Increase in the relative amount of bilirubin diconjugates in rat bile and serum under infusion of phthaleins and indocyanine green.

The effect of organic anions on bilirubin metabolism and excretion was investigated in rats. Biliary excretion of bromosulfophthalein, bromcresol green and indocyanine green led to a significant decrease in excretion of bilirubin pigments and to an increase of their concentration in serum. This was concomitant with a marked increase in the ratio of bilirubin diconjugates to monoconjugates in bile and serum. These changes were unrelated to either bile flow, biliary lipid output, or hepatic activity of bilirubin UDP-glucuronyl transferase. Following bolus injection of [14C]bilirubin, peak excretory rate of radioactivity was markedly delayed in rats infused with bromosulfophthalein, as compared to controls. It is concluded that administration of the organic anions increased the ratio of bilirubin diconjugates to monoconjugates in bile and serum, by slowing down the intrahepatic transit of bilirubin pigments. This, in turn, allowed more efficient enzyme-catalyzed formation of bilirubin diconjugates from the intermediate bilirubin monoconjugates.