Melatonin inhibits vasopastic action of hydrogen peroxide in human umbilical artery

We evaluated the antioxidant property of melatonin as it relates to the vasospastic effect of hydrogen peroxide (H2O2) on the human umbilical artery. Helical sections of umbilical arteries were obtained from healthy pregnant women who were delivered between weeks 37 and 39 of gestation. Changes in maximal potassium chloride (KCl)‐induced tension were measured in arterial segments with intact endothelium. Segments were treated with H2O2 alone, or were pretreated either with an H2O2scavenger (catalase, 2000 IU), a hydroxyl radical scavenger (mannitol, 10−2M), a nitric oxide‐synthesis inhibitor (L‐NG‐monomethyl arginine, LNMA, 2 × 10−4M), or melatonin (10−6M to 10−4M). The effect of H2O2(10−4M) on the relaxation induced by the calcium ionophore A23187 was also determined in arterial segments, with or without pretreatment with melatonin (10−6M, 10−4M). H2O2 (10−6 M to 10−4 M) potentiated vascular tension in a concentration‐dependent manner (P < 0.0001). Pretreatment with LNMA significantly suppressed the vasospastic effect of H2O2 (P < 0.0001). Pretreatment with either catalase or mannitol significantly reduced the vasospastic effect of H2O2 (P < 0.005, P < 0.002, respectively). Melatonin also significantly reduced the vasospastic effect of H2O2 in a concentration‐dependent manner (H2O2 10−6 M, P < 0.0001: H2O2 10−5 M, P