Improvements in risk assessment require better linkage of exposure to response by the determination of target tissue dose. The relative sensitivity of several responses in female B6C3F1 mice was compared on the basis of administered and target tissue dose spanning 3 orders of magnitude. Twenty-four hours after administration, [3H]TCDD was detected in the heart, spleen, kidney, uterus, thymus, lung, and liver, and the highest concentrations were noted in the liver, uterus, and lung. At doses from 5 to 25 ng/kg, hepatic [3H]TCDD levels associated with the cytosolic and nuclear subcellular fractions increased from 12 to 62% of the total liver levels and then decreased at higher doses. At the two lowest doses used in the enzyme induction study, 5 and 10 ng/kg, the levels of specifically bound nuclear Ah receptor complex liganded with [3H]TCDD were 2.3 and 2.5 fmol/mg protein. Slightly higher levels of nuclear Ah receptor complex were observed at doses between 25 and 100 ng/kg (i.e., 3.6 to 4.2 fmol/mg protein) and a steep dose-dependent increase in nuclear Ah receptor levels was noted at doses of 500, 1000, and 5000 ng/kg (8.0, 39.3, and 92.8 fmol/mg protein, respectively). The dose-dependent effects of [3H]TCDD on hepatic Cypla-1 and Cypla-2 mRNA levels, ethoxyresorufin O-deethylase (EROD) activity, and the splenic antibody plaqueforming cell (PFC) response to sheep red blood cells were also determined; the latter response was determined 9 days after administration of TCDD. Statistically significant induction of hepatic Cypla-1 was observed at lower doses (25 ng/kg) than any other marker, followed by induction of EROD and PFCs expressed per spleen or per 106 cells which was observed at 100 ng TCDD/kg and at higher doses. Cypla-2 was elevated significantly relative to control at doses <1000 ng/kg. The ED50 value for PFCs/10 cells was the lowest of the variables analyzed and was not statistically significantly different from control (91 ± 92 ng/kg). A 50% increase in Cypla-2 and Cypla-1 mRNA levels was observed at doses of 736 ± 132 and 1630 ± 431 ng/kg, respectively. Due to variability in response in PFCs/spleen and the submaximal induction of EROD activity, ED50 values could not be calculated for these responses. The analyses indicate that the immunosuppressive response (when normalized for the number of spleen cells) may be depressed by administered doses as low as 90 ng TCDD/kg body weight. A 50% increase in Cypla-1 or Cypla-2 was observed at higher administered doses (1630 or 736 ng/kg, respectively). This suggests that the immunosuppressive response is depressed at lower doses of TCDD than the other variables studied.
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