Sex Differences of Hepatic Conjugation of Bilirubin Determine its Maximal Biliary Excretion in Non-Anaesthetized Male and Female Rats

Abstract

Hepatic bilirubin UDP-glucuronosyltransferase activity was higher in female than in male rats: gonadectomy decreased enzyme activity in females and increased it in males. This sex difference in bilirubin conjugation was further used to evaluate the effect of differences in conjugation on the maximal biliary excretion of bilirubin in the nonanesthetized rat. After infusion of bilirubin, the maximal biliary excretory rate (Tm) and maximal concentration of bilirubin in bile were 70 and 40% higher in female than in male rats, respectively; these values were decreased in females after ovariectomy and increased in males after orchidectomy. A linear relationship was found (r = 0.86; P < 0.001) between bilirubin Tm and hepatic bilirubin UDP-glucuronosyltransferase activity in the 4 groups of rats, suggesting that conjugation was the rate-limiting step for the maximal hepatic transport of bilirubin. At the end of bilirubin infusion, bilirubin conjugates in serum, determined by alkaline methanolysis and high-performance liquid chromatography, ranged from 0.5-1.4% of total bilirubin. Therefore no significant reflux of conjugated bilirubin occurred during saturation of the hepatic transport of the pigment, once more suggesting that the secretory step was not rate-limiting. The composition of bilirubin conjugates in bile was similar in the 4 groups of rats despite significant differences in transferase activity. The relative proportion of bilirubin mono- and diconjugates in bile is probably affected by factors other than transferase activity alone. Relatively more monoconjugates were excreted under the bilirubin load than in basal conditions.