Imaging exocytosis of single glucagon-like peptide-1 containing granules in a murine enteroendocrine cell line with total internal reflection fluorescent microscopy

To analyze the exocytosis of glucagon-like peptide-1 (GLP-1) granules, we imaged the motion of GLP-1 granules labeled with enhanced yellow fluorescent protein (Venus) fused to human growth hormone (hGH–Venus) in an enteroendocrine cell line, STC-1 cells, by total internal reflection fluorescent (TIRF) microscopy. We found glucose stimulation caused biphasic GLP-1 granule exocytosis: during the first phase, fusion events occurred from two types of granules (previously docked granules and newcomers), and thereafter continuous fusion was observed mostly from newcomers during the second phase. Closely similar to the insulin granule fusion from pancreatic β cells, the regulated biphasic exocytosis from two types of granules may be a common mechanism in glucose-evoked hormone release from endocrine cells.