Tributyltin stimulates apoptosis in rat thymocytes

Treatment of rat thymocytes with micromolar concentrations of tributyltin caused a rapid increase in the cytosolic free Ca²⁺ concentration that was inhibited by Ni²⁺, which blocks Ca²⁺ influx through membrane channels. The elevation of cytosolic Ca²⁺ was associated with extensive DNA fragmentation, which was prevented by pretreatment of the cells with either of the intracellular Ca²⁺ chelators quin-2 or 1,2-bis(2-amino-phenoxy) ethane-N′,N′,N′,N′,-tetraacetic acid. Loss of thymocyte viability, which followed DNA fragmentation, was also prevented by the two Ca²⁺ chelators or by removing extracellular Ca²⁺ with ethylene glycol bis(β-aminoethyl ether)N,N′-tetraacetic acid. The pattern of DNA fragmentation was characteristic of that produced by agents which activate a Ca²⁺- and Mg²⁺-dependent endogenous endonuclease during apoptosis or programmed cell death. Additional studies showed that other organotin compounds, including trimethyltin, triphenyltin, and dibutyltin had minimal effects on cytosolic Ca²⁺, DNA fragmentation, and cell viability. These results are consistent with a greater susceptibility of thymocytes to tributyltin and provide a basis for understanding its selective immunotoxicity in vivo.