Calcium regulation of the platelet membrane glycoprotein IIb-IIIa complex

Platelet membrane glycoproteins (GP) IIb and IIIa form a Ca2+-dependent, heterodimer complex (GP IIb-IIIa) in detergent solution. To determine whether these glycoproteins were complexed or dissociated in intact human platelets, 125I-labeled whole platelets were lysed with an EDTA/Triton X-100 buffer, which stabilized both the complexed and dissociated forms of GP IIb and GP IIIa. The percentage of the heterodimer complex present after lysis was determined by quantitating the sedimentation of GP IIIa in sucrose gradients. The distribution of GP IIIa in sucrose gradient fractions from unlabeled platelets was detected by incubation of 125I-concanavalin A with Western blots. It was determined that GP IIb and GP IIIa existed predominantly as the GP IIb-IIIa complex in either unstimulated (92% complexed) or thrombin-stimulated (90% complexed) platelets. The GP IIb-IIIa complex in unstimulated platelets could be reversibly dissociated by brief (up to 10-min) incubations at 37 degrees C with EDTA, while longer (30-min) incubations with EDTA resulted in irreversible polymerization of GP IIb and GP IIIa. The dissociation of the GP IIb-IIIa complex by EDTA in intact cells was temperature-dependent, and the critical extracellular Ca2+ concentration for dissociation was 10(-5) to 10(-6) M. These results demonstrate that GP IIb and GP IIIa exist as a Ca2+-dependent, heterodimer complex in intact platelets and that treatment of platelets with EDTA can have multiple effects on this complex.