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 orVmaxmet/Kmmetof 0.008 ml/min/mg of S9 protein,Vmaxmet of 21 ± 6 nmol/min/mg of S9 protein, and Kmmet of 2612 ± 236 μM) was greater in PV than in PP (Vmaxmet of 5.5 ± 3.1 nmol/min/mg of S9 protein and Kmmet of 1049 ± 335 μM; 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.
Footnotes
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Send reprint requests to: Dr. K. S. Pang, Faculty of Pharmacy, University of Toronto, 19 Russell Street, Toronto, Ontario, Canada M5S 2S2. E-mail: ks.pang{at}utoronto.ca
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This work was supported by the Medical Research Council of Canada (MT15657, MOP36457) and a grant from the Merck Sharp and Dohme Research Laboratories, West Point, PA.
- Abbreviations used are::
- PV
- perivenous
- PP
- periportal
- MZ
- midzonal
- Ess
- steady-state extraction ratio
- HAPV
- portal venous flow
- HAHV
- hepatic venous flow
- S9
- 9000g supernatant
- TLC
- thin-layer chromatography
- Received December 28, 1999.
- Accepted March 22, 2000.
- The American Society for Pharmacology and Experimental Therapeutics
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