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The kinetics of ADP-induced human platelet shape change have been examined. Initial velocities of platelet shape change were estimated by two methods: 1) the slope of the initial decrease in light transmission through stirred, citrated platelet-rich plasma, and 2) direct examination of platelet morphologies by phase-contrast microscopy. In both cases, a value of the Hill coefficient, n, significantly greater than 1 is obtained (2.0 +/- 0.2 and 1.8 +/- 0.2, respectively). The observed elevated value of n is not due to a substantial fraction of the ADP being platelet bound, the presence of factors in the plasma, platelet heterogeneity, of the influence of the rate of platelet shape change reversion. Our observations suggest that ADP-induced platelet shape change may be a positive cooperative or "threshold" type response.


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