Bile salt hydrophobicity controls vesicle secretion rates and transformations in native bile.

After drainage of the bile salt pool, we infused unanesthetized bile fistula prairie dogs (Cynomys ludovicianus) intravenously with taurine-conjugated chenodeoxycholate (TCDC), cholate (TC), ursodeoxycholate (TUDC), and ursocholate (TUC) in concentrations that attained greater than 94% enrichment of biliary bile salts. With decreases in bile salt hydrophobicity, maximum steady state lecithin and to a lesser extent cholesterol secretion rates decreased in the rank order, TCDC greater than TC greater than TUDC greater than TUC. By phase analysis, TCDC-rich and TC-rich biles plotted inside their respective micellar zones, whereas TUDC-rich and TUC-rich biles plotted outside and were so-called "supersaturated" with cholesterol. Quasi-elastic light scattering and electron microscopy, when performed within 30 min of collection, revealed unilamellar vesicles in all biles. By 24 h, vesicles in TCDC-rich and TC-rich biles had dissolved into mixed micelles, whereas vesicles in TUDC-rich biles formed mixed micelles plus multilamellar liquid crystals, and vesicles in TUC-rich biles formed multilamellar liquid crystals exclusively. Because cholesterol/phospholipid molar ratios of multilamellar liquid crystals were less than or equal to 1, cholesterol monohydrate crystals did not form in these biles. We conclude that, despite drastic alterations in bile salt detergency, unilamellar vesicles are the final common pathway for lecithin and cholesterol secretion into bile. During equilibration of bile, the fate of unilamellar vesicles may be micellar, micellar plus liquid crystalline, or liquid crystalline only depending on the detergency (i.e., hydrophobic-hydrophilic balance) of the secreted bile salt.