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 gallstone model consisting of cholesterol pellets.

METHODS

Cholesterol was compressed into pellets at 2500 psi, producing a pellet that behaved like human cholesterol gallstones in regard to penetration of solutes into the stone. Pellets were implanted into gallbladders of pigs and harvested after 4 weeks. Bilirubin species were measured by high-performance liquid chromatography.

RESULTS

The proportions of bilirubin species in bile were not changed by the presence of pellets, i.e., diconjugates (mean +/- SD, 1.9% +/- 1.0% vs. 0.7% +/- 0.8%), monoconjugates (83.8% +/- 5.5% vs. 87.8% +/- 6.6%), and unconjugated bilirubin (14.2% +/- 5.3% vs. 11.5% +/- 5.6%) were similar at the time of implantation and removal. The cut surfaces of the pellets were pigmented. Pellets contained 5.46 +/- 1.38 micrograms 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 bile. Pellets also became pigmented in vitro, but there was considerable variability in the bilirubin species present in the pellets.

CONCLUSIONS

Unconjugated bilirubin accumulates in cholesterol pellets and pigments them. This provides a mechanism by which cholesterol gallstones could become secondarily pigmented.