Fig. 32.10 ). Useful solutes (e.g., glucose, amino acids) are reclaimed by the activity of specific transporters. Chloride ions in bile are also exchanged for HCO3 − , rendering the bile slightly alkaline and reducing the risk of precipitation of Ca++ . Glutathione is broken down on the surface of cholangiocytes into its constituent amino acids by the enzyme γ-glutamyl transpeptidase (GGT), and the products are reabsorbed. Bile is also diluted at this site, in concert with ingestion of a meal, in response to hormones such as secretin that increase HCO3 − secretion and stimulate insertion of aquaporin water channels into the cholangiocyte’s apical membrane. Flow of bile is thereby increased during the postprandial period when bile acids are needed to aid in assimilation of lipid.
Role of the Gallbladder
Finally, bile enters the ducts and is conveyed toward the intestine. However, in the period between meals, outflow is blocked by constriction of the sphincter of Oddi, and thus bile is redirected to the gallbladder. The gallbladder is a muscular sac lined with high-resistance epithelial cells. During gallbladder storage, bile becomes concentrated because sodium ions are actively absorbed in exchange for protons, and bile acids, as the major anions, are too large to exit across the gallbladder epithelial tight junctions (Fig.
32.11 ). However, although the concentration of bile acids can rise more than 10-fold, bile remains isotonic because a •Fig. 32.11 Mechanismsaccountingforconcentrationofbileduringstorageinthegallbladder.NHE,sodium/hydrogenexchanger;BA−,bileacidanion;AQP1,aquaporin1.
single micelle acts as only one osmotically active particle. Any additional bile acid monomers that become available as a result of concentration are thus immediately incorporated into existing mixed micelles. This also reduces to some extent the risk that cholesterol will precipitate from bile. However, cholesterol is supersaturated in the bile of many adults, with precipitation normally being inhibited by the presence of antinucleating proteins. Prolonged storage of bile increases the chance that nucleation can occur, thus making a good case for never skipping breakfast and perhaps explaining why gallstone disease is relatively prevalent in humans.