Background: Human polymorphonuclear granulocytes (PMN) are an essential component in the immunological defense network against a variety of harmful pathogens. We have studied the effects of the airborne pollutant sulfite on the calcium metabolism and respiratory burst of these cells simultaneously. Methods: A flow cytometric method was developed using the fluochromes Indo-1 and DHR-123. This method allowed us to investigate the real-time kinetics of intracellular free calcium and reactive oxygen intermediates in viable cells with a temporal resolution of 1 s over a time course of 17 min. An additional feature was the possibility to discriminate between reacting and nonreacting cells after treatment with defined stimuli, thus gaining additional insight into the behavior of cell subpopulations. Results: We analyzed the effects of sulfite on PMN before and after stimulation with formyl-Met-Leu-Phe (FMLP). Treatment with sulfite alone (0.001-1 mM) caused a small, nontransient increase in intracellular calcium. Preincubation with sulfite reduced the maximal calcium response elicited by FMLP. A significant increase in steady- state calcium levels after stimulation with FMLP was observed after treatment with sulfite in concentrations of 10 and 100 mM. Regarding the respiratory burst, treatment with sulfite alone in concentrations of 0.00l-1 mM induced a significant increase in DHR-123-derived fluorescence, whereas concentrations of 5 and 10 mM caused a significant depression of this fluorescence below baseline values. Sulfite caused a maximal twofold increase of DHR123-derived fluorescence compared with the FMLP response. Similar results were obtained after preincubation with sulfite before treatment with FMLP, showing that the effect of sulfite on the respiratory burst was additive to the FMLP response. Regarding the fractions of responding cells, treatment with sulfite up to 1 mM induced a concentration-dependent increase of burst-reactive PMN, whereas preincubation before stimulation with FMLP showed no correlation between sulfite concentration and fraction of burst-reacting cells. Conclusions: By simultaneous registration of [Ca2+](i) and [H2O2](i) of PMN after treatment with FMLP and sulfite, the essential responses were already observed within a short time interval (15 min). Striking differences were found in the response of calcium as second messenger and respiratory burst in PMN treated with sulfite. Until a critical concentration (0.5-1 mM), sulfite caused a concentration-dependent increase of [H2O2](i), in addition to the FMLP-induced response. The [Ca2+](i) changes induced by sulfite alone, however, were found to be small and showed no correlation with the respiratory burst response. (C) 2000 Wiley-Liss, Inc.