Regulation of dense cove release from neuroendocrine cells revealed by imaging single exocytic events

Using FM1-43 fluorescence, we have optically detected single exocytic and e ndocytic events in rat pituitary lactotrophs. About fifty discrete fluoresc ent spots abruptly appear around the entire surface of a cell bathed in FM1 -43 and high-potassium saline. The spots, which also immunostain for prolac tin, reflect the labeling of dense cores as well as membranes of exocytosed secretory granules. Stained cores are not released, but remain attached to the cell and are eventually endocytosed. However, in cells exposed to dopa mine (or an analog, bromocriptine), the cores dissolve and are secreted aft er several seconds. Solubilization of dense cores is mediated through a red uction in cytoplasmic cyclic AMP. Thus, the composition of secretions from individual secretory granules is regulated.