Tempol and vasoconstriction of isolated vessels in vitro
In this issue of the journal, Shastri et al. report on the effects of tempol on the vasoconstrictor response to angiotensin II in vitro . Together with his colleagues, Shailesh Shastri, whose premature death during the completion of this study must be mourned, demonstrated that the superoxide dismutase mimetic, tempol, attenuated the exaggerated vasoconstrictor response to angiotensin II of spontaneously hypertensive rat vessels. The effects of tempol were specific for angiotensin II and were not observed with other vasoconstrictors. Finally, the action of tempol was observed only with an intact endothelium, and was abolished by pretreatment with a nitric oxide synthase inhibitor . Shastri et al. focus on different vasoconstrictors, differences between rat strains and superoxide generation.
Here, we will draw attention to the complexity of the interaction of angiotensin II with endothelial mediators. According to the type of vessel studied, the endothelial mediators induced by angiotensin II may include superoxide anions [2,3] and, under some circumstances, the vasoconstrictor action of angiotensin II may even be mediated by the endothelium [4,5].
Angiotensin II and superoxide anions
During the last 5 years, the role of superoxide in mediating some effects of angiotensin II has increasingly gained recognition [6,7]. The generation of superoxide anions, presumably by a membrane-bound NADH/NADPH oxidase system [3,7] but possibly by other enzyme systems as well , contributes to the vasoconstriction  as well as to the trophic effects  induced by angiotensin II. Furthermore, superoxide anions may play a key role in the pro-inflammatory actions of the octapeptide [8,9]. In-vivo experiments suggested that stimulation of superoxide production is not a general phenomenon caused by hypertension per se but is dependent on the presence of angiotensin II . The type 1 angiotensin II receptor, AT1, mediates superoxide production [6,11]. The type 2 receptor, AT2, may oppose the effects of AT1 . In the vasculature, angiotensin II can induce superoxide production in all cell types, including adventitial fibroblasts , vascular smooth muscle cells  and endothelial cells [3,11]. Angiotensin II supplied from the plasma, or generated in the vascular wall , could thus induce superoxide production in the endothelium, or in the smooth muscle layer (Fig. 1).
Superoxide anions and endothelial vasodilators
Superoxide may inactivate the vasodilator action of nitric oxide by binding it, as discussed by Shastri et al. . Peroxynitrite which may be formed by the nitric oxide–superoxide reaction, may also inhibit the action of other endothelial vasodilators (e.g. prostaglandin I2) . Superoxide may interfere with other signal transduction pathways as well , even in the absence of nitric oxide . Although the binding of nitric oxide is certainly an important component of the vasoconstrictor action of superoxide anions (Fig. 1), this may not be the ‘end of the story'. The superoxide anion may be considered as an endothelial mediator in its own right [2,3]. The biological effects of superoxide and its mechanism of action are still a focus of current research. For the purpose of our discussion, it is important to understand that the vascular effects of superoxide anions are not necessarily restricted to scavenging nitric oxide.
However, some caveats should also be considered when interpreting in-vitro studies, such as the one performed by Shastri et al. . For example, the artificial buffer which lacks haemoglobin, proteins and many other potential radical scavengers, together with the high oxygen tension induced by gassing the buffer, could conceivably lead to increased superoxide levels. The experimental setup might lead to an overestimation of the role of superoxide anions. Furthermore, the endothelial nitric oxide synthase itself may switch to production of superoxide anions if certain factors are lacking . Whether or not such a ‘switch’ occurs in the settings used by Shastri et al.  is unknown. Finally, the authors studied angiotensin II-induced vasoconstriction under conditions which allowed the occurrence of tachyphylaxis, a rapid desensitization to the agonist that is most likely an in-vitro artifact and which can be avoided by using preconstricted vessels . Therefore, the possibility remains that superoxide anions affect the desensitization to the effects of angiotensin II rather than the vasoconstrictor action itself.
Modulation of the effects of angiotensin II by the endothelium
In isolated rings of the aorta, the most widely used model to study endothelium-dependent vasomotor effects, endothelial nitric oxide (and possibly additional endothelial vasodilators as well) buffers angiotensin II-induced vasoconstriction, as described by Shastri et al. . The endothelium-dependent effects of tempol are explained by the increased bioavailability of nitric oxide after tempol administration ameliorating the pressor effects of angiotensin II, but only if the endothelium and the endothelial nitric oxide synthase are present (Fig. 1). Their experiments with endothelium denudation and endothelial nitric oxide synthase inhibition certainly support this notion . Furthermore, recent in-vivo studies suggest that this mechanism also occurs in the intact animal .
However, a quite different interaction between angiotensin II and the endothelium may occur in other vessels. Chen et al.  described that, in the rat tail artery, the contractile response to angiotensin II was suppressed in the absence of the endothelium. The authors ascribed the endothelium-dependent, angiotensin II-induced vasoconstriction to the formation of endothelin-1, a mechanism which was also described in mesenteric vessels . We recently reported that the angiotensin II-induced vasoconstriction in the isolated, perfused rat hindquarter, which accounts for one- quarter of the total peripheral resistance, also depended on an intact endothelium . Our unpublished observations indicate that neither endothelin-1, nor thromboxane accounted for this endothelium-dependent vasoconstriction. We hypothesize that endothelial release of superoxide anions  (Fig. 1) may contribute to the pressor effects of angiotensin II in this important vascular bed.
These considerations emphasize the increasing complexity of the interaction of peptide hormones with local vasoactive mediators. In some vessels, tonic generation of nitric oxide by the endothelium buffers the pressor effects of angiotensin II. In turn, the peptide may induce production of superoxide anions which inactivate nitric oxide. This effect of superoxide anions is now well documented but may only be a part of the entire picture. In other vessels, or under pathophysiological conditions, endothelial mediators, presumably including superoxide anions, may promote the vasoconstrictor actions of angiotensin II.
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