Characteristics of the synergistic actions of platelet agonists.

Platelets are activated by many different agonists that act synergistically. Since there is a characteristic pattern of responses to each agonist, and since there is a clear distinction between weak and strong agonists, understanding the nature of the synergism and its physiologic significance requires characterization of the pattern of responses to the synergistic action of the various agonists. Shape change, aggregation, and secretion of ATP by human platelets in citrated plasma were analyzed after activation by ADP, epinephrine, arachidonic acid, gamma-thrombin, or collagen, either singly or in pairs. The patterns of responses were characteristic of the agonist in higher concentration relative to its threshold concentration; if neither was clearly higher, the pattern of responses was intermediate between the responses characteristic of each agonist. No combination of weak agonists had the characteristics of a strong agonist. These results help define the extent to which platelet responses can be attributed to the synergistic actions of weak agonists.