Enhancement by secretin of the apparently maximal hepatic transport of bilirubin in the rat.

The effect of secretin (0.4 C.U. per hr per 100 gm body weight) on bile flow and the apparent maximal hepatic transport of bilirubin (Tm) was investigated in the rat. When secretin was administered during an already established bilirubin-Tm condition, it increased bile flow and bilirubin-Tm by 15 to 20% over a 30- to 50-min period. Enhancement of bilirubin output correlated with augmented flow and was sustained by an increased rate of excretion of monoglucuronides. When secretin was given for 90 min before bilirubin loading, it enhanced biliary bilirubin concentration and output, largely as diglucuronides. Bilirubin-Tm correlated positively with glururonyltransferase activity in liver homogenates. In the isolated perfused rat liver, injection of secretin in the portal cannula failed to produce choleresis. Bilirubin uridine diphosphate-glucuronyltransferase activity was lower than in intact rats and higher after treatment than in controls. The effect of secretin had an early effect on hepatocytic bile flow and a later effect on conjugation. Maximal hepatic bilirubin output was modulated both by flow and conjugation rate; the two mechanism may act independently.