ToxSci Advance Access originally published online on November 24, 2004
Toxicological Sciences 2005 83(2):363-371; doi:10.1093/toxsci/kfi041
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Toxicological Sciences vol. 83 no. 2 © Society of Toxicology 2005; all rights reserved.
Early Developmental 2,3,7,8-Tetrachlorodibenzo-p-Dioxin Exposure Decreases Chick Embryo Heart Chronotropic Response to Isoproterenol but Not to Agents Affecting Signals Downstream of the Beta-Adrenergic Receptor
* Bates College, Department of Biology, Lewiston, Maine 04240-6018, and College of Pharmacy, University of New Mexico Health Sciences Center, Albuquerque, New Mexico 87131-5691
Received August 12, 2004; accepted November 17, 2004
2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) causes cardiovascular toxicity in laboratory animals, including alteration in several processes in which beta-adrenergic receptor (ß-AR) signaling plays important roles. Thus, our laboratory investigated the effects of TCDD on ß-AR expression and signal transduction. Fertile chicken eggs were injected with vehicle (corn oil), 0.24 or 0.3 pmol TCDD/g egg on incubation day 0 (D0) or D5. On D10, heart function was assessed by ECG in ovo. Exposure to TCDD increased the incidence of arrhythmias and decreased the positive chronotropic responsiveness of the heart to isoproterenol. The reduced ß-AR responsiveness was, in part, independent of any overt morphological changes in the heart as chick embryos exposed to TCDD on D5 displayed an intermediate responsiveness to ß-AR agonist in the absence of the dilated cardiomyopathy observed in chick embryos exposed to TCDD on D0. TCDD did not decrease the chronotropic response of the heart to agents that stimulate signals downstream of the ß-AR. In fact, TCDD-exposed embryos were more sensitive than controls to forskolin, increasing heart rates (HR) 21.8 ± 3.5 beats per min (bpm) above baseline versus control values at 6.3 ± 2.7 bpm above baseline. TCDD exposure also augmented the negative chronotropic response of the heart to verapamil, decreasing HR –23.2 ± 7.4 bpm relative to baseline versus control embryos at –12.7 ± 5.9 bpm below baseline. Finally, the mean cardiac ß1-AR mRNA expression in D10 embryos was not significantly altered by exposure to TCDD on D0. These findings establish that a functional end point of the developing chick heart is sensitive to TCDD exposure and that the TCDD-induced reduction in ß-AR responsiveness may result from alterations in signal transduction upstream of adenylyl cyclase.
Key Words: TCDD; dioxin; developmental cardiovascular toxicity; beta-adrenergic receptor; ECG, arrhythmia.
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