The long term goal is to develop a thorough understanding of the processes of exocytosis and granule recycling in neuroendocrine cells. These processes are essential for the secretion of regulatory hormones from the pituitary gland. Additionally, regulation of exocytosis and endocytosis in neuroendocrine cells is a representative model of secretion from other cell types, including peptide secretion from neurons. The large size of granules in neuroendocrine cells makes it possible to study secretion at the level of individual exocytic events, a task which is difficult to achieve in neurons. The proposed experiments utilize three techniques that are in routine use in the sponsor's laboratory: electrophysiological measurements of cell capacitance, optical measurements of membrane fluorescence and photoconversion of fluorophores. Pilot studies have shown that two neuroendocrine cells, lactotrophs and somatotrophs, demonstrate remarkable differences in capacitance and membrane staining in response to stimulation. When stimulated in the presence of FM1-43, lactotrophs demonstrate clear spots of exocytic activity and somatotrophs show no punctate brightening. When examined in electron micrographs, stimulated lactotrophs show abundant "omega figures;" stimulated somatotrophs show none. Lactotrophs show robust increases in membrane capacitance when stimulated; somatotrophs show little if any change in capacitance. The proposed experiments will attempt to resolve these differences by investigating the hypothesis that lactotrophs employ "full fusion" exocytosis and somatotrophs utilize "kiss and run" exocytosis.