Zn2+, derived from cell preparation, partly attenuates Ca2+-dependent cell death induced by A23187, calcium ionophore, in rat thymocytes

A23187, a calcium ionophore, is used to induce Ca2+-dependent cell death by increasing intracellular Ca2+ concentration ([Ca2+]i) under in vitro condition. Since this ionophore also increases membrane permeability of metal divalent cations such as Zn2+ and Fe2+ rather than Ca2+, trace metal cations in cell suspension may affect Ca2+-dependent cell death induced by A23187. Therefore, the effects of chelators for divalent metal cations, EDTA and TPEN, on the A23187-induced cytotoxicity were cytometrically examined in rat thymocytes. The cytotoxicity of A23187 was attenuated by 1 mM EDTA while it was augmented by 50 μM EDTA and 10 μM TPEN. These changes were statistically significant. The A23187-induced increase in Fluo-3 fluorescence intensity, a parameter for [Ca2+]i, was significantly reduced by 1 mM EDTA while it was not the case for 50 μM EDTA and 10 μM TPEN. The intensity of FluoZin-3 fluorescence, a parameter for [Zn2+]i, increased by A23187 was respectively reduced by 50 μM EDTA and 10 μM TPEN. It is suggested that the attenuation of A23187-induced cytotoxicity by 1 mM EDTA is due to the chelation of extracellular Ca2+ and Zn2+ while the augmentation by 50 μM ETDA or 10 μM TPEN is due to the chelation of extracellular Zn2+. The Tyrode’s solution without thymocytes contained 32.4 nM of zinc while it was 216.9 nM in the cell suspension. In conclusion, trace Zn2+, derived from cell preparation, partly attenuates the Ca2+-dependent cell death induced by A23187.