Figure 2. Quantal secretion of serotonin from human platelets.

(A) An electron micrograph shows the “bull’s eye”-shaped dense-body granules (arrows). (B) A false-color plot obtained using FSCV shows quantal serotonin release from an individual thrombin-stimulated human platelet. Waveform update frequency is 10 Hz. (C) Cyclic voltammograms obtained from a secretion event (indicated by the * on the color plot) and a standard serotonin solution (dotted line) are overlaid. (D) Cumulative granular serotonin secretion (dotted green line) based on single platelet amperometry measurements (solid green representative amperometric trace at the bottom) was plotted on the left y-axis, while the serotonin release measured from the same serotonin-loaded platelet suspension (1.0 × 10⁷ platelets/mL) was plotted on the right y-axis. Thrombin stimulation times were represented by the symbol ▼and a 3-sec solid bar for platelet suspension and single platelet measurements, respectively. The cumulative release line was constructed using the spike time of 221 spikes collected from 42 single platelets. All measurements were performed at room temperature.