Effect of secretin on bile osmolality

During a series of investigations upon the physiologic action of secretin on bile composition and flow from the perfused pig liver, we noted that in 7 of 8 experiments, bile osmolality increased with secretin stimulation. This increase implied that the secretin-stimulated fraction of bile was hypertonic to the basal or control fraction of bile. However, the nonideal osmotic behavior of bile salts upon dilution with electrolyte solution could explain the higher osmolality of secretin-stimulated bile. Bile salts above a certain critical micellar concentration (CMC) aggregate to form micelles which are in equilibrium with the monomer at the CMC. Dilution even with a solution that is isosmotic by virtue of another solute causes dissociation of micelles so that the concentration of the monomer is maintained constant at a concentration equal to the CMC. This results in more osmotically active particles than predicted from simple linear dilution of an ideal solution. We tested this theory by mixing solutions of bile salt (sodium cholate or taurocholate) and of pig bile with pure electrolyte solutions of approximately equal osmolality. The resultant osmolality was always greater than that predicted from ideal behavior. We mixed control bile samples from a perfused pig liver with an artificial electrolyte solution made to approximate the calculated secretin fraction from the same liver. The increase in osmolality with mixing was similar to that observed in bile collected from the liver during secretin stimulation. We conclude that the nonideal osmotic behavior of bile salt solutions can account for the increase in bile osmolality observed in our preparation during secretin stimulation. Future studies involving bile osmolality must take into account this behavior of bile salts.