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 th e constraints this places on clearance predictions, the kinetics of ethoxyc oumarin O-deethylation have been determined under varying conditions of hep atic cytochrome P-450 activity. Liver slices, isolated hepatocytes, and mic rosomes were obtained from rats treated with the inducers phenobarbital (PB ) and beta-naphthoflavone (beta NF) and the inhibitor aminobenzotriazole (A BT). In hepatocytes and microsomes, a two-site kinetic model with a high-af finity, low-capacity site and an unsaturated low-affinity, high-capacity si te described the hydroxycoumarin formation data. There were marked increase s in V-max (2- to 8-fold and 50- to 70-fold for PB and beta NF, respectivel y) in both systems and in CLint (3- and 9-fold for PB and beta NF, respecti vely) in hepatocytes and substantial decreases in both parameters (3-8 and 12-23% of control, respectively) in ABT hepatocytes and microsomes, A quali tatively similar response was evident in slices obtained from livers of rat s treated with phenobarbital and ABT, but although slices from PNF livers p roduced high metabolic rates (comparable to slices obtained from livers of rats treated with phenobarbital), these showed a linear increase with subst rate concentration without indication of a high-affinity site. The intrinsi c clearance parameters were scaled to full liver capacity using hepatocellu larities and microsomal recovery indices to allow direct comparison of thes e responses. The slice system consistently underestimated the effects of th e modifiers. When compared with hepatocytes, estimates of 30, 15, and 1% fo r ABT, PB, and PNF, respectively, were observed and the degree of underesti mation was dependent on the magnitude of intrinsic clearance and was consis tent with the above theory.