Mucus is secreted from airway epithelial cells via exocytosis of mucus-secreting vesicles. The human pulmonary mucoepidermoid cell line NCI-H292 is a common model to investigate secretion of MUC5AC mucin. The process of exocytotic vesicle fusion, however, has not yet been directly investigated in these cells.

In analogy to methods used to visualize the exocytotic process of lamellar bodies from type II pneumocytes (Haller et al., PNAS 95, 1579-1584, 1998), we stained acidic vesicles in NCI-H292 cells with LysoTracker Green DND-26 (LTG), and investigated instantaneous, localized, loss of LTG fluorescence as a parameter for exocytotic vesicle fusion. We found that the NCI-H292 cell line consists of at least two subpopulations of cells with different staining patterns. Only the subtype showing highest LTG accumulation was highly sensitive to stimulation with the Ca2+ ionophore ionomycin, resulting in fusion of about 78% of its stored vesicles within 15 min. In the presence of extracellular Ba2+ the ionomycin induced vesicle fusion was reduced to 26%. These findings demonstrate that Ca2+ -concentration stimulated exocytotic fusion activity in a subset of NCI H292 cells with large exocytotic granules is inhibited by barium.

Supported by the DFG, grant D1402, the FWF, grant P15743 and the EU ("Pulmo-Net")