Membrane-limited hepatic transport of the conjugative metabolites of 4-methylumbelliferone in rats

Abstract

The hepatic transports of 4-methylumbelliferone (4-MU) and its conjugative metabolites, the glucuronide (4-MUG) and sulfate (4-MUS), were investigated in rats with various methods. The extraction ratio (E) was estimated with the multiple indicator dilution (MID) method using isolated perfused rat liver. The values of E for 4-MUG and 4-MUS were much lower (less than 0.2) than that for the parent compound, 4-MU (0.89). In addition, we examined the simulation of the outflow curves of conjugates based on the "distributed" model in which we varied the permeability between the blood and hepatocytes. When the permeability was much smaller relative to the hepatic blood flow, the simulated curve was superimposed on the dilution curve. These results suggest that the influx permeabilities of these conjugates are so low that little extraction occurs during the passage through the liver. Measuring the unidirectional uptake of these conjugates into the liver with the in vitro centrifugal filtration method using isolated hepatocytes, we determined the influx permeabilities (PSinf(total] for the total ligands. The value of PSinf(total) determined with the in vitro method was extrapolated to that per gram of liver, assuming 1 g of liver has 1.3 X 10(8) cells. The values of PSinf(total) for 4-MU, 4-MUG, and 4-MUS were 4.8, 0.06, and 0.11 mL/min/g liver, respectively. Thus, the influx permeabilities for 4-MUG and 4-MUS were much smaller than the hepatic blood flow (1.6 mL/min/g liver), confirming the results of MID method.