Inhibitory effects of extracellular Mg2+ on intracellular Ca2+ dynamic changes and thapsigargin-induced apoptosis in human cancer MCF7 cells

The effects of extracellular Mg2+ on both dynamic changes of [Ca2+](i) and apoptosis rate were analysed. The consequences of spatial and temporal dynamic changes of intracellular Ca2+ on apoptosis, in thapsigargin- and the calcium-ionophore 4BrA23187-treated MCF7 cells were first determined. Both 4BrA23187 and thapsigargin induced an instant increase of intracellular Ca2+ concentrations ([Ca2+](i)) which remained quite elevated (> 150 nM) and lasted for several hours. [Ca2+](i) increases were equivalent in the cytosol and the nucleus. The treatments that induced apoptosis in MCF7 cells were systematically associated with high and sustained [Ca2+](i) (150 nM) for several hours. The initial [Ca2+](i) increase was not determinant in the events triggering apoptosis. Thapsigargin-mediated apoptosis and [Ca2+](i) rise were abrogated when cells were pretreated with the calcium chelator BAPTA. The role of the extracellular Mg2+ concentration has been studied in thapsigargin treated cells. High (10 mM) extracellular Mg2+, caused an increase in basal [Mg2+](i) from 0.8 +/- 0.3 to 1.6 +/- 0.5 mM. As compared to 1.4 mM extracellular Mg2+, 1 muM thapsigargin induces, in 10 mM Mg2+, a reduced percentage from 22 to 11% of fragmented nuclei, a lower sustained [Ca2+](i) and a lower Ca2+ influx through the plasma membrane. In conclusion, the cell death induced by thapsigargin was dependent on high and sustained [Ca2+](i) which was inhibited by high extracellular and intracellular Mg2+.