Improvement of endothelium-derived hyperpolarizing factor function by renin–angiotensin system inhibition: paving the way towards prevention of age-related endothelial dysfunction

The occurrence of cardiovascular disease, unless of congenital origin, depends on a number of well-known risk factors [1]. Smoking, hypertension, hyperlipidemia and diabetes top the list. Apart from these clearly identified risk factors, age plays a pivotal role. On the one hand, age is a factor that determines the contribution of a risk factor to the development of cardiovascular disease. On the other hand, age also serves as an independent risk factor. Although every researcher in the field of cardiology is aware of the influence of age as a risk factor, it is hard to determine the mechanisms underlying this phenomenon, let alone the extent to which age contributes to cardiovascular disease and how we should intervene in this process. One can easily argue that ageing is associated with a prolonged exposure to many different external and internal risk factors that integrate to an unidentifiable cause for cardiovascular disease. The problem remains that a preventive treatment for cardiovascular disease may not be found because ageing becomes a non-modifiable risk factor. However, downstream of the ageing process, specific mechanisms have been identified that may contribute to cardiovascular disease. Research in this field is important because the identification and characterization of such specific mechanisms might provide an anchor for the development of preventive treatment against cardiovascular disease. Age-related endothelial dysfunction may be the most important reason why ageing is a risk factor. Endothelial dysfunction in itself has been identified as a cause of cardiovascular disease, and it has been suggested that patients may benefit from protection against endothelial dysfunction [2]. Endothelial dysfunction may consist of a broad variety of functional disorders of the endothelial cells but, most frequently it is measured as a loss of endothelium-dependent relaxation (EDR). There is evidence that, during ageing, EDR decreases independent of the known risk factors for endothelial dysfunction [3]. In normotensive subjects, it was shown that relaxations to acetylcholine decrease as age increases. This process, in which oxidative stress may play a role, is accelerated in hypertensive subjects [4]. Insight into the mechanisms for decreased EDR may enhance the development of preventive therapy against age-related cardiovascular disease. This means that a suitable model is needed to research age-related endothelial dysfunction. Such a model must include the basic systems for EDR, i.e. release of nitric oxide (NO), vasodilating prostaglandins and endothelium-derived hyperpolarizing factor (EDHF). The rat seems to be a suitable model to explore these mechanisms, as illustrated in the article by Kansui et al. in this issue of the journal [5]. Furthermore, the results of this and another previous study suggest that inhibition of the renin–angiotensin system (RAS) improves age-related impairment of EDR, independent of a lowering of blood pressure [6]. This is an important contribution to what is known with respect to the beneficial effects of RAS inhibition on endothelial dysfunction. The beneficial effects of RAS inhibition on disease-related endothelial dysfunction is well documented [7]. Inhibition of angiotensin converting enzyme (ACE) promotes the activity of nitric oxide (NO) [8]. The explanations for this phenomenon are straightforward. ACE binds NO, and ACE inhibition may directly increase NO availability [9]. Furthermore, ACE activation decreases the activity of bradykinin, leading to reduced NO release [7]. Moreover, angiotensin II promotes the production of reactive superoxide species that scavenge NO [10]. Finally, there is an inverse relationship between ACE and endothelial NO synthase activity and expression [11]. This knowledge of the relationship between NO production pathways and effects of RAS inhibition may contribute to the design of new therapeutic strategies to improve endothelial dysfunction and reduce the risk for cardiovascular disease. However, next to NO, EDHF may also play an important role. If this is the case, it is also important to search for the mechanisms that influence EDHF activity. The study by Kansui et al. completes the picture of how RAS inhibition leads to improved endothelial function by placing EDHF next to NO. Together with previous work by this group, it has been shown that EDHF is attenuated in both hypertension and aged rats. Moreover, the authors show that EDHF can be improved by RAS inhibition, and that we have the tools available to effectively search for the mechanisms responsible. Unlike its effect on NO, the effect of RAS inhibition on EDHF is less easy to explain. That is not surprising in view of the still unknown chemical identity of EDHF. Nevertheless, Kansui et al. provide an anatomical hypothesis: RAS inhibition decreases subendothelial thickening, thereby improving transfer of EDHF to the vascular smooth muscle cells in the media. However, pharmacological mechanisms may also be postulated to account for the observed inverse relationship between EDHF and RAS activation. There are basically two pharmacological mechanisms through which RAS inhibition can lead to improved EDHF function: altered activity of (i) peptide hormones and (ii) second messenger systems. Alteration of peptide hormone activity may be the simplest explanation for improvement of EDHF by RAS inhibition. Bradykinin and angiotensin-(1-7) are vasodilator hormones which are stimulated by ACE inhibition, and which may improve EDHF activity [12,13]. Furthermore, AT1 receptor antagonists may divert angiotensin II to AT2 receptors, which may be involved in vasodilation [14]. However, there is no evidence for the involvement of AT2 receptors in EDHF release. Alteration of second messenger systems may represent another site of interaction between the RAS and EDHF. EDHF induces vasorelaxation through opening of calcium-activated potassium channels (KCa2+) and, although still controversial, of ATP-sensitive potassium channels (KATP) [15]. Remarkably, angiotensin II inhibits opening of KCa2+ and KATP through AT1 receptor activation [16–18], providing a possible explanation for the effects observed by Kansui et al. However, there was no difference in activity of the KATP opener levcromakalim between the groups. Two explanations can be proposed. First, KATP may indeed not play an important role in the vessel types used by Kansui et al. Second, EDHF-mediated opening of KATP may depend on activation of cofactors that are apparently enhanced by chronic RAS inhibition. Another second messenger that is shared by the RAS and EDHF is phospholipase A2 (PLA2) but, for this signalling system, it is more difficult to determine how EDHF and the RAS interact. PLA2 activation leads to the formation of arachidonic acid that may, amongst others, be used as a substrate for the synthesis of vasodilating or vasoconstricting prostaglandins, and possibly also of EDHF [15]. PLA2 is activated by angiotensin II [15,19]. Strikingly, vasoconstriction induced by angiotenisn II depends on the release of vasoconstrictive prostaglandins (personal observation), and it is conceivable that this is mediated by PLA2 activation. Hypothetically, inhibition of angiotensin II may unmask an EDHF effect by increasing the availability of PLA2 for EDHF production. In agreement with this suggestion, Kansui et al. found that indomethacin, an aspecific inhibitor of prostaglandins, increases responses to acetylcholine. Hence, their results suggest the release of vasoconstrictive prostaglandins, and a decrease of this release due to chronic RAS inhibition. In summary, the interaction between EDHF and the RAS may result from changes in peptide hormone activity, such as increase of bradykinin or alteration of angiotensin peptide function. Alternatively, second messenger systems such as K+ channels and PLA2 may play a role. Of course, the suggested effects on anatomical barriers, as proposed by Kansui et al. could also contribute. As described above, there are multiple, putative interaction sites for EDHF and the RAS. Future studies should provide more evidence for such interactions in age-related endothelial dysfunction. Insight into these interactions may also lead to new therapies for cardiovascular diseases that depend on endothelial dysfunction, such as heart failure. Another point that deserves attention is whether the observed effect on EDHF is limited to RAS inhibition. Although combined treatment with hydralazine and the antidiuretic, hydrochlorothiazide, did not change age-related decrease of EDHF function [6], there are other antihypertensive compounds that need to be tested. While an obvious candidate is the group of β-adrenergic receptor antagonists, a more challenging one is cholecalciferol. This vitamin D3 derivative lowers blood pressure and induces recovery of impaired KCa2+ channels in hypertensive rats [20]. Hence, it appears to be an attractive drug for treatment of age-related endothelial dysfunction. In summary, age is an important cause of the development of endothelial dysfunction and, hence, cardiovascular disease. Drugs that restore EDHF function, such as RAS inhibitors, may alleviate age-related endothelial dysfunction. Whether these compounds can be used for prevention of age-related cardiovascular disease remains to be established but, clearly, important steps have been made in that direction. Presently, however, the only preventive measure is to adopt a life style that avoids the remaining risk factors.