DIVALENT-CATION REQUIREMENTS FOR AGGREGATION OF HUMAN-BLOOD PLATELETS AND THE ROLE OF THE ANTI-COAGULANT

Aggregation of human platelets induced by ADP or vasopressin is partially inhibited by EGTA in heparinised platelet-rich plasma but completely inhibited in citrated platelet-rich plasma. Secretion induced by these agonists and by adrenaline is completely inhibited by EGTA in either system. Aggregation induced by adrenaline in heparinised platelet-rich plasma is completely inhibited by EGTA but only at higher concentrations of this chelating agent. The effect of EGTA on ADP-induced aggregation in heparinised plateletrich plasma can be potentiated to total inhibition by addition of citrate or other polycarboxylic acids in an order of potency correlating with their affinity for Mg2+. In divalent cation-free platelet preparations prepared by gel filtration Ca2+ is more effective than Mg2+ in restoring the aggregation response to ADP. Our data suggest that at least two different extracellular divalent cation sites participate in the platelet response. One of these sites which is normally occupied by Ca2+ is involved in secretion, and hence secondary aggregation, induced by weak agonists. The other site which is normally occupied by Mg2+ is implicated in the primary aggregation response. © 1979.