Cytochrome P-450-dependent biotransformation of QA 208-199 in cultured rat conceptuses

It has been shown, using the method of rat post-implantation embryo culture, that the rat conceptus metabolizes the lipoxygenase inhibitor N-hydroxy-N-methyl-7-propoxy-2-naphthalenethan-amine (QA 208-199, QAB) in vitro. The capacity to metabolize QAB and accumulate its main metabolites depends on the developmental stage and length of exposure. In the present study, pregnant Han Wistar dams were given ip injections of either Aroclor 1254 (AC), 3-methylcholanthrene (3-MC) or phenobarbital (PB) before the dissection and culture of 9.5- and 10.5-day-old conceptuses in order to identify possible inducible conceptal cytochrome P-450 species. Conceptuses preinduced in utero were exposed to QAB in vitro for 48 hr, after which metabolites in the culture medium and conceptual tissues were determined with HPLC. Embryotoxic effects were evaluated in the same conceptuses. Preinduction in utero with AC, 3-MC or PB revealed that pretreatment with PB led, after 48 hr of culture, to a statistically significantly increased impairment of embryonic differentiation in 9.5-day-old conceptuses in vitro in comparison with vehicle-induced QAB-exposed conceptuses. Preinduction with PB led to higher levels of QAA, the main metabolite in vivo, in the culture medium. In the conceptal tissues, metabolite levels were significantly lower after preinduction with AC. In vehicle-induced 10.5-day-old embryos QAB caused less embryotoxicity in comparison with 9.5-day-old ones. In all preinduced conceptuses cultured from 10.5 to 12.5 days, yolk-sac vascularization was less affected and fewer embryonic anomalies were found in comparison with conceptuses cultured from 9.5 to 11.5 days. AC and PB pretreatment caused a decreased morphological score without changes in overall growth. Analysis of QAB and its metabolites in culture media and conceptal tissues at 12.5 days showed different levels of metabolites depending on the inducer used. To find further evidence for the involvement of cytochrome P-450 enzymes in the conceptal metabolism of QAB, conceptuses preinduced in utero (3-MC or PB) were exposed to QAB in vitro and gassed during the second half of the culture period with a mixture containing 35% carbon monoxide (CO), an inhibitor of cytochrome P-450 enzymes. CO treatment led to an inhibition of conceptal metabolism of QAB in comparison with that in conceptuses cultured under normal gassing conditions (without CO). These results strongly suggest the involvement of cytochrome P-450-dependent monooxygenases in the conceptal metabolism of QAB in vitro. 10.5-day-old conceptuses, after preexposure in utero to PB, were less susceptible to QAB embryotoxicity in vitro than were 9.5-day-old ones. In addition, the response to preinduction appeared to be age dependent.