Phospholipids and bile acids as diffusional carriers of Na+ across nonpolar media

Calcium affinity for biliary lipid aggregates in model biles: complementary importance of bile salts and lecithin

BACKGROUND/AIMS

Despite putative roles of calcium in biliary physiology and gallstone formation, quantitative aspects of calcium binding to bile salt (BS) monomers, simple micelles, mixed micelles, and vesicles, which constitute the lipid aggregates in bile, remain unexplored.

METHODS

Calcium activity was measured using the calcium electrode in pathophysiologically relevant model biles composed of either individual BS species or a physiological mixture of glycine and taurine conjugates, as functions of lecithin and cholesterol contents and total lipid concentration.

RESULTS

Calcium binding increased with increasing BS concentrations and lecithin contents and varied with species (dihydroxy > trihydroxy BS) and with conjugation (unconjugated > glycine conjugates > taurine conjugates). Although lecithin/cholesterol vesicles did not bind detectable calcium, when taurocholate was incorporated into membrane bilayers, calcium binding was substantially greater than with equimolar BS alone. Added cholesterol did not alter calcium binding, despite cholesterol saturation of biliary lipid aggregates and induction of liquid crystalline and solid crystalline-phase transitions.

CONCLUSIONS

In model biles, most calcium is bound to mixed micelles, with minor contributions by BS monomers, simple micelles, and vesicles. It is proposed that BS-induced binding of calcium to vesicles and mixed micelles may be important in nucleation of cholesterol and bilirubinates from native bile.

The influence of bile salts on small intestinal motility in the guinea pig in vitro

The effect of bile salts on intestinal motility is unclear. In the current study, isometric contractions of the guinea pig terminal ileum were examined in vitro. Dose-response curves to known agonists cholecystokinin (CCK), bethanechol, and KCl were constructed alone and in the presence of atropine (10(-6) mol/L), tetrodotoxin (10(-6) mol/L), and different bile salts, namely, taurodeoxycholate, tauroursodeoxycholate, taurocholate, glycodeoxycholate, and glycoursodeoxycholate. These bile salts, at levels as low as 5 and 50 mumol/L, significantly depressed (P less than 0.05) CCK-induced contractions throughout the dose-response curves and were concentration dependent. This depressant effect was not dependent on the bile salt species or any apparent physicochemical differences between them. The inhibitory effect was also specific for certain agonists such as CCK (the action of which was partially mediated by cholinergic nerves, being depressed by atropine and abolished by tetrodotoxin), field stimulation, and nicotine. Bile salts had no effect on either bethanechol- or KCl-induced contractions. Such bile salt inhibition of excitatory, cholinergic, enteric neurons may slow transit through the ileum, enhancing the time for absorption and conserving the bile salt pool.

Taurine-conjugated bile acids act as Ca2+ ionophores

The ionophoretic properties of several taurine-conjugated bile acids have been investigated in two experimental systems: in a two-phase bulk partitioning system and in proteoliposomes. In the former, a bile acid/Ca2+ complex was extracted into the bulk organic phase and had an experimental stoichiometry of 1.75. Extraction was specific for Ca2+ over Mg2+; Na+ and K+ did not compete with the extraction of Ca2+. In the second system, bile acids at concentrations as low as 5-100 molecules/vesicle lowered the steady-state Ca2+ gradient maintained by a reconstituted sarcoplasmic reticulum Ca(2+)-ATPase. The effect was not due to nonspecific membrane perturbation. In addition to releasing intravesicular Ca2+ in a transmembraneous process, bile acids caused partition of Ca2+/bile acid complexes into the hydrophobic core of the bilayer. In both experimental systems, the Ca2+ ionophoretic activity correlated well with the concentration and the hydrophobicity of the bile acid. Taurolithocholate was most active, with a significant effect measurable at 10 microM in either system. Since bile acid concentrations equal to those used in our experiments can occur in the blood in certain liver diseases, the results support the notion that bile acids can increase the intracellular Ca2+ concentration bypassing the regulatory systems that maintain cellular Ca2+ homeostasis.