FIGURE 5.

Differential effects of Ca²⁺, Sr²⁺, or Ba²⁺ on depolarization-induced C_m changes, and vesicle depletion by repetitive stimulation (A) Differential effects of Ca²⁺, Sr²⁺, or Ba²⁺ on depolarization-induced exocytosis. Oocytes expressing Ca_v1.2, syntaxin 1A, syt-1, and SNAP-25 (excitosome). (Left panel) Original traces showing time course of C_m upon depolarization, fitted by simple exponentials; (middle panel) instantaneous depolarization-induced C_m increase in Ca²⁺, Sr²⁺, and Ba²⁺ (mean ± SE, n = 10, each); and (right panel) rate of compensatory C_m decrease (mean ± SE, n = 10, each). As compared to Ca²⁺, the instantaneous C_m increase was greater in Ba²⁺ (middle panel), and the compensatory C_m decrease was slower (right panel). In Sr²⁺, the respective values were between those of Ca²⁺ and Ba²⁺. (B) Current amplitude during repeated trains of membrane depolarization. Oocytes were depolarized by three trains composed of 10 depolarizations from −80 to 0 mV 1 s each (upper panel). Depolarization-induced inward current inactivated during trains whereas no run-down of current amplitude was observed from one train to the other (lower panel). (C) Exhaustibility of depolarization-induced capacitance changes. Repeated depolarization pulses from −80 mV to 0 mV for 1 s each (upper trace) and the associated C_m changes (lower trace). Data are consistent with depletion and only partial replenishing of a pool of releasable vesicles.