Selective permeabilization of the endoplasmic reticulum by monohydroxylated bile acids in liver

Contributions of bile acids to gastrointestinal physiology as receptor agonists and modifiers of ion channels

Bile acids (BAs) are known to be important regulators of intestinal motility and epithelial fluid and electrolyte transport. Over the past two decades, significant advances in identifying and characterizing the receptors, transporters, and ion channels targeted by BAs have led to exciting new insights into the molecular mechanisms involved in these processes. Our appreciation of BAs, their receptors, and BA-modulated ion channels as potential targets for the development of new approaches to treat intestinal motility and transport disorders is increasing. In the current review, we aim to summarize recent advances in our knowledge of the different BA receptors and BA-modulated ion channels present in the gastrointestinal system. We discuss how they regulate motility and epithelial transport, their roles in pathogenesis, and their therapeutic potential in a range of gastrointestinal diseases.

Lithocholyltaurine interacts with cholinergic receptors on dispersed chief cells from guinea pig stomach

Although bile acids damage gastric mucosa, the mechanisms underlying tissue injury induced by these agents are not well understood. To determine whether bile acids alter gastric secretory function, we investigated the actions of sodium cholate, deoxycholate, lithocholate, and their taurine and glycine conjugates on a highly homogeneous population of gastric chief cells. Lithocholyltaurine (LCT), a particularly injurious bile acid, caused a threefold increase in pepsinogen secretion (detectable with 100 nM and maximal with 10 microM LCT). When combined with other secretagogues, increasing concentrations of LCT caused progressive inhibition of carbamylcholine (carbachol)-induced pepsinogen secretion but did not alter CCK- or 8-bromo-cAMP-induced secretion. Taurine and unconjugated lithocholate did not alter basal or carbachol-induced secretion. These observations suggested that LCT is a partial cholinergic agonist. To test this hypothesis, we examined the actions of the cholinergic antagonist atropine on LCT-induced pepsinogen secretion. Atropine (10 microM) abolished carbachol- and LCT-induced pepsinogen secretion. Likewise, carbachol (0.1 mM) and LCT (1 mM) induced an atropine-sensitive, two- to threefold increase in cellular levels of inositol 1,4,5-trisphosphate. We examined the actions of LCT on binding of the cholinergic radioligand [N-methyl-3H]scopolamine ([3H]NMS) to chief cells. Half-maximal inhibition of [3H]NMS binding was observed with approximately 0.5 mM carbachol and 1 mM LCT. These results indicate that the bile acid LCT is a partial agonist for muscarinic cholinergic receptors on gastric chief cells.

Conjugates of ursodeoxycholate protect against cytotoxicity of more hydrophobic bile salts: in vitro studies in rat hepatocytes and human erythrocytes

Intraduodenal infusion of hydrophobic bile salts to bile-fistula rats leads within hours to severe hepatocellular necrosis and cholestasis; simultaneous administration of conjugates of ursodeoxycholate, either intraduodenally or intravenously, reduces or prevents liver injury. To evaluate the short-term protective effects of ursodeoxycholate at the cellular level, we incubated primary monolayer cultures of adult rat hepatocytes or freshly isolated washed human erythrocytes for 1 to 240 min with varying defined concentrations of different bile salts in the presence or absence of ursodeoxycholate. Cytolysis was quantified by measuring the release into the medium of cytosolic lactate dehydrogenase (hepatocytes) or hemoglobin (erythrocytes). In both systems, cytolysis increased sigmoidally with increasing bile salt concentration, and the relative toxicity of different bile salts proceeded in the following order: tauroursodeoxycholate was less toxic than taurocholate, which was less toxic than taurodeoxycholate. Taurochenodeoxycholate was more toxic to erythrocytes than taurodeoxycholate; the two were equally toxic to rat hepatocytes. Unconjugated bile salts were more toxic than their conjugates. The addition of tauroursodeoxycholate to taurochenodeoxycholate or taurodeoxycholate led to time-dependent and concentration-dependent reduction or elimination of the toxicity of the more hydrophobic component. Protection was evident within minutes. With respect to hemolysis, at pH 8.5 glyco was less protective than tauroursodeoxycholate, and free ursodeoxycholate was only minimally protective. We conclude that the hepatocytotoxicity of hydrophobic bile salts at millimolar concentrations is markedly reduced in the presence of tauroursodeoxycholate. Conjugates of ursodeoxycholate also prevented disruption of erythrocytes by bile salts, suggesting that protection does not depend on liver-specific pathways of bile salt uptake, compartmentation, transport or metabolism.(ABSTRACT TRUNCATED AT 250 WORDS)