Two actions of calcium regulate the supply of releasable vesicles at the ribbon synapse of retinal bipolar cells

Ribbon synapses of sensory neurons are able to sustain high rates of exocyt osis in response to maintained depolarization, but it is not known how this is achieved. Using the capacitance technique, we have found that Ca2+ regu lates the supply of releasable vesicles at the ribbon synapse of depolarizi ng bipolar cells from the retina of goldfish. Ca2+ had two actions that cou ld be differentiated by introduction of the Ca2+ chelator EGTA; one action stimulated refilling of the rapidly releasable pool of vesicles from a rese rve pool, and a second action stimulated recruitment of vesicles to the res erve pool. The capacity of the reserve pool was similar to 3500 vesicles, w hich is similar to the number that can attach to the ribbons. These results suggest that continuous exocytosis at ribbon synapses is maintained by the Ca2+-dependent translocation of vesicles from the cytoplasm, through the r ibbon, to release sites on the plasma membrane.