Adrenaline and adenosine diphosphate-induced platelet aggregation require shape change. Importance of pseudopods.

The requirement of platelet shape change for early platelet aggregation measured from the disappearance of singlet platelets is compared for ADP and adrenaline. Platelet shape change is measured from pseudopod formation or from a change in main body volume or axial ratio. In the absence of shape change, platelets have a smooth discoid form and are referred to as discocytes. Citrated platelet-rich plasma prepared at 37 degrees C contains approximately 65% discocytes and approximately 35% discoid platelets, which possess pseudopods. The addition of ADP causes all platelets to change shape (pseudopods axial ratio and body volume) and 80% to 90% platelet aggregation. On the other hand, adrenaline causes only an approximately 15% increase in shape-changed platelets (pseudopods only) and approximately 33% platelet aggregation, in the presence of apyrase to minimize the effects of released ADP. Measurements of the shape of the nonaggregated platelets in the presence and absence of ongoing platelet aggregation induced by either ADP or adrenaline indicate that discocytes are not recruited into aggregates. For both ADP and adrenaline, the extent of platelet aggregation exceeds the extent of platelet shape change as measured from the disappearance of discocytes. However, in both cases, platelet aggregation does not exceed the total number of shape-changed platelets (i.e., pseudopods plus newly shape-changed discocytes), indicating that the pseudopods are also participating in platelet aggregation. Our results suggest that pseudopod formation is a common requirement for ADP- and adrenaline-induced platelet aggregation and is of greater importance than either changes in main body volume or axial ratio.