Weak vasoconstrictor activity of melatonin in human umbilical artery: relation to nitric oxide-scavenging action

We evaluated the nitric oxide (NO)-scavenging property of melatonin, demons trated in a recent in vitro study, on vascular reactivity in the human umbi lical artery. Helical sections of human umbilical artery were prepared foll owing elective Cesarean deliveries near term. Changes in maximal tension in duced by prostaglandin F-2 alpha (5 X 10(-5) M) were measured in artery sec tions with an intact endothelium. Melatonin at concentrations higher than 1 0(-6) M increased prostaglandin F-2 alpha-induced vascular tension. The vas ospastic effect of melatonin was much less than that of L-N-G-monomethylarg inine (L-NMA, 2 X 10(-4) M), an inhibitor of NO synthesis (2.8 +/- 1.4%, 9. 1 +/- 1.7%, 16.5 +/- 2.5%, and 29.6 +/- 5.9% of the L-NMA effect at melaton in concentrations of 10(-8), 10(-7), 10(-6), and 10(-5) M, respectively). R emoval of the mdothelium significantly reduced the vasoconstrictive effect of melatonin. Treatment with.-NMA (2 x 10(-4) M) prior to addition of prost aglandin F-2 alpha also significantly reduced the vasoconstrictive effect o f melatonin (10(-5) M). Treatments with melatonin (10(-5) M) did not affect calcium ionophore A 23187-induced relaxation or 5-hydroxytryptamine-induce d constriction. The findings indicate that melatonin may potentiate vascula r tension in human umbilical artery by scavenging endogenous endothelial NO , but not by inhibiting NO synthesis. However, the NO-scavenging vasoconstr ictive effect of melatonin may be negligible at physiologic concentrations and very weak at pharmacologic concentrations below 10(-7) M. (C) 2001 Else vier Science B.V. All rights reserved.