Retrograde perfusion to probe the heterogeneous distribution of hepatic drug metabolizing enzymes in rats.

The elimination of [14C]acetaminophen which was formed from [14C]phenacetin was slower than that for the preformed metabolite, [3H]acetaminophen. The observation had been attributed to the uneven distribution of enzyme systems for O-deethylation in the formation of acetaminophen and sulfate-conjugation of acetaminophen in the liver parenchyma. In this study, retrograde perfusion which reversed not only the direction of hepatic flow into the liver but also the location of enzyme systems with respect to the flow path, was used to examine the elimination kinetics of [14C]acetaminophen and [3H]acetaminophen. In the same rat liver preparation, both [14C[phenacetin and [3H]acetaminophen in tracer concentrations were delivered simultaneously into the perfused rat liver in situ preparation with normal directional flow (into the portal vein and out of the hepatic vein). The direction of flow was then reversed to retrograde perfusion (into the hepatic vein and out of the portal vein) which was later reverted back to normal perfusion. The elimination of [14C]acetaminophen was again slower than the elimination of [3H]acetaminophen during normal perfusion, but the elimination kinetics were virtually identical for both metabolite species during retrograde perfusion. This finding confirmed our previous hypothesis that sulfate-conjugation occurred predominantly in the periportal region while O-deethylation occurred preferentially in the centrilobular region of the liver and showed that retrograde perfusion was a useful probe in the investigation of the uneven distribution of hepatic drug metabolizing enzyme systems.