Factors Affecting the Observation of a Synergistic Response in Platelet Aggregation Induced by Pairs of Excitatory Agonists

When aggregation is measured as the disappearance of single platelets synergistic interaction between excitatory agonist pairs can be observed using washed platelets in a modified Tyrode's medium or platelet-rich plasma anticoagulated with hirudin; but not using citrated platelet-rich plasma. For aggregation induced by the ADP/adrenaline agonist pair, both the observation of synergistic interaction and the sensitivity of the platelets to these agonists, is a function of extracellular [Ca2+]. Synergistic interaction and reduced sensitivity to the individual agonists, especially adrenaline, is observed when extracellular [Ca2+] > 100 µM. The data suggest that lower affinity binding of Ca2+ to the glycoprotein IIb/IIIa complex may modulate platelet sensitivity to these excitatory agonists. The conditions used to resuspend the platelets also influences the nature of the response to the ADP/adrenaline agonist pair and the sensitivity of the platelets to these agonists. A synergistic response and/or reduced sensitivity to ADP is observed on resuspension in modified Tyrode's medium but does not occur on resuspension in citrated plasma or in plasma anticoagulated with hirudin. The factor responsible for enhancing sensitivity, and hence abolishing the synergistic response, is a species of low molecular weight (Mr less than 25 KDa). It is neither citrate nor Ca2+