Abstract
The drug metabolising capability of the fetus has been investigated using freshly isolated and cultured fetal rat hepatocytes. Phase I metabolism has been investigated using 7-ethoxycoumarin and 7-ethoxyresorufin as substrates and Phase II using 7-hydroxycoumarin. 7-Ethoxycoumarin 0-deethylase and 7-ethoxyresorufin 0-deethylase activities increased with increasing time in culture. 7-Ethoxycoumarin 0-deethylase activity in freshly isolated hepatocytes increased with increasing fetal age between 17 and 22 days of gestation. alpha-Naphthoflavone inhibited 7-ethoxycoumarin 0-deethylase activity poorly in the freshly isolated hepatocytes and strongly in the cultured hepatocytes. The pattern of metabolism of some alkyl analogues of 7-ethoxyresorufin in cultured fetal hepatocytes suggested that a form of cytochrome P-450 was present resembling that form inducible in adult rat by 3-methylcholanthrene. Cytochrome P-450 concentration was very low in the freshly isolated fetal hepatocytes and increased approximately 2-fold after 48 hours of culture. Studies with inhibitors of RNA and protein synthesis indicated that translation was a prerequisite for the culture-induced increase in 7-ethoxycoumarin 0-deethylase. Glucuronyl transferase and sulphotransferase showed increased 7-ethoxycoumarin 0-deethylase activity in cultured fetal hepatocytes compared with the freshly isolated cells. This increase was considerably less that that seen for phase I metabolism. 7-Ethoxycoumarin 0-deethylase activity in the fetal rat was very susceptible to induction by 3-methylcholanthrene, Arochlor and alpha-naphthoflavone. This induction was reflected in an increased inhibition by alpha-naphthoflavene and by a large increase in the amount of material immunoprecipitated by an anti-cytochrome P-448 immunoglobulin in an Ouchterlony double diffusion assay and in immunochemically stained sections of fetal liver. Fetal rat 7-ethoxycoumarin 0-deethylase activity appeared unresponsive to induction by phenobarbitone.