Inhibition of platelet aggregation by activation of platelet intermediate conductance Ca -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 Ca -activated K (IK ) channels and generate nitric oxide (NO). Although NO limits platelet aggregation, the role of IK channels in platelet function and NO generation has not yet been explored.

OBJECTIVES

We investigated whether IK 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 /β activation, granule release, and procoagulant platelet formation.

RESULTS

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

CONCLUSIONS

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