ENHANCED ASSOCIATION OF FIBRINOGEN WITH ITS PLATELET RECEPTOR DUE TO SODIUM CITRATE INDICATION FOR ONE FIBRINOGEN RECEPTOR ONLY

Washed platelet suspensions are almost always prepared from blood anticoagulated with sodium citrate. As citrate has been implicated to affect platelet function and as estimates of the number of fibrinogen (Fb^g) bindingsites on platelets range from 4,700 -82,500, we examined the involvement of citrate on the platelet-fibrinogen interaction. The binding of 125-1-fibrinogen to washed platelets from citrated blood (PCB) and thosefrom gel-filtered non-anticoagulated blood (PNB) from the same donor showed linear Scatchard plots for PNB and curvilinear plots for PCB. Assuming the presence of two classes of bindingsites, a high_.affinity site (5,540±760 molecules/ platelets; K_d, 1.29 x 10^-7 M) and low affinity site (32,070 α 6,520 molecules/platelet; K_d 1.02 x 10^-6 M) were determined for PCB. However, PNB indicated a single class of binding sites with 16,480α 2,800 Fbg molecules/platelet.When blood from one donor was collected into 10 mM and 20 mM citrate, increased binding of Fbg was observed onplatelets exposed to 20 mM citrate. The effects of citrate appear to be on theplatelet Fbg receptor since non-specific binding was not affected by the citrate. As no 14-C-citric acid binding to platelets was observed, citrate may affect the receptors without binding to the platelets. The dependence of binding on the pHof thecitrate suggests ionic interactions between platelets and citrate. Different amounts of Fbg were bound when four different preparations of sodium citrate varying in concentration and pH were used. Ourdata suggest that acombination of thedirect effects of citrate on the Fbg bindingof platelets, together with the variable concentrations and pH of the different citrate preparations routinelyused toanticoagulate blood, may explain why some investigators obtain upwardly concave Scatchard plotswhile others obtain linear plots. Ourresultsindicate that the major reasonfor the disparities in the Fbg binding datais due to the effects of citrate on platelets.