Probability density function (pdf) of the frequency distribution of K_ex values. Human platelets (1.2 × 10⁹ /mL) were prepared as described and were stimulated with thrombin (0.3 U/mL; n = 3), convulxin (0.3 μg/mL; n = 2), PAR1-agonist (50μM; n = 2), or PAR4-agonist (500μM; n = 2) for increasing times. Releasates were probed using the micro-ELISA arrays, and the rate constant, K_ex of release was calculated for each cargo molecule. The pool of K_ex values with MSE < 150 was used for constructing the pdf. The best-fit curve is seen in black corresponding to the existence of 3 different subclasses. The dashed blue, green and red curves are the Gaussian functions that represent fast, intermediate, and slow classes, respectively.