Canga L, Paroli L, Blanck T J, Silver R B, Rifkind A B
Department of Pharmacology, Cornell University Medical College, New York, New York 10021.
Mol Pharmacol. 1993 Dec;44(6):1142-51.
Binding by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) to the Ah receptor leads to transcriptional activation of several genes and a toxicity syndrome that includes tumor promotion, wasting, hormonal and immune system dysfunction, and death. Recent findings indicate that TCDD may also affect cardiac function. Here, we used the chick embryo, a TCDD-sensitive species, to further characterize the effects of TCDD on ventricular muscle contraction and on cardiac myocyte [Ca2+]i assessed with fura 2. The results show that TCDD causes an evolving sequence of contractile defects, independent of changes in diet, first impairing cAMP-modulated contraction (after 48 hr) and later (by seven days) decreasing responses to [Ca2+]o. Phenobarbital, even at high doses, failed to affect the inotropic response to isoproterenol, supporting the specificity of the ventricular contractile effects of TCDD. TCDD treatment also depressed inotropic responses to theophylline and forskolin, indicating that it has a post-beta-adrenergic receptor effect on cAMP action. In contrast to its depression of responses to beta-adrenergic stimuli and to [Ca2+]o, TCDD did not affect initial tensions of ventricular muscle stimulated at 1 Hz or the force-frequency response up to 1 Hz, indicating that TCDD-treated ventricles can respond normally at slow rates of stimulation. TCDD treatment depressed lusitropic (relaxation) responses to isoproterenol and to increasing [Ca2+]o indicating that it impairs the ability of the sarcoplasmic reticulum to sequester Ca2+. Fura 2-based measurements showed that [Ca2+]i was nearly doubled after TCDD treatment. The increase in [Ca2+]i is consistent with the decrease in the contractile response to [Ca2+]o, amelioration of the response to isoproterenol by subphysiologic concentrations of [Ca2+]o, and intermittent lack of response to electrical stimulation in high K+ observed in ventricles from TCDD-treated embryos. TCDD treatment also depressed the initial increase in [Ca2+]i by isoproterenol, consistent with the decreased contractile response to isoproterenol. The findings show that TCDD causes well defined, progressive impairment of avian ventricular responses to inotropic stimuli, providing new evidence that the heart is a target of TCDD action and that TCDD disturbs intracellular calcium processing.
2,3,7,8-四氯二苯并对二恶英（TCDD）与芳烃受体（Ah受体）结合会导致多个基因的转录激活以及一系列毒性反应，包括肿瘤促进、消瘦、激素和免疫系统功能紊乱以及死亡。最近的研究结果表明，TCDD可能还会影响心脏功能。在此，我们使用鸡胚这一对TCDD敏感的物种，进一步研究TCDD对心室肌收缩以及用fura 2评估的心肌细胞胞内钙离子浓度（[Ca2+]i）的影响。结果显示，TCDD会引发一系列不断演变的收缩缺陷，且与饮食变化无关，首先损害cAMP调节的收缩功能（48小时后），随后（7天后）降低对细胞外钙离子浓度（[Ca2+]o）的反应。即使高剂量的苯巴比妥也未能影响对异丙肾上腺素的变力反应，这支持了TCDD对心室收缩作用的特异性。TCDD处理还抑制了对茶碱和福斯高林的变力反应，表明它对cAMP作用具有β-肾上腺素能受体后效应。与它对β-肾上腺素能刺激和[Ca2+]o反应的抑制作用相反，TCDD并不影响以1Hz频率刺激时心室肌的初始张力或高达1Hz的力-频率反应，这表明经TCDD处理的心室在缓慢刺激频率下能够正常反应。TCDD处理抑制了对异丙肾上腺素和升高的[Ca2+]o的舒张反应，表明它损害了肌浆网摄取钙离子的能力。基于fura 2的测量结果显示，TCDD处理后[Ca2+]i几乎增加了一倍。[Ca2+]i的增加与对[Ca2+]o收缩反应的降低、亚生理浓度的[Ca2+]o对异丙肾上腺素反应的改善以及在经TCDD处理胚胎的心室中观察到的在高钾环境下对电刺激间歇性无反应相一致。TCDD处理还抑制了异丙肾上腺素引起的[Ca2+]i的初始增加，这与对异丙肾上腺素收缩反应的降低相一致。这些研究结果表明，TCDD会导致鸟类心室对变力刺激产生明确的、渐进性的损害，为心脏是TCDD作用靶点以及TCDD扰乱细胞内钙离子处理提供了新的证据。
