Calcium influx selects the fast mode of endocytosis in the synaptic terminal of retinal bipolar cells

To investigate the regulation of endocytosis by Ca2+, we have made capacitance measurements in the synaptic terminal of depolarizing bipolar cells from the retina of goldfish. After a brief depolarization, all of the excess membrane was retrieved rapidly (tau approximate to 1 s). But when the rise in free [Ca2+] was reduced by the introduction of Ca2+ buffers [1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetate (BAPTA) or EGTA], a large fraction of the membrane was retrieved by a second, slower mechanism (tau greater than or equal to 10 s). The block of fast endocytosis by EGTA could be overcome by increasing the amplitude of the Ca2+ current, demonstrating that Ca2+ influx was the trigger for fast endocytosis. These manipulations of the Ca2+ signal altered the relative proportions of fast and slow endocytosis but did not modulate the rate constants of these processes. A brief stimulus that triggered fast endocytosis did not generate a significant rise in the spatially averaged [Ca2+], indicating that Ca2+ regulated endocytosis through an action close to the active zone. The slow mode of retrieval occurred at the resting [Ca2+]. These results demonstrate that Ca2+ influx couples fast endocytosis and exocytosis at this synapse.