Roles of the two active sites of somatic angiotensin-converting enzyme in the cleavage of angiotensin I and bradykinin: insights from selective inhibitors

Somatic angiotensin-converting enzyme (ACE) contains two homologous domains, each bearing a functional active site. The in vivo contribution of each active site to the release of angiotensin II (Ang II) and the inactivation of bradykinin (BK) is still unknown. To gain insights into the functional roles of these two active sites, the in vitro and in vivo effects of compounds able to selectively inhibit only one active site of ACE were determined, using radiolabeled Ang I or BK, as physiological substrates of ACE. In vitro studies indicated that a full inhibition of the Ang I and BK cleavage requires a blockade of the two ACE active sites. In contrast, in vivo experiments in mice demonstrated that the selective inhibition of either the N-domain or the C-domain of ACE by these inhibitors prevents the conversion of Ang I to Ang II, while BK protection requires the inhibition of the two ACE active sites. Thus, in vivo, the cleavage of Ang I and BK by ACE appears to obey to different mechanisms. Remarkably, in vivo the conversion of Ang I seems to involve the two active sites of ACE, free of inhibitor. Based on these findings, it might be suggested that the gene duplication of ACE in vertebrates may represent a means for regulating the cleavage of Ang I differently from that of BK.     

Hydrolysis of angiotensin peptides by human angiotensin I-converting enzyme and the resensitization of B2 kinin receptors

We measured the cleavage of angiotensin I (Ang I) metabolites by angiotensin I-converting enzyme (ACE) in cultured cells and examined how they augment actions of bradykinin B2 receptor agonists. Monolayers of Chinese hamster ovary cells transfected to stably express human ACE and bradykinin B2 receptors coupled to green fluorescent protein (B2GFP) or to express only coupled B2GFP receptors. We used 2 ACE-resistant bradykinin analogues to activate the B2 receptors. We used high-performance liquid chromatography to analyze the peptides cleaved by ACE on cell monolayers and found that Ang 1-9 was hydrolyzed 18x slower than Ang I and &30% slower than Ang 1-7. Ang 1-7 was cleaved to Ang 1-5. Although micromol/L concentrations of slowly cleaved substrates Ang 1-7 and Ang 1-9 inhibit ACE, they resensitize the desensitized B2GFP receptors in nmol/L concentration, independent of ACE inhibition. This is reflected by release of arachidonic acid through a mechanism involving cross-talk between ACE and B2 receptors. When ACE was not expressed, the Ang 1-9, Ang 1-7 peptides were inactive. Inhibitors of protein kinase C-alpha, phosphatases and Tyr-kinase blocked this resensitization activity, but not basal B2 activation by bradykinin. Ang 1-9 and Ang 1-7 enhance bradykinin activity, probably by acting as endogenous allosteric modifiers of the ACE and B2 receptor complex. Consequently, when ACE inhibitors block conversion of Ang I, other enzymes can still release Ang I metabolites to enhance the efficacy of ACE inhibitors.     

Comparison of the antihypertensive effects of the new angiotensin II (AT1) receptor antagonist candesartan cilexetil (TCV-116) and the angiotensin converting enzyme inhibitor enalapril in rats.

Antihypertensive effects of an angiotensin (Ang) II receptor antagonist, candesartan cilexetil (TCV-116), were compared with those of an angiotensin converting enzyme (ACE) inhibitor, enalapril, in spontaneously hypertensive rats (SHR), 2-kidney, 1-clip hypertensive rats (2K, 1C-HR) and 1-kidney, 1-clip hypertensive rats (1K, 1C-HR). CV-11974, the active form of TCV-116, had no inhibitory activity for plasma ACE. In rats, TCV-116 inhibited the pressor responses to Ang I, Ang II, and Ang III without an effect on the bradykinin (BK)-induced depressor response. Enalapril inhibited only the Ang I-response and potentiated the BK-response. In SHR, the antihypertensive effect of TCV-116 (10 mg/kg) was larger than the maximum antihypertensive effect of enalapril and was not intensified by combination with enalapril. Administration of CV-11974 potentiated the maximum antihypertensive effect of enalapril. Although both agents reduced blood pressure in 2K, 1C-HR, only TCV-116 had a marked antihypertensive effect in 1K, 1C-HR. These findings indicate that TCV-116 is more effective than enalapril in reducing blood pressure in SHR and 1K, 1C-HR, and that the BK- and/or prostaglandin-potentiating effect of enalapril contributes little to its antihypertensive mechanism in SHR.     

Effect of dietary sodium on angiotensin-converting enzyme (ACE) inhibition and the acute hypotensive effect of enalapril (MK-421) in essential hypertension

The hormonal and hypotensive effects of a single oral 10 mg dose of enalapril (MK-421), were assessed by a double-blind randomized trial in 12 subjects with essential hypertension, during a 100 and 40 mmol/day sodium intake. Peak serum MK-421 appeared 1 h following oral dosage. The bioactive conversion product of MK-421 (the parent diacid MK-422) appeared later, was maximal 4 h following dosage, and was still detectable 24 and 32 h later. Serum angiotensin-converting enzyme (ACE) activity was inhibited maximally at 4 h (by 57 +/- 4% of control activity) and had a similar time course to the serum MK-422 level. Plasma angiotensin II and aldosterone fell during ACE inhibition, but no change in bradykinin was detected. Reciprocal rises in plasma renin and angiotensin I occurred with a similar time course to ACE inhibition. Sodium depletion did not alter drug levels, basal serum ACE nor the time course of its inhibition. The initial blood pressure was however significantly lower when the subjects had been on the 40 mmol/day sodium diet. Blood pressure fell in all subjects and the fall was maximal 4-8 h following MK-421. There was a close correlation between plasma drug level, ACE inhibition and the hypotensive effect. These results suggest that regardless of the final mechanism for the antihypertensive action of MK-421 it is a consequence of its inhibition of 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.     

Effect of atrial natriuretic factor on angiotensin converting enzyme

We studied the effects of atrial natriuretic factor (ANF) on the conversion of angiotensin I to angiotensin II. Pulmonary artery endothelial cells converted 1.22 nmol/min per dish [125I]angiotensin I to angiotensin II, but ANF suppressed the conversion by 0.475 nmol/min per dish. The maximum rate of inhibition of angiotensin converting enzyme (ACE) activity by ANF was 60% at 10(-6) mol/l ANF compared with control conditions. The amino acid fragments of ANF were studied to determine whether its specific structure was necessary for inhibiting ACE in endothelial cells. We found that atriopeptin II (fragment 5-27) inhibited ACE activity as much as ANF. Fragment 7-25 had the same rate of inhibition, but fragment 13-27 had no effect on the conversion of angiotensin I to angiotensin II. These results suggest that the amino acid sequence of Cys (7)-Cys (23) in ANF is important for the inhibition of ACE in pulmonary endothelial cells.     

Differential effect of low-dose thiazides on the renin angiotensin system in genetically hypertensive and normotensive rats

Fifty years since thiazide diuretics were introduced, they are established as first-line antihypertensive therapy. Because the thiazide dosing profile lessened, the blood pressure lowering mechanism may lie outside their diuretic properties. We evaluated this mechanism in spontaneously hypertensive rats (SHR) and normotensive Wistar Kyoto rats (WKY) by examining the effects of low-dose hydrochlorothiazide (HCTZ) administration on renin-angiotensin system components. The 7-day, 1.5 mg/kg per day HCTZ did not change systolic pressure (SBP) in WKY, but decreased SBP by 41 ± 2 mm Hg (P < .0001) in SHR, independent of increased water intake, urine output, or alterations in electrolyte excretion. HCTZ significantly increased the plasma concentrations of angiotensin I (Ang I) and angiotensin II (Ang II) in both WKY and SHR while reducing angiotensin-converting enzyme (ACE) activity and the Ang II/Ang I ratio (17.1 ± 2.9 before vs. 10.3 ± 2.9 after, P < .05) only in SHR. HCTZ increased cardiac ACE2 mRNA and activity, and neprilysin mRNA in WKY. Conversely in SHR, ACE2 activity was decreased and aside from a 75% increase in AT₁ mRNA in the HCTZ-treated SHR, the other variables remained unaltered. Measures of cardiac mas receptor mRNA showed no changes in response to treatment in both strains, although it was significantly lower in untreated SHR. These data, which document for the first time the effect of low-dose thiazide on the activity of the ACE2/Ang-(1-7)/mas receptor axis, suggest that the opposing arm of the system does not substantially contribute to the antihypertensive effect of thiazides.     

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.     

Local angiotensin II and transforming growth factor-beta1 in renal fibrosis of rats

Studies have demonstrated that local angiotensin II (Ang II) generation is enhanced in repairing kidney and that ACE inhibition or AT(1) receptor blockade attenuates renal fibrosis. The localization of ACE and Ang II receptors and their relationship to collagen synthesis in the injured kidney, however, remain uncertain. Using a rat model of renal injury with subsequent fibrosis created with chronic elevations in circulating aldosterone (ALDO), we examined the distribution and binding density of ACE and Ang II receptors in repairing kidneys, as well as their anatomic relationship to transforming growth factor-beta1 (TGF-beta1) mRNA, type I collagen mRNA, collagen accumulation, and myofibroblasts. Two groups of animals (n=7 in each group) were studied: (1) normal rats served as controls, and (2) uninephrectomized rats received ALDO (0.75 microg/h SC) and 1% NaCl in drinking water for 6 weeks. Compared with control rats, in ALDO-treated rats we found (1) significantly (P<0.01) increased blood pressure, reduced plasma renin activity, and increased plasma creatinine levels, (2) diffuse fibrosis in both renal cortex and medulla, (3) abundant myofibroblasts at these sites of fibrosis, (4) significantly increased (P<0.01) binding density of ACE and Ang II receptors (60% AT(1), 40% AT(2)) at the sites of fibrosis, and (5) markedly increased (P<0.01) expression of TGF-beta1 and type I collagen mRNAs at these same sites. Thus, in this rat model of renal repair, the enhanced expression of ACE, Ang II receptors, and TGF-beta1 is associated with renal fibrosis. Ang II generated at the sites of repair appears to have autocrine/paracrine functions in the regulation of renal fibrous tissue formation alone or through its stimulation of TGF-beta1 synthesis.     

Angiotensin Converting Enzyme Inhibition Reduces ACTH Release due to Hypoglycaemia

To investigate the role of Angiotensin II in the release of ACTH, the response of adrenocorticotrophic hormone to hypoglycaemia was studied before and during treatment with an angiotensin converting enzyme inhibitor, enalapril, in 15 male patients with essential hypertension. Plasma levels of ACTH were measured before and 60, 90 and 120 min after an i.v. bolus of normal saline, as placebo, and, 3 days later, after an i.v. bolus of regular insulin (0.15 U/Kg b.w.). Enalapril treatment was then started and both placebo and hypoglycaemic tests were repeated 15 days thereafter. No changes in ACTH plasma levels were observed after acute normal saline either before or during enalapril treatment. On the contrary, hypoglycaemia induced a sharp increase of ACTH before enalapril (from 19.5±4.1 to 74.4±13.0 pg/ml, p < 0.01 60 min after insulin) but not during ACE inhibition (from 26.1±6.2 to 34.6±5.9 pg/ml, NS, at min 60 of the study). The present data confirm our previous observation on the reduction of the hypoglycaemic-induced ACTH release during ACE inhibition with captopril and support the hypothesis that circulating Ang II may exert a facilitating role on adrenocorticotrophic hormone release.     

Angiotensin-converting enzyme inhibition potentiates angiotensin II type 1 receptor effects on renal bradykinin and cGMP

Angiotensin (Ang) receptor blockers (ARBs) increase bradykinin (BK) by antagonizing Ang II at its type 1 (AT(1)) receptors and diverting Ang II to its counterregulatory type 2 (AT(2)) receptors. Because the effect of ARBs on BK is constrained by the short half-life of BK and because ACE inhibitors block the degradation of BK, this study was designed to test the hypothesis that an ACE inhibitor can potentiate ARB-induced increases in renal interstitial fluid (RIF) BK levels. We used a microdialysis technique to recover BK and cGMP in vivo from the RIF of sodium-depleted, conscious Sprague-Dawley rats infused for 60 minutes with the AT(1) receptor blocker valsartan (0.17 mg/kg per minute), with the active metabolite of the ACE inhibitor benazepril (benazeprilate, 0.05 mg/kg per minute), or with the specific AT(2) receptor blocker PD 123,319 (50 microg/kg per minute) alone or combined. Each animal served as its own control. RIF BK and cGMP levels increased significantly over 1 hour in response to valsartan, benazeprilate, or both but not to a vehicle control (P<0.01). The combined benazeprilate-valsartan effect was greater than the sum of their individual effects, suggesting potentiation rather than addition, and was abolished by PD 123,319. We demonstrate for the first time that an ACE inhibitor (benazepril) and an ARB (valsartan) potentiate each other, and we postulate that such combinations may be beneficial in clinical states marked by Ang II elevation, such as chronic heart failure, postinfarction left ventricular dysfunction, and hypertension.     

Angiotensin-converting enzyme inhibition, but not AT(1) receptor blockade, in the solitary tract nucleus improves baroreflex sensitivity in anesthetized transgenic hypertensive (mRen2)27 rats

Transgenic hypertensive (mRen2)27 rats overexpress the murine Ren2 gene and have impaired baroreflex sensitivity (BRS) for control of the heart rate. Removal of endogenous angiotensin (Ang)-(1-7) tone using a receptor blocker does not further lower BRS. Therefore, we assessed whether blockade of Ang II with a receptor antagonist or combined reduction in Ang II and restoration of endogenous Ang-(1-7) levels with Ang-converting enzyme (ACE) inhibition will improve BRS in these animals. Bilateral solitary tract nucleus (nTS) microinjections of the AT(1) receptor blocker, candesartan (CAN, 24?pmol in 120?nl, n=9), or a peptidic ACE inhibitor, bradykinin (BK) potentiating nonapeptide (Pyr-Trp-Pro-Arg-Pro-Gln-Ile-Pro-Pro; BPP9α, 9?nmol in 60?nl, n=12), in anesthetized male (mRen2)27 rats (15-25 weeks of age) show that AT(1) receptor blockade had no significant effect on BRS, whereas microinjection of BPP9α improved BRS over 60-120?min. To determine whether Ang-(1-7) or BK contribute to the increase in BRS, separate experiments using the Ang-(1-7) receptor antagonist D-Ala(7)-Ang-(1-7) or the BK antagonist HOE-140 showed that only the Ang-(1-7) receptor blocker completely reversed the BRS improvement. Thus, acute AT(1) blockade is unable to reverse the effects of long-term Ang II overexpression on BRS, whereas ACE inhibition restores BRS over this same time frame. As the BPP9α potentiation of BK actions is a rapid phenomenon, the likely mechanism for the observed delayed increase in BRS is through ACE inhibition and elevation of endogenous Ang-(1-7).     

The renal antifibrotic effects of angiotensin-converting enzyme inhibition involve bradykinin B2 receptor activation in angiotensin II-dependent hypertension

OBJECTIVE: The renoprotective action of angiotensin I-converting enzyme inhibitors (ACE-Is) is well established, but the role played by bradykinin (BK) remains unclear. We therefore investigated whether an enhanced BK effect on B2 receptor subtype mediated the antifibrotic effect of ACE-Is and whether neutral endopeptidase (NEP) inhibition, which can blunt BK degradation more effectively than ACE inhibition, provided further renoprotection in a rat model of angiotensin (Ang) II-dependent renal damage. METHODS: Five-week-old Ren-2 transgenic rats (TGRen2) received, for 8 weeks, a placebo, ramipril (5 mg/kg body weight) or the dual ACE + NEP inhibitor MDL 100,240 (MDL) (40 mg/kg body weight). After 4 weeks, the B2 receptor antagonist icatibant (0.5 mg/kg body weight) was administered on top of active treatment for 4 weeks to 50% of the TGRen2 rats. Blood pressure was measured weekly by a tail-cuff method and, after sacrifice, kidney weight, glomerular volume, density of glomerular profiles were measured; tubulo-interstitial fibrosis, glomerular and perivascular fibrosis were quantified by histomorphometry. RESULTS: The development of hypertension and tubulo-interstitial fibrosis was prevented by both ramipril and MDL (P = 0.0001 versus placebo); icatibant annulled the latter effect. Glomerular and perivascular fibrosis were unaffected by either ramipril or MDL alone; however, combined treatment with icatibant enhanced glomerular fibrosis (P = 0.0001 versus placebo). CONCLUSION: Enhanced BK effect on B2 subtype receptors is essential for the prevention of tubulo-interstitial fibrosis with ACE or dual ACE + NEP inhibition in TGRen2 rats.     

Potentiation of bradykinin effect by angiotensin-converting enzyme inhibition does not correlate with angiotensin-converting enzyme activity in the rat mesenteric arteries

Angiotensin-converting enzyme (kininase II [ACE]) inhibitors are capable of potentiating bradykinin (BK) effects by enhancing the actions of bradykinin on B(2) receptors independent of blocking its inactivation. To investigate further the importance of ACE kininase activity on BK-induced vasodilation, we investigated the effect of inhibiting ACE, as well as other kininases, on both BK metabolism and vasodilator effect in preparations that exhibit increased ACE activity. Mesenteric arterial beds obtained from 1-kidney, 1-clip hypertensive rats presented augmented ACE and angiotensin I converting activities compared with normotensive rats. The isolated and perfused mesenteric beds were exposed to BK for 15 minutes in the absence or in the presence of kininase inhibitors; then, the perfusate was collected for analysis of the products of BK metabolism by high-performance liquid chromatography. BK was metabolized to the fragments BK(1-8), BK(1-7), and BK(1-5), and the recovery of intact BK was reduced by 47% in the hypertensive group. Recovery of BK was increased in both groups in the presence of a kininase I inhibitor and in the hypertensive group by neutral endopeptidase 24.11 inhibitor; however, ACE inhibition did not affect BK metabolism in both groups. In contrast, only the ACE inhibitor potentiated the vasodilator effect of BK in a mesenteric bed preconstricted with phenylephrine; the increase in BK effect, nevertheless, was not greater in arteries from hypertensive rats that presented an increased ACE activity when compared with those in the normotensive group. These data demonstrated that ACE inhibitor-induced potentiation of BK vasodilator effects is not related to their actions on BK degradation.     

Mechanism for Hypotensive Action of Angiotensin Converting Enzyme Inhibitors

The mechanism(s) for the hypotensive effect of Angiotensin Converting Enzyme (ACE) inhibitors remains elusive. This is because of the multiplicity of the biological actions of angiotensin, the dual role of ACE and the ability of the inhibitors to induce the enzyme. After a single dose of enalapril (MK421), a new ACE inhibitor, in patients with essential hypertension a close linear relationship between the plasma level of enalaprilic acid (MK422) and the degree of ACE inhibition could be demonstrated. Furthermore the degree of ACE inhibition was linearly related to the hormonal changes and to the fall in blood pressure. After chronic administration of enalapril the plasma levels of MK422 were found to be dose dependent. As in the acute study there was also a linear relationship between the plasma level of MK422 and the degree of ACE inhibition. However, the plasma enalaprilic acid level - ACE inhibition dose response curve after chronic administration was shifted to the right, compared to the dose response curve after acute administration suggesting that ACE had been induced during chronic administration of enalapril in humans. There were direct linear relationships between both the degree of ACE inhibition the plasma and enalaprilic acid (MK422) level to the fall in mean arterial pressure. These results suggest that regardless of the final mechanism for the hypotensive action of ACE inhibitors it is a consequence of their inhibition of the enzyme.     

Regional renal haemodynamics of angiotensin II infusion under prostaglandin, kinin or converting enzyme inhibition in the Wistar rat

AIMS: Renal medullary blood flow is important in blood pressure regulation and is surprisingly unaffected by the vasoconstrictor action of angiotensin II (Ang II). This study tested if the effect of Ang II on the renal papillary circulation is modulated by bradykinins, prostaglandins or NO (NO). In anaesthetised Wistar rats, total renal blood flow (RBF) was measured, as was cortical (CBF) and papillary (PBF) blood flow, using the laser-Doppler technique, in responses to Ang II (30 ng kg(-1) min(-1)) alone and after ACE inhibition (enalapril) or bradykinin/prostaglandin synthesis inhibition (ketoprofen, aprotinin). PBF was also measured after blockade of NO formation with or without pretreatment with an Ang II receptor antagonist (losartan). MAJOR FINDINGS: (i) PBF did not change in response to Ang II infusion but MAP increased (+ 10%) and RBF and CBF decreased. (ii) Treatment with aprotinin and ketoprofen left MAP, RBF and CBF unchanged but decreased PBF. Ang II did not decrease PBF further but a significant increase in MAP was seen. (iii) Enalapril treatment left PBF unchanged but decreased MAP and increased RBF and CBF. When Ang II was infused PBF and MAP increased markedly. (iv) L-NAME reduced PBF independently of losartan treatment. PRINCIPAL CONCLUSION: Bradykinin and prostaglandins do not appear to cause the lack of renal papillary vasoconstriction to Ang II. However, the increase in PBF to Ang II seen after enalapril treatment suggests that enalapril treatment, possibly via its effects on kinin breakdown and subsequent NO formation, might affect the sensitivity of renal papillary autoregulation. This may be an important aspect of the blood pressure lowering effect of ACE inhibitors.     

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.     

多巴酚丁胺与环磷酸腺苷葡甲胺或咪利酮联用对实验性心力衰竭大鼠的作用

目的　对比研究实验性心力衰竭 (CHF)大鼠在用多巴酚丁胺治疗之后 ,继用咪利酮或环磷酸腺苷葡甲胺对左心室舒缩功能 ,血浆血管紧张素 (Ang )和红细胞内 Ca2 +的影响。方法　采用腹腔注射阿霉素复制大鼠 CHF模型 ,将 4 0只 CHF大鼠随机分成 (1) CHF组 (2 )多巴酚丁胺治疗组 (3)多巴酚丁胺 +环磷酸腺苷葡甲胺治疗组 (4 )多巴酚丁胺 +咪利酮治疗组 ,另设 (5 )正常对照组。给药治疗 10 d后测定血流动力学指标 ,血浆 Ang 及红细胞内Ca2 +。结果　与正常组相比 ,CHF组 ,左心室压力上升和下降最大速度 (± dp/dtmax) ,左心室收缩压 (L VSP)显著下降 ,左心室舒张末期压 (L VEDP)、等容舒张期左心室压力下降的时间常数 (T)、血浆 Ang 及红细胞内 Ca2 +显著增加 (P均 <0 .0 1)。与 CHF组相比 ,多巴酚丁胺 +环磷酸腺苷葡甲胺组 ,± dp/dtmax,L VSP升高 ,L VEDP降低 (P<0 .0 5～ 0 .0 1) ,血浆 Ang 明显降低 (P<0 .0 5 ) ,红细胞内 Ca2 + 下降。多巴酚丁胺 +咪利酮组 ,+dp/dtmax及 L VSP升高 (P<0 .0 5～ 0 .0 1) ,血浆 Ang 降低 (P<0 .0 5 )。各用药组相比 ,多巴酚丁胺 +环磷酸腺苷葡甲胺组 L VSP,- dp/dtmax,L VEDP,Ang 浓度及红细胞内 Ca2 +与多巴酚丁胺组相比差异有显著性 (P均 <0 .0 5 )。结论　多巴酚丁胺与?     

Left ventricular eccentric remodeling and matrix loss are mediated by bradykinin and precede cardiomyocyte elongation in rats with volume overload.

OBJECTIVES: We hypothesized that left ventricular (LV) remodeling and matrix loss in volume overload (VO) are mediated by bradykinin (BK) and exacerbated by chronic angiotensin-converting enzyme (ACE) inhibition. BACKGROUND: Chronic ACE inhibition increases anti-fibrotic BK and does not attenuate LV remodeling in pure VO. The relative contribution of changes in extracellular matrix versus cardiomyocyte elongation in acute and chronic LV chamber remodeling during VO is unknown. METHODS: Echocardiography, LV collagen content, and isolated cardiomyocytes were studied in rats after aortocaval fistula (ACF) of 12 h, 2 and 5 days, and 4, 8, and 15 weeks. We also studied ACF rats after BK2 receptor (BK2R) blockade (2 days) or ACE inhibition (4 weeks). RESULTS: At 2 days after ACF, LV end-diastolic dimension (LVEDD)/wall thickness was increased, and LV interstitial collagen was decreased by 50% without cardiomyocyte elongation. The BK2R blockade prevented collagen loss and normalized LVEDD/wall thickness. From 4 to 15 weeks after ACF, interstitial collagen decreased by 30% and left ventricular end-systolic (LVES) dimension increased despite normal LVES pressure and isolated cardiomyocyte function. The ACE inhibition did not decrease LVEDD/wall thickness, further decreased LV interstitial collagen, and did not improve LV fractional shortening despite decreased LVES pressure. CONCLUSIONS: Immediately after ACF induction, eccentric LV remodeling is mediated by interstitial collagen loss without cardiomyocyte elongation. Acute BK2R blockade prevents eccentric LV remodeling and improves function. Chronic ACE inhibition does not prevent eccentric LV remodeling or improve function. These findings suggest that ACE inhibitor-mediated increase in LV BK exacerbates matrix loss and explains why ACE inhibition is ineffective in VO.     

Functional importance of angiotensin-converting enzyme-dependent in situ angiotensin II generation in the human forearm

To assess the importance for vasoconstriction of in situ angiotensin (Ang) II generation, as opposed to Ang II delivery via the circulation, we determined forearm vasoconstriction in response to Ang I (0.1 to 10 ng. kg(-1). min(-1)) and Ang II (0.1 to 5 ng. kg(-1). min(-1)) in 14 normotensive male volunteers (age 18 to 67 years). Changes in forearm blood flow (FBF) were registered with venous occlusion plethysmography. Arterial and venous blood samples were collected under steady-state conditions to quantify forearm fractional Ang I-to-II conversion. Ang I and II exerted the same maximal effect (mean+/-SEM 71+/-4% and 75+/-4% decrease in FBF, respectively), with similar potencies (mean EC(50) [range] 5.6 [0.30 to 12.0] nmol/L for Ang I and 3.6 [0.37 to 7.1] nmol/L for Ang II). Forearm fractional Ang I-to-II conversion was 36% (range 18% to 57%). The angiotensin-converting enzyme (ACE) inhibitor enalaprilat (80 ng. kg(-1). min(-1)) inhibited the contractile effects of Ang I and reduced fractional conversion to 1% (0.1% to 8%), thereby excluding a role for Ang I-to-II converting enzymes other than ACE (eg, chymase). The Ang II type 1 receptor antagonist losartan (3 mg. kg(-1). min(-1)) inhibited the vasoconstrictor effects of Ang II. In conclusion, the similar potencies of Ang I and II in the forearm, combined with the fact that only one third of arterially delivered Ang I is converted to Ang II, suggest that in situ-generated Ang II is more important for vasoconstriction than circulating Ang II. Local Ang II generation in the forearm depends on ACE exclusively and results in vasoconstriction via Ang II type 1 receptors.