Effects of the Reactive Oxygen Species Hydrogen Peroxide and Hypochlorite on Endothelial Nitric Oxide Production

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Abstract
Reactive oxygen species (ROS) hydrogen peroxide (H 2 O 2 ) and hypochlorite (HOCl) participate in the pathogenesis of ischemia/reperfusion injury, inflammation, and atherosclerosis. Both NO and ROS are important modulators of vascular tone and architecture and of adhesive interactions between leukocytes, platelets, and vascular endothelium. We studied the effect of H 2 O 2 and HOCl on receptor-dependent (bradykinin [10 −6 mol/L] and ADP [10 −4 mol/L]) and receptor-independent mechanisms (calcium ionophore A23187 [10 −6 mol/L]) of NO production by porcine aortic endothelial cells (ECs). Changes in the level of EC cGMP (the second messenger of NO) were used as a surrogate of NO production. EC cGMP increased 300% in response to bradykinin and A23187 and 200% in response to ADP. Exposure of ECs to H 2 O 2 (50 μmol/L) for 30 minutes significantly impaired cGMP levels in response to ADP, bradykinin, and the receptor-independent NO agonist A23187. In contrast, preincubation with HOCl (50 μmol/L) impaired cGMP production only in response to ADP and bradykinin but not A23187. These concentrations of H 2 O 2 and HOCl did not result in increased EC lethality as assessed by lactate dehydrogenase release. Neither H 2 O 2 nor HOCl affected EC cGMP production in response to NO donor sodium nitroprusside, which suggests that guanylate cyclase is resistant to these oxidants. We also demonstrated that neither H 2 O 2 nor HOCl affects endothelial NO synthase (eNOS) catalytic activity as measured by conversion of l -arginine to l -citrulline in EC homogenates supplemented with eNOS cofactors. The present studies show that H 2 O 2 impairs NO production in response to both receptor-dependent and receptor-independent agonists and that these effects are due, at least in part, to inactivation of eNOS cofactors, whereas HOCl inhibits NO production by interfering with receptor-operated mechanisms at the level of the cell membrane. Concentrations of H 2 O 2 and HOCl used in the present studies have been shown to be generated in vivo during inflammation and ischemia/reperfusion. Therefore, we infer that these effects of H 2 O 2 and HOCl on EC NO production may contribute to disregulated vascular tone and altered leukocyte-EC interactions that occur in vascular injury as a result of those causes in which ROS generation is involved.

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