Kinetics of Hepatic Transport of 4-Methylumbelliferone in Rats. Analysis by Multiple Indicator Dilution Method

Hepatic elimination of 4-methylumbelliferone (4MU), which has been used as a model compound for conjugative metabolism, was studied by means of a multiple indicator dilution (MID) method in the isolated perfused rat liver. Using this method, three intrinsic hepatic clearances, CL int,inf , CL int,eff, and CL int,seq, which represent the influx, efflux, and sequestration processes, respectively, were obtained. When the dose was increased from a low dose (50 μg/rat liver) to a high dose (3000 μg/rat liver), the hepatic availability of 4MU increased from 0.11 to 0.73. With increasing dose, the CL int,eff value increased approximately two times, while the CL int,seq value decreased to approximately one-third. The remarkable dose dependence of hepatic availability was due to nonlinearity in both CL int,eff and CL int,seq values. However, the CLint,inf value was almost independent of dose. The dose-dependent change in CLint,seq might be explained by the saturation of conjugative metabolism of 4-MU, while the increase in the CL int,eff value with increasing dose might be partly explained by the nonlinear tissue binding of 4-MU, since the tissue unbound fraction determined by an ultrafiltration method using liver homogenate increased approximately 1.5 times at higher concentration of 4-MU compared to that at lower concentrations. In addition, based on a comparison of the individual intrinsic clearances, i.e., CL int,inf , CL int,eff, and CL int,seq, the major determining process of the apparent hepatic intrinsic clearance of 4MU is thought to be the sequestration process at the high dose. However, at the low dose, the membrane transport process (influx and efflux processes) as well as the sequestration process also determine the apparent hepatic intrinsic clearance.