SExocytosis to the extracellular space

s-Exocytosis can be regulated or constitutive. Regulated cxocytosis occurs on demand via secretagogues and signal transduction. Constitutive exocytosis is continuous; no secretagogue is involved, there are no storage vesicles, and the products are released apace upon synthesis. s-Exocytosed products of nonneu-ronal cells (digestive proenzymes, peptide hormones, immunoglobulins, chylomicra lipoproteins) originate in the RER, are carried by vesicles to and through the Golgi apparatus, then by vesicles or membrane-bounded granules to the plasmalemma, and are finally extruded by fusion of the vesicles with the cell membrane. For that entire transit they remain confined topologically within the cisternal space (Figure 1). In neurons some vesicles recycle and reload near the synapse. In pancreatic acini, fusion of progressively deeper granules with more peripheral ones, called sequential or compound exocytosis, forms extended extrusion channels, also said to occur in mast cells. Various agents elicit release of mast cell mediators: Ca2+ ionophores, cross-linkcrs of immunoglobulin E (IgE) receptors on the cell surface by antigen, polycations or some lectins. Phagocytosis of negatively-charged thorium dioxide particles or plastic beads by rat serosal mast cells delivers them over time via endosomes to young, then mature granules, without causing degranulation. The metal is retained for months and migrates between granules, which change their staining properties as the cell ages. They have in situ enzymatic activity land ipso facto function?), and their mediators have long half-lives. Polylysine causes degranulation. Cationic (polylysine-coated) beads breach the granular sac by a different path that bypasses endosome vesicles, and they contact mature granules directly without delay. All this argues for a transient or permanent membranous sac enfolding multiple granules in a way unrelated, if morphologically resembling, sequential exocytosis. Could IgE antigens act as pathologic mast cell secretagogues? This paradoxical dichotomy suggests that mast cell degranulation interferes with a primary intracellular function, whatever that might be, and that evidence for compound exocytosis must be interpreted cautiously.

At plasmalemmal docking sites, a series of biochemical signaling reactions involving G protein(s), enzymes, phosphoinositol metabolites, etc. may lead to transient shifts of ionic calcium into cells and/or between some intracellular compartments. These Ca2+ shifts may not be obligate steps for all cells or all s-exocytotic processes. As fusion of a vesicle with the cell membrane establishes continuity between vesicular contents and extracellular space, all the contents stored within are disgorged simultaneously (epinephrine, enkephalins, chromogranins, nucleotides, etc., by adrenal chromaffin cells), faster and in much higher concentrations than possible via active transport mechanisms. Vesicular pathways can couple endocytosis with s-exocytosis to transport material across a cell (endothelium, kidney tubules) by transcytosis. Though uptake at one surface may be selective, there need be no modification of the vesicular contents before s-exocytosis at another surface. Capillary endothelium transfers bulk plasma into tissue spaces, and selected substances across the blood-brain barrier. IgA and IgM are moved across enterocytes in the opposite direction, from basal to apical surfaces.

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