Elevations in cytosolic free Ca2+ are not required to trigger apoptosis in human leukaemia cells
Previous studies have indicated that Ca2+ is a trigger Tor apoptosis (programmed cell death) in thymocytes and related cell lines. Recently we have shown that levels of apoptosis in leukacmic cells are diminished in Ca2+‐deficient conditions, indicating that Ca2+ may be important in the mechanism of apoptosis in these cells. In the present study we investigated the possibility that Ca2+ serves as a trigger for apoptosis in the human leukaemic cell line, HL‐60. Using fura‐2 to measure cytosolic free Ca2+ concentrations, [Ca2+]i, in cell suspensions, and by using ratio imaging of fura‐2 in single cells, we did not observe an early significant increase in [Ca2+]f, in HL‐60 cells undergoing apoptosis. The latter stages of apoptosis were, however, accompanied by increasing [Ca2+]f, these increases were apparently a result of, rather than a cause of, apoptosis. Furthermore, apoptosis could be induced in HL‐60 cells under conditions of vastly reduced [Ca2+]i achieved by loading these cells with fura‐2 in the presence of EGTA. These results indicate that elevation of [Ca2+]i is not a prerequisite for apoptosis in HL‐60 cells and that apoptosis can occur in these cells in the presence of low[Ca2+]i.