Effect of Calcium and magnesium on Receptor Mediated Binding of Fibrinogen by Platelets

When washed human platelets are stimulated by ADP, specific and saturable binding of fibrinogen is rapidly induced, consistent with a discrete cellular receptor. In the absence of divalent cations (<10^-6 M), stimulation of plateles by ADP resulted in a well defined shape change but aggregation occurred only when Ca⁺⁺ or Mg⁺⁺ were added at mM levels. Deaggergation occurred only in the presence of Ca⁺⁺ and apyrase. Binding studies demonstrated that ¹²⁵I-fibrinogen did not interact with platelets in the absence of divalentations. Both Ca⁺⁺ and Mg⁺⁺ supported the ADP induced binding, but approximf~ely half as nany ¹²⁵I-fibrinogen molecules were bound at optimal Mg⁺⁺ as opposed to optimal Ca⁺⁺(1 mM). Ca⁺⁺ and Mg⁺⁺ did not produce actitive bipping; and affinity of the platelet receptor for ¹²⁵I-fibrinogen in the presence of either Ca⁺⁺ or Mg⁺⁺ was similar suggesting induction by both divalent ions of a single class of receptor. Under optimal conditions of ADP and Ca⁺⁺ the maximum number of receptors per platelet differed for individuals with a mean of 49,826 ± 22,321 in 12 normal donors with a range of 17,500 to 82,500. ¹²⁵I-fibrinogen did not dissociate from the platelet unless apyrase and Ca⁺⁺ were present, paralleling the aggregation studies. The binding or dissociation of fibrinogen and platelet aggregation or deaggregation are consistent with the hypotpesis that induction of the fibrinogen receptor may be implicated in ADP dependent platelet aggregation. Supported by NIH grant HL-16411.