The actions of barium and strontium on exocytosis and endocytosis in the synaptic terminal of goldfish bipolar cells.
1. We investigated the properties of Ca2+-sensitive steps in the cycling of synaptic vesicles by comparing the actions of Ca2+, Ba2+ and Sr2+ 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 Ca2+ was replaced by either Ba2+ or Sr2+. 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 mM Ca2+, but only 0.4 % in 2.5 mM Sr2+. The relative sizes of the RRP in Ca2+, Sr2+ and Ba2+ 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 Sr2+ or Ba2+ compared to Ca2+. 3. The slow phase of exocytosis was not altered when Ca2+ was replaced by Ba2+, but it was accelerated 1.6-fold in Sr2+. The peak concentrations of Ca2+, Sr2+ and Ba2+ (measured using Mag-fura-5) were approximately 4, approximately 14 and approximately 60 microM, respectively. The order of efficiency for the stimulation of slow exocytosis was Ca2+ approximately Sr2+ > Ba2+. 4. Exocytosis was prolonged after the influx of Sr2+ and Ba2+. Sr2+ was cleared from the synaptic terminal with the same time constant as Ca2+ (1.3 s), but Ba2+ 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 Ca2+ or Sr2+. 5. The recovery of the membrane capacitance was equally rapid in Sr2+ and Ca2+, demonstrating that the fast mode of endocytosis could be triggered by either cation.
Neves
G
G
MRC Laboratory of Molecular Biology, Hills Road, Cambridge CB2 2QH, UK.
Neef
A
A
Lagnado
L
L
D000818:Animals; D001464:Barium / Q000378:metabolism / Q000494:pharmacology*; D002118:Calcium / Q000378:metabolism; D025802:Electric Capacitance; D004705:Endocytosis / Q000187:drug effects*; D005089:Exocytosis / Q000187:drug effects*; D005456:Fluorescent Dyes; D006054:Goldfish; D066298:In Vitro Techniques; D007473:Ion Channels / Q000187:drug effects; D009474:Neurons / Q000187:drug effects / Q000378:metabolism*; D017729:Presynaptic Terminals / Q000187:drug effects*; D011726:Pyridinium Compounds; D000644:Quaternary Ammonium Compounds; D013324:Strontium / Q000378:metabolism / Q000494:pharmacology*; D013572:Synaptic Vesicles / Q000187:drug effects / Q000378:metabolism
C073804:FM1 43; D005456:Fluorescent Dyes; D007473:Ion Channels; D011726:Pyridinium Compounds; D000644:Quaternary Ammonium Compounds; D001464:Barium; D002118:Calcium; D013324:Strontium