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 generation of nitric oxide (NO) modulates platelet adhesion to human umbilical vein endothelial cells (HUVEC), Methods: The adhesion of In-111 labelled human platelets to transfected HUVEC (SGHEC-7) was measured either alone or after incubation of SGHEC-7 cells for 18 h with interleukin- 1beta (IL-1beta) and/or tumour necrosis factor alpha (TNFalpha). The activity of NO synthase in these cells was measured by formation of citrulline. The effects of dexamethasone (0.3 muM) and N(G)-monomethyl-L-arginine (L-NMMA, 100 muM) on these two variables were determined. 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(SEM 0.2)% by L-NMMA but was not affected by dexamethasone. Stimulation of the cells with IL-1beta and TNFalpha increased platelet adhesion to a maximum of 2.2(0.4)%. This increase was enhanced by both dexamethasone and L-NMMA. The effect of L-NMMA was prevented by L-arginine. Conclusions: Inhibition of NO synthesis by L-NMMA potentiates platelet adhesion to unstimulated SGHEC-7 cells, showing that basally released NO regulates platelet adhesion. Stimulation of SGHEC-7 cells by cytokines increases their adhesive properties but at the same time causes them to express the inducible NO synthase. Nitric oxide generated by this enzyme contributes to the modulation of the adhesive properties of the endothelial cells. Thus both constitutive and inducible NO synthases modulate endothelial cell thrombogenicity.