Compensatory endocytosis in chromaffin cells
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Abstract Exocytosis occurs via fusion of secretory granules with the cell membrane, whereupon the granule content is at least partially released and the granule membrane is temporarily added to the plasma membrane. Exocytosis is balanced by compensatory endocytosis to achieve net equilibrium of the cell surface area and to recycle and redistribute components of the exocytosis machinery. The underlying molecular mechanisms remain a matter of debate. In this review, we summarize and discuss recent progress in the understanding of compensatory endocytosis, with the focus on chromaffin cells as a useful model for studying mechanisms of regulated secretion.Keywords:
Granule (geology)
Chromaffin cell
1. Changes in membrane capacitance evoked by the rapid photolysis of a caged Ca2+ compound, DM‐nitrophen or nitrophenyl‐EGTA, were investigated in undifferentiated PC12 cells. They were interpreted as representing exocytosis and endocytosis. 2. The Ca2+ jumps evoked two components of exocytosis. Slow exocytosis was selectively evoked with small increases in intracellular Ca2+ concentration between 5 and 10 microM, while fast exocytosis preceded the slow one at [Ca2+]i greater than 10 microM. 3. The release rates of the two components of exocytosis depended steeply on [Ca2+]i. A half‐maximal release rate was achieved at 8 and 24 microM for the slow and fast exocytoses, respectively. 4. Prior Ca2+ rises did not augment the fast exocytosis. 5. The fast exocytosis was often followed by a rapid decrease in membrane capacitance, representing endocytosis, after a delay of 0.5‐2 s. The speed and delay in the fast endocytosis were Ca2+ dependent. Amounts of the fast endocytosis tended to balance with those of the fast exocytosis evoked by the same Ca2+ jumps. 6. The slow exocytosis was followed by a sluggish endocytosis that was associated with large capacitance steps indicative of secretory processes involving large dense‐core vesicles. The onset of the slow endocytosis exhibited a complex Ca2+ dependence. The amounts of the slow endocytosis appeared to parallel those of the slow exocytosis. Prior induction of the slow exocytosis gave rise to selective excess retrieval of membrane during the slow endocytosis. 7. These data indicate the existence of two distinct populations of secretory vesicles in PC12 cells. They seem to couple selectively with specific endocytotic mechanisms. Our data suggest that the two vesicles belong to two distinct secretory pathways.
Bulk endocytosis
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We studied endocytosis in chromaffin cells with both perforated patch and whole cell configurations of the patch clamp technique using cell capacitance measurements in combination with amperometric catecholamine detection. We found that chromaffin cells exhibit two relatively rapid, kinetically distinct forms of stimulus-coupled endocytosis. A more prevalent “compensatory” retrieval occurs reproducibly after stimulation, recovering an approximately equivalent amount of membrane as added through the immediately preceding exocytosis. Membrane is retrieved through compensatory endocytosis at an initial rate of ∼6 fF/s. Compensatory endocytotic activity vanishes within a few minutes in the whole cell configuration. A second form of triggered membrane retrieval, termed “excess” retrieval, occurs only above a certain stimulus threshold and proceeds at a faster initial rate of ∼248 fF/s. It typically undershoots the capacitance value preceding the stimulus, and its magnitude has no clear relationship to the amount of membrane added through the immediately preceding exocytotic event. Excess endocytotic activity persists in the whole cell configuration. Thus, two kinetically distinct forms of endocytosis coexist in intact cells during perforated patch recording. Both are fast enough to retrieve membrane after exocytosis within a few seconds. We argue that the slower one, termed compensatory endocytosis, exhibits properties that make it the most likely mechanism for membrane recycling during normal secretory activity.
Chromaffin cell
Stimulus (psychology)
Cell membrane
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Evoked exocytosis in excitable cells is fast and spatially confined and must be followed by coupled endocytosis to enable sustained exocytosis while maintaining the balance of the vesicle pool and the plasma membrane. Various types of exocytosis and endocytosis exist in these excitable cells, as those has been found from different types of experiments conducted in different cell types. Correlating these diversified types of exocytosis and endocytosis is problematic. By providing an outline of different exocytosis and endocytosis processes and possible coupling mechanisms here, we emphasize that the endocytic pathway is pre-determined at the time the vesicle chooses to fuse with the plasma membrane in one specific mode. Therefore, understanding the early intermediate stages of vesicle exocytosis may be instrumental in exploring the mechanism of tailing endocytosis.
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The effects of osmolar and ionic factors on endocytosis and exocytosis were investigated using rabbit reticulocytes and 125I-59Fe labelled transferrin. Endocytosis and exocytosis of transferrin and the uptake of iron were inhibited by increasing the osmolality or decreasing the ionic strength or pH of the cell incubation medium. However, elevation of the pH above 8.0 inhibited endocytosis but not exocytosis. Replacement of the NaCl in the incubation medium by Nal, NaF, NaSCN, NaCIO4, Na2SO4, Na phosphate, or Na Hepes inhibited endocytosis and iron uptake but only Nal, NaF, and NaSCN inhibited exocytosis. Transferrin exocytosis was insensitive to inhibitors of anion or cation transport, but endocytosis and iron uptake were inhibited by several anion transport inhibitors. Overall, transferrin endocytosis was more sensitive than exocytosis to most of the factors which were investigated, and the effects on the rates of endocytosis and iron uptake were quantitatively very similar. The results provide strong support for the concept that transferrin endocytosis is a necessary step in iron uptake by reticulocytes. They do not support the chemiosmotic models of exocytosis in their present formulations, out do not rule out the possible role of an osmotic event in exocytosis.
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Abstract Exocytosis occurs via fusion of secretory granules with the cell membrane, whereupon the granule content is at least partially released and the granule membrane is temporarily added to the plasma membrane. Exocytosis is balanced by compensatory endocytosis to achieve net equilibrium of the cell surface area and to recycle and redistribute components of the exocytosis machinery. The underlying molecular mechanisms remain a matter of debate. In this review, we summarize and discuss recent progress in the understanding of compensatory endocytosis, with the focus on chromaffin cells as a useful model for studying mechanisms of regulated secretion.
Granule (geology)
Chromaffin cell
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Granule (geology)
Chromaffin cell
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Transmission electron microscopy was used to study the characteristics of exocytosis of chromaffin cells in adrenal medulla during emergency reaction of restrained rats. It was found that three types of exocytosis existed: single granule exocytosis, multigranule successive exocytosis and exocytosis through a temporary fusion pore. Two kinds of granule were estimated by using stereological morphometry. The results showed: the number of dense core granules was more than that of shallow core granules ( P 0 01). When the rats were restrained for 8h, we found that only the dense core granules decreased. When the restrained time was extended to 24h, both two kinds of granule decreased. It is suggested that these two kinds of granules may be the same secretory granules of chromaffin cells, which are in the different developmental stages.
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Chromaffin cell
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