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The aryl hydrocarbon receptor (AhR) shares a common PAS domain with a number of genes that exhibit a pronounced circadian rhythm. Therefore, this study examined the daily cycle of AhR and AhR nuclear translocator (Arnt) protein expression in multiple tissues of female Sprague-Dawley rats. Rats were euthanized at 4, 7, and 11 am and 4, 7, and 11 pm after which whole tissue homogenates were made from multiple tissues. Western blot analysis showed that the daily cycle of relative AhR protein expression exhibits a similar oscillation pattern in the liver, lungs, and thymus. The daily cycle of relative Arnt protein expression exhibits a similar oscillation pattern in the liver and lungs. The apparent daily cycle of AhR and Arnt protein expression in multiple tissues was not observed within the spleen. This preliminary report is the first study to suggest that the PAS proteins, AhR and Arnt, exhibit a daily oscillation pattern within multiple target tissues which may give insight into the tissue-specific toxic and biochemical responses mediated through this dimerization pair, as well as the physiological function of these proteins., (Copyright 1998 Academic Press.)

There is currently little information concerning the time-dependent relationship between 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure and aryl hydrocarbon receptor (AHR) and aryl hydrocarbon receptor nuclear translocator (ARNT) protein concentration in vivo. Therefore, female Sprague-Dawley rats were given a single oral dose of TCDD (10 micrograms/kg), and the AHR and ARNT protein concentrations in liver, spleen, thymus, and lung determined by Western blotting. In liver, the concentration of AHR protein was significantly reduced 8 and 24 h postdosing as compared to time-matched controls. In spleen and lung, the concentration of AHR protein was reduced 3, 8, 24, and 168 h posttreatment compared to time-matched controls but returned to control levels by 336 h. In thymus, reductions in AHR protein concentration were observed 8, 24, 168, and 336 h postdosing as compared to time-matched controls. Significant reductions in the concentration of ARNT protein were not observed in any of the TCDD-exposed tissues. Functional studies in cell culture showed that exposure of a mouse hepatoma cell line (Hepa-1c1c7) and a rat smooth muscle cell line (A-7) to TCDD (1 nM) for 12 days resulted in a 50% reduction in TCDD-inducible reporter gene expression following subsequent challenge by an additional dose of TCDD (1 nM). Collectively, these results show that (i) TCDD-mediated depletion of AHR occurs in vivo, (ii) AHR protein does not rapidly recover to pretreatment levels even though the tissue concentration of TCDD has fallen, and (iii) reduction in AHR protein concentration correlates with reduction in TCDD-mediated reporter gene expression in mammalian culture cells.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is the most toxic member of a class of planar and halogenated chemicals. Improvements in exposure assessment of TCDD require scientific information on the distribution of TCDD in target tissues and cellular responses induced by TCDD. Since 1980, several physiologically based pharmacokinetic (PBPK) models for TCDD and related compounds have been reported. Some of these models incorporated the induction of a hepatic binding protein in response to interactions of TCDD, the Ah receptor, and DNA binding sites and described the TCDD disposition in a biological system for certain data sets. Due to the limitations of the available experimental data, different values for the same physical parameters of these models were obtained from the different studies. The inconsistencies of the parameter values limit the application of PBPK models to risk assessment. Therefore, further refinement of previous models is necessary. This paper develops an improved PBPK model to describe TCDD disposition in eight target tissues. The interaction of TCDD with the Ah receptor and with hepatic inducible CYP1A2 were also incorporated into the model. This model accurately described the time course distribution of TCDD following a single oral dose of 10 microg/kg, as well as the TCDD concentration on Day 3 after six different doses, 0.01, 0.1, 0.3, 1, 10, and 30 microg TCDD/kg, in target tissues. This study extends previous TCDD models by illustrating the validity and the limitation of the model and providing further confirmation of the potential PBPK model for us in optimal experimental design and extrapolation across doses and routes of exposure. In addition, this study demonstrated some critical issues in PBPK modeling., (Copyright 1997 Academic Press.)