Membrane fusion of secretory vesicles and liposomes Two different types of fusion

Abstract

Secretory vesieles isolated from adrenal medulla were found to fuse in vitro in response to incubation with Ca 2 +. Intervesicular fusion was detected by electron microscopy and was indicated by the appearance of twinned vesieies in freeze-fractured suspensions of vesieles and in thin-sectioned pellet. Two types of fusion could be distinguished: Type I, occurring between 10-7 M and 10-4 M Ca 2 +, was specific for ea 2 +, was inhibited by other divalent cations and was abolished by pretreatment of vesieles with glutaraldehyde, neuraminidase or trypsin. Fusion type I was linear with temperature. A second type of intervesicular fusion was elicited by Ca 2 + in concentrations higher than 2.5 mM and was morphologically characterized by multiple fusions of secretory vesicles. This type of fusion was found to be similar to fusion of liposomes prepared from the membrane lipids of adrenal medullary secretory vesieles: ea 2 + could be replaced by other divalent cations, the effect of different divalent cations was additive and pretreatments attacking membrane proteins were ineffective. Fusion type 11 of intact secretory vesieies as weil as liposome fusion was discontinuous with temperature. Liposome fusion could be detected within 35 ms and persisted for 180 min. Using liposomes containing defined Ca 2 + concentrations we have not found a major influence of Ca 2 + asymmetry on fusion. Incorporation of the ganglioside GM 3 , which is present in the membranes of intact adrenal medullary secretory vesicles did not change the properties of liposomes fusion. Using a Ca 2 +-selective electrode we have identified in secretory vesiele membranes both high affinity binding sites for Ca 2 + (/(} = 1.6 . 10-6 M) and low affinity sites (/(} = 1.2· 10-4 M).

Insulin-Induced Hypoglycemia Stimulates Both Adrenaline and Noradrenaline Release from Adrenal Medulla in 21-Day-Old Rats

“…Therefore, the granules seen in the present study were probably not vacuoles but chromaffin granules. Fusion of several chromaffin granules at high Ca 21 concentration (about 1 1tM) has been reported in in vitro studies (20,21). These large chromaffin granules, observed in the present study, may be produced by a …”

Uptake of Ca2+ by isolated secretory vesicles from adrenal medulla

“…An inherent puzzle, which is not yet solved, is that secretory vesicles fuse upon addition of Ca2+ [2] and that the same organelles are, in addition, able to take up Ca2+ . That means the same membrane, within the sequence of events between stimulus and secretion by exocytosis, acts as an acceptor of the signal as weIl as a 'scavenger' of the signal.…”

“…In earlier work [2] high affinity-binding sites for Ca2+ have been identified on the secretory vesicle membrane. Occupation of these sites has been found to parallel the fusion of secretory vesicles.…”

“…One 'of the key events triggered by Ca 2 + is the process of membrane fusion. This has been demonstrated with secretory vesicles from adrenal medulla and other types of tissue [2,3].…”

Ca2+-ATPase activity in the gastric parietal cells of rat during perinatal development.

“…It is well known that mitochondria are the site of accumulation of calcium (30,32), and that A23187 induces the calcium release from the intracellular stores (38). Kurihara and Fujita (29)) have reported that calcium ions necessary for exocytosis and membrane fusion (8,12,18,37) are derived not only from the extracellular fluid but also from the mitochondrial matrix. Moreover several workers (28, 33-35, 47, 51-53) have noticed Ca2+-ATPase activity in the mitochondria of different types of secretory cells and they proposed that mitochondria are exclusively concerned with the Ca2+ accumulation and that Ca2+-ATPase activity in mitochondria may help to decrease intracellular Ca2-concentrations in cells.…”