Vesicular calcium transport shapes rapid acetylcholine secretion.

Rapid secretion relies on the occurrence of spike-like Ca2+ transients in active zones (Llinás et al., 1992; Yazejian et al., 2000; Dunant and Bloc, 2003). Presynaptic Ca2+ nanodomains are to be restricted both in time and in space as to assure rapid onset and termination of transmitter release (Llinás et al., 1992; Pozzan et al., 1994; Yazejian et al., 2000; Dunant and Bloc, 2003). A very fast Ca2+-buffering mechanism should allow Ca2+ rise above approximately 100 microM for less than approximately 250 micros and then rapid reduction of Ca2+ to subthreshold levels of release (Llinás et al., 1992; Pozzan et al., 1994; Yazejian et al., 2000; Dunant and Bloc, 2003). Swift Ca2+ clearance by vesicular Ca2+/H+ antiport as a low-affinity, high-capacity extrusion mechanism was postulated in the past (Pozzan et al., 1994; Dunant and Bloc, 2003). We demonstrated pH gradient (DeltapH)-dependent Ca2+ uptake by mammalian brain synaptic vesicles (Gonçalves et al., 1998, 2000). Moreover, this antiport activity is effective at [Ca2+] ranging from approximately 100 to 800 microM (max. at approximately 500 microM) (Gonçalves et al., 1998, 2000). We now show that the time course of acetylcholine (ACh) secretion in Torpedo neuroelectrocytic synapse is modified by bafilomycin A1 (baf.), which compromises antiport activity. Along with this mechanism, synaptic vesicles also have a P-type Ca2+ ATPase, exhibiting half-maximal activation for 0.6 microM Ca2+ (Gonçalves et al., 2000). Here, we demonstrate the role of P-type Ca2+ ATPase in preventing desensitization of the release mechanism by inhibiting it with orthovanadate.