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 μm 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.