FIGURE 4.

Quantitative characterization of reconstituted depolarization-induced exocytosis. (A) Dependence of depolarization-induced ΔC_m on extracellular Ba²⁺ concentration. Depolarization from −80 mV to 0 mV for 2 × 500 ms 100 ms apart in oocytes expressing Lc-type channel without (lower trace) or with (upper trace) the synaptic proteins syntaxin 1A, SNAP-25, and synaptotagmin I, as in Fig. 1. (B) Hill plot. Data (from A) show saturation of the C_m effect >2 mM, and a nonlinear concentration-dependence of ΔC_m with an estimated Hill coefficient of n_H ≈ 2.8. (C) Dependence of depolarization-induced ΔC_m on depolarization duration. Depolarization from −80 mV to 0 mV for indicated times in 5 mM Ba²⁺. A linear correlation observed with pulse duration for longer depolarization periods, saturation at ∼400 ms and half-maximal capacitance changes are reached at a depolarization time of ∼250 ms. A straight line fitted to the data between 50 and 400 ms had a slope of 7.7 pF ms⁻¹ (R² = 0.998) corresponding to ∼600,000 vesicles at saturation. The results could be fitted less well with a single exponent (inset) showing a time constant of 101.05 ± 12 ms corresponding to an initial rate of 6 × 10⁶ vesicles/s (inset). The saturation in the capacitance jumps defines a readily releasable pool of ∼10⁶–10⁷ vesicles (see 31).