Effects of gallbladder function on human bile: compositional and structural changes.

Abstract

Water and electrolyte absorption leading to increased intraluminal concentrations of lipids and other solutes comprise the primary physiologic effect of the gallbladder. The dynamics of enterohepatic circulation can lead to confinement of up to 60% of the bile acid pool within the gallbladder during prolonged fasting. This bile acid sequestration markedly reduces both the bile salt secretory rate and total solute concentration in comparison with the nonfasting state producing hepatic bile that is more than normally dilute especially in comparison with gallbladder bile. These conditions favor formation of small low-density vesicles in hepatic bile which are more buoyant than the concentrated biliary micelles found in anatomically dependent regions of the gallbladder. Vesicle formation in hepatic bile may explain the "stratification" phenomenon often noted within the gallbladder. This observation could be accounted for by inhomogeneous mixing of the dissimilar lipid particles, i.e., vesicles and micelles. If vesicles are present and stratified as presumed, the overall cholesterol transporting capability of the total lipid particles, including these regionalized vesicles, would be considerably enhanced. The net effect of this process would lead to an overestimation of the true degree of metastable cholesterol supersaturation for the biliary micellar system within the gallbladder on the incorrect assumption of homogeneous mixing of lipids.