Background-Selective inhibitors of cyclooxygenase-2 (Cox-2) are reported to cause cardiovascular side effects in patients at risk. However, direct proof of prothrombotic effects of these drugs is lacking. We investigated in the microcirculation in vivo whether selective inhibition of Cox-2 induces platelet activation. Methods and Results-The behavior of fluorescence-labeled human platelets was studied in hamster arterioles (dorsal skinfold chamber) by intravital microscopy. Transient platelet-vessel wall interactions (PVWIs), firm platelet adhesion to the vessel wall, and vessel occlusion after FeCl 3 -induced wall injury were analyzed as platelet activation parameters.In vitro experiments in human umbilical vein endothelial cells (HUVECs) were performed to assess specific effects of Cox-2 inhibition on platelet adhesion under shear stress (16 dyn/cm 2 ) and on endothelial release of 6-ketoprostaglandin (PG) F 1␣ . Selective inhibition of Cox-2 (NS-398, 0.5 mg/kg) increased platelet adhesion to the vessel wall in vivo (11.9Ϯ3.9 platelets/mm 2 ; controls, 1.4Ϯ1.4 platelets/mm 2 , PϽ0.05) and platelet adhesion after ADP stimulation in vitro. PVWIs were significantly enhanced in NS-398 -treated animals, which were reduced by platelet pretreatment with aspirin (5 mg/kg) or iloprost (1 nmol/L). Inhibition of Cox-2 reduced levels of 6-keto-PGF 1␣ in vivo and in HUVEC supernatants. Time to occlusion after vessel wall injury was significantly shortened by NS-398 (125.4Ϯ13.6 seconds in NS-398 -treated animals versus 270.8Ϯ46 seconds in controls; PϽ0.01). Conclusions-Selective inhibition of Cox-2 reduces 6-keto-PGF 1␣ endothelial release, increases PVWIs, and increases firm platelet adhesion in hamster arterioles. Moreover, it leads to faster occlusion of damaged microvessels. Thus, selective inhibition of Cox-2 may trigger thrombotic events by diminishing the antiplatelet properties of the endothelium.

The arteriolar endothelium in vivo continuously releases a CYP2C9-derived EDHF. This EDHF exerts its effects by hyperpolarization of platelets through activation of K(Ca) channels and reduction of platelet adhesion molecule expression, indicating that hyperpolarization reduces platelet activation. This demonstrates that EDHF is part of the antithrombotic properties of healthy endothelium in vivo.

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