Evidence of increased association of fibrinogen with platelets caused by sodium citrate.

Washed platelet suspensions are almost always prepared from blood anticoagulated with sodium citrate. Because citrate has been reported to affect platelet function, we examined the involvement of citrate on the platelet-fibrinogen interactions. More iodine 125-labeled fibrinogen was bound on washed platelets from citrate-anticoagulated blood (CP) than on washed platelets from non-anticoagulated blood (PNB) obtained by the rapid gel filtration of native blood from the same donor. Scatchard analyses of the equilibrium binding data gave linear plots for PNB indicating a single class of binding sites with 16,480 +/- 2,800 fibrinogen molecules bound per platelet (Kd 5.6 x 10(-7) mol/L); CP gave curvilinear plots that, when resolved into two components assuming two classes of binding sites, gave a high-affinity site (4,950 +/- 970 molecules per platelet; Kd 1.29 x 10(-7) mol/L) and a low-affinity site (25,660 +/- 4,600 molecules per platelet; Kd 1.02 x 10(-6) mol/L). When blood from one donor was collected into 10 mmol/L and 20 mmol/L citrate, increased binding of 125I-fibrinogen was observed on platelets exposed to 20 mmol/L citrate. Exposure of PNB to varying citrate concentrations showed enhanced fibrinogen binding with increasing citrate concentrations; differences in the Kd values between non-citrate-treated PNB and citrate-treated PNB were apparent at about 7.5 mmol/L of citrate. The effects of citrate in increasing the association of fibrinogen with platelets were not caused by variations in the pH; although fibrinogen binding was diminished at low pH of the citrate used, more binding was observed in the presence of citrate than with buffer of the same pH. The effects of citrate appear to be on the platelet fibrinogen receptor because nonspecific binding was not affected by the citrate. Inasmuch as no carbon 14-labeled citric acid binding to platelets was observed, citrate may affect the platelet-fibrinogen interactions without binding to the platelets. We conclude that platelet exposure to citrate increases the fibrinogen binding and may lead to the appearance of curved Scatchard plots.