Stochastic simulation of a single platelet. (A) A single, fura-2–loaded platelet was immobilized on a fibrinogen-coated coverslip and activated with 40 μM ADP at t = 90. Ca²⁺_i trace was obtained from Heemskerk et al.⁴⁷  (B) After 90 se-conds of simulated rest, the platelet model was activated by setting extracellular [ADP] to 40 μM. [Ca²⁺]_i was sampled every 0.1 second. (C) Single platelets were simulated for 10 minutes, and [Ca²⁺]_i was sampled once per second. Ca²⁺ peaks were identified as described by Heemskerk et al.¹⁸  Peak intervals between 5 and 30 seconds were counted from 10 simulations as described for video-imaging studies (Supplemental data). Interval times were binned in 2-second increments for direct comparison with experiment (inset). (D) The platelet model compartment sizes were artificially scaled to 1, 10, or 100 times their original sizes. Simulation results are shown for 300-second stochastic simulations with 20 μM of ADP added at 90 seconds. The bottom plot shows results for the deterministic simulation.