute alcohol intoxication (94%) while total serum bile acids were elevated in all patients with alcoholic liver cirrhosis. Total serum bile acid levels were found to discriminate most efficiently between acute alcohol intoxication and liver cirrhosis. The combined determination of serum beta-hexosaminidase and total serum bile acids is proposed for evaluating alcoholic liver disease.     

Regulation of the cerebral circulation by endothelium.

Endothelium exerts an important influence on cerebral vascular tone through the production and release of a diverse group of vasoactive factors. Relaxing factors produced by endothelium include nitric oxide (or a nitric oxide-containing compound), a hyperpolarizing factor, and prostacyclin. Endothelium-derived contracting factors include cyclooxygenase products of arachidonic acid and endothelins. Several pathophysiological conditions are associated with increased formation of endothelium-derived contracting factors. Such endothelial dysfunction in the cerebral circulation may shift the balance of vascular tone toward constriction and may potentially contribute to the onset or maintainance of cerebral ischemia and stroke.     

Effects of controlled interruption of the enterohepatic circulation of bile salts by biliary diversion and by ileal resection on bile salt secretion, synthesis, and pool size in the rhesus monkey

The effects of controlled interruption of the enterohepatic circulation (EHC) of bile salts by biliary diversion on bile volume, bile salt secretion and synthesis rates, bile salt pool size, and the relationship to fecal fat excretion were studied in 16 rhesus monkeys.Bile from a chronic bile fistula was returned to the intestine through an electronic stream-splitter which, by diverting different percentages of bile to a collecting system, provided graded and controlled interruption of the EHC.The increase in hepatic bile salt synthesis in response to interruption of the EHC was limited and reached a maximum rate at 20% interruption of the EHC. Up to this level of biliary diversion, the increased hepatic synthesis compensated for bile salt loss so that bile salt secretion and pool size were maintained at normal levels. With diversion of 33% or more, there was no further increase in hepatic bile salt synthesis to compensate for external loss, and as a result there was diminished bile salt secretion, a reduction in bile salt pool size, and steatorrhea was observed.The effects of interruption of the EHC by the streamsplitter were compared with those produced by resection of the distal one-third or two-thirds of small bowel. While ileal resection appreciably reduced bile salt secretion, the EHC was by no means abolished. Bile salt reabsorption from the residual intestine was greater after one-third than after two-thirds small bowel resection. These observations suggest that jejunal reabsorption of bile salts occurs and may well contribute to the normal EHC.     

Serotonin increases the velocity of propagation and frequency of the migrating myoelectric complexes

The effect of serotonin on the myoelectric activity of the gastrointestinal tract was evaluated in seven opossums. Continuous intravenous administration of serotonin reduced the cycle duration of the migrating myoelectric complex and increased the velocity of propagation of the phase III. These changes were dose-dependent and were observed only with high doses of serotonin of 0.1 mg kg-1 h-1 or more. Infusion of 0.01 mg kd-1 h-1 had no effect on the motility of the gastrointestinal tract. The motility changes occurred in all segments of the gastrointestinal tract studied and were characterized by a continuous and organized increase in the velocity of propagation of the activity front (phase III) of the migrating myoelectric complex from the antrum to the terminal ileum. The cycle duration of the two migrating myoelectric complexes following administration of methysergide at 1.0 mg kg-1 was similar to the control migrating myoelectric complexes. We concluded that continuous infusion of serotonin in the opossum increases the velocity of propagation of the phase III of the migrating myoelectric complex from the antrum to the terminal ileum.     

Lipoprotein-bound bile acids in serum from healthy men, postprandially and during fasting

Individual bile acids were determined by gas-liquid chromatography in, very low density, low density and high density lipoprotein fractions obtained by sequential ultracentrifugation of serum from normal adults, both postprandially and during fasting. The lipoproteins were found to contain 22-34% of fasting serum bile acids. The observed postprandial increase in bile acids did not exhibit any shift in the ratio between lipoprotein bound- and non-lipoprotein-bound bile acids. Bile acids were present in all isolated lipoprotein fractions, high density lipoproteins containing the highest amounts. In the lipoprotein fraction, a higher percentage of cholate than of chenodeoxycholate was found.     

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.     

Interruption of the enterohepatic circulation of digitoxin by cholestyramine: I. Protection against lethal digitoxin intoxication

Previous studies have demonstrated that considerable amounts of parenterally administered cardiac glycosides are excreted in the bile and reabsorbed across the intestinal mucosa in several species. It is currently believed that the more prolonged action of nonpolar digitalis glycosides is due to their retention and recycling in the enterohepatic circulation. This report describes studies carried out to evaluate the effects of pharmacologic interruption of this enterohepatic cycle with the intraluminal sequestering agent cholestyramine.Cholestyramine was found to bind substantial quantities of digitoxin-(3)H and digoxin-(3)H in vitro and this binding was only modestly inhibited by the presence of bile. Administration of cholestyramine to rats by intragastric catheter before the subcutaneous injection of the LD(100) dose of digitoxin (10 mg/kg) resulted in a 70% survival rate. Further, oral administration of cholestyramine to rats before the subcutaneous injection of digitoxin-(3)H resulted in accelerated fecal excretion of radioactivity and lower levels of digitoxin-(3)H and metabolites in brain tissue compared to controls. Similarly, pretreatment of guinea pigs with cholestyramine orally before the injection of digitoxin in dosages of 10.0 and 4.0 mg/kg resulted in a 25 and 70% survival rate respectively as compared to survival rates of 0 and 30% in control animals. Cholestyramine pretreatment of guinea pigs was also accompanied by lower levels of digitoxin-(3)H and metabolites in heart and liver 90 min after injection of digitoxin-(3)H. Cholestyramine therapy did not result in significant changes in serum potassium levels excluding the possibility that drug-induced hyperkalemia might have affected the cardiac uptake of digitoxin.The data obtained in this study indicate that cholestyramine treatment affords a significant degree of protection against lethal digitoxin intoxication in rats and guinea pigs. It is suggested that cholestyramine binds appreciable amounts of digitoxin in the intestinal lumen resulting in reduced reabsorption, increased fecal excretion, and lower tissue levels of glycoside in critical organs. The protective effects of cholestyramine appear to be mediated by interruption of the enterohepatic circulation of digitoxin.     

Radioimmunoassay of bile acids in serum.

We developed four radioimmunoassay procedures for the determination of glycine-conjugated bile acids in serum. Antibodies to two primary bile acids, cholylgycine and chenodeoxycholylglycine, and to two secondary bile acids, sulfolithocholylglycine and deoxycholylglycine, were raised in rabbits after the acids were covalently linked to albumin by use of the carbodiimide reaction. Assay sensitivity for each of these bile acids is in the picomole range with the standard curve extending from 10-80 pmol. The concentration of bile acids in serum increased in various states of liver disease and its measurement appears to be an extremely sensitive indicator of liver function.