HEMOSTASIS, THROMBOSIS, AND VASCULAR BIOLOGY Reversible inhibition of the platelet procoagulant response through manipulation of the Gardos channel

The platelet procoagulant response requires a sustained elevation of the intracellular Ca2 concentration, [Ca2 ]i, causing exposure of phosphatidylserine (PS) at the outer surface of the plasma membrane. An increased [Ca2 ]i also activates Ca2 -dependent K channels. Here, we investigated the contribution of the efflux of K ions on the platelet procoagulant response in collagen-thrombin–activated platelets using selective K channel blockers. The Gardos channel blockers clotrimazol, charybdotoxin, and quinine caused a similar decrease in prothrombinase activity as well as in the number of PS-exposing platelets detected by fluorescence-conjugated annexin A5. Apamin and iberiotoxin, inhibitors of other K channels, were without effect. Only clotrimazol showed a significant inhibition of the collagen-plus-thrombin–induced intracellular calcium response. Clotrimazol and charybdotoxin did not inhibit aggregation and release under the conditions used. Inhibition by Gardos channel blockers was reversed by valinomycin, a selective K ionophore. The impaired procoagulant response of platelets from a patient with Scott syndrome was partially restored by pretreatment with valinomycin, suggesting a possible defect of the Gardos channel in this syndrome. Collectively, these results provide evidence for the involvement of efflux of K ions through Ca2 -activated K channels in the procoagulant response of platelets, opening potential strategies for therapeutic interventions. (Blood. 2006;108: 2223-2228)