Relationship between the murine Ah phenotype and the hepatic uptake and metabolism of 2,3,7,8-tetrachlorodibenzo-p-dioxin

The influence of the Ah locus on the hepatic uptake and metabolism of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was studied using isolated hepatocytes from Ah responsive C57BL/6J (C57) and nonresponsive DBA/2J (DBA) mice. Hepatocytes from control and TCDD-pretreated C57 and DBA mice were incubated with purified [¹⁴C] TCDD (2.2 μM0 for 8 hr in the metabolism studies or 2 hr in the uptake studies. Mice were pretreated 7 days prior to hepatocyte isolation with TCDD at doses that maximally induce aryl hydrocarbon hydroxylase activity (C57: 3 μg/kg, ip; DBA: 30 μg/kg, ip) or at doses approaching the LD₅₀ value (C57: 150 μg/kg, ip; DBA: 600 μg/kg, ip). Hepatocytes isolated from untreated C57 and DBA mice had similar uptake of [¹⁴C]TCDD, and, at all doses, TCDD pretreatment increased [¹⁴C]TCDD uptake. The rates of hepatic TCDD metabolism over the first 2 hr of incubation were similar for control C57 and DBA mice, although some qualitative differences in metabolites were detected by HPLC. TCDD pretreatment at doses of 3 and 30 μg/kg for C57 and DBA mice, respectively, produced no detectable quantitative or qualitative changes in TCDD metabolism, despite increases in cytochrome P-450 content, 7-ethoxyresorufin O-deethylase (EROD) activity, and benzo[a]pyrene (BaP) metabolism. Pretreatment of C57 and DBA mice with the respective LD₅₀ doses of TCDD decreased the rate of TCDD metabolism by hepatocytes, although cytochrome P-450 content, EROD activity, and BaP metabolism were increased. These results suggest that the uptake and the rate of hepatic metabolism of TCDD do not correlate with genetic differences at the murine Ah locus.