Products of angiotensin I hydrolysis by human cardiac enzymes potentiate bradykinin

Some beneficial effects of ACE inhibitors are attributed to potentiation of bradykinin's actions exerted through its B2 receptor. We investigated them on cultured cells transfected or constitutively expressing both ACE and B2 receptor. The potentiation of bradykinin was indirect and attributed to a crosstalk induced between enzyme and receptor via ACE, a heterodimer formation. While looking for endogenous activators, we investigated the split products of angiotensin I (Ang) Ang 1-9 and 1-7, peptides released by enzymes of human atria and ventricle. Ang 1-9 was liberated by a cathepsin A-type enzyme, Ang 1-7 by a different metallopeptidase-protease. Cathepsin A's presence in heart tissue was shown by deamidating enkephalinamide substrate, by immunoprecipitation and by immunohistochemistry. In immunohistochemistry, cathepsin A was detected in myocytes of atrial tissue. Ang 1-9 and Ang 1-7 potentiated the effect of an ACE-resistant bradykinin analogue on the B2 receptor in transfected cells expressing human ACE and B2, and in human endothelial cells. Ang 1-9 and 1-7 augmented arachidonic acid and NO release by bradykinin. NO liberation by bradykinin from endothelial cells was potentiated at 10nmol/L concentration by Ang 1-9 and Ang 1-7; at higher concentrations, Ang 1-9 was significantly more active. Both peptides had little activity in absence of bradykinin or ACE. Ang 1-9 and 1-7 potentiated bradykinin action on its B2 receptor at much lower concentrations than their IC50 values with ACE. They probably induce conformational changes in the ACE/B2 receptor complex via interaction with ACE.     

Angiotensin-converting enzyme inhibition modifies angiotensin but not kinin peptide levels in human atrial tissue.

Angiotensin-converting enzyme (ACE) converts angiotensin I (Ang I) to angiotensin II (Ang II) and metabolizes bradykinin and kallidin peptides. Decreased Ang II levels and increased kinin peptide levels are implicated in the mediation of the therapeutic effects of ACE inhibition. However, alternative non-ACE pathways of Ang II formation have been proposed to predominate in human heart. We investigated the effects of ACE inhibition on cardiac tissue levels of angiotensin and kinin peptides. High-performance liquid chromatography-based radioimmunoassays were used to measure angiotensin peptides and hydroxylated and nonhydroxylated bradykinin and kallidin peptides in right atrial appendages of subjects who had been prepared for cardiopulmonary bypass. Peptide levels in subjects who received ACE inhibitor therapy were compared with those who did not receive ACE inhibitor therapy. ACE inhibition reduced Ang II levels, which was associated with an 80% reduction in the Ang II/Ang I ratio. ACE inhibition did not modify either bradykinin or kallidin peptide levels or the bradykinin-(1-7)/bradykinin-(1-9) ratio. The 80% reduction in the Ang II/Ang I ratio by ACE inhibition indicated a primary role for ACE in the conversion of Ang I to Ang II in atrial tissue. These data support a role for reduced Ang II levels but do not support a role for increased kinin peptide levels in mediating the direct cardiac effects of ACE inhibition.     

An alternative strategy for the radioimmunoassay of angiotensin peptides using amino-terminal-directed antisera: measurement of eight angiotensin peptides in human plasma

We describe here a method of measuring angiotensin peptides and their carboxy-truncated metabolites in human plasma using N-terminal-directed antisera. Antisera raised against N-acetylated angiotensin (Ang) II and N-acetylated Ang III analogues were used to develop two radioimmunoassays. Extracted plasma samples were acetylated prior to separation of cross-reacting angiotensin peptides by high-performance liquid chromatography (HPLC). Fractions were assayed with both antisera to obtain measurements for eight angiotensin peptides. Angiotensin levels measured in normal males were (fmol/ml plasma, mean +/- s.e.m., n = 14): Ang-(1-7) 1.0 +/- 0.2, Ang II 13.9 +/- 2.0, Ang-(1-9) less than 0.4, Ang I 19.5 +/- 2.4, Ang-(2-7) less than 1.1, Ang III 2.9 +/- 1.0, Ang-(2-9) less than 2.1, Ang-(2-10) 2.4 +/- 0.8. Hypertensive patients receiving angiotensin converting enzyme (ACE) inhibitor therapy (n = 8) had an increase in Ang I to 187.3 +/- 107.2 fmol/ml (P = 0.002), and a reduction in Ang II to 4.8 +/- 1.2 fmol/ml (P less than 0.001). Furthermore, these patients showed a ninefold increase in Ang-(1-7) to 9.7 +/- 4.3 fmol/ml (P less than 0.001), indicating a role for prolylendopeptidase in the metabolism of Ang I in vivo. These N-terminal assays have demonstrated that carboxy-truncated metabolites of Ang I and Ang II make little contribution to plasma angiotensin peptides, except during ACE inhibitor therapy. Furthermore, these antisera allow the measurement of Ang I and Ang II in the same radioimmunoassay of fractions from HPLC, providing a highly reliable estimate of the Ang II:Ang I ratio.     

Angiotensin 1-9 and 1-7 release in human heart: role of cathepsin A

Human heart tissue enzymes cleave angiotensin (Ang) I to release Ang 1-9, Ang II, or Ang 1-7. In atrial homogenate preparations, cathepsin A (deamidase) is responsible for 65% of the liberated Ang 1-9. Ang 1-7 was released (88% to 100%) by a metallopeptidase, as established with peptidase inhibitors. Ang II was liberated to about equal degrees by ACE and chymase-type enzymes. Cathepsin A's presence in heart tissue was also proven because it deamidated enkephalinamide substrate by immunoprecipitation of cathepsin A with antiserum to human recombinant enzyme and by immunohistochemistry. In immunohistochemistry, cathepsin A was detected in myocytes of atrial tissue. The products of Ang I cleavage, Ang 1-9 and Ang 1-7, potentiated the effect of an ACE-resistant bradykinin analog and enhanced kinin effect on the B(2) receptor in Chinese hamster ovary cells transfected to express human ACE and B(2) (CHO/AB), and in human pulmonary arterial endothelial cells. Ang 1-9 and 1-7 augmented arachidonic acid and nitric oxide (NO) release by kinin. Direct assay of NO liberation by bradykinin from endothelial cells was potentiated at 10 nmol/L concentration, 2.4-fold (Ang 1-9) and 2.1-fold (Ang 1-7); in higher concentrations, Ang 1-9 was significantly more active than Ang 1-7. Both peptides had traces of activity in the absence of bradykinin. Ang 1-9 and Ang 1-7 potentiated bradykinin action on the B(2) receptor by raising arachidonic acid and NO release at much lower concentrations than their 50% inhibition concentrations (IC(50)s) with ACE. They probably induce conformational changes in the ACE/B(2) receptor complex via interaction with ACE.     

Angiotensin II binding to angiotensin I-converting enzyme triggers calcium signaling

Angiotensin (Ang) I-converting enzyme (ACE) is involved in the control of blood pressure by catalyzing the conversion of Ang I into the vasoconstrictor Ang II and degrading the vasodilator peptide bradykinin. Human ACE also functions as a signal transduction molecule, and the binding of ACE substrates or its inhibitors initiates a series of events. In this study, we examined whether Ang II could bind to ACE generating calcium signaling. Chinese hamster ovary cells transfected with an ACE expression vector reveal that Ang II is able to bind with high affinity to ACE in the absence of the Ang II type 1 and type 2 receptors and to activate intracellular signaling pathways, such as inositol 1,4,5-trisphosphate and calcium. These effects could be blocked by the ACE inhibitor, lisinopril. Calcium mobilization was specific for Ang II, because other ACE substrates or products, namely Ang 1-7, bradykinin, bradykinin 1-5, and N-acetyl-seryl-aspartyl-lysyl-proline, did not trigger this signaling pathway. Moreover, in Tm5, a mouse melanoma cell line endogenously expressing ACE but not Ang II type 1 or type 2 receptors, Ang II increased intracellular calcium and reactive oxygen species. In conclusion, we describe for the first time that Ang II can interact with ACE and evoke calcium and other signaling molecules in cells expressing only ACE. These findings uncover a new mechanism of Ang II action and have implications for the understanding of the renin-Ang system.     

Effect of reduced angiotensin-converting enzyme gene expression and angiotensin-converting enzyme inhibition on angiotensin and bradykinin peptide levels in mice

There is uncertainty about the contribution of angiotensin-converting enzyme (ACE) to angiotensin II formation, with recent studies suggesting that non-ACE enzymes may be the predominant pathway of angiotensin II formation in kidney, heart, and lung. To investigate the role of ACE in angiotensin II formation, we measured angiotensin I and II levels in blood, kidney, and heart of 2 mouse genetic models (ACE.1 and ACE.4) of reduced somatic ACE gene expression and in blood, kidney, heart, lung, adrenal, and brain of mice administered the ACE inhibitor lisinopril. We also measured the levels of bradykinin (1-9) and its ACE metabolite bradykinin (1-7). Reduced ACE gene expression and ACE inhibition had similar effects on angiotensin and bradykinin peptide levels. Angiotensin II levels were reduced by 70% to 97% in blood, 92% to 99% in kidney, 93% to 99% in heart, 97% in lung, and 85% in adrenal and brain. The marked reductions in angiotensin II/angiotensin I ratio indicated that ACE was responsible for at least 90% of angiotensin I conversion to angiotensin II in blood, kidney, heart, lung, and brain, and at least 77% in adrenal. Blood bradykinin (1-9) levels were increased 6.4-fold to 8.4-fold. Heart bradykinin (1-9) levels were increased in ACE.4 mice and the bradykinin (1-7)/bradykinin (1-9) ratio was reduced in kidney and heart of ACE.4 mice and heart of lisinopril-treated mice. These studies demonstrate that ACE is the predominant pathway of angiotensin II formation in blood and tissues of mice and plays a major role in bradykinin (1-9) metabolism in blood and, to a lesser extent, in kidney and heart.     

New mass spectrometric assay for angiotensin-converting enzyme 2 activity

A novel assay was developed for evaluation of mouse angiotensin-converting enzyme (ACE) 2 and recombinant human ACE2 (rACE2) activity. Using surface-enhanced laser desorption/ionization time of flight mass spectrometry (MS) with ProteinChip Array technology, ACE1 and ACE2 activity could be measured using natural peptide substrates. Plasma from C57BL/6 mice, kidney from wild-type and ACE2 knockout mice, and rACE2 were used for assay validation. Plasma or tissue extracts were incubated with angiotensin I (Ang I; 1296 m/z) or angiotensin II (Ang II; 1045 m/z). Reaction mixtures were spotted onto the ProteinChips WCX2 and peptides detected using surface-enhanced laser desorption/ionization time of flight MS. MS peaks for the substrates, Ang I and Ang II, and the generated peptides, Ang (1-7) and Ang (1-9), were monitored. The ACE2 inhibitor MLN 4760 (0.01 to 100 micromol/L) significantly inhibited rACE2 activity (IC50=3 nmol/L). Ang II was preferably cleaved by rACE2 (km=5 mumol/L), whereas Ang I was not a good substrate for rACE2. There was no detectable ACE2 activity in plasma. Assay specificity was validated in a model of ACE2 gene deletion. In kidney extract from ACE2-deficient mice, there was no generation of Ang (1-7) from Ang II. However, Ang (1-7) was produced when Ang I was used as a substrate. In conclusion, we developed a specific and sensitive assay for ACE2 activity, which used the natural endogenous peptide substrate Ang II. This approach allows for the rapid screening for ACE2, which has applications in drug testing, high-throughput enzymatic assays, and identification of novel substrates/inhibitors of the renin-angiotensin system.     

Selective angiotensin-converting enzyme C-domain inhibition is sufficient to prevent angiotensin I-induced vasoconstriction

Somatic angiotensin-converting enzyme (ACE) contains 2 domains (C-domain and N-domain) capable of hydrolyzing angiotensin I (Ang I) and bradykinin. Here we investigated the effect of the selective C-domain and N-domain inhibitors RXPA380 and RXP407 on Ang I-induced vasoconstriction of porcine femoral arteries (PFAs) and bradykinin-induced vasodilation of preconstricted porcine coronary microarteries (PCMAs). Ang I concentration-dependently constricted PFAs. RXPA380, at concentrations >1 mumol/L, shifted the Ang I concentration-response curve (CRC) 10-fold to the right. This was comparable to the maximal shift observed with the ACE inhibitors (ACEi) quinaprilat and captopril. RXP407 did not affect Ang I at concentrations < or =0.1 mmol/L. Bradykinin concentration-dependently relaxed PCMAs. RXPA380 (10 micromol/L) and RXP407 (0.1 mmol/L) potentiated bradykinin, both inducing a leftward shift of the bradykinin CRC that equaled approximately 50% of the maximal shift observed with quinaprilat. Ang I added to blood plasma disappeared with a half life (t(1/2)) of 42+/-3 minutes. Quinaprilat increased the t(1/2) approximately 4-fold, indicating that 71+/-6% of Ang I metabolism was attributable to ACE. RXPA380 (10 micromol/L) and RXP407 (0.1 mmol/L) increased the t(1/2) approximately 2-fold, thereby suggesting that both domains contribute to conversion in plasma. In conclusion, tissue Ang I-II conversion depends exclusively on the ACE C-domain, whereas both domains contribute to conversion by soluble ACE and to bradykinin degradation at tissue sites. Because tissue ACE (and not plasma ACE) determines the hypertensive effects of Ang I, these data not only explain why N-domain inhibition does not affect Ang I-induced vasoconstriction in vivo but also why ACEi exert blood pressure-independent effects at low (C-domain-blocking) doses.     

Evidence against a major role for angiotensin converting enzyme-related carboxypeptidase (ACE2) in angiotensin peptide metabolism in the human coronary circulation

OBJECTIVE: To investigate the role of angiotensin-converting enzyme-related carboxypeptidase (ACE2) in angiotensin peptide metabolism in the human coronary circulation. METHODS: Angiotensin I and angiotensin II, and their respective carboxypeptidase metabolites, angiotensin-(1-9) and angiotensin-(1-7), were measured in arterial and coronary sinus blood of heart failure subjects receiving angiotensin-converting enzyme (ACE) inhibitor therapy and in normal subjects not receiving ACE inhibitor therapy. In addition, angiotensin I, angiotensin II and angiotensin-(1-7) were measured in arterial and coronary sinus blood of subjects with coronary artery disease before, and at 2, 5 and 10 min after, intravenous administration of ACE inhibitor. RESULTS: In comparison with normal subjects, heart failure subjects receiving ACE inhibitor therapy had a greater than 40-fold increase in angiotensin I levels, but angiotensin-(1-9) levels were low (1-2 fmol/ml), and similar to those of normal subjects. Moreover, angiotensin-(1-7) levels increased in parallel with angiotensin I levels and the angiotensin-(1-7)/angiotensin II ratio increased by 7.5-fold in coronary sinus blood. Intravenous administration of ACE inhibitor to subjects with coronary artery disease rapidly decreased angiotensin II levels by 54-58% and increased angiotensin I levels by 2.4- to 2.8-fold, but did not alter angiotensin-(1-7) levels or net angiotensin-(1-7) production across the myocardial vascular bed. CONCLUSIONS: The failure of angiotensin-(1-9) levels to increase in response to increased angiotensin I levels indicated little role for ACE2 in angiotensin I metabolism. Additionally, the levels of angiotensin-(1-7) were more linked to those of angiotensin I than angiotensin II, consistent with its formation by endopeptidase-mediated metabolism of angiotensin I, rather than by ACE2-mediated metabolism of angiotensin II.     

Enhancement of bradykinin and resensitization of its B2 receptor

We studied the enhancement of the effects of bradykinin B2 receptor agonists by agents that react with active centers of angiotensin-converting enzyme (ACE) independent of enzymatic inactivation. The potentiation and the desensitization and resensitization of B2 receptor were assessed by measuring [3H]arachidonic acid release and [Ca2+]i mobilization in Chinese hamster ovary cells transfected to express human ACE and B2 receptor, or in endothelial cells with constitutively expressed ACE and receptor. Administration of bradykinin or its ACE-resistant analogue desensitized the receptor, but it was resensitized (arachidonic acid release or [Ca2+]i mobilization) by agents such as enalaprilat (1 micromol/L). Enalaprilat was inactive in the absence of ACE expression. La3+ (100 micromol/L) inhibited the apparent resensitization, probably by blocking the entry of extracellular calcium. Enalaprilat resensitized the receptor via ACE to release arachidonic acid by bradykinin at a lower concentration (5 nmol/L) than required to mobilize [Ca2+]i (1 micromol/L). Monoclonal antibodies inhibiting the ACE N-domain active center and polyclonal antiserum potentiated bradykinin. The snake venom peptide BPP5a and metabolites of angiotensin and bradykinin (angiotensin-[1-9], angiotensin-[1-7], bradykinin-[1-8]; 1 micromol/L) enhanced arachidonic acid release by bradykinin. Angiotensin-(1-9) and -(1-7) also resensitized the receptor. Enalaprilat potentiated the bradykinin effect in cells expressing a mutant ACE with a single N-domain active site. Agents that reacted with a single active site, on the N-domain or on the C-domain, potentiated bradykinin not by blocking its inactivation but by inducing crosstalk between ACE and the receptor. Enalaprilat enhanced signaling via ACE by Galphai in lower concentration than by Galphaq-coupled receptor.     

Pathways of hepatic and renal damage through non‐classical activation of the renin‐angiotensin system in chronic liver disease

In liver cirrhosis, renin‐angiotensin system (RAS) activation sustains renal sodium retention and hepatic fibrogenesis. New information has recently enlivened the traditional concept of RAS. For instance, renin and prorenin bind their ubiquitous receptors, resulting in the local production of angiotensin (Ang) II; increased serum calcium and calcimimetic agents, through stimulation of extracellular calcium‐sensing receptors (CaSR), blunt renin production and lead to natriuretic effects in human and experimental cirrhosis. Alongside systemic production, there is Ang II tissue production within various organs through RAS enzymes different from angiotensin‐converting enzyme (ACE), that is chymase, tissue plasminogen activator and several cathepsins. In experimental cirrhosis, inhibition of chymase leads to natriuretic and hepatic antifibrotic effects, without changes in systemic haemodynamics. In the kidney, local RAS coordinates proximal and distal tubular sodium reabsorption. However, renalase, whose plasma and tissue levels are severely altered in experimental cirrhosis, degrades systemic and renal tubule catecholamines, antagonizing the effects of renal RAS. Angiotensinogen‐derived natriuretic and vasodilating peptides (Ang1‐9, Ang1‐7, Ang3‐8) and their receptors have been described. Receptor agonists or antagonists are available to affect portal hypertension and sodium retention in cirrhosis. ACE2‐dependent generation of Ang1‐7 may inhibit experimental liver fibrosis. inhibition of Ang1‐7 clearance by means of neprilysin blockade has portal hypotensive and natriuretic effects. Ang1‐12, whose production renin does not regulate, is converted to several different angiotensin peptides via chymase. Finally, Ang II behaves as either an antinatriuretic or a natriuretic agent, based on the tissue content of AT₁R and AT₂R receptors, their ratio being prone to pharmacological modulation.     

血管紧张素1-7对培养心肌细胞血管紧张素转换酶2基因表达的影响

目的探讨血管紧张素1-7(Ang1-7)对培养心肌细胞血管紧张素转换酶2(ACE2)基因表达的影响。方法将体外原代培养的新生SD大鼠心肌细胞,随机分为对照组和Ang1-7刺激组。测量心肌细胞直径和表面积,用RT-PCR方法检测心肌细胞ACE2mRNA表达,激光共聚焦显微镜检测ACE2蛋白表达部位。结果Ang1-7对心肌细胞直径和表面积没有影响(P>0.05);Ang1-7刺激组较对照组ACE2mRNA表达显著升高(P<0.01);激光共聚焦显微镜显示,ACE2在心肌细胞膜表达。结论外源性Ang1-7对心肌细胞形态无影响,Ang1-7刺激心肌细胞ACE2mRNA表达上调,其意义在于加速血管紧张素Ⅱ降解为Ang1-7,发挥心肌保护效应。     

Olmesartan is an angiotensin II receptor blocker with an inhibitory effect on angiotensin-converting enzyme.

Angiotensin II receptor blockers (ARBs) are widely used for the treatment of hypertension. It is believed that treatment with an ARB increases the level of plasma angiotensin II (Ang II) because of a lack of negative feedback on renin activity. However, Ichikawa (Hypertens Res 2001; 24: 641-646) reported that long-term treatment of hypertensive patients with olmesartan resulted in a reduction in plasma Ang II level, though the mechanism was not determined. It has been reported that angiotensin 1-7 (Ang-(1-7)) potentiates the effect of bradykinin and acts as an angiotensin-converting enzyme (ACE) inhibitor. It is known that ACE2, which was discovered as a novel ACE-related carboxypeptidase in 2000, hydrolyzes Ang I to Ang-(1-9) and also Ang II to Ang-(1-7). It has recently been reported that olmesartan increases plasma Ang-(1-7) through an increase in ACE2 expression in rats with myocardial infarction. We hypothesized that over-expression of ACE2 may be related to a reduction in Ang II level and the cardioprotective effect of olmesartan. Administration of 0.5 mg/kg/day of olmesartan for 4 weeks to 12-week-old stroke-prone spontaneously hypertensive rats (SHRSP) significantly reduced blood pressure and left ventricular weight compared to those in SHRSP given a vehicle. Co-administration of olmesartan and (D-Ala7)-Ang-(1-7), a selective Ang-(1-7) antagonist, partially inhibited the effect of olmesartan on blood pressure and left ventricular weight. Interestingly, co-administration of (D-Ala7)-Ang-(1-7) with olmesartan significantly increased the plasma Ang II level (453.2+/-113.8 pg/ml) compared to olmesartan alone (144.9+/-27.0 pg/ml, p<0.05). Moreover, olmesartan significantly increased the cardiac ACE2 expression level compared to that in Wistar Kyoto rats and SHRSP treated with a vehicle. Olmesartan significantly improved cardiovascular remodeling and cardiac nitrite/ nitrate content, but co-administration of olmesartan and (D-Ala7)-Ang-(1-7) partially reversed this anti-remodeling effect and the increase in nitrite/nitrate. These findings suggest that olmesartan may exhibit an ACE inhibitory action in addition to an Ang II receptor blocking action, prevent an increase in Ang II level, and protect cardiovascular remodeling through an increase in cardiac nitric oxide production and endogenous Ang-(1-7) via over-expression of ACE2.     

Effect of chronic angiotensin converting enzyme inhibition on angiotensin I and bradykinin metabolism in rats

We determined the effect of chronic administration of the angiotensin converting enzyme (ACE) inhibitor, enalapril, on the in vivo pulmonary inactivation of bradykinin (BK) and conversion of angiotensin I (Ang I). In addition we assessed whether chronic ACE inhibition influenced the activity of prolylendopeptidase (PEP), which metabolizes Ang I to generate angiotensin-(1-7) (Ang-[1-7]) and inactivates BK. Male Wistar rats were treated orally with enalapril (10 mg/kg once a day) for 7 to 15 days (n = 20) and 21 to 30 days (n = 11). Vehicle-treated rats (7 to 30 days, n = 11) were used as controls. Pulmonary inactivation of BK and conversion of Ang I were determined in conscious enalapril- or vehicle-treated rats before and after intravenous administration of the ACE inhibitor enalaprilat (MK-422, 10 mg/kg). Pulmonary inactivation of BK (%) was determined by comparing equipotent doses of BK injected by the intravenous and intraaortic routes, and Ang I conversion (%) by comparing the pressor effect of Ang I and Ang II injected intravenously. PEP-like activity in plasma and lung homogenates was determined fluorometrically using the synthetic substrate Suc-Gly-Pro-MCA. In control rats, pulmonary BK inactivation averaged 97.6% ± 0.54%. Acute ACE inhibition with MK-422 reduced BK inactivation to 42.0% ± 2.7%. However, in rats treated chronically with enalapril, BK inactivation was increased as compared with acute ACE inhibition, averaging 58.8% ± 3.7% at 7 to 15 days and 58.8% ± 4.5% at 21 to 30 days of treatment. Intravenous administration of MK-422 to the enalapril-treated rats did not return the increased BK inactivation to the level observed during acute ACE inhibition. In contrast, Ang I conversion was significantly reduced from 46.7% ± 6.5% to 0.9% ± 0.2% by MK-422, and this inhibition remained essentially unchanged during chronic treatment. PEP-like activity in plasma and lung homogenates of control rats was 4.4 ± 0.3 nmol MCA/min/mL and 11.4 ± 0.9 nmol MCA/min/mg protein, respectively. After chronic treatment with enalapril there was a progressive increase of PEP-like activity in both plasma and lung, which after 21 to 30 days of treatment averaged 10.7 ± 1.7 nmol MCA/min/mL and 29.2 ± 2.8 nmol MCA/min/mg protein, respectively. These data indicate that chronic ACE blockade induces alternative BK-inactivating mechanisms and increases Ang-(1-7)-generating mechanisms.     

Renin-angiotensin system revisited

New components and functions of the renin-angiotensin system (RAS) are still being unravelled. The classical RAS as it looked in the middle 1970s consisted of circulating renin, acting on angiotensinogen to produce angiotensin I, which in turn was converted into angiotensin II (Ang II) by angiotensin-converting enzyme (ACE). Ang II, still considered the main effector of RAS was believed to act only as a circulating hormone via angiotensin receptors, AT1 and AT2. Since then, an expanded view of RAS has gradually emerged. Local tissue RAS systems have been identified in most organs. Recently, evidence for an intracellular RAS has been reported. The new expanded view of RAS therefore covers both endocrine, paracrine and intracrine functions. Other peptides of RAS have been shown to have biological actions; angiotensin 2-8 heptapeptide (Ang III) has actions similar to those of Ang II. Further, the angiotensin 3-8 hexapeptide (Ang IV) exerts its actions via insulin-regulated amino peptidase receptors. Finally, angiotensin 1-7 (Ang 1-7) acts via mas receptors. The discovery of another ACE2 was an important complement to this picture. The recent discovery of renin receptors has made our view of RAS unexpectedly complex and multilayered. The importance of RAS in cardiovascular disease has been demonstrated by the clinical benefits of ACE inhibitors and AT1 receptor blockers. Great expectations are now generated by the introduction of renin inhibitors. Indeed, RAS regulates much more and diverse physiological functions than previously believed.     

Angiotensinogen and angiotensin-converting enzyme gene copy number and angiotensin and bradykinin peptide levels in mice

OBJECTIVE: To test the hypothesis that changes in gene expression that may accompany angiotensinogen (AGT) and angiotensin-converting enzyme (ACE) gene polymorphism cause alteration in angiotensin and bradykinin peptide levels. DESIGN: Mice with one or two genes for AGT and ACE allow assessment of the effects of modest alteration in AGT and ACE gene expression on angiotensin and bradykinin peptide levels. METHODS: Angiotensin and bradykinin peptides were measured in the blood, kidney, heart, lung, adrenal, brain, and aorta of mice that were either wild-type (+/+), heterozygous (+/-) or null (-/-) for either the AGT or ACE gene. RESULTS: Angiotensin I and angiotensin II were not detectable in blood or tissues of AGT -/- mice, which had increased bradykinin levels in kidney and lung. ACE -/- mice had markedly reduced angiotensin II levels and increased bradykinin levels in blood and tissues. However, despite reduced AGT and ACE gene expression, angiotensin and bradykinin peptide levels in AGT and ACE +/- mice were no different from the levels in wild-type mice. CONCLUSION: Although the AGT and ACE genes are fundamental determinants of angiotensin and bradykinin peptide levels, compensatory mechanisms attenuate the effect of modest change in AGT and ACE gene expression on the levels of these peptides. Identification of these compensatory mechanisms may provide new candidate genes for investigation in humans.     

ACE2 of the heart: From angiotensin I to angiotensin (1-7)

Angiotensin II (Ang II), a bioactive peptide of the renin-angiotensin system (RAAS), plays an important role in the development of cardiovascular diseases (CVD). Pharmacological inhibition of angiotensin-converting enzyme (ACE), the Ang II forming enzyme, or specific blockade of Ang II binding to angiotensin type 1 receptor (AT1R) through which it exerts its deleterious effects, were shown to provide some protection against progression of CVD. The ACE-Ang II-AT1R axis has been challenged over the last few years with RAAS components able to counterbalance the effects of the main axis. The ACE homologue ACE2 efficiently hydrolyses Ang II to form Ang (1-7), a peptide that exerts actions opposite to those of Ang II. In contrast to the Ang II axis, the role of the ACE2-Ang (1-7) axis in cardiac function is largely obscure. Ang (1-7) is present in the viable myocardium, and its formation depends on Ang II as a substrate. The expression of this peptide is associated with cardiac remodeling: it is lost in the infarcted area and significantly increased in the border area. Low doses of Ang (1-7) improve cardiac output and antagonize Ang II-induced vasoconstriction. The type of Ang (1-7) biological activity is tissue specific and dose dependent. These findings point to a possible protective role for Ang (1-7) in abating the Ang II-induced actions. The elevated expression of Ang (1-7) in failing heart tissue paralleled the expression of its forming enzyme, ACE2. Several observations and experimental evidence suggest a beneficial role for ACE2 in cardiovascular function. Elevated ACE2 expression at the initial stage of several pathologies which decline with progression of disease might indicate a protective role for ACE2. Genetic manipulation of ACE2 expression, either targeted disruption or overexpression, point to the possible significance of this enzyme in cardiac function. Based on the above, a therapeutic approach that will amplify the ACE2-Ang (1-7) axis could provide further protection against the development of CVD. It turns out that the merits of currently used drugs--ACE inhibitors, AT1R blockers and mineralocorticoid receptor blockers (MRB) - lay beyond their direct effects on suppression of the ACE-Ang II-AT1R axis as they also increase cardiac ACE2 and Ang (1-7) significantly.     

Murine Recombinant Angiotensin-Converting Enzyme 2: Effect on Angiotensin II-Dependent Hypertension and Distinctive Angiotensin-Converting Enzyme 2 Inhibitor Characteristics on Rodent and Human Angiotensin-Converting Enzyme 2

A newly produced murine recombinant angiotensin (Ang)-converting enzyme 2 (ACE2) was characterized in vivo and in vitro. The effects of available ACE2 inhibitors (MLN-4760 and 2 conformational variants of DX600, linear and cyclic) were also examined. When murine ACE2 was given to mice for 4 weeks, a marked increase in serum ACE2 activity was sustainable. In acute studies, mouse ACE2 (1 mg/kg) obliterated hypertension induced by Ang II infusion by rapidly decreasing plasma Ang II. These effects were blocked by MLN-4760 but not by either form of DX600. In vitro, conversion from Ang II to Ang-(1-7) by mouse ACE2 was blocked by MLN-4760 (10(-6) m) but not by either form of DX600 (10(-5) m). Quantitative analysis of multiple Ang peptides in plasma ex vivo revealed formation of Ang-(1-9) from Ang I by human but not by mouse ACE2. Both human and mouse ACE2 led to the dissipation of Ang II with formation of Ang (1-7). By contrast, mouse ACE2-driven Ang-(1-7) formation from Ang II was blocked by MLN-4760 but not by either linear or cyclic DX600. In conclusion, sustained elevations in serum ACE2 activity can be accomplished with murine ACE2 administration, thereby providing a strategy for ACE2 amplification in chronic studies using rodent models of hypertension and cardiovascular disease. Human but not mouse ACE2 degrades Ang I to form Ang-(1-9). There are also species differences regarding rodent and human ACE2 inhibition by known inhibitors such that MLN-4760 inhibits both human and mouse ACE2, whereas DX600 only blocks human ACE2 activity.