Phenacetin and acetaminophen metabolism in the isolated perfused rat liver. Precursor concentration influences the selection of kinetic parameters to assess hypoxic impairment

Abstract

Subnormal oxygen concentrations affect a host of cellular functions and can exist under normal or pathological conditions. The present study examined the effects of hypoxia on phenacetin O-deethylation by the mixed-function oxidase system and the subsequent sulfation and glucuronidation of the generated metabolite acetaminophen, compared to hypoxic alterations in conjugation of administered acetaminophen, in isolated perfused rat livers. A recirculating perfusion system containing either 20% (normoxic conditions) or 2.5% (hypoxic conditions) donor rat blood delivered 4.46 and 1.47 mumol/min/g liver oxygen, respectively, resulting in a 44% reduction in oxygen consumption during hypoxia. The total hepatic clearance of phenacetin was diminished significantly during hypoxia, due to a 60% decrease in the formation clearance of acetaminophen. Hypoxia did not influence significantly the total hepatic clearance of either administered or generated acetaminophen. Although the formation clearance for acetaminophen sulfate (AS) remained unchanged, the Vmax for sulfate formation was diminished 35%. The formation clearance (mean +/- SD; N = 4/group) of acetaminophen glucuronide (AG) was greater from administered compared to generated acetaminophen during normoxia (0.47 +/- 0.15 vs. 0.22 +/- 0.06 ml/min, p less than 0.05), and was decreased 2- to 3-fold during hypoxia (0.14 +/- 0.08 vs. 0.11 +/- 0.07 ml/min). Hypoxic conditions did not affect differentially the time lag for the appearance of AG in perfusate, and did not appear to alter the diffusional barrier for AS and AG from perfusate into the hepatocyte.