Fluorometric Na+ Evaluation in Single Cells Using Flow Cytometry: Comparison with Flame Emission Assay

Abstract

Sodium is a key player in the fundamental cell functions. Fluorescent probes are indispensable tools for monitoring intracellular sodium levels in single living cells. 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 indirectly using ionophores for equalizing external and intracellular ion concentration. Attempts to compare data obtained using fluorescent probes and by direct flame emission analysis are sparse and results are inaccurate. We determined the intracellular sodium concentration in U937 cells by flow cytometry using the Na+-sensitive probe Asante Natrium Green-2 (ANG), and by standard flame emission photometry combined with the cellular water determination by cell density in Percoll gradient. The intracellular Na+ concentrations was modified using known ionophores or, alternatively, by blocking the sodium pump with ouabain or by causing cell apoptosis with staurosporine. It is revealed that both methods are comparable when intracellular sodium concentration was modified by ouabain-mediated blockage of the sodium pump or staurosporine-induced apoptosis. 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. The sodium sensitive dye ANG-2 is a sensitive and useful probe for determination changes in Na+ content and concentration both in single cells and subcellular microparticles. The ANG fluorescence determined in the studied cells in the absence of ionophores, cannot be used as a measure of the real intracellular concentration of Na+ if calibration was carried out in the presence of ionophores.