beta(2)-adrenoceptors activate nitric oxide synthase in human platelets

Nitric oxide (NO), generated by platelets through stimulation of nitric oxi de synthase (NOS), limits platelet adhesion and aggregation after a prothro mbotic stimulus. Platelet beta-adrenoceptors (beta ARs) mediate inhibition of aggregation, but no direct link has been shown between these receptors a nd platelet adhesion or NO production. We examined NOS activity in human pl atelets from the conversion of L-[H-3]-arginine to L-[H-3]-citrulline, afte r beta AR stimulation or cAMP elevation. Basal NOS activity was 0.11+/-0.03 pmol L-citrulline/10(8) platelets. The beta AR agonist isoproterenol 1 mu mol/L and the adenylyl cyclase activator forskolin 1 mu mol/L each increase d NOS activity, to 0.26+/-0.04 and 0.23+/-0.03 pmol L-citrulline/10(8) plat elets, respectively (P<0.01 for each). Both responses were abolished by the adenylyl cyclase inhibitor SQ22536 50 mu mol/L. NOS activation by isoprote renol or forskolin was not associated with a change in intracellular Ca2+. In functional studies, isoproterenol inhibited U46619-induced platelet aggr egation in a concentration-dependent manner, but this effect was not signif icantly diminished by NOS inhibition. In contrast, thrombin-stimulated plat elet adhesion to cultured human umbilical vein endothelial cell monolayers was inhibited by isoproterenol, and this effect was abolished by NOS inhibi tion (1.3+/-0.2% versus 2.6+/-0.2% respectively; P<0.001). Effects of isopr oterenol on NOS activity, platelet aggregation, and adhesion were mediated exclusively through beta(2)ARs, as determined by coincubation with beta AR subtype-selective antagonists. We conclude that beta(2)ARs activate platele t NOS by increasing cAMP, and that this activation is Ca2+-independent. bet a(2)ARs may contribute to modulation of platelet aggregation and adhesion t o endothelium, and our findings suggest that activation of the L-arginine/N O system mediates the effects of beta(2)ARs on adhesion but not aggregation .