It is often assumed that during exocytic secretion from dense core vesicles of endocrine cells, these exocytic organelles release all of their content on exposure to the extracellular medium. Now, however, Joseph Angleson and colleagues reveal that some of the peptide hormone prolactin contained in the dense core vesicles of lactotrophs in the anterior pituitary is retained, processed, and recycled (see ). Using a combination of fluorescence imaging and electron microscopy, the authors follow the fate of vesicles containing non-secreted prolactin as they undergo intracellular processing. They show that the non-secreted prolactin does not traffic to lysosomes but instead colocalizes with transferrin, suggesting that the endosomal recycling compartment is involved in this intracellular processing. Finally, Angleson and colleagues report that the vesicles containing the retained prolactin are preferentially released during subsequent exocytic stimulation. They propose that these vesicles have a functional role and that recycling of retained hormone may be a common feature of neuroendocrine secretion.
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