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KS Pang, F Barker , WF Cherry and CA Goresky
Faculty of Pharmacy, University of Toronto, Montreal, Canada.
Perfusion of substrate via only the hepatic artery with simultaneous substrate-free perfusion of portal vein or hepatic vein [hepatic artery portal vein-hepatic artery hepatic vein] was used to examine the enzymic distribution of carboxylesterases towards the hydrolysis of enalapril to enalaprilat in the perfused rat liver preparation. In this single-pass method, [14C]enalapril was delivered by the hepatic artery at 2 ml/min into the liver, whereas drug-free perfusate entered the portal vein or hepatic vein at 10 ml/min for HAPV and HAHV perfusions, respectively. During steady state, a multiple indicator dose of 51Cr- labeled red blood cells (vascular marker), [58Co]EDTA (interstitial space marker, which behaves similarly to labeled tracer sucrose), and 3H2O was given into the hepatic artery. Labeled enalapril and the reference indicators entering via the hepatic artery will reach virtually all sinusoidal spaces during HAPV, and will be confined to the peripheral region during HAHV. By defining the steady-state extraction ratios of enalapril (Etot) and segregating the components of biliary excretion and metabolism, and by assessing the intracellular water spaces and the corresponding transit times during HPAV and HAHV, the metabolic sequestration rate constant (metabolic intrinsic clearance per unit accessible cell water space) for the periportal region/whole liver (HAHV/HAPV) was 0.344. The data suggest that the carboxylesterase activity for enalapril conversion to enalaprilat is primarily localized in the perihepatic venous region of the rat liver.
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