ACCUMULATION OF POLYCHLORINATED DIBENZO-P-DIOXINS AND DIBENZOFURANS IN LIVER OF CONTROL LABORATORY RATS

Polychlorinated dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs), and biphenyls belong to a class of compounds, the polyhalogenated aromatic hydrocarbons (PHAHs), which are ubiquitous environmental contaminants . Due to the existence of a common mechanism of action, i.e., binding to the Ah receptor, the activity of members of this class of compounds is generally expressed relative to the prototypical 2,3,7,8-tetrachlo rodibenzo-p-dioxin (TCDD) as toxic equivalency factors (TEFs). In the present studies we examined the presence of PCDDs and PCDFs in standar d laboratory feed and in the liver of untreated rats at three differen t ages (60, 140, and 200 days) in terms of concentration and in toxic equivalents (TEQs, TEF x concentration). Feed was shown to contain tra ce amounts of PCDDs and PCDFs and control rat liver was shown to conta in several PCDD and PCDF congeners in terms of concentration of congen er and concentration of TEQs contributed by that congener. The total c oncentration of TEQs increased with increasing age in rat liver, going from 20 ppt TEQ at 60 days to 78 ppt TEQ at 200 days of age. This acc umulation in dioxin-like activity was due primarily to PCDFs. In parti cular the congener 2,3,4,7,8-pentachlorodibenzofuran accrued in untrea ted rat liver accounting for approximately 80% of the total TEQ at 200 days of age. These studies affirm the pervasive presence of PHAHs and suggest prudence in evaluating chronic rat studies in which interfere nce from background levels of PCDDs and PCDFs may be a factor. (C) 199 4 Society of Toxicology.