Bile secretory characteristics of beta-muricholate and its taurine conjugate are similar to those of ursodeoxycholate in the rat

Hepatic bile formation: bile acid transport and water flow into the canalicular conduit

Advances in molecular biology identifying the many carrier-mediated organic anion transporters and advances in microscopy that have provided a more detailed anatomy of the canalicular conduit make updating the concept of osmotically determined canalicular flow possible. For the most part water flow is not transmembrane but via specific pore proteins in both the hepatocyte and the tight junction. These pores independently regulate the rate at which water flows in response to an osmotic gradient and therefore are determinants of canalicular bile acid concentration. Review of the literature indicates that the initial effect on hepatic bile flow of cholestatic agents such as Thorazine and estradiol 17β-glucuronide are on water flow and not bile salt export pump-mediated bile acid transport and thus provides new approaches to the pathogenesis of drug-induced liver injury. Attaining a micellar concentration of bile acids in the canaliculus is essential to the formation of cholesterol-lecithin vesicles, which mostly occur in the periportal region of the canalicular conduit. The other regions, midcentral and pericentral, may transport lesser amounts of bile acid but augment water flow. Broadening the concept of how hepatic bile flow is initiated, provides new insights into the pathogenesis of canalicular cholestasis.

Enterohepatic circulation: physiological, pharmacokinetic and clinical implications

Enterohepatic recycling occurs by biliary excretion and intestinal reabsorption of a solute, sometimes with hepatic conjugation and intestinal deconjugation. Cycling is often associated with multiple peaks and a longer apparent half-life in a plasma concentration-time profile. Factors affecting biliary excretion include drug characteristics (chemical structure, polarity and molecular size), transport across sinusoidal plasma membrane and canniculae membranes, biotransformation and possible reabsorption from intrahepatic bile ductules. Intestinal reabsorption to complete the enterohepatic cycle may depend on hydrolysis of a drug conjugate by gut bacteria. Bioavailability is also affected by the extent of intestinal absorption, gut-wall P-glycoprotein efflux and gut-wall metabolism. Recently, there has been a considerable increase in our understanding of the role of transporters, of gene expression of intestinal and hepatic enzymes, and of hepatic zonation. Drugs, disease and genetics may result in induced or inhibited activity of transporters and metabolising enzymes. Reduced expression of one transporter, for example hepatic canalicular multidrug resistance-associated protein (MRP) 2, is often associated with enhanced expression of others, for example the usually quiescent basolateral efflux MRP3, to limit hepatic toxicity. In addition, physiologically relevant pharmacokinetic models, which describe enterohepatic recirculation in terms of its determinants (such as sporadic gall bladder emptying), have been developed. In general, enterohepatic recirculation may prolong the pharmacological effect of certain drugs and drug metabolites. Of particular importance is the potential amplifying effect of enterohepatic variability in defining differences in the bioavailability, apparent volume of distribution and clearance of a given compound. Genetic abnormalities, disease states, orally administered adsorbents and certain coadministered drugs all affect enterohepatic recycling.

Ursodeoxycholic acid and cholesterol induce enterohepatic cycling of bilirubin in rodents

BACKGROUND & AIMS

Oral administration of ursodeoxycholic acid (UDCA) and cholesterol causes bile salt malabsorption; the former by competition for and the latter by down-regulation of ileal bile acid transporters. Because ileectomy in rats induces enterohepatic cycling of bilirubin, the hypothesis that dietary steroids might have the same effect was tested.

METHODS

Male inbred C57L/J mice and Sprague-Dawley rats were fed low doses of UDCA, chenodeoxycholic acid (CDCA), or cholesterol added to laboratory chow with simultaneous chow-fed controls. After 1 week (mice) or 2 weeks (rats), indices of bile salt malabsorption and enterohepatic cycling of bilirubin were measured, including bilirubin secretion rates into bile, serum and intestinal bilirubin and bile salt levels, and urobilinogen levels in cecum, large intestine, and feces.

RESULTS

Dietary UDCA and cholesterol, but not CDCA, significantly increased bilirubin secretion rates into bile. In UDCA-fed mice, gallbladder biles contained increased levels of bilirubin conjugates and unconjugated bilirubin, and in 60%, granules of amorphous calcium bilirubinate precipitated. Dietary cholesterol and bile acids, particularly UDCA, increased cecal bile salt levels, unconjugated bilirubin and urobilinogen concentrations, and decreased fecal bilirubin outputs, consistent with colonic absorption.

CONCLUSIONS

By causing bile salt malabsorption, dietary UDCA and cholesterol induce enterohepatic cycling of bilirubin.

Metabolic fate and hepatocyte toxicity of reverse amide analogs of conjugated ursodeoxycholate in the rat

Reverse amide analogs of conjugated bile acids were tested for their effects on the viability of cultured primary rat hepatocytes, for their transport and metabolism in the intact rat, and for their susceptibility to hydrolysis by intestinal bacteria. Succinylnorursodeoxycholanylamide (SNUDCN) and its parent C23 amine showed the same general lack of toxicity toward hepatocytes as the normal conjugates of ursodeoxycholic acid, at concentrations up to 500 microM. The 3alpha,7alpha,12alpha-trihydroxy analog and its parent amine were more toxic than the corresponding dihydroxy compounds, although their effects were similar to those observed for the normal conjugates of cholic acid. Following intraduodenal infusion, greater than 80% of administered SNUDCN appeared in the bile of bile fistula rats. Analysis of bile fractions indicated the presence of SNUDCN (81.5 mol% of original amount) and two metabolites, the taurine conjugate of SNUDCN (9.4 mol%) and SNUDCN containing an additional hydroxy group (9.1 mol%). Although SNUDCN underwent an efficient first pass enterohepatic circulation, it displayed a shorter biological half life than taurocholate (T1/2: 8.9 h vs 39.6 h, respectively). The reverse amide analogs were not hydrolyzed by any of a variety of intestinal bacteria known to hydrolyze normal conjugated bile acids. Despite the shorter half-life, the reverse amide analogs may be of potential use in the targeting of therapeutic bile acids to the colon.

Tauro alpha-muricholate has a biliary transport maximum (Tm) value equivalent to that for tauroursodeoxycholate and tauro beta-muricholate in the rat

Our previous studies have shown that Tm values for tauroursodeoxycholate (TUDC) and tauro beta-muricholate (T beta-MC) are more than two-fold higher than that for taurocholate (TC) in the rat. The present study attempted to clarify whether tauro alpha-muricholate (T alpha-MC) also has such an unusually large Tm value in the rat. Under nembutal anesthesia, male Wistar derived rats (body weight 280-300 g, 13 wks in age) were continuously infused with T alpha-MC solution. The infusion rate was raised stepwise every 20 min, until the bile flow began to decline. Bile was collected every 10 min and bile salt excretion rate was determined. The average of the highest three excretion values was assumed to be the Tm in each animal. The Tm value of T alpha-MC was found to be 2.86 +/- 0.36 mumol/min/100 g (mean +/- SD, n = 4), which was even greater than Tm values for TUDC (2.59 +/- 0.39 mumol/min/100 g, n = 4) and T beta-MC (1.93 +/- 0.31 mumol/min/100 g, n = 4) as we reported previously. The relationship between the bile flow rate (microliter/min/100 g, Y axis) and bile salt excretion rate (mumol/min/100 g, X axis) was highly linear [Y = (6.00 +/- 0.29) x +(6.60 +/- 1.88), P < 0.001, r = 0.95, n = 54]. The slope value for T alpha-MC (6.00 +/- 0.29 microliters/mumol) was significantly higher than that for TUDC (4.76 +/- 0.71 microliters/mumol) and was comparable to that for T beta-MC as we previously found for these bile salts in this rat strain. The results suggest that T alpha-MC has a very efficient transport system in this species as was observed for the other two bile salts that have a 7 beta-hydroxy group (TUDC and T beta-MC). This efficient transport system thus appears to be shared not only by bile salts specifically having a 7 beta-hydroxy group, but also by other bile salts such as T alpha-MC that have a 6 beta-hydroxy group but not a 7 beta-hydroxy group.