NADPH‐induced contractions of mouse aorta do not involve NADPH oxidase: A role for P2X receptors

Abstract

Reactive oxygen species elicit vascular effects ranging from dilatation due to hydrogen peroxide-mediated opening of K+ channels, to contraction arising from superoxide (O2−) -dependent inactivation of endothelium-derived nitric oxide. Given that NADPH oxidases are major sources of O2− in the vascular wall, this study examined the effects of exogenous NADPH, a substrate of these enzymes, on O2− generation (lucigenin) and vascular tone (isometric tension) in mouse isolated aortic rings. NADPH caused concentration-dependent increases in O2− generation and vascular tone. However, while oxidised NADP+ was unable to support O2− production, it was equally as effective as reduced NADPH at stimulating contraction. An NADPH oxidase inhibitor, diphenyleneiodonium, markedly attenuated NADPH-induced O2− production, yet had no effect on contractions to NADPH. In contrast, two P2X receptor antagonists, PPADS and NF023, markedly attenuated both endothelium-dependent and -independent contractions to NADPH, as did the P2X desensitising agent, α,β-methylene-ATP. Importantly, α,β-methylene-ATP had no effect on O2− production induced by NADPH. In conclusion, these findings suggest little role for NADPH oxidase-derived O2− in the contractile effects of NADPH in the mouse aorta. Rather, NADPH acts as an agonist at two P2X receptor populations; one located on the endothelium and the other on smooth muscle layer.