Regulation of intracellular glutathione in rat embryos and visceral yolk sacs and its effect on 2-nitrosofluorene-induced malformations in the whole embryo culture system.

The dysmorphogenic effects of 2-nitrosofluorene (NF) in vitro were modulated in Day 10 rat embryos by agents which regulate intracellular glutathione (GSH) levels. The incidence of abnormal axial rotation caused by NF alone increased in a dose-dependent manner at NF concentrations in excess of 25 microM. No effects were observed at 15 microM NF and doses of 100 microM resulted in a 100% incidence of mortality. L-Buthionine-S,R-sulfoximine (BSO), an inhibitor of GSH synthesis, produced malformations (50%) in embryos exposed to 15 microM NF but produced no additional effects on embryos at higher NF concentrations. BSO treatment alone resulted in a greater than 50% decrease in GSH content in visceral yolk sacs and had a lesser but likewise significant effect (15% decrease) on the GSH content of embryos. Protein content was inversely affected as embryonic levels were increased by 20% and yolk sac levels were unchanged. When BSO was added in combination with NF at the onset of the culture period, embryonic GSH decreased in a dose-dependent manner, suggesting a relatively low rate of embryonic GSH turnover that could be increased by addition of an exogenous substrate capable of forming adducts with and removing GSH from the cells. 2-Oxothiazolidine-4-carboxylate (OTC), a compound which is enzymatically modified to provide an additional source of intracellular cysteine and increase GSH synthesis, produced no significant changes in embryonic or yolk sac GSH when added alone to the culture medium. When OTC (5 mM) was added in combination with NF, however, NF-elicited malformations were eliminated. This was also the case at 100 microM NF in which OTC not only prevented malformations but completely protected embryos against the loss in viability. The GSH and protein levels were indistinguishable from controls when OTC and NF were added simultaneously except for the 41 microM NF dose at which a highly significant increase in both embryonic and yolk sac protein was observed. This study clearly demonstrates the potential importance of GSH in the modulation of chemical dysmorphogenesis and provides an important new tool for the study of mechanisms of developmental toxicity.