The effect of bilirubin on the surface charge and aggregation tendency of platelets in cord blood

Unbound unconjugated bilirubin markedly increased the negative electrophoretic mobility of washed platelets from cord blood, even at concentration as low as 1.5 mg/dl. The increase in negativity could be abolished by washing the platelets. Aggregation of platelets was observed in parallel with the increase in negativity. These actions of bilirubin required calcium ions, which could not be replaced by magnesium ions. The results suggest attachment of the negative bilirubin molecules onto the platelet surface, probably through calcium ions, leading to platelet aggregation.Plasma components inhibited the actions of bilirubin on platelets up to 19.5 mg/dl bilirubin concentration.The effect of bilirubin on platelets was also investigated in the presence of albumin. When the saturation of albumin with bilirubin was exceeded, platelet negativity increased. However, the fraction of free bilirubin exceeding the albumin saturation point has not the same effect as truly free bilirubin at that concentration in the complete absence of albumin. The results indicate that albumin molecules loosly adsorbed onto the surface may protect the platelets against the attachment of free bilirubin. This protection, however, might be impaired by acidosis, which is frequently combined with hyperbilirubinaemia in the sick newborn. It is suggested that bilirubin could contribute to the haemostatic abnormalities in sick babies by acting on the platelet surface.