title: Effect of zonal transport and metabolism on hepatic removal: enalapril hydrolysis in zonal, isolated rat hepatocytes in vitro and correlation with perfusion data.
abstract: Previous studies showed that the transport of enalapril occurred homogeneously among zonal rat hepatocytes. However, the metabolism of hepatic arterially delivered enalapril, swept into the rat liver by the portal or hepatic venous flow (HAPV and HAHV perfusion), was more abundant in the perivenous (PV) than the periportal (PP) region. Hence, metabolic activities toward enalapril in 9000g supernatant (S9) fractions of enriched rat PP and PV hepatocytes were examined. Although Michaelis-Menten kinetics were invariably observed, the metabolic activity toward enalapril (intrinsic clearance or V(max)(met)/K(m)(met) of 0.008 ml/min/mg of S9 protein, V(max)(met) of 21 +/- 6 nmol/min/mg of S9 protein, and K(m)(met) of 2612 +/- 236 microM) was greater in PV than in PP (V(max)(met) of 5.5 +/- 3.1 nmol/min/mg of S9 protein and K(m)(met) of 1049 +/- 335 microM; intrinsic clearance of 0.0052 ml/min/mg of S9 protein) hepatocytes. These metabolic intrinsic clearances were much lower than the sinusoidal influx clearances observed from previous transport studies, revealing metabolism as the rate-limiting step. Substitution of the scaled-up transport and metabolic intrinsic clearances into the "well stirred", "parallel-tube", and "dispersion" models predicted higher steady-state extraction ratios for HAHV perfusion. By contrast, integration of the scaled-up in vitro parameters on zonal metabolism and homogeneous transport into a "zonal-compartment" model of three zonal subcompartments (1, 2, and 3) provided an improved description of the extraction ratios during HAPV and HAHV. Zonal factors are important for the scale-up of data in vitro to the whole organ.
authors.affiliation: Faculty of Pharmacy and Department of Pharmacology, Faculty of Medicine, University of Toronto, Toronto, Ontario, Canada.
authors.forename: T N
authors.initials: TN
authors.lastname: Abu-Zahra
authors.forename: K S
authors.initials: KS
authors.lastname: Pang
mesh_terms: D000806:Angiotensin-Converting Enzyme Inhibitors / Q000493:pharmacokinetics*; D000818:Animals; D001711:Biotransformation; D004656:Enalapril / Q000493:pharmacokinetics*; D066298:In Vitro Techniques; D008099:Liver / Q000378:metabolism*; D008297:Male; D010477:Perfusion; D051381:Rats; D017207:Rats, Sprague-Dawley
chemical_list: D000806:Angiotensin-Converting Enzyme Inhibitors; D004656:Enalapril