Biliary excretion of injected conjugated and unconjugated bilirubin by normal and Gunn rats

Abstract

Conjugated and unconjugated bilirubin were injected intravenously at different times into normal rats and homozygous, jaundiced Gunn rats. As was expected, Gunn rats did not excrete injected unconjugated bilirubin in the bile. After the intravenous injection of conjugated bilirubin into normal rats, approximately twice as much bilirubin appeared in the bile per 100 g of body wt within 10 min than after the injection of comparable amounts of unconjugated bilirubin. This difference probably reflects the time required for cellular uptake and conjugation of bilirubin prior to excretion. The maximal biliary bilirubin excretory rate in Gunn rats following the administration of conjugated bilirubin was 56±8.4 (S.D.) µg of bilirubin excreted/100 g body wt/min. This does not differ significantly from the maximal biliary bilirubin excretory rate observed in normal rats after infusions of either conjugated or unconjugated bilirubin. This demonstrates that conjugation alone does not limit metabolism of bilirubin by normal rat liver. These studies, when considered in the light of other investigations, suggest that the ability to excrete conjugated bilirubin is the limiting factor in metabolism of bilirubin by normal rat liver.