Modulation of platelet shape and membrane receptor binding by Ca²⁺–calmodulin complex

Abstract

Interaction between factor VIII/von Willebrand factor, a plasma glycoprotein, and platelets is essential for platelet adhesion and subsequent platelet plug formation at sites of vascular injury during primary phase haemostasis^1,2. We now report that binding of ¹²⁵I-labelled factor VIII/von Willebrand factor to its platelet membrane receptors is inhibited by preincubation of washed human platelets with inhibitors of Ca²⁺–calmodulin complex (for example, 50 µM trifluoperazine or chlorpromazine) at 37 °C for 2 min. Scatchard analysis of the binding data indicates that the total number of accessible binding sites on each pretreated platelet is significantly reduced to ∼50% of the original value. There is also negative cooperativity of binding or reduced binding affinity in a major portion of binding sites. Platelet shape changes from diskoid to spherical after treatment. The results suggest that Ca²⁺–calmodulin complex modulates platelet shape and membrane receptor behaviour.