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

To investigate the regulation of endocytosis by Ca(2+), 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 (τ ≈1 s). But when the rise in free [Ca(2+)] was reduced by the introduction of Ca(2+) 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 (τ ≥ 10 s). The block of fast endocytosis by EGTA could be overcome by increasing the amplitude of the Ca(2+) current, demonstrating that Ca(2+) influx was the trigger for fast endocytosis. These manipulations of the Ca(2+) 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 [Ca(2+)], indicating that Ca(2+) regulated endocytosis through an action close to the active zone. The slow mode of retrieval occurred at the resting [Ca(2+)]. These results demonstrate that Ca(2+) influx couples fast endocytosis and exocytosis at this synapse.