Platelet adhesion to collagen is inhibited by adenosine diphosphate but unaffected by cell shape.

The effect of limited platelet activation, in the absence of aggregation, on the subsequent ability of rabbit platelets to adhere to collagen was studied in vitro. ADP at concentrations that initiate shape change ( greater than 0.01 micro M) reduce platelet adhesion. Shape change per se however, was not responsible since the time-dependence of the effect of ADP on shape change and adhesion is different and ADP induces reduced adhesion even when shape change is prevented with PGE or fixatives. Platelets shape-changed without ADP addition, e.g. by chilling or by low ethanol concentrations, display normal adhesion to collagen. It appears likely that upon binding to the platelet ADP induces a time-dependent alteration in the membrane, akin to refractoriness, that influences binding sites for collagen. The effect of ADP can be blocked by prior addition of AMP but not if the additions are reversed. The implications of the present finding for platelet adhesion studies are discussed.