Fig. 6. The time course of Ca2+recovery predicts the time course of endocytosis. A,B, Top panels show the capacitance records from synaptic terminals of bipolar neurons dialyzed with internal solution containing 10 mm ATP and stimulated to exocytose with either a 500 msec (A) or 1 sec (B) depolarization from −60 to 0 mV.Bottom panels, The ratiometrically calculated intraterminal Ca2+ determined with fura-2.Insets show the corresponding Ca2+current. The thickness of the trace that precedes and follows the inward current carried by Ca2+ is attributable to the superimposed sine wave stimulus used to monitor membrane capacitance. Curves in the top panels are the time course of membrane recovery predicted from the recovery time course of intraterminal Ca2+. Curves were drawn according to the following:Cm(t) =a + be−αt, where α(t) = 0.54(1 − 1/(1 + (0.460/[Ca2+])4)), from von Gersdorff and Matthews (1994b). C, Black bars show the mean rate of endocytosis experimentally observed in terminals with 10 mm ATP after depolarizations of the indicated durations from −60 to 0 mV. Gray bars show the rate of endocytosis predicted from the recovery time course of intraterminal Ca2+. For mild to moderate duration depolarizations, the time course of endocytosis matched the time course of endocytosis predicted by the time course of recovery of intraterminal Ca2+ in terminals with 10 ATP. However, after a 2 sec depolarization, there was a statistically significant difference between the measured and the predicted rates of endocytosis (p < 0.005). After a 5 sec depolarization, the measured and predicted rates were also significantly different (p < 0.07). For ≤500 msec,n = 4 responses, 2 terminals. For 1 sec,n = 2 responses, 1 terminal. For 2 sec,n = 4 responses, 3 terminals. For 5 sec,n = 5 responses, 3 terminals. Data are expressed as mean ± SEM.