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Measurements of the intracellular free calcium concentration [Ca2+]i in single cells of the human tracheal gland cell line MM 39 demonstrate dynamic changes in [Ca2+]i after their exposure to human neutrophil elastase (HNE). A heterogeneity in [Ca2+]i responses measured cell to cell in monolayer culture is evident: cells generate an initial [Ca2+]i peak rise with or without a delayed time (up to 180 sec) followed either by a rapid return to baseline, asynchronous oscillations or a sustained plateau phase. From basal concentration of 85 +/- 15 nM, HNE (1 microM) produces a [Ca2+]i increase of 91 +/- 66 nM in about 50% of responding cells. At lower concentrations of HNE (0.1 microM, 0.01 microM), the [Ca2+]i rise remains similar, but only 30-40% of the cells are responding. Pretreatment of cells with the recombinant elafin protein, a specific elastase inhibitor, reduces both the [Ca2+]i response to HNE and the number of responding cells. Electron microscopy observations reveal an increased number of secretory granules located beneath the cell plasma membrane after HNE treatment. These results suggest that intracellular [Ca2+]i changes may be associated to the HNE-induced exocytosis in human tracheal gland cells. These findings could have implications with regard to the pathogenesis of increased mucus secretion in human airway diseases.