Kinetics of Adp-Induced Human Platelet Shape Change: Apparent Positive Cooperativity

Platelet shape change from smooth dlscocytes to more spherical forms ulth pseudopods (eehlnocytes) induced by adenosine diphosphate CADP) has been regarded to be a receptor mediated process following “Mlchaelis-Henton” kinetics. Here we examine the kinetics of ADP-induced platelet shape change over a wide concentration range. ADP (<10 ul) was injected into human citrated platelet rich plasma, stirred at 1000 rpm, 37°C, pH 7.4, and initial velocity of shape change (V) determined as eithera) initial slope of the decrease In light transmission monitored by an aggregometer, orb) the fraction of eehlnocytes determined by microscopic observation of PRP fixed 5 sees after ADP addition with 4/3% glutaraldehyde (4 vols). Both methods gave essentially the same results (for a plot of V-l versus [ADP]^-1): (i) for [ADP] = 0.4-20 μM, the plot is approximately linear and yields K’ (apparent Michaelis-Menton constant = 4.7 ± 2.1 × 10 × M (14 trials), a value in agreement with that observed by other investigators, and (ii) for [ADP] < 4 × 10^-4M included, the plot Is non-linear, while a Hill plot yields a Hill coefficient, N_H, = 1.9 + 0.3. N_H was shown to be independent of plasma factors, of platelet number, and did not arise from contributions due to echinocyte to discocyte reversion. These results Indicate that ADP-induced platelet shape change may be either a positively cooperative or threshold-type process.