Purpose: The present study examined the underlying mechanism by which 4-hydroxyacetophenone (4-HA), a bioactive compound found in several medicinal herbs, exerts its potent stimulatory effects on hepatic bile secretion.
Methods: Bile flow, and biliary excretion of 4-HA, its metabolites, and inorganic electrolytes was examined in both normal Wistar rats and in TR(-) Wistar rats that have a congenital defect in the multidrug resistance-associated protein-2, Mrp2/Abcc2. The effects of 4-HA were also examined in animals treated with buthionine sulfoximine to decrease hepatic glutathione (GSH) levels.
Results: In normal rats, 4-HA dramatically increased bile flow rate, whereas it failed to exert a choleretic effect in TR(-) rats. This choleresis was not explained by increased biliary output of Na(+), K(+), Cl(-) or HCO(3) (-), or by increased biliary GSH excretion. Depletion of hepatic GSH with buthionine sulfoximine had no effect on the 4-HA-induced choleresis. HPLC analysis revealed that a single major compound was present in bile, namely.4-hydroxyacetophenone-4-O-beta-glucuronide, and that the parent compound was not detected in bile. Biliary excretion of the glucuronide was directly correlated with the increases in bile flow. In contrast to normal rats, this 4-HA metabolite was not present in bile of TR(-) rats.
Conclusions: These results demonstrate that the major biliary metabolite of 4-HA in rats is the 4-O-beta-glucuronide, a compound that is secreted into bile at high concentrations, and may thus account in large part for the choleretic effects of 4-HA. Transport of this metabolite across the canalicular membrane into bile requires expression of the Mrp2 transport protein.