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(Circulation Research. 2000;87:39.)
© 2000 American Heart Association, Inc.

Cellular Biology

ß₂-Adrenoceptors Activate Nitric Oxide Synthase in Human Platelets

Lindsay R. Queen, Biao Xu, Kazumi Horinouchi, Ian Fisher, Albert Ferro

From the Department of Clinical Pharmacology, Center for Cardiovascular Biology & Medicine, King’s College London, St Thomas’ Hospital, London, UK.

Correspondence to Lindsay R. Queen, Department of Clinical Pharmacology, Center for Cardiovascular Biology & Medicine, King’s College London, St Thomas’ Hospital, Lambeth Palace Road, London SE1 7EH, UK. E-mail lindsay.queen{at}kcl.ac.uk

Abstract—Nitric oxide (NO), generated by platelets through stimulation of nitric oxide synthase (NOS), limits platelet adhesion and aggregation after a prothrombotic stimulus. Platelet ß-adrenoceptors (ßARs) mediate inhibition of aggregation, but no direct link has been shown between these receptors and platelet adhesion or NO production. We examined NOS activity in human platelets from the conversion of L-[³H]-arginine to L-[³H]-citrulline, after ßAR stimulation or cAMP elevation. Basal NOS activity was 0.11±0.03 pmol L-citrulline/10⁸ platelets. The ßAR agonist isoproterenol 1 µmol/L and the adenylyl cyclase activator forskolin 1 µmol/L each increased NOS activity, to 0.26±0.04 and 0.23±0.03 pmol L-citrulline/10⁸ platelets, respectively (P<0.01 for each). Both responses were abolished by the adenylyl cyclase inhibitor SQ22536 50 µmol/L. NOS activation by isoproterenol or forskolin was not associated with a change in intracellular Ca²⁺. In functional studies, isoproterenol inhibited U46619-induced platelet aggregation in a concentration-dependent manner, but this effect was not significantly diminished by NOS inhibition. In contrast, thrombin-stimulated platelet adhesion to cultured human umbilical vein endothelial cell monolayers was inhibited by isoproterenol, and this effect was abolished by NOS inhibition (1.3±0.2% versus 2.6±0.2% respectively; P<0.001). Effects of isoproterenol on NOS activity, platelet aggregation, and adhesion were mediated exclusively through ß₂ARs, as determined by coincubation with ßAR subtype-selective antagonists. We conclude that ß₂ARs activate platelet NOS by increasing cAMP, and that this activation is Ca²⁺-independent. ß₂ARs may contribute to modulation of platelet aggregation and adhesion to endothelium, and our findings suggest that activation of the L-arginine/NO system mediates the effects of ß₂ARs on adhesion but not aggregation. (Circ Res. 2000;87:39-44.)

Key Words: blood platelets • nitric oxide synthase • adrenoceptors • adenosine cyclic monophosphate

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