The use of isolated perfused liver in studies of biological transport processes

Phloracetophenone-induced choleresis in rats is mediated through Mrp2

Phloracetophenone (2,4,6-trihydroxyacetophenone, THA) is a potent choleretic in the bile fistula rat, although the mechanism is unknown. In the present study, we examined how THA enhances bile secretion. Stepwise infusions of THA (1-4 micromol/min) in the isolated perfused rat liver resulted in an immediate and dose-dependent increase in bile flow (BF), which reached saturation. The increase in BF was not associated with a change in the excretion of bile acids, suggesting that THA stimulated bile acid-independent bile flow. To further define the mechanism, the effect of THA on the excretion of sulfobromophthalein (BSP) and disulfobromophthalein (DBSP), typical multidrug resistance protein-2 (Mrp2) substrates was examined. THA inhibited the biliary excretion of both substrates. Because DBSP is excreted without conjugation to glutathione, in contrast to BSP, the findings suggest that THA might compete with DBSP and BSP metabolites at a common canalicular transport site, presumably Mrp2. THA infusions had no effect on the subcellular localization and distribution of either Mrp2 or the bile salt export pump (Bsep), nor the integrity of the tight junction. In contrast, the choleretic activity of THA was completely absent in the TR(-) rat, an animal model that lacks Mrp2, directly implicating this canalicular export pump as the mechanisms by which THA is excreted in bile. THA also partially reversed the cholestatic effects of estradiol-17beta-D-glucuronide, a process also dependent on Mrp2. In conclusion, the choleretic activity of THA and its possible metabolites is dependent on Mrp2. THA appears to stimulate BF by its osmotic effects and may attenuate the cholestatic effects of hepatotoxins undergoing biotransformation and excretion via similar pathways.

Identification of proteins mediating clearance of liposomes using a liver perfusion system

The objective of this paper is to identify the principal blood components governing the fate of liposomes in blood circulation. Information based on an isolated perfused liver system in rats has revealed the central role of the complement system in enhancing the uptake of liposomes by the liver. A species difference was an important factor in determining the uptake mechanisms of liposomes by the liver. Limited evidence revealed the tendency that opsonin-dependent hepatic uptake is principal in rats, while opsonin-independent or dysopsonin-dependent uptake governs in mice, although there are some exceptions. These studies provide us with important information for understanding the uptake mechanisms of liposomes by the liver, and useful insights in predicting the in vivo disposition of liposomes in humans.