Evaluating toxic endpoints of exposure to a commercial PCB mixture: an in vivo laboratory study

Polychlorinated biphenyls (PCBs) are ubiquitous environmental contaminants that produce a wide range of toxic effects. To determine sensitive endpoints in various organ systems, the effects of Aroclor 1260 on immune, endocrine, and hepatic systems were evaluated in a dose-response study. Nine-week old male rats were treated with Aroclor 1260 by oral gavage at dosages ranging from 0.025 to 156 mg/kg/day for 10 consecutive days and killed two days after the last treatment. Eight days prior to sacrifice, rats were injected i.v. with sheep red blood cells (SRBC) for determination of humoral immunity. No observable adverse effect level (NOAEL) and lowest observable adverse effect level (LOAEL) were determined for liver, thymus and genital organ weights, body weight, serum luteinizing hormone (LH), testosterone, thyroxine and thyroid-stimulating hormone (TSH) concentrations, hepatic microsomal testosterone hydroxylase activities, and hepatic microsomal cytochrome P450 (CYP) 1A1, CYP1A2, CYP2B1 and CYP2B2 protein levels. Treatment with Aroclor 1260, at all dosages, had no effect on testis, seminal vesicle or ventral prostate weights, on thymus weight or on serum LH or testosterone levels. Among the endpoints altered by Aroclor 1260, the most sensitive, with a LOAEL of 1.25 mg/kg/day, were increased testosterone 16β-hydroxylase activity and androstenedione formation. The LOAEL for increased liver weight, testosterone 16α-hydroxylase activity and CYP2B1 protein content was 3.13 mg/kg/day, while the LOAEL for decreased serum thyroxine levels and anti-SRBC IgM titer was 6.25 mg/kg/day. Less sensitive responses, as reflected by larger LOAEL values, included CYP1A enzyme induction and decreased body weight. In summary, comparison of NOAEL and LOAEL values indicated that hepatic CYP2B-mediated activities were a more sensitive response to Aroclor 1260 exposure in male rats than immune or endocrine endpoints.