Fusion of neurohypophyseal membranes in vitro

Abstract

(In suspensions containing secretory vesicles as well as sheets of cell membrane, release of vasopressin parallel to intervesicular fusion and fusion of secretory vesicles with sheets of cell membrane was observed after exposure to Ca 2+ . Mg 2+ and Sr 2+ were ineffective in replacing Ca 2+ as trigger for fusion or vasopressin release.Intervesicular fusion and exocytotic profiles were observed when isolated neurohypophyses or neurosecretosomes were exposed to cold. IntroductionIt is well established that the passage of.Ca 2+ across the cell membrane is a critical step in stimulus-secretion coupling in the neurohypophysis [1,2,3]. Although Ca 2+ influx is apparently too small to be reliably quantitated [4], experiments with Ca 2+ transport inhibitors [2] bear out this contention. Also, treatment of isolated neurohypophyses with ionophores, which increase intracellular, free Ca 2+ concentration by increasing transmembrane transport, can induce secretion [5,6]. Both ultrastructural [7] and biochemical [8,9] studies suggest that secretion in the neurohypophysis as in various other endocrine systems [10], occurs by exocytosis. However, the events that are triggered by

Synaptic transmission at retinal ribbon synapses

“…An alternative explanation is that in the presence of high, global calcium, a different form of compound fusion takes place, in which docked vesicles fuse with the plasma membrane and the next row of vesicles fuses with the docked vesicles, and so on up the ribbon. High calcium concentrations are known to facilitate vesicle-vesicle fusion in other systems and conceivably could do so in neurons (Gratzl et al, 1977). With this scenario (Fig.…”

Fusion of secretory vesicles isolated from rat liver

“…Fusion of secretory vesicles isolated from hepatocytes as well as those of pancreatic islet cell [16] and the neurohypophysis [33,74] was CaZ+-specific; other cations were ineffective in triggering the fusion. Furthermore, the fusion of secretory vesicles was half maximal at 10 -6 M Ca 2+.…”

“…In these experiments, as well in others, where the ionic composition of a cell was changed by injection of ions, use of ionophores, or replacement of ions in the extracellular fluid, the results are difficult to interpret, since an intracellular redistribution of Ca 2 + by other cations cannot be ruled out. In this context it is of interest to note that release of intravesicular vasopressin in a suspension containing sheets of cell membrane [33] resembles closely the ionic requirements for intervesicular fusion of secretory vesicles from rat liver and other tissues [16,17,33,74].…”

“…Studies with intact cells have not yielded data which lead to ready interpretation concerning subcellular processes. On the other hand, studies with artificial membranes may be only tangentially related to the fusion of biological membranes.Recently our group investigated the fusion of isolated biological membranes [16,17,31,33,69,74]. We observed that Ca 2+ specifically induces the fusion of isolated secretory vesicles in a buffered sucrose medium.…”

“…Recently our group investigated the fusion of isolated biological membranes [16,17,31,33,69,74]. We observed that Ca 2+ specifically induces the fusion of isolated secretory vesicles in a buffered sucrose medium.…”