Fig. 6. Ca²⁺ activity and SNARE density. The midpoint of the Ca²⁺ activity curve following papain treatment at either 1 hour (blue) or 4 hours (red) is correlated with SNARE density (Copy #/CV). The data were fit using the relationship Axexp(–density/B)+C, where A+C (µM) is the midpoint of the Ca²⁺ activity curve in the absence of SNAREs (248±10, 433±66 and 305±22) and B is the density decay constant (212±63, 42±10 and 11±3) for VAMP, SNAP-25 and syntaxin, respectively. When SNARE density was normalized by the density decay constant, the pooled transformed data were described by the relationship Axexp(–normalized density/B)+7.1; the midpoint of the Ca²⁺ activity curve in the absence of SNAREs (A+7.1) is 260±22 µM.