Fig. 10. The model describes how moderate and intense stimulation of exocytosis might affect SDS-resistant SNARE complexes in neuroendocrine cells. (Left) Ca²⁺ elevations to submicromolar concentrations (e.g. by 57 mM K⁺) stimulate vesicle priming and support low rates of vesicle exocytosis (left drawing: (1) unprimed vesicle, (2) primed vesicle). As a consequence, the amount of fusion-competent SDS-resistant SNARE complexes is increased. This is illustrated in the lower panel showing cells permeabilized in the presence of 100 nM (1) and 1000 nM (2) free Ca²⁺. (Right) Ca²⁺ elevations to micromolar concentrations (e.g. by 110 mM K⁺) support vesicle priming and high rates of vesicle exocytosis [right drawing, (3)]. The immediate fusion of primed vesicles leads to a reduction of fusion-competent SDS-resistant SNARE complexes via a post-fusion action of NSF [right drawing, (4)] [shown in the lower panel for a 5 minute stimulation of intact cells with 110 mM K⁺ (4) compared with SNAP-25-immunoreactive protein bands obtained from non-depolarized control cells (3)].