Effects of ions on ADP-induced aggregation of bovine or human platelets

Effects of divalent cations on ADP-induced aggregation response were examined. Bovine platelets were transferred by Sepharose 2B gel filtration from citrate-PRP into citrate free buffer (buffer-GFP). Response increases, reaches a maximum and decreases with increasing calcium and/or magnesium concentration. For either calcium or magnesium alone, increasing response is proportional to a rate coefficient and, through an apparent ion-platelet association constant, to the fraction of platelet critical sites bound to cation. With both ions present, bound magnesium appears to inhibit bound calcium in excess of that accounted for by competition and a lower rate coefficient for bound magnesium. With citrate present in buffer-GFP, apparent association constants increase, excess magnesium inhibition is present, but systems are path dependent. Initial conditions appear to establish a response which is thereafter immutable to environmental magnesium alteration. Citrate-PRP resembles buffer-GFP: response is sensitive to the selective removal of calcium and excess magnesium inhibition is present. With heparin-PRP, response is immutable to the selective removal of approximately 90% of initial calcium. The dependency of response inhibition observed at high divalent cation concentrations indicates that aggregation is not due to interplatelet cross linking by ions. Ion effects are similar for bovine and human platelets.