Stabilization of platelet-fibrinogen interactions: Modulation by divalent cations

The binding of fibrinogen to its GPIIb-IIIa receptor is divalent-cation dependent. In addition to Ca⁺² and Mg⁺², Mn⁺² has been shown to modulate adhesive protein interactions with integrins. This study examined the effect of Mn⁺² on fibrinogen interactions with intact platelets. Compared with that of control platelets in buffer containing 1 mmol/L Mg⁺², fibrinogen binding to adenosine diphosphate- or thrombin-stimulated platelets decreased 23% ± 12% and 15% ± 9% (mean ± SD, n = 4), respectively, after addition of 1 mmol/L Mn⁺². No change in binding affinity was noted, but the stability of platelet-fibrinogen interactions was diminished markedly. Ethylenediaminetetraacetic acid dissociated 68% ± 8% of fibrinogen bound to ADP-treated platelets (p < 0.05) during a 60-minute incubation with fibrinogen and 1 mmol/L Mn⁺², compared with 40% ± 13% of fibrinogen bo und to control platelets and 29% ± 8% of fibrinogen bound in the presence of Ca⁺² (mean ± SD, n = 6). Mn⁺² also diminished the stabilization of fibrinogen interaction with thrombin-stimulated platelets and inhibited the recovery of bound fibrinogen with the Triton X-100 (Union Carbide Corp., Danbury, Conn.) insoluble cytoskeleton. Only 31% ± 10% of fibrinogen bound to thrombin-stimulated platelets for 60 minutes in the presence of Mn⁺² associated with the cytoskeleton (p < 0.05), compared with 61% ± 14% and 75% ± 20% of fibrinogen bound to control platelets incubated with and without Ca⁺², respectively. Mn⁺² further inhibited large adenosine diphosphate- or thrombin-induced platelet aggregate formation and reduced the ability of platelets to retract fibrin clots. These data suggest that Mn⁺² alters GPIIb-IIIa function relative to native fibrinogen and support a role for the stabilization of platelet-fibrinogen interactions in platelet aggregation and clot retraction.