Activated oxygen metabolites: do they really play a role in angiotensin II-regulated vascular tone?

Angiotensin II, originally described as a potent vasoconstrictor, is now well recognized as having pleitropic actions in multiple organ systems. In the vasculature, angiotensin II, through the AT1 receptor, induces contraction, cell growth, migration, differentiation and secretion [1]. It is also pro-inflammatory and pro-fibrotic and plays a major role in maintaining vascular integrity [2]. At the subcellular level, these processes are highly regulated by complex networks of interacting signalling pathways [1]. Among the numerous signalling molecules involved in angiotensin II-mediated vascular actions, reactive oxygen species appear to play a pivotal role [3,4]. Angiotensin II stimulates production of reactive oxygen species in all vascular cell types, primarily through activation of cell membrane-associated NAD(P)H oxidase [5]. Although xanthine oxidase has also been implicated in vascular generation of free radicals, the contribution of this enzyme appears to be minor. Vascular NAD(P)H oxidase is a neutrophil-like NAD(P)H oxidase comprising, to varying degrees, the major phagocytic NADPH oxidase subunits; cell-membrane-associated p22phox and gp91phox (also known as nox2) or its homologues nox1 or nox4, and cytosolicassociated p47phox, p67phox and p40phox [6,7]. Angiotensin II-activated NAD(P)H oxidase, in the presence of molecular O2 and the substrate NAD(P), generates superoxide anion (dO2 ), which is spontaneously or enzymatically [by superoxide dismutase (SOD)] dismutated to hydrogen peroxide (H2O2). Unlike phagocytic NAD(P)H oxidase, which generates dO2 – extracellulary in a burst-like pattern, vascular NAD(P)H oxidasemediated production is constitutive and inducible and is generated intracellularly in a slow and sustained manner [5]. Whereas dO2 – is highly unstable and cell membrane impermeable, H2O2 is more stable, has a longer half-life and easily crosses cell membranes. In addition to the mitogenic, proinflammatory and profibrotic characteristics of angiotensin II-generated reactive oxygen species, emerging evidence indicates that oxygen-derived molecules regulate vascular reactivity. Studies performed as early as the 1980s reported that H2O2 is a potent vasodilator in cerebral arteries [8]. More recent studies suggest that H2O2 induces vasoconstriction in various vascular beds including rat mesenteric arteries [9], rat aorta [10], porcine pulmonary arteries [11] and canine cerebral arteries [12]. Superoxide has also been shown to induce vasoconstriction in certain vascular beds and to reduce relaxation to endothelium-dependent vasodilators [13]. This may be either the result of nitric oxide (NO) scavenging or the result of a direct contractile effect on vascular smooth muscle [14]. However, not all reports are in agreement with these findings. Indeed, studies of the effects of oxygen free radicals on vascular tone often report contradictory results, as demonstrated by Schuijt et al. [15] in the present issue of the journal. In their study, the role of endogenous reactive oxygen species in contractile responses to angiotensin II was questioned. Studies were performed in isolated human and porcine coronary or femoral arteries in organ baths and the bioavailability of reactive oxygen species was modulated using various pharmacological tools. Under these experimental conditions dO2 – did not play a role in angiotensin IIinduced vasoconstriction and, if anything, it resulted in vasodilation through H2O2 formation. These results support those previously found in cerebral arteries [16], coronary arteries [17] and pulmonary arteries [18], but are in contrast to those of Torrecillas et al. [19], who demonstrated that H2O2 is a critical intracellular mediator in vascular contractile responses to angiotensin II.