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 s ynaptic terminal of depolarizing bipolar cells isolated from the retina of goldfish. FM1-43 fluorescence and capacitance measurements demonstrated tha t exocytosis, endocytosis and vesicle mobilization were maintained when ext ernal 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+, bu t only 0.4 % in 2.5 mM Sr2+. The relative sizes of the RRP in Ca2+, Sr2+ an d Ba2+ were 1.0, 0.28 and 0.1, respectively. We conclude that a smaller pro portion of docked vesicles are available for fast exocytosis triggered by t he influx of Sr2+ or Ba2+ compared to Ca2+. 3. The slow phase of exocytosis was not altered when Ca2+ was replaced by B a2+, but it was accelerated 1.6-fold in Sr2+. The peak concentrations of Ca 2+, Sr2+ and Ba2+ (measured using Mag-fura-5) were similar to4, similar to 14 and similar to 60 mum, respectively. The order of efficiency for the sti mulation of slow exocytosis was Ca(2+)approximate to Sr2+ > Ba2+. 4. Exocytosis was prolonged after the influx of Sr2+ and Ba2+. Sr2+ was cle ared from the synaptic terminal with the same time constant as Ca2+ (1.3 s) , but Ba2+ was cleared 10-100 times more slowly. Although Ba2+ stimulates t he slow release of a large number of vesicles, it did so less efficiently t han Ca2+ or Sr2+. 5. The recovery of the membrane capacitance was equally rapid in Sr2+ and C a2+, demonstrating that the fast mode of endocytosis could be triggered by either cation.