Role of neutral endopeptidase and angiotensin-converting enzyme in deoxycorticosterone acetate-salt hypertension

The deoxycorticosterone acetate (DOCA)-salt hypertensive rat represents a volume-dependent, low renin model of hypertension. Therefore, inhibitors of the renin–angiotensin system are not very effective in lowering blood pressure in these animals. A number of studies have demonstrated that neither treatment with angiotensin-converting enzyme (ACE) inhibitors nor AT1 receptor antagonists lowered blood pressure in DOCA-salt hypertensive rats [1–3]. In contrast, inhibition of neutral endopeptidase reduced blood pressure in these animals indicating that the activity of the enzyme is crucial for maintaining high blood pressure in these rats [1,4]. Neutral endopeptidase is a protease that is involved in the metabolism of several peptides, some of which are crucially involved in cardiovascular control, such as bradykinin, endothelin and the natriuretic peptides atrial natriuretic peptide (ANP), brain natriuretic peptide and C-natriuretic peptide. Similarly, ACE participates in the degradation of peptides, such as bradykinin or substance P, in addition to the conversion of angiotensin I to angiotensin II. To evaluate the cardiovascular effects of inhibitors of neutral endopeptidase and ACE, it is important to understand the role of neutral endopeptidase and ACE in the metabolism of these peptides in different target organs. To date, no information was available on the importance of neutral endopeptidase compared to ACE in small resistance vessels derived from DOCA-salt hypertensive rats. This question is addressed by Pu et al. [5] in this issue of the journal. In their study, the authors investigated the contribution of neutral endopeptidase and ACE to the antihypertensive and vascular actions of the dual neutral endopeptidase/ACE inhibitor, omapatrilat, in DOCA-salt hypertensive rats. The effects of omapatrilat were compared with those of the selective neutral endopeptidase inhibitor, CGS 25462, and the ACE inhibitor, enalapril. As expected, the ACE inhibitor had no effect on blood pressure in DOCA-salt hypertensive rats, although the dose used was effective in lowering blood pressure in other animal models of hypertension, such as in spontaneously hypertensive rats (SHR). Moreover, enalapril did not alter the structure and the function of small mesenteric resistance vessels. In contrast, both the neutral endopeptidase inhibitor and omapatrilat markedly prevented the rise in blood pressure, improved vascular remodelling (e.g. decreased media-to-lumen ratio and collagen deposition) and increased the relaxation of small mesenteric arteries to acetylcholine, indicating an improved vascular function. These results strongly suggest that neutral endopeptidase inhibition and not ACE inhibition mediated these effects of omapatrilat in DOCA-salt hypertensive rats. The effects of omapatrilat and the neutral endopeptidase inhibitor CGS 25462 on vascular remodelling and endothelial function in small mesenteric resistance vessels may be explained by their blood pressure-lowering effects rather than by a direct action of these drugs. In this respect, it is predictable that the ACE inhibitor, which did not alter blood pressure in DOCA-salt hypertensive rats, will fail to exert similar effects. However, it is often difficult to distinguish between blood pressure dependent and blood pressure independent, direct vascular actions of the particular drugs. With respect to ACE inhibitors, several studies have demonstrated cardiovascular effects which are independent of their blood pressure-lowering action. For example, in rats made hypertensive by aortic banding, ACE inhibition prevented the development of left ventricular hypertrophy. This effect was independent of blood pressure reduction and was abolished by bradykinin B2 receptor blockade [6,7]. Furthermore, treatment over 1 year with the ACE inhibitor, ramipril, prevented myocardial fibrosis without blood pressure reduction (7). In SHR and stroke-prone SHR, long-term ACE inhibition improved cardiac function and metabolism, increased cardiac prostacyclin generation and enhanced the production of nitric oxide (NO) in compliance vessels [8–10]. All these effects were achieved with doses of the ACE inhibitor which did not lower blood pressure. Moreover, these blood pressure-independent effects of ACE inhibition appeared to be related to the potentiation of endogenous bradykinin actions, as they were inhibited by bradykinin B2 receptor blockade. In contrast, structural changes in mesenteric arteries of SHR, such as a decrease in media-to-lumen ratio or a decrease in the number of smooth muscle cell layers, were closely associated with the antihypertensive actions of the ACE inhibitors [11]. Blood pressure-independent cardiac, renal and cerebral effects of inhibitors of the renin–angiotensin system have also been demonstrated in DOCA-salt hypertensive rats [2,3]. Treatment of DOCA-salt hypertensive rats with captopril reversed the increased expression of collagen I as well as the deposition of perivascular and interstitial collagen [3]. The ACE inhibitor further improved myocardial function by attenuating the increased diastolic stiffness. Similar results were obtained with the AT1 receptor antagonist, candesartan cilexetil, and with spironolactone, suggesting that inhibition of the renin–angiotensin–aldosterone system rather than a potentiation of bradykinin accounts for the ACE inhibitor effects in these studies. In a further study, ACE inhibition partially prevented myocardial remodelling by attenuating the wall-to-lumen ratio and perivascular fibrosis of small coronary arteries and decreasing myocardial fibrosis without affecting blood pressure [2]. In addition, the ACE inhibitor decreased the expression of ACE, the AT1 receptor and type I collagen and increased the expression of endothelial NO synthase in the left ventricle. Myocardial NO production in ACE inhibitor-treated DOCA-salt hypertensive rats was also enhanced. Treatment with enalapril and candesartan cilexetil, both prevented renal damage and brain oedema measured after 3 weeks of treatment. Again, the effects of the inhibitors were not dependent on blood pressure reduction. In contrast, in the study by Pu et al. [5], ACE inhibition not only failed to lower blood pressure in DOCA-salt hypertensive rats, but was also unable to exert organ protective actions, such as improving endothelial dysfunction or preventing vascular hypertrophy and fibrosis. The reasons for the discrepancies observed in these studies in DOCA-salt hypertensive rats may be related to differences in the sensitivity of cardiac tissue and small coronary arteries to ACE inhibition compared to small mesenteric resistance vessels. Furthermore, the failure of ACE inhibition to exert organ protective effects in mesenteric arteries from DOCA-salt hypertensive rats might be explained by differences in the regulatory function of bradykinin in this vascular bed compared to other animal models of hypertension, such as SHR and aortic banding rats. Indeed, the role of bradykinin in the cardiovascular control in DOCA-salt hypertensive rats appears to be controversial. A depressed endogenous kinin system in DOCA-salt hypertensive rats has been suggested based on the finding that the rise in blood pressure in response to bradykinin receptor blockade was markedly reduced compared to control rats [12]. Furthermore, kinin concentrations in arterial plasma were lower in DOCA-salt hypertensive rats compared to control rats, probably as a result of an enhanced kinin degradation [12]. Kinin infusion can produce a hypotensive action in DOCA-salt hypertensive rats. However, the kinin levels in plasma which were necessary to achieve these effects were 110-fold higher than those attained following ACE inhibitor treatment [13]. Therefore, the ACE inhibitor-induced kinin potentiation is not sufficient to contribute to blood pressure reduction in these rats. On the other hand, chronic blockade of bradykinin B2 receptors in DOCA-salt hypertensive rats facilitated the development of hypertension, suggesting that endogenous kinins limit the DOCA-salt-induced increase in blood pressure [14]. This conclusion is also supported by the finding that blood pressure increased more rapidly in kinin-deficient rats (Brown Norway Katholiek rats) than in control rats [15]. As mentioned above, neutral endopeptidase is involved in the degradation of bradykinin and ANP. The effects of both bradykinin and ANP are mainly mediated by an increase in the intracellular cGMP levels involving different pathways. Bradykinin stimulates NO production leading to a stimulation of soluble guanylate cyclase, while ANP activates the guanylate cyclase-coupled natriuretic receptors A and B. Moreover, ANP may also increase NO production [16]. In this respect, it is important to note that the guanylate cyclase-cGMP system which mediates the effects of NO in the vasculature is not impaired in DOCA-salt hypertension. This can be concluded from studies in different vascular preparations, such as cremaster muscle arterioles and coronary vasculature of isolated hearts, demonstrating that the vasodilatory effects of sodium nitroprusside were not impaired in DOCA-salt hypertensive rats [17,18]. The study by Pu et al. [5] demonstrates that the same holds true for mesenteric resistance vessels [5]. However, the basal production of NO by the vascular endothelium may be impaired in some vascular beds in DOCA-salt hypertensive rats, as suggested by a reduced constriction of cremaster muscle arterioles and a decreased coronary vasoconstriction in response to blockade of the NO synthase [17,18]. Differences may exist between different vascular beds regarding the responsiveness to the NO-dependent vasodilation in response to acetylcholine. In mesenteric arteries, Pu et al. [5] observed an impaired relaxation in response to acetylcholine, suggesting the presence of endothelial dysfunction in DOCA-salt hypertensive rats. In contrast, in arterioles derived from the cremaster muscle, the vasodilatory responses to acetylcholine were not altered [17]. In conclusion, the study of Pu et al. in the DOCA-salt hypertensive rat strongly suggests that the inhibition of neutral endopeptidase is mainly responsible for the blood pressure-lowering action of the dual neutral endopeptidase/ACE inhibitor, omapatrilat. Furthermore, neutral endopeptidase inhibition, but not ACE inhibition, mediates the beneficial effects on the structure and function of small mesenteric resistance vessel in these rats. Although the ACE inhibitory part of the dual neutral endopeptidase/ACE inhibitor contributes neither to the antihypertensive action nor to the prevention of vascular remodelling of small resistance vessels, it may well add to the prevention of cardiac and renal damage in DOCA-salt hypertension. Therefore, in this particular model of hypertension, dual neutral endopeptidase/ACE inhibition offer advantages over selective neutral endopeptidase or ACE inhibition alone.