Similarities and Differences among Neuroendocrine, Exocrine, and Endocytic Vesiclesa

Abstract

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.

Human Endothelial Cell Storage Granules

“…Although the internal pH of Weibel-Palade bodies has not been determined to our knowledge, other storage granules, eg, granules in parotid acinar cells, have previously been characterized and found to be weakly acidic (internal pH of 6.8). 27 We therefore speculate that big ET-1 is translocated from the trans-Golgi network into cell storage granules and is converted to ET-1 by both ECE-1␣ and ECE-1␤. This hypothesis is supported by the localization of big ET-1-like immunoreactive staining in HUVEC Weibel-Palade bodies (present study) and by our recent findings showing the localization of ET-like immunoreactivity within storage granules in human coronary artery endothelium.…”

Characterization of Phospholipids in Insulin Secretory Granules and Mitochondria in Pancreatic Beta Cells and Their Changes with Glucose Stimulation

“…The percentage of PS (10%) in ISG is very similar to the percentage of PS in bovine, rat, and human chromaffin granules (9 -11%) and brain synaptic vesicles (12%) and much higher than in pancreatic granules (2%), whereas the percentages of PE and PC in ISG are about 30% lower than in chromaffin granules, pancreatic granules, and brain synaptic vesicles. The percentage of PI in ISG (21%) is much higher than in chromaffin granules and brain synaptic vesicles (1-3%) but is about the same as in pancreatic granules (15%) (53,54). The percentage of LPC in ISG (19%) is very similar to the percentage in chromaffin granules but much higher than the percentage in brain synaptic vesicles (1%).…”

Localization of Salivary Proteins in Granules of Human Parotid and Submandibular Acinar Cells

“…Several possible mechanisms have been proposed, such as selfor coaggregation (Burgess and Kelly, 1987;Stoller and Shields, 1989), including the formation of crystalline cores (Michael et al, 1987), specific binding proteins (Chung et al, 1989), and complexes of secretory proteins with calcium (Clemente and Meldolesi, 1975) or sulfated macromolecules (Palade, 1975). Studies of salivary gland granules in situ (using radioautography and electron probe X-ray microanalysis) and in vitro, showing the presence of substantial amounts of sulfur, presumably as chondroitin sulfate, and calcium (Berg and Austin, 1976;Iversen et al, 1987;Izutsu and Johnson, 1986;Izutsu et al, 1991;Wallach and Schramm, 1971;Castle et al, 1987;Muller and Roomans, 1984), are consistent with a mechanism involving complex formation with divalent cations or sulfated macromolecules, as mentioned earlier.…”