Analysis on various toxic actions of organotins including tri-n-butyltin

(A)三丁基锡是一种环境污染物，随着其在野生动物体内的积累，人们对三丁基锡对野生动物和人类的毒性越来越关注。据报道，TBT在几种类型的细胞中增加细胞内Ca2+浓度。为了研究Ca2+如何参与TBT诱导的细胞死亡，使用流式细胞仪和荧光探针比较了TBT与钙离子载体A23187在不同浓度的外部Ca2+下对大鼠胸腺细胞的影响。虽然TBT和A23187在正常Ca2+条件下对细胞都有毒性，但在外部Ca2+无条件下，TBT的细胞毒性作用在不改变阈值浓度的情况下被增强，而A23187的细胞毒性作用被完全消除。在正常Ca2+浓度下，A23187可以减弱tbt诱导的细胞活力下降，尽管细胞内Ca2+浓度升高。随着外源Ca2+浓度的升高，tbt诱导的死亡细胞数量逐渐减少，而早期凋亡细胞数量逐渐增加。结果表明，Ca2+在tbt诱导的大鼠胸腺细胞死亡过程中具有相互矛盾的作用。(B)在大鼠迷走神经背侧运动核急性分离的神经元中，采用制氨抑素穿孔贴片记录模式，研究了环境污染物三丁基丁（TBT）对n-甲基- d -天冬氨酸（NMDA）引发的膜电流的影响，以预测TBT对人类和野生动物的神经毒性作用。环境相关浓度（100 nM）的TBT显著增加了nmda诱导的向外电流幅值，但对nmda诱导的内向电流幅值没有显著影响。nmda诱导的向外电流的反转电位接近理论K+平衡电位-82 mV。5 mM四乙基氯化铵和300 nM肉毒杆菌毒素对TBT增强的nmda诱导的外向电流有明显的抑制作用。胞外Ca2+的去除消除了向外电流，表明向外电流是由于Ca2+通过NMDA通道激活Ca2+活化的K+通道所致。nmda诱导的向外电流被Cs+内溶液完全阻断。在此条件下，TBT对nmda诱导的内向电流没有影响。目前的研究结果表明，TBT增强了NMDA诱导的Ca2+激活的K+电流，而不增加NMDA诱导的内向电流。少

(A) As tri-n-butyltin (TBT), one of environmental pollutants, is accumulated in wild animals, concern regarding the toxicity of TBT in both wildlife and humans is increasing. TBT has been reported to increase intracellular Ca2+ concentration in several types of cells. In order to examine how Ca2+ is involved in TBT-induced cell death, the effect of TBT on rat thymocytes has been compared with that of A23187, a calcium ionophore, under various concentrations of external Ca2+ using a flow cytometer and fluorescent probes. Although both TBT and A23187 were toxic to cells under normal Ca2+ condition, under external Ca2+-free condition the cytotoxic action of TBT was potentiated without changing the threshold concentration while that of A23187 was completely abolished. A23187 attenuated the TBT-induced descent in cell viability under normal Ca2+ concentration despite intracellular Ca2+ concentration was increased. As external Ca2+ concentration increased, the TBT-induced increase in number … More of dead cells gradually decreased whereas the number of cells in an early stage of apoptosis increased. Results suggest that Ca2+ has contradictory actions on the process of TBT-induced cell death in rat thymocytes.(B) The effects of tri-n-butyltin (TBT), an environmental pollutant, on membrane currents elicited by N-methyl-D-asparlate (NMDA) were examined in the neurons acutely dissociated from the dorsal motor nucleus of vagus of rats, using a nystatin-perforated patch recording mode under the voltage-clamp conditions, in order to predict a neurotoxic action of TBT on humans and wild animals. TBT at an environmentally-relevant concentration (l00 nM) greatly increased the amplitude of NMDA-induced outward current, but not that of NMDA-induced inward current. The reversal potential of the NMDA-induced outward current was close to the theoretical K+ equilibrium potential of -82 mV. The NMDA-induced outward current augmented by TBT was markedly inhibited by 5 mM tetraethylammonium chloride and 300 nM charybdotoxin. The outward current was abolished by removal of extracellular Ca2+, suggesting that the outward current was due to the activation of Ca2+-activated K+ channels by Ca2+ passing through NMDA channels. The NMDA-induced outward current was completely blocked by Cs+-internal solution. Under this condition, TBT had no effect on the NMDA-induced inward current. The present results suggest that TBT potentiates Ca2+-activated K+ current induced by NMDA without augmenting the NMDA-induced inward current. Less