Transport functions of the gallbladder.

The absorptive functions of the gallbladder are responsible for concentrating the Na+ salts of bile acids during interprandial periods. This can be attributed entirely to its ability to absorb NaCl (and NaHCO3) and water in isotonic proportions, thus reducing the volume of hepatic bile by 80%--90%. The results of studies employing gallbladders of several species are consistent with the presence of neutral NaCl (and NaHCO3) absorption that is due to the presence of a coupled (one-for-one) NaCl entry process at the mucosal membrane. Active Na+ extrusion from cell to serosal solution appears to provide the energy for cellular Cl- accumulation, and thus for transepithelial Cl- transport. The mechanism of Cl- exit from the cell to serosal solution is uncertain andrequires further study. Rabbit gallbladder provided an ideal preparation for the characterization of NaCl cotransport and continues to be the tissue of choice for further study of this mechanism. Electrophysiological studies support the concept of nonconductive NaCl cotransport and also suggest that departures from the process of strictly neutral salt absorption may be related to the presence of additional (diffusional) pathways for Na+ and/or Cl- movement across the mucosal membrane so that the mechanistic constraint of neutral copuling between the absorptive movements of these ions is removed. Under these conditions, a significant serosa-positive transepithelial PD is observed and a fraction of Cl- absorption may be electrically coupled to that of Na+. Water is absorbed passively by virtue of osmotic coupling to electrolyte transport. A region of hypertonicity generated within the epithelium, at the level of the lateral intercellular space, provides the driving force for osmotic water flow. In view of the high osmotic water permeability of the gallbladder, the degree of hypertonicity required to account for the rate of water absorption is probably smaller than originally anticipated and is likely to be difficult to detect experimentally. Recent studies of the effects of humoral and pharmacological agents on electrolyte and water transport suggest that the rate of fluid absorption may be subject to physiological regulation. For example, secretin, which stimulates a HCO3--rich biliary secretion, also inhibits the reabsorption of this HCO3--rich fluid by the gallbladder, and in this manner may expedite the neutralization of the duodenal lumen. Inquires aimed at defining the physiological control of the absorptive functions of the gallbladder should provide an exciting avenue for future studies.