PLATELET-ADHESION TO HUMAN VASCULAR ENDOTHELIUM IS MODULATED BY CONSTITUTIVE AND CYTOKINE-INDUCED NITRIC-OXIDE

Objective: The aim was to study whether basal or cytokine stimulated g eneration of nitric oxide (NO) modulates platelet adhesion to human um bilical vein endothelial cells (HUVEC), Methods: The adhesion of In-11 1 labelled human platelets to transfected HUVEC (SGHEC-7) was measured either alone or after incubation of SGHEC-7 cells for 18 h with inter leukin- 1beta (IL-1beta) and/or tumour necrosis factor alpha (TNFalpha ). The activity of NO synthase in these cells was measured by formatio n of citrulline. The effects of dexamethasone (0.3 muM) and N(G)-monom ethyl-L-arginine (L-NMMA, 100 muM) on these two variables were determi ned. Results: Stimulation of SGHEC-7 cells with IL-1beta or TNFalpha ( each at 1-30 ng.ml-1) caused them to express the inducible NO synthase , an effect that was prevented by dexamethasone. Platelet adhesion to unstimulated SGHEC-7 cells was <0.1% (n=3) and was increased to 0.7(SE M 0.2)% by L-NMMA but was not affected by dexamethasone. Stimulation o f the cells with IL-1beta and TNFalpha increased platelet adhesion to a maximum of 2.2(0.4)%. This increase was enhanced by both dexamethaso ne and L-NMMA. The effect of L-NMMA was prevented by L-arginine. Concl usions: Inhibition of NO synthesis by L-NMMA potentiates platelet adhe sion to unstimulated SGHEC-7 cells, showing that basally released NO r egulates platelet adhesion. Stimulation of SGHEC-7 cells by cytokines increases their adhesive properties but at the same time causes them t o express the inducible NO synthase. Nitric oxide generated by this en zyme contributes to the modulation of the adhesive properties of the e ndothelial cells. Thus both constitutive and inducible NO synthases mo dulate endothelial cell thrombogenicity.