ACCUMULATION OF UNCONJUGATED BILIRUBIN IN CHOLESTEROL PELLETS IMPLANTED IN SWINE GALLBLADDERS

Background & Aims: Most cholesterol gallstones have a pigmented center , but it is unclear whether its presence is primary or secondary, This study was performed to determine if bilirubin would accumulate in a g allstone model consisting of cholesterol pellets, Methods: Cholesterol was compressed into pellets at 2500 psi, producing a pellet that beha ved like human cholesterol gallstones in regard to penetration of solu tes into the stone, Pellets were implanted into gallbladders of pigs a nd harvested after 4 weeks, Bilirubin species were measured by high-pe rformance liquid chromatography, Results: The proportions of bilirubin species in bile were not changed by the presence of pellets, i.e., di conjugates (mean +/- SD, 1.9% +/- 1.0% vs, 0.7% +/- 0.8%), monoconjuga tes (83.8% +/- 5.5% vs. 87.8% +/- 6.6%), and unconjugated bilirubin (1 4.2% +/- 5.3% vs, 11.5% +/- 5.6%) were similar at the time of implanta tion and removal, The cut surfaces of the pellets were pigmented, Pell ets contained 5.46 +/- 1.38 mu g bilirubin/g sample at harvesting, and 98.6% +/- 2.3% of bilirubin in pellets was unconjugated, In in vitro studies, there was a large increase in unconjugated bilirubin in the b ile, Pellets also became pigmented in vitro, but there was considerabl e variability in the bilirubin species present in the pellets, Conclus ions: Unconjugated bilirubin accumulates in cholesterol pellets and pi gments them, This provides a mechanism by which cholesterol gallstones could become secondarily pigmented.