Compound Exocytosis in Pituitary Cells

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Neurotransmitter and hormone release from vesicles involves fusion between the vesicle and the plasma membranes, a process termed exocytosis. Recently we reported that most of the spontaneous and stimulated exocytotic events in pituitary lactotrophs are transient and repetitive, appearing in bursts lasting more than 100 s. However, whether this is also the case in compound vesicle-to-vesicle exocytosis is unknown. Here we investigated compound exocytotic events in resting and stimulated lactotrophs by using optical and cell-attached patch-clamp capacitance measurements. Elementary compound exocytotic events were characterized by multiple-amplitude on-steps in synaptopHluorin fluorescence and in membrane capacitance signals. Multiple-amplitude on-steps appeared either as a relatively large upward step, indicating that vesicles were fused with each other prior to fusion of the vesicle membrane with the plasma membrane (multivesicular exocytosis), or as a time-dependent stepwise signal increase, indicating sequential fusion of two or more vesicles with the plasma membrane (sequential exocytosis). In the majority of membrane capacitance recordings (>90%), multiple-amplitude on-steps terminated as multiple-amplitude off-steps. These complex amplitude events were repetitive, indicating that transient fusion pore openings reflect repetitive interactions of a single vesicle or vesicles in a cluster with the plasma membrane. Out of many mechanisms, these interactions may enable the diffusion of fusion proteins from the plasma membrane to the membrane of the primary fused vesicles, consequently enabling vesicle-to-vesicle fusion. The incidence of compound exocytotic events increased by 33% after stimulation, which is consistent with the enhanced efficiency of hormone secretion after the stimulus.

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