Fig. 2. Exocytosis of endothelial secretory organelles. (A,B) Individual fusion events of a Rab27a-GFP-positive organelle (A) and a tPA-GFP vesicle (B) stimulated by 100 µM histamine. Rab27a-GFP diffuses in the plasma membrane, whereas tPA-GFP remains at the fusion site after exocytosis. Numbers indicate time (in seconds) relative to the moment of fusion. Lower panels show three-dimensional luminance plots of four successive frames starting one frame before fusion. Bars, 1 µm. (C) Plot of the half-width R_fluo²(t) obtained by a Gaussian fit of the distribution of fluorescence intensities from the images shown in A. A linear fit (grey line) yields the diffusion coefficient of Rab27a-GFP in the membrane D_fluo=0.12±0.01 µm²/second. (D) Time course of the fluorescence intensity of the tPA-GFP vesicle shown in B. The characteristic decay time of the fluorescence is given by an exponential fit (grey line): =13.7±0.9 s.