Computer simulation of portal venous shunting and other isolated hepatobiliary defects of the enterohepatic circulation of bile acids using a physiological pharmacokinetic model.

Abstract

The effect of three isolated defects in the enterohepatic circulation of bile acids on the size and distribution of the bile acid pool, plasma bile acid levels and bile acid secretion into the intestine was simulated using a linear multicompartmental physiological pharmacokinetic model previously used to simulate these aspects of bile acid metabolism in healthy man. Stepwise increases in portal-systemic shunting (with a reciprocal decrease in hepatic blood flow) caused an exponential increase in systemic plasma concentrations of bile acids, but no other major changes in bile acid metabolism. When the effect of varying fractional hepatic extraction was simulated, it was found that the greater the fractional hepatic extraction, the greater the elevation observed for systemic plasma bile acid levels for a given degree of portal-systemic shunting. When total hepatic blood flow was restored to normal by simulating "arterialization," systemic plasma levels of bile acids decreased strikingly, yet remained elevated. For cholate with a fractional hepatic extraction of 0.9 and 100% portal-systemic shunting, arterialization caused a decrease from a 20-fold elevation to a 5-fold elevation. This simulation thus defined the effect of the presence of the portal venous system per se on plasma bile acid levels and also quantified the circulatory route by which substances reach the liver when portal-systemic shunting is present. An isolated defect in hepatic uptake of bile acids caused little change in overall bile acid metabolism other than modestly increased plasma levels. Loss of bile acid storage by the gallbladder caused the majority of the bile acid pool to move from the gallbladder compartments to the proximal small intestine during fasting but had little effect on the dynamics of the enterohepatic circulation during eating. The results of these novel simulations of isolated defects in bile acid transport should aid in the interpretation of the more complex changes in bile acid metabolism which are likely to occur in hepatic or biliary disease.