Ac. Petty et Mc. Scrutton, FACTORS AFFECTING THE OBSERVATION OF A SYNERGISTIC RESPONSE IN PLATELET-AGGREGATION INDUCED BY PAIRS OF EXCITATORY AGONISTS, Platelets, 4(3), 1993, pp. 159-166
When aggregation is measured as the disappearance of single platelets
synergistic interaction between excitatory agonist pairs can be observ
ed using washed platelets in a modified Tyrode's medium or platelet-ri
ch plasma anticoagulated with hirudin; but not using citrated platelet
-rich plasma. For aggregation induced by the ADP/adrenaline agonist pa
ir, both the observation of synergistic interaction and the sensitivit
y of the platelets to these agonists, is a function of extracellular [
Ca2+]. Synergistic interaction and reduced sensitivity to the individu
al agonists, especially adrenaline, is observed when extracellular [Ca
2+] > 100 muM. The data suggest that lower affinity binding of Ca2+ to
the glycoprotein IIb/IIIa complex may modulate platelet sensitivity t
o these excitatory agonists. The conditions used to resuspend the plat
elets 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 o
n resuspension in modified Tyrode's medium but does not occur on resus
pension in citrated plasma or in plasma anticoagulated with hirudin. T
he factor responsible for enhancing sensitivity, and hence abolishing
the synergistic response, is a species of low molecular weight (M(r) l
ess than 25 KDa). It is neither citrate nor Ca2+.