Combined hepatic arterial-portal venous and hepatic arterial-hepatic venous perfusions to probe the abundance of drug metabolizing activities: perihepatic venous O-deethylation activity for phenacetin and periportal sulfation activity for acetaminophen in the once-through rat liver preparation.

Although normal-retrograde liver perfusion allows definition of relative distribution patterns of multienzyme systems involved in parallel, competing and sequential metabolic pathways, the technique fails to describe localization patterns for unienzyme systems. A novel method of perfusion, which can describe the relative zonal enrichment of drug metabolizing activity of unienzyme systems in periportal region and in the entire liver, is described. The rat liver was perfused once-through with combined hepatic arterial (2 ml/min/liver-containing drug) and portal or hepatic venous (10 ml/min/liver, without drug) flows [HAPV and HAHV]. Because of known enrichment of phenacetin O-deethylation and acetaminophen sulfation activities in the perihepatic venous and periportal regions, respectively, metabolic data derived with these compounds were used for validation of the method. The overall steady-state hepatic extraction ratio of [3H]phenacetin during HAPV (0.88-0.98) was decreased markedly during HAHV (0.06-0.47), but was reduced to a lesser extent for preformed [3H]acetaminophen (0.57-0.86 for HAPV to 0.23-0.62 for HAHV). Because a comparison of metabolic activities encountered by substrate(s) [estimated from extraction ratios] required normalization to the intracellular water spaces accessed during both HAPV and HAHV, the multiple indicator dilution technique (a bolus injection of noneliminated reference materials: 51Cr-labeled RBC, 125I-labeled albumin, [14C]sucrose and 3H2O) into the hepatic artery during both HAPV and HAHV was used, with outflow profiles from the hepatic vein or portal vein characterized by rapid serial collections. Sinusoidal blood volume, albumin accessible Disse space, sucrose accessible Disse space and the estimated intracellular water space reached during HAHV were 48, 56, 53 and 33% those during HAPV. The ratios of intrinsic clearances per volume of cell water space for HAHV/HAPV, representing periportal activity/whole liver activity were: phenacetin O-deethylation, 0.34 and acetaminophen sulfation, 1.13. These findings validate the use of HAHV and HAPV perfusion for the examination of relative abundances of enzymic activities across the liver acinus for substrates metabolized by unienzyme systems.