Prostacyclin rather than nitric oxide lowers human umbilical artery tone in vitro

This study was designed to determine vasodilator activities of two endothelium-derived relaxing factors: prostacyclin (PGI₂) and nitric oxide (NO) in human umbilical arteries. Isolated vessel segments were contracted by submaximal concentrations of serotonin and bradykinin. These contractions were enhanced after inhibition of prostaglandin formation by the cyclooxygenase inhibitor indomethacin and after removal of the endothelium, both resulting in a pronounced decrease in PGI₂ formation. Contractions remained unchanged after treatment of the vessels with nitro-l-arginine, a selective inhibitor of endogenous NO biosynthesis. The efficacy of inhibition of NO biosynthesis was established by a more than 60% reduction in cyclic GMP accumulation. Even inhibition of stimulated NO formation by histamine did not change vascular tone. These data suggest that PGI₂ rather than NO is an endothelium-derived relaxing factor in human umbilical arteries.