Similarities and differences among neuroendocrine, exocrine, and endocytic vesicles.

Secretory and endocytic vesicles have analogous functions as cyclic carriers between specific cellular compartments. The centrifugally functioning secretory system operates from the Golgi complex, whereas the centripetally functioning endocytic system operates from the cell surface. Further, within polarized epithelial cells the export traffic can be directed to a distinct plasmalemmal domain which distinguishes exocrine from endocrine secretion and import traffic can be directed transcellularly. These shuttle operations involve a special class of lipid-rich, protein-poor membranes that appear to use an inwardly directed H+-translocase activity to varying extents for pH-dependent sorting and for accumulation and concentration of transported molecules. Comparative analyses of purified membrane preparations from exocrine and endocrine sources identify compositional overlap between different types of shuttle membrane. However, the structural elements that specify a particular origin or destination for a given carrier or determine function in storage and stimulus-dependent shuttling remain unknown.