Slp4-a enhances WPB probability of release through a region distal to the SHD domain. (A) HUVEC expressing EGFP-Slp4-a-SHD (green) and immunolabeled for VWF (red). Grayscale images are from the region indicated by the white box. Bar represents 20 μm. (B) Fura-2 fluorescence ratio showing a representative record of the increase in fluorescence ratio (intracellular Ca²⁺) upon addition of 1 μM ionomycin. (C) Cumulative plot of WPB fusion times, normalized by their total number, in Proregion-EGFP– (black), EGFP-Slp4-a FL– (gray), and EGFP-Slp4-a-SHD–expressing cells (red). (Di-iii) Mean delays (seconds), maximal rates of exocytosis (WPBs/second), and probabilities of WPB exocytosis, P_r, (%) for Proregion-EGFP– (black), EGFP-Slp4-a FL– (gray), and EGFP-Slp4-a-SHD–expressing cells (red). Mean % degranulation WPB Proregion-EGFP = 65.8 ± 1.5% n = 46 cells, 2151 fusion events; mean % degranulation WPB EGFP-Slp4-a FL = 85.8 ± 2.6%, n = 15 cells, 849 fusion events; mean % degranulation WPB EGFP-Slp4-a-SHD = 65.8 ± 2.2%, n = 22 cells, 694 fusion events. *P < .05, ***P < .005