title: Melatonin inhibits endothelial nitric oxide production in vitro.
abstract: Endothelial cell function is a major player on the regulation of both vascular tonus and permeability. Activation of nitric oxide synthase (NOS) by bradykinin is one physiological pathway for the well-known vascular relaxation mediated by endothelial-derived nitric oxide (NO). In this study we investigated if melatonin, which is known to modulate endothelial cell function and NO production in other tissues, is able to impair bradykinin-induced NO production in vitro. Rat microvascular endothelial cells were incubated with fluorescent dyes to detect either NO or Ca2+. In addition, cGMP levels were measured by enzyme immunoassay. We found that while bradykinin (1-100 nm) increased both cytosolic Ca2+ and NO production, melatonin (1 nm) abolished this NO production but not cytosolic Ca2+ elevation. N-acetylserotonin (0.1 and 1 nm) had the same effect, while the selective agonist for MT3 receptors (5-MCA-NAT, 1 nm) had no effect. Moreover, nonselective and MT2-selective antagonists did not alter the effect of melatonin, suggesting that it is not mediated by MT melatonin receptors. A possible direct inhibition of calmodulin was also discarded as melatonin did not mimic the effect of calmidazolium on cytosolic Ca2+. Melatonin also abolished cGMP production induced by 1 microm bradykinin, indicating that the NO downstream effect is impaired. Thus, here we show that melatonin reduces NO production induced by bradykinin by a mechanism upstream to the interaction of Ca2+ -calmodulin with NOS. Moreover, this effect might be the basis of the diurnal variation in endothelial cell function.
authors.affiliation: Departamento de Fisiologia, Instituto de Biociências, Universidade de São Paulo, São Paulo, Brazil.
authors.forename: Eduardo K
authors.initials: EK
authors.lastname: Tamura
authors.forename: Claudia L M
authors.initials: CL
authors.lastname: Silva
authors.forename: Regina P
authors.initials: RP
authors.lastname: Markus
mesh_terms: D000818:Animals; D002478:Cells, Cultured; D042783:Endothelial Cells / Q000378:metabolism; D004730:Endothelium, Vascular / Q000378:metabolism*; D008297:Male; D008550:Melatonin / Q000502:physiology*; D009569:Nitric Oxide / Q000037:antagonists & inhibitors* / Q000096:biosynthesis*; D051381:Rats; D017208:Rats, Wistar
chemical_list: D009569:Nitric Oxide; D008550:Melatonin