Kinetics of drug metabolism in rat liver slices: IV. Comparison of ethoxycoumarin clearance by liver slices, isolated hepatocytes, and hepatic microsomes from rats pretreated with known modifiers of cytochrome P-450 activity.

To evaluate the theory that within precision-cut liver slices intercellular transport occurs in parallel with cellular metabolism and to illustrate the constraints this places on clearance predictions, the kinetics of ethoxycoumarin O-deethylation have been determined under varying conditions of hepatic cytochrome P-450 activity. Liver slices, isolated hepatocytes, and microsomes were obtained from rats treated with the inducers phenobarbital (PB) and beta-naphthoflavone (betaNF) and the inhibitor aminobenzotriazole (ABT). In hepatocytes and microsomes, a two-site kinetic model with a high-affinity, low-capacity site and an unsaturated low-affinity, high-capacity site described the hydroxycoumarin formation data. There were marked increases in Vmax (2- to 5-fold and 50- to 70-fold for PB and betaNF, respectively) in both systems and in CLint (3- and 9-fold for PB and betaNF, respectively) in hepatocytes and substantial decreases in both parameters (3-8 and 12-23% of control, respectively) in ABT hepatocytes and microsomes. A qualitatively similar response was evident in slices obtained from livers of rats treated with phenobarbital and ABT, but although slices from betaNF livers produced high metabolic rates (comparable to slices obtained from livers of rats treated with phenobarbital), these showed a linear increase with substrate concentration without indication of a high-affinity site. The intrinsic clearance parameters were scaled to full liver capacity using hepatocellularities and microsomal recovery indices to allow direct comparison of these responses. The slice system consistently underestimated the effects of the modifiers. When compared with hepatocytes, estimates of 30, 15, and 1% for ABT, PB, and betaNF, respectively, were observed and the degree of underestimation was dependent on the magnitude of intrinsic clearance and was consistent with the above theory.