Selectively reduced biliary excretion of cholyldiglycylhistamine but not of cholyltetraglycylhistamine in ethinyl estradiol-treated rats. A possible indicator of increased bile canalicular permeability

Structural biology and function of solute transporters: implications for identifying and designing substrates

Solute carrier (SLC) proteins have critical physiological roles in nutrient transport and may be utilized as a mechanism to increase drug absorption. However, we have little understanding of these proteins at the molecular level due to the absence of high-resolution crystal structures. Numerous efforts have been made in characterizing the peptide transporter (PepT1) and the apical sodium dependent bile acid transporter (ASBT) that are important for both their native transporter function as well as targets to increase absorption and act as therapeutic targets. In vitro and computational approaches have been applied to gain some insight into these transporters with some success. This represents an opportunity for optimizing molecules as substrates for the solute transporters and providing a further screening system for drug discovery. Clearly the future growth in knowledge of SLC function will be led by integrated in vitro and in silico approaches.

Modeling of active transport systems

Transport proteins have critical physiological roles in nutrient transport and may be utilized as a mechanism to increase drug absorption. However, we have little understanding of these proteins at the molecular level due to the absence of high-resolution crystal structures. Numerous efforts have been made to characterize the P-glycoprotein efflux pump, the peptide transporter (PepT1) and the apical sodium-dependent transporter (ASBT) which are important not only for their native transporter function but also as drug targets to increase absorption and bioactivity. In vitro and computational approaches have been applied to gain some insight into these transporters with some success. This represents an opportunity for optimizing molecules as substrates for the solute transporters and providing a further screening system for drug discovery. Clearly the future growth in knowledge of transporter function will be led by integrated in vitro and in silico approaches.

Different lobular distributions of altered hepatocyte tight junctions in rat models of intrahepatic and extrahepatic cholestasis

Hepatocyte tight junctions (TJs), the only intercellular barrier between the sinusoidal and the canalicular spaces, play a key role in bile formation. Although hepatocyte TJs are impaired in cholestasis, attempts to localize the precise site of hepatocyte TJ damage by freeze-fracture electron microscopy have produced limited information. Recently, several TJ-associated proteins like ZO-1 and 7H6 have been identified and characterized. Immunolocalization of 7H6 appears to closely correlate with paracellular permeability. We used rat models of intrahepatic cholestasis by ethinyl estradiol (EE) treatment and extrahepatic cholestasis by bile duct ligation (BDL) to precisely determine the site of TJ damage. Alterations in hepatocyte TJs were assessed by double-immunolabeling for 7H6 and ZO-1 using a confocal laser scanning microscope. In control rats, immunostaining for 7H6 and ZO-1 colocalized to outline bile canaliculi in a continuous fashion. In contrast, 7H6 and ZO-1 immunostaining was more discontinuous, outlining the bile canaliculi after BDL. Immunostaining for 7H6, not ZO-1, decreased and predominantly appeared as discrete signals in the submembranous cytoplasm of periportal hepatocytes after BDL. After EE treatment, changes in immunostaining for 7H6 and ZO-1 were similar to those seen in periportal hepatocytes after BDL, but distributed more diffusely throughout the lobule. This study is the first to demonstrate that impairment of hepatocyte TJs occurs heterogenously in the liver lobule after BDL and suggests that BDL and EE treatments produce different lobular distributions of increased paracellular permeability.

Biliary excretion of chenodeoxycholyllysylrhodamine in Wistar rats: a possible role of a bile acid as a carrier for drugs

The effect on biliary excretion of rhodamine after its conjugation to give chenodeoxycholyl-lysyl-rhodamine (cheno-lys-R) was studied in male Wistar rats. Following its intravenous injection via the jugular vein of animals cheno-lys-R was efficiently excreted into bile with a peak biliary excretion of 31.6 +/- 1.2% dose 5 min-1 and a cumulative biliary excretion of 96.4 +/- 2.0% in 30 min of the total dose administered. Unlike cheno-lys-R, rhodamine had a poor biliary excretion of 1.0 +/- 0.1% dose 5 min-1 and a cumulative biliary excretion of 3.3 +/- 0.6% in 30 min. Cheno-lys-R had a short plasma half-life (t1/2 alpha) of 4.0 +/- 0.5 min, whereas free rhodamine had a longer half life (t1/2 alpha) of 82.1 +/- 1.4 min. The plasma clearances of cheno-lys-R and rhodamine were 41.2 +/- 6.5 and 9.0 +/- 1.2 ml/min per kg, respectively. The data indicate that the cationic fluorescent xenobiotic, rhodamine, when conjugated to the bile salt analogue, greatly increased the biliary excretion of rhodamine and that cheno-lys acted as a carrier for hepatic uptake of rhodamine. Thus, an appropriate bile salt derivative may be used to target a drug to the liver.

Synthesis and biliary excretion of tyrosine-conjugated bile salts in Wistar rats

Tyrosine-labelled free and glycine-conjugated bile acids were synthesized and radiolabelled with 125I to high purity. The synthetic method utilized excess tyrosine methyl ester hydrochloride (1.4 equiv.) and bile acid (one equiv.) via dicyclohexylcarbodiimide (1.4 equiv.) with yields of 90-93% for tyrosine bile acid conjugates and glycyltyrosine conjugates and 56-60% yields for the glycylglycyltyrosine conjugates. All of the eight iodinated tyrosine bile acids tested were rapidly excreted into bile following intravenous injection. In bile duct-cannulated rats with ligated renal pedicles under pentobarbital anaesthesia the percentages of injected dose recovered from bile within 20 min were as follows: cholylglycine ([14C]cholylGly), 81.2 +/- 1.3%; taurocholate ([14C]taurocholate), 94.3 +/- 1.0%; cholyltyrosine (125I-cholylTyr), 85.5 +/- 3.3%; deoxycholyltyrosine (125I-deoxycholylTyr), 87.9 +/- 6.3%; chenodeoxycholyltyrosine (125I-chenodeoxycholylTyr), 93.4 +/- 2.9; cholylglycyltyrosine (125I-cholylGlyTyr), 95.7 +/- 6.7%; deoxycholylglycyltyrosine (125I-deoxylcholylGlyTyr), 92.5 +/- 3.2%; chenodeoxycholylglycyltyrosine (125I-chenodeoxycholylGlyTyr), 94.1 +/- 3.1%; cholyldiglycyltyrosine (125I-cholylGlyGlyTyr), 85.2 +/- 3.6%, and deoxycholyldiglycyltyrosine (125I-deoxycholylGlyGlyTyr), 85.5 +/- 2.7%. Values are means +/- SD. Thus the biliary excretion of 125I-chenodeoxycholylGlyTyr, 125I-chenodeoxycholylTyr, 125I-deoxycholylGlyTyr and 125I-cholylGlyTyr was similar to that of [14C]taurocholate, the major naturally occurring bile acid in the rat, and the biliary excretion of all the tyrosine conjugates was similar to or exceeded that of [14C]cholylglycine. Conjugation with tyrosine enhanced the efficiency of plasma-to-bile transport of most naturally occurring bile acids. Comparison of glycyltyrosine conjugates with glycylglycyltyrosine conjugates suggests that any additional benefit derived by elongation of the side-chain is probably negated by obscuring the 12 alpha-hydroxyl function on the steroid nucleus in the bile acid glycylglycyltyrosine conjugates.

Biliary permeability during ethinyl estradiol-induced cholestasis studied by segmented retrograde intrabiliary injections in rats

The influence of ethinyl estradiol (EE) treatment on the permeability of the biliary tree in rats has been assessed by the segmented retrograde intrabiliary injection (SRII) technique. Three pairs of compounds were studied, inert [14C]sucrose and [3H]inulin; [14C]taurocholic acid (TC) and [14C]glycocholic acid (GC) and the non-ionic bile acid derivatives [125I]cholyldiglycylhistamine (CG2H) and [131I]cholyltetraglycylhistamine (CG4H). In control rats recovery in bile after SRII was always greater for the larger of any pair of compounds, confirming that the biliary tree acts as a filter, and that decreased recovery from bile during this technique is an index of greater biliary permeability. After EE treatment recovery of all compounds was significantly reduced, thus confirming that EE increases biliary permeability. Recovery of sucrose and inulin fell from 55-65% of the administered dose in controls to 8-9% in EE rats. Recoveries of TC, GC, CG2H and CG4H was also reduced, but their biliary recovery profiles were consistent with marked re-excretion into bile of that portion which had initially passed out of the biliary system by filtration. During the later phase of the experiment excretion of the negatively charged bile acids TC and GC was greater than that of the non-ionic bile acid derivatives CG2H and CG4H. Although the site at which these permeability changes have occurred is unknown, our results are compatible with previous data implicating increased tight junction permeability as a mechanism of EE-induced cholestasis.(ABSTRACT TRUNCATED AT 250 WORDS)