PLATELET INHIBITION BY AN L-ARGININE-DERIVED SUBSTANCE RELEASED BY IL-1-BETA-TREATED VASCULAR SMOOTH-MUSCLE CELLS

Experiments were performed to examine whether stimulation of cultured vascular smooth muscle cells by interleukin (IL)-1-beta would induce platelet inhibitory properties of these cells. Incubation of platelets with untreated rat aortic smooth muscle cells had no effect on thrombin-induced platelet aggregation. In contrast, incubation of platelets with IL-1-beta-pretreated smooth muscle cells or the perfusate from such cells resulted in the inhibition of thrombin-induced platelet aggregation. This effect was potentiated by superoxide dismutase and reversed by incubating the IL-1-beta-treated smooth muscle cells with N(G)-nitro-L-arginine (L-NNA) or by treating the platelets with methylene blue. Cytokine-treated smooth muscle cells inhibited thrombin-stimulated changes in platelet cytosolic ionized calcium, whereas untreated cells were without effect. Incubating platelets with IL-1-beta-treated smooth muscle cells resulted in a 10-fold increase in platelet guanosine 3',5'-cyclic monophosphate (cGMP) levels, whereas untreated smooth muscle cells had no effect. The elevation of platelet cGMP induced by the IL-1-beta-treated smooth muscle cells was prevented by exposing the cytokine-treated cells to L-NNA or by treating platelets with methylene blue. Treatment of smooth muscle cells with IL-1-beta also resulted in an eightfold increase in nitrite production, which was blocked when the cells were incubated with L-NNA. The addition of cycloheximide to smooth muscle cells during their incubation with IL-1-beta completely inhibited smooth muscle cell nitrite production, the effects of the smooth muscle cells on platelet cGMP levels, and platelet responses to thrombin. These results demonstrate that vascular smooth muscle cells exposed to inflammatory mediators can release platelet inhibitory nitric oxide.