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The actions of barium and strontium on exocytosis and endocytosis in the synaptic terminal of goldfish bipolar cells
Author(s) -
Neves Guilherme,
Neef Andreas,
Lagnado Leon
Publication year - 2001
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1111/j.1469-7793.2001.t01-1-00809.x
Subject(s) - exocytosis , synaptic vesicle , chemistry , biophysics , vesicle , endocytosis , membrane potential , membrane , biology , biochemistry , cell
1 We investigated the properties of Ca 2+ ‐sensitive steps in the cycling of synaptic vesicles by comparing the actions of Ca 2+ , Ba 2+ and Sr 2+ in the synaptic terminal of depolarizing bipolar cells isolated from the retina of goldfish. FM1‐43 fluorescence and capacitance measurements demonstrated that exocytosis, endocytosis and vesicle mobilization were maintained when external Ca 2+ was replaced by either Ba 2+ or Sr 2+ . 2 The rapidly releasable pool of vesicles (RRP) was equivalent to 1.5 % of the membrane surface area when measured in the presence of 2.5 m m Ca 2+ , but only 0.4 % in 2.5 m m Sr 2+ . The relative sizes of the RRP in Ca 2+ , Sr 2+ and Ba 2+ were 1.0, 0.28 and 0.1, respectively. We conclude that a smaller proportion of docked vesicles are available for fast exocytosis triggered by the influx of Sr 2+ or Ba 2+ compared to Ca 2+ . 3 The slow phase of exocytosis was not altered when Ca 2+ was replaced by Ba 2+ , but it was accelerated 1.6‐fold in Sr 2+ . The peak concentrations of Ca 2+ , Sr 2+ and Ba 2+ (measured using Mag‐fura‐5) were ≈4, ≈14 and ≈60 μ m , respectively. The order of efficiency for the stimulation of slow exocytosis was Ca 2+ ≈ Sr 2+ > Ba 2+ . 4 Exocytosis was prolonged after the influx of Sr 2+ and Ba 2+ . Sr 2+ was cleared from the synaptic terminal with the same time constant as Ca 2+ (1.3 s), but Ba 2+ was cleared 10‐100 times more slowly. Although Ba 2+ stimulates the slow release of a large number of vesicles, it did so less efficiently than Ca 2+ or Sr 2+ . 5 The recovery of the membrane capacitance was equally rapid in Sr 2+ and Ca 2+ , demonstrating that the fast mode of endocytosis could be triggered by either cation.