Quantitative analysis of exocytosis visualized by a video-enhanced light/fluorescence microscope reveals two distinct components of exocytosis in RBL-2H3 cells

Rat basophilic leukemia (RBL-2H3) cells, which exhibit Ca 2+-dependent secretion of granules when stimulated with antigen or the Ca 2+-ionophore A23187, were observed under a video-enhanced light/fluorescence microscope. Exocytotic events of individual granules were visualized in individual cells stimulated with antigen or A23187. Exocytosis of granules stimulated with A23187 showed two peaks in the time course. The earlier one was inhibited by selective inhibitors of protein kinase C (Ro31–8425, Ro31–8220, and chelerythrine) and the other was inhibited by an inhibitor of phosphatidate hydrolase, propranolol. Exocytosis by antigen stimulation, however, showed only one peak, which was inhibited by the selective inhibitors of protein kinase C, but not by propranolol. These results indicate that at least two distinct components of exocytosis exist in RBL2H3 cells.