Detection of apoptosis by flow cytometry of cells simultaneously stained for intracellular pH (carboxy SNARF-1) and membrane permeability (Hoechst 33342).

Intracellular acidification appears to be a frequent and possibly universal occurrence during apoptosis. We have used carboxy SNARF-1 to measure intracellular pH by flow cytometry. We report here that the addition of Hoechst 33342 concurrently with carboxy SNARF-1 provides a clearer discrimination of the apoptotic population. Apoptotic cells accumulate Hoechst 33342 more rapidly than do viable nonapoptotic cells. The cells with greater Hoechst 33342 fluorescence are the same cells as those with decreased intracellular pH; these cells also have decreased volume. The different parameters in this analysis are presented for three models of apoptosis: human myeloblastoid ML-1 cells incubated with etoposide, murine cytotoxic T cells following withdrawal of interleukin-2, and Chinese hamster ovary cells incubated with staurosporine. In most circumstances, carboxy SNARF-1 provides excellent resolution of viable and apoptotic cells. However, the addition of Hoechst 33342 is particularly valuable when carboxy SNARF-1 alone can not fully discriminate the two cell populations. This situation occurs in Chinese hamster ovary cells, which undergo a smaller degree of acidification than do the other cell models. This situation also occurs when the extracellular pH is experimentally reduced to investigate the mechanism of pH dysregulation; the apoptotic cells appear to retain the ability to regulate intracellular pH at low pH, although they are defective in proton export at neutral pH.