Comparison of cellular Na+ assays by flow fluorometry with ANG-2 and flame emission: Pitfalls in using ionophores for calibrating fluorescent probes

Monovalent ions, sodium in particular, are involved in fundamental cell functions, such as water balance and electric processes, intra- and intercellular signaling, cell movement, pH regulation and metabolite transport into and out of cells. Fluorescent probes are indispensable tools for monitoring intracellular sodium levels in single living cells in heterogeneous cell populations and tissues. Since the fluorescence of sodium-sensitive dyes in cells is significantly different from that in an aqueous solution, the fluorescence signal is calibrated in situ by changing the concentration of extracellular sodium in the presence of ionophores, making the membrane permeable to sodium and equilibrating its intra- and extracellular concentrations. The reliability of this calibration method has not been well studied. Here, we compare the determinations of the intracellular sodium concentration by flame emission photometry and flow cytometry using the Na+-sensitive probe Asante Natrium Green-2 (ANG). The intracellular Na+ concentration was altered using known ionophores or, alternatively, by blocking the sodium pump with ouabain or by causing cell apoptosis with staurosporine. The use of U937 cells cultured in suspension allowed the fluorometry of single cells by flow cytometry and flame emission analysis of samples checked for uniform cell populations. It is revealed that the ANG fluorescence of cells treated with ionophores is approximately two times lower than that in cells with the same Na+ concentration but not treated with ionophores. Although the mechanism is still unknown, this effect should be taken into account when a quantitative assessment of the concentration of intracellular sodium is required. Sodium sensitive fluorescent dyes are widely used at present, and the problem is practically significant.