Regulation of nitric oxide‐like activity by prostanoids in smooth muscle of the canine saphenous vein

1 Organ bath experiments and measurements of prostanoids were performed to investigate the presence of nitric oxide synthase in venous smooth muscle and its interaction with cyclo‐oxygenase. 2 In rings of canine saphenous vein without endothelium, the inhibitor of cyclo‐oxygenase, indomethacin (10 μm), induced contraction. NG‐nitro‐L‐arginine (100 μm) (L‐NOARG), an inhibitor of nitric oxide synthase did not affect the tone of rings of canine saphenous vein when administered alone. However, in the presence of indomethacin L‐NOARG (100 μm) induced further contraction. 3 Similar results were obtained in response to NG‐monomethyl‐L‐arginine (L‐NMMA)(300 μm or NG‐nitro‐L‐arginine methylester (L‐NAME)(100 μm). 4 When rings of canine saphenous vein without endothelium were contracted with phenylephrine (1 μm) instead of indomethacin, neither L‐NOARG or L‐NMMA induced further contraction. 5 When rings of canine saphenous vein without endothelium were contracted with noradrenaline (0.3 μm) in the presence of indomethacin (10 μm) plus L‐NOARG (100 μm), a relaxation to L‐arginine was observed. Transient relaxations to superoxide dismutase (150 u ml−1) were observed in all rings. 6 When rings of saphenous vein without endothelium were incubated with lipopolysaccharide (LPS) (100 μg ml−1) or interleukin −1β (10 u ml−1) the concentration‐contraction curve to noradrenaline was not affected. 7 Rings without endothelium released prostaglandin E2 and prostaglandin I2, as measured by radiommunoassay. The basal production was abolished by indomethacin and not affected by L‐NOARG. 8 These results suggest that when cyclo‐oxygenase is inhibited, a nitric oxide synthase activity is revealed in rings of canine saphenous vein without endothelium.