Summary
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 ~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.
