Inhibition of platelet aggregation by activation of platelet intermediate conductance Ca²⁺ -activated potassium channels

Background: Within the vasculature platelets and endothelial cells play crucial roles in hemostasis and thrombosis. Platelets, like endothelial cells, possess intermediate conductance Ca²⁺ -activated K⁺ (IK_Ca ) channels and generate nitric oxide (NO). Although NO limits platelet aggregation, the role of IK_Ca channels in platelet function and NO generation has not yet been explored.

Objectives: We investigated whether IK_Ca channel activation inhibits platelet aggregation, and per endothelial cells, enhances platelet NO production.

Methods: Platelets were isolated from human volunteers. Aggregometry, confocal microscopy, and a novel flow chamber model, the Quartz Crystal Microbalance (QCM) were used to assess platelet function. Flow cytometry was used to measure platelet NO production, calcium signaling, membrane potential, integrin α_IIb /β₃ activation, granule release, and procoagulant platelet formation.

Results: Platelet IK_Ca channel activation with SKA-31 inhibited aggregation in a concentration-dependent manner, an effect reversed by the selective IK_Ca channel blocker TRAM-34. The QCM model along with confocal microscopy demonstrated that SKA-31 inhibited platelet aggregation under flow conditions. Surprisingly, IK_Ca activation by SKA-31 inhibited platelet NO generation, but this could be explained by a concomitant reduction in platelet calcium signaling. IK_Ca activation by SKA-31 also inhibited dense and alpha-granule secretion and integrin α_IIb /β₃ activation, but maintained platelet phosphatidylserine surface exposure as a measure of procoagulant response.

Conclusions: Platelet IK_Ca channel activation inhibits aggregation by reducing calcium-signaling and granule secretion, but not by enhancing platelet NO generation. IK_Ca channels may be novel targets for the development of antiplatelet drugs that limit atherothrombosis, but not coagulation.