Angiotensin I converting enzyme

Hemodynamic and humoral effects of the new renin inhibitor enalkiren in normal humans

The effect of the renin inhibitor enalkiren (Abbott-64662) was evaluated in eight normal volunteer subjects on a standardized sodium diet (100 mmol/day) by measurement of various components of the renin-angiotensin system and drug levels in plasma. On day 1, vehicle and doses of 0.001, 0.003, and 0.01 mg/kg i.v. were administered within 2 minutes at 90-minute intervals. On day 2, vehicle and doses of 0.01, 0.03, and 0.1 mg/kg i.v. were given. With the higher doses, blood pressure tended to decrease slightly with no change in heart rate. Plasma renin activity and plasma angiotensin-(1-8)octapeptide (angiotensin II) fell markedly in a dose-dependent manner. Inhibition of plasma renin activity was maximal 5 minutes after administration of the drug and persisted 90 minutes after the doses of 0.03 and 0.1 mg/kg. Not surprisingly, there was a close correlation between plasma renin activity and plasma angiotensin II levels (r = 0.81, n = 28, p less than 0.001). In contrast, active and total renin measured directly by monoclonal antibodies rose in dose-related fashion in response to renin inhibition. Pharmacokinetic parameters were calculated using the plasma drug concentrations obtained up to 6 hours after the 0.1 mg/kg dose. By means of a two-compartment model, plasma mean half-life of the drug was estimated at 1.60 +/- 0.43 hours.

Thrombin and bradykinin enhance prostaglandin production in human peripheral blood monocytes

Thrombin and bradykinin stimulate production of prostaglandin E2 (PGE2), and 6-keto-prostaglandin F1 alpha (the stable breakdown product of prostacyclin) in isolated human peripheral blood monocytes in a dose- and time-dependent manner. Since PGE2 and prostacyclin can affect the activity of immunocompetent cells and bone resorbing osteoclasts, our finding indicates that thrombin and bradykinin, both of which are formed in inflammatory processes as a consequence of activation of the Hageman factor (coagulation factor XII), may have important roles in the modulation of the inflammatory response and the loss of alveolar bone in periodontitis.

Kinetic and inhibitory characteristics of serum angiotensin-converting enzyme from nine mammalian species

1. Serum angiotensin-converting enzyme activities were obtained from nine mammalian species: rat, mouse, horse, sheep, guinea pig, hamster, rabbit, dog and man. 2. Kinetic constants (Km and Vmax) using hippuryl-L-histidyl-L-leucine as substrate and inhibitory constants (I50 and Ki) for captopril were determined for the serum ACE of each species. 3. There were important differences in the kinetic and inhibitory constants (Kms went from 6.6 mM to 1.21 mM for hamster and guinea pig; I50 ranged from 2100 nM to 3 nM for mouse and sheep) as well as differences in enzyme activity of the different species (values varied from 938 to 13 nmol hippuric acid/ml/min for guinea pig and dog serum).

Involvement of the kallikrein-kinin system in the antihypertensive effect of the angiotensin converting enzyme inhibitors

1. Studies were performed in normal subjects and in rats to assess the effect of angiotensin converting enzyme (ACE) inhibition on the kallikrein-kinin system. As ACE is identical to kininase II, one of the enzymes physiologically involved in bradykinin degradation, bradykinin may be expected to accumulate during ACE inhibition. 2. A competitive antagonist of bradykinin was used to explore in unanaesthetized rats the contribution of circulating bradykinin to blood pressure control under ACE inhibition. 3. No evidence was found for a role of this vasodilating peptide in the blood pressure lowering effect of acute ACE inhibition. 4. The plasma activity of carboxypeptidase N (= kininase I), another pathway of bradykinin degradation, remained intact during a 1 week course of treatment with an ACE inhibitor in normal subjects. This therefore indicates that bradykinin formed during ACE inhibition can still be metabolized.

Serum angiotensin converting enzyme in sarcoidosis--its value in present clinical practice

Angiotensin converting enzyme (ACE) has a central role in blood pressure homeostasis. It is present in low and fairly constant concentration in the serum and in high concentration in the pulmonary capillary bed. Unusually high serum angiotensin converting enzyme (SACE) activity is present in active sarcoidosis, an observation now confirmed by many investigators. In spite of its lack of specificity as a test for sarcoidosis, SACE provides a good monitor of disease activity which clinicians find useful in the management of patients with sarcoidosis. There continues to be considerable interest in SACE in sarcoidosis and with the recent development of simpler assays, more centres may be expected to offer SACE measurements as a service. In this paper we discuss the indications for estimating SACE in sarcoidosis and its relevance to current clinical practice.

Interactions between angiotensin II, sympathetic nerve-mediated pressor response and cyclo-oxygenase products in the pithed rat

1. The influence of angiotensin II (AII) on resting blood pressure and on sympathetic nerve-mediated pressor responses in the pithed rat was investigated either by inhibiting the renin-angiotensin system or by infusing AII. 2. Plasma AII levels in the pithed rat were approximately 20 fold higher than in normotensive rats. 3. Infusion of a subpressor dose of AII (50 ng kg-1 min-1) had no effect on sympathetic nerve mediated pressor responses but a pressor dose of AII, (200 ng kg-1 min-1) facilitated nerve-mediated pressor responses. 4. The angiotensin converting enzyme inhibitor, teprotide, and the AII-receptor antagonist, saralasin, lowered the diastolic blood pressure and attenuated sympathetic nerve-mediated pressor responses. There was no difference in the effects of teprotide at 1 mg kg-1 and 10 mg kg-1. Infusion of sodium nitroprusside at concentrations producing a fall in diastolic blood pressure of similar magnitude to that produced by teprotide and saralasin significantly attenuated nerve-mediated pressor responses. 5. After teprotide, AII 50 mg kg-1 min-1 increased diastolic blood pressure. The inhibitory effect of teprotide on nerve-mediated pressor responses was antagonized by this infusion of AII only if the rats were pretreated with the cyclo-oxygenase inhibitor, flurbiprofen. 6. It is concluded that AII is a major determinant of vascular tone in the pithed rat and that inhibition of the renin-angiotensin system attenuates sympathetic nerve-mediated pressor responses at least in part through the fall in blood pressure per se. The demonstration of this is complicated by an excessive release of vasodilator prostaglandins possibly due to the infused AII. Since plasma All levels are high, the effects of blockade of the renin-angiotensin system will be exaggerated and so the importance of All as a modulator of sympathetic responses will be overestimated in this model.

Comparison of the effects of calcium antagonists and converting enzyme inhibitors on renal function under normal and hypertensive conditions

Calcium antagonists decrease the ability of the kidney to autoregulate renal blood flow (RBF) and glomerular filtration rate (GFR). Therefore, when afferent renovascular resistance is elevated, as in essential hypertension, there is a resultant increase in RBF and GFR with the administration of calcium antagonists. These agents also induce a marked natriuresis because of direct tubular action through unknown mechanisms. The natriuresis can be dissociated from renal and systemic hemodynamic actions, indicating that the decreased sodium reabsorption could override other compensatory mechanisms explaining the absence of sodium retention during the treatment. The renal effects of converting enzyme inhibitors (CEIs) can be explained by the reduction of intrarenal formation in angiotensin II. Because the activation of the renin-angiotensin system is mainly responsible for inducing sodium retention during a decrease in systemic blood pressure, CEIs could have a protecting effect without disturbing other homeostatic mechanisms. CEIs decrease efferent glomerular resistance, reducing capillary pressure and thereby reducing GFR. This effect is not translated in sodium retention because the reduction of GFR is mild during captopril administration in kidneys with normal or increased renal perfusion pressure. At low renal perfusion pressure, the reduced glomerular afferent vasoconstriction can compromise GFR, leading to renal insufficiency. Although these situations are not likely to be encountered during the treatment of uncomplicated essential hypertension, in severe hypertension with hypertrophy of pre-glomerular vessels, glomerular perfusion may decrease. Combination therapy of calcium antagonists and CEIs has been reported to be an effective treatment of severe hypertension. Currently, little information is available on the manner in which renal function is affected by simultaneous administration of both drugs.

Oxidation-induced increase in activity of angiotensin converting enzyme in the rat kidney

The activity of angiotensin converting enzyme(ACE) in crude extracts of the rat renal cortex was increased when the oxidizing agent diamide was added to the extract. The maximal activity was obtained at concentrations over 1 mM, and the value was twice or more the activity in the absence of the pretreatment. The activity of ACE was also increased by the diamide-pretreatment of the isolated membrane fraction of the renal cortex, thereby indicating that the increase in activity was not due to oxidation of endogenous glutathione (GSH) that may lower the ACE activity, but rather that ACE itself was oxidized. When O2 was included in the extract for 2 h, the ACE activity also increased to about twice the original activity. Lineweaver-Burk plots analysis demonstrated that, after oxidation with diamide and O2, the Vmax was increased but the Km remained unchanged. We conclude that the action of ACE in the kidney functions may differ in relation to oxidation of the tissue.

Effects of interrupting the renin-angiotensin system on sodium excretion in man

1. Twelve normal volunteers were studied on 2 separate days, having taken a range of diets providing 50-300 mmol sodium per day for 3 days and having been dehydrated overnight. Each volunteer received a background intravenous infusion of arginine vasopressin (5 x 10(-7) i.u. kg-1 min-1) on both days, and also received 6 mg captopril orally on one day and a placebo tablet on the other. The ensuing changes in arterial pressure, and in urinary solute and solute-free water excretion were recorded. 2. Captopril did not significantly alter arterial pressure. It increased the rate of excretion of sodium but not of potassium, and it did not significantly change urinary osmolality or creatinine clearance. 3. Captopril increased the rate of solute-free water reabsorption and did so in direct proportion to its effect of increasing sodium excretion. 4. A further twelve normal, dehydrated volunteers on free diets were studied on each of 2 days, after taking 500 mg lithium carbonate on the previous evening. On each day, they also received a loading dose and maintenance infusion of inulin. On one day they received 50 mg captopril orally, and, on the other, they received a placebo tablet. The arterial pressure, urinary excretion of electrolytes, and inulin clearance were recorded. 5. Captopril increased the rates of excretion of sodium, lithium and potassium, but there were no significant changes in inulin clearance or arterial pressure. 6. The natriuretic effect of captopril in either group of twelve volunteers was not significantly less in those volunteers who were already excreting more sodium, at least over the range of dietary sodium loading to which the volunteers were subjected. 7. Six normal volunteers were studied on a further 2 days, having taken a diet supplying 30 mmol sodium per day for 3 days and being dehydrated overnight. Each volunteer received a background intravenous infusion of arginine vasopressin (5 x 10(-7) i.u. kg-1 min-1) on both days and also received an intravenous infusion of saralasin acetate (50 ng kg-1 min-1) plus carrier on one day and carrier alone on the other. The ensuing changes in arterial pressure, and in urinary solute and solute-free water excretion were recorded. 8. There was a small but significant fall in systolic arterial pressure during the infusion of saralasin.(ABSTRACT TRUNCATED AT 400 WORDS)

Regulation of angiotensin converting enzyme activity in cultured human vascular endothelial cells

Angiotensin converting enzyme (ACE) of vascular endothelial cells is suggested to control vascular wall tonus through the conversion of angiotensin I (AI) to angiotensin II (AII) and the degradation of bradykinin. To obtain more insight into the pathophysiological significance of ACE of vascular endothelial cells, we studied the regulation of ACE produced by cultured human umbilical vein endothelial cells (EC). Phorbol 12-myristate 13-acetate (PMA) increased the cellular and medium ACE activity, accompanied by a marked morphological change in EC. N'-O'-dibutylyladenosine 3';5'-cyclic monophosphate (db-cAMP) increased only the cellular ACE activity and not the medium ACE activity. The effect of isoproterenol with 0.1mM theophylline mimicked that of db-cAMP. These findings suggest that PMA and cAMP-related agents participate in the control of vascular wall tonus through the positive regulation of ACE produced by vascular endothelial cells.

Changes in the biological activity of autacoids during passage through the human perfused fetoplacental lobule

Changes in the biological activities of a number of autacoids after a single passage through the human perfused fetoplacental lobule have been assessed by bioassay, whilst recording fetal vascular resistance. Bradykinin did not affect vascular resistance, but its biological activity on the superfused bioassay tissues fell by approximately 98%, whereas des-Asp1-angiotensin I activity increased at least 80-fold and the vascular resistance rose. All these effects were inhibited by captopril. Angiotensin II increased vascular resistance but its activity on the bioassay tissues was not changed. 5-Hydroxytryptamine activity was reduced by 67-90% and resistance to flow was not affected. The activities of prostaglandins D2, E2, and F2 alpha were slightly reduced. Prostaglandins D2 and F2 alpha caused vasoconstriction, their maximum effects being greater than those of either of the angiotensins. The TxA2-mimetic U46619 was approximately 90 times more potent than PGF2 alpha, as a vasoconstrictor, but the maximal effects were comparable. Thus, autacoid activity can be reduced, augmented or not affected during passage through the human perfused fetal placental vasculature.

Asthma and diabetes mellitus: a biochemical basis for antithetical features

Diabetes mellitus and asthma have many antipodal features. Although both are common disorders, concurrence occurs less often than would be predicted. When co-existence does occur, the cases are generally mild, and effective treatment of one disease frequently exacerbates the other. The hypothesis is advanced that basilar membrane concentrations of heparan sulfate differ in these two diseases and that this difference may account for the antithetical features. An experimental basis for postulating increased concentrations of extracellular heparan sulfate in asthma and diminished concentrations in diabetes is cited. A rationale for tying these differences to the polar activities of cholinergic transmission and atherogenesis in the two diseases is advanced. Diminished heparan sulfate concentrations in diabetes may down-regulate the transmission of vagal impulses to insulin-producing pancreatic cells, and thereby impair both the continued vitality of these cells, and the acetylcholine modulated potentiation of glucose-induced insulin release.

Serum angiotensin converting enzyme activity in ex-smokers

A low activity of angiotensin converting enzyme (ACE) has been reported in people who smoke. To determine whether this low ACE activity would be reversible on cessation of smoking, we measured serum ACE activity in 107 healthy male volunteers. They included 27 active cigarette smokers, 28 non-smokers, 24 ex-smokers who had stopped smoking for less than 10 yr, and 28 ex-smokers who had stopped smoking for more than 10 yr. The mean value (+/-SD) of serum ACE in those who had stopped smoking for more than 10 yr was comparable to that of non-smokers: 23.2 +/- 5.1 and 23.5 +/- 4.5, respectively. ACE activity in smokers and the ex-smokers who had stopped smoking for less than 10 yr was significantly lower (17.8 +/- 4.5 and 17.8 +/- 3.9, respectively) than values obtained in non-smokers and the group who had not smoked for more than 10 yr (p less than 0.001). These findings suggest that the effect of chronic smoking on the serum ACE activity may be reversible.

Angiotensin I converting enzyme inhibitors containing unnatural alpha-amino acid analogues of phenylalanine

The activity of three angiotensin I converting enzyme (ACE) inhibitors with unique related structures was assessed in vitro and in vivo. The three compounds were (S)(-)-1,2,3,4-tetrahydro-2-(3-mercapto-1-oxopropyl)-3-isoquinoline carboxylic acid (EU-4865), 1,2,3,4-tetrahydro-2-(3-mercapto-1-oxopropyl)-1- isoquinolinecarboxylic acid (EU-4881), and (S)(-)-1,2,3,4-tetrahydro-1-(3-mercapto-1-oxopropyl)-2- quinolinecarboxylic acid (EU-5031). In vitro EU-4881 was a competitive inhibitor that lacked potency (IC50 = 1980 nM) against purified ACE. The other two compounds were equipotent (IC50 = 41 nM) against purified ACE but differed in their inhibition kinetics. EU-4865 (Ki = 38 nM) was a noncompetitive inhibitor, and EU-5031 (Ki = 6.9 nM) was a competitive inhibitor. Against caveolae membrane-bound ACE EU-4881 also lacked potency (IC50 = 2852 nM). In vivo in the conscious acute aortic coarctate (AAC) rat it also lacked potency, having an ED30 (oral dose decreasing blood pressure 30 mmHg) greater than 100 mg/kg. The activity of EU-4865 and EU-5031 in the caveolae membrane-bound ACE and AAC rat reflected their different Ki values rather than their similar IC50 values. In vitro, EU-4865 and EU-5031 had IC50 values of 19 and 6.7 nM, respectively, and in vivo, they had ED30 values of 52 and 1.1 mg/kg, respectively. These results suggest that ACE has a binding requirement for a carboxy-terminus, hydrophobic amino acid that is important for in vivo activity.

The broad substrate specificity of human angiotensin I converting enzyme

Angiotensin I converting enzyme (kininase II; ACE) has been described as a peptidyldipeptidase or dipeptidyl carboxypeptidase (EC 3.4.15.1) of the pulmonary endothelial cells, which liberates angiotensin II or inactivates kinins. However, ACE has a much wider distribution and substrate specifity; it is concentrated in human epithelial cells (e.g. brush border of the kidney, placenta, intestine and choroid plexus), neuroepithelial cells (subfornical organ, pallidonigral dendrites, median eminence) and male genital tract (testes, prostate, epididymides, seminal plasma). Its substrates include enkaphalins, the C-terminal extended proenkephalins and a protected chemotactic tripeptide. Recent, mostly in vitro studies with purified ACE, indicate that ACE also cleaves peptides by other than peptidyldipeptidase action. Homogeneous human ACE inactivated substance P in spite of its blocked C-terminus (Met11-NH2) primarily by releasing the C-terminal tripeptide. A blocked C-terminal tripeptide, Arg-Pro-Gly-NH2 was also released from the luteinizing hormone releasing hormone (LHRH). Although ACE shares many properties with carboxypeptidases, it surprisingly cleaves the N-terminal tripeptide greater than Glu1-His2-Trp3 from LHRH. Because human ACE hydrolyzes a variety of peptide hormones, actions of its inhibitors may go well beyond blocking the conversion of angiotensin I.

The effect of chronic captopril therapy on platelet angiotensin II receptor density and vascular responsiveness to angiotensin II infusion

The density of angiotensin II (Ang II) receptors on the platelet and the vascular responsiveness to infused angiotensin II before and after two weeks of captopril therapy were examined in ten healthy male volunteers. There was a significant increase in blood flow to the forearm, but no significant changes in either the density of angiotensin II receptors or the pressor response to infused angiotensin II following captopril therapy. The study demonstrates that long term reduction of angiotensin II formation by captopril in man does not increase the responsiveness of the receptors to infused angiotensin, nor results in an "up regulation" of the angiotensin receptors. It also provides further evidence that some of the long term vasodilator effects of captopril may be mediated by mechanisms other than inhibition of angiotensin I (Ang I) converting enzyme.

Preclinical studies on angiotensin converting enzyme inhibitors

The identification of the renin-angiotensin-aldosterone system in the control of blood pressure, and the preclinical development of the angiotensin converting enzyme inhibitors for therapeutic use are reviewed. The properties of these compounds are discussed with respect to their in vitro enzyme inhibitory potency; prevention of the pharmacological effects of angiotensin I; potentiation of those of bradykinin; tissue enzyme inhibition; mechanism of effect on blood pressure both alone and in combination with other antihypertensive agents; and effect on cardiac parameters.

Mild but prolonged elevation of serum angiotensin converting enzyme (ACE) activity in alcoholics

Serum activity of angiotensin converting enzyme (ACE) was serially measured in 47 hospitalized chronic alcoholics with liver disease. Compared to healthy controls, ACE activity, on admission, in the serum of alcoholics was significantly elevated (42.5 +/- 16.6 U/ml vs. 32.4 +/- 9.6 U/ml; p less than 0.005). About 36% of the patients had an elevated ACE level exceeding an upper normal value of 42 U/ml (mean +/- SD). In contrast to the rapid normalization of such enzymes as aspartate transaminase (AST), alanine transaminase (ALT) and lactic dehydrogenase (LDH) which represent parenchymal liver cell injury, the activity of ACE remained elevated over a period of 4 weeks even with abstinence. The serum level of ACE was significantly correlated with levels of alkaline phosphatase, gamma-glutamyltranspeptidase and monoamine oxidase, but not with those of AST, ALT and LDH. These data suggest increased ACE activity in alcoholics may be related to the influence of chronic consumption of alcohol on hepatic nonparenchymal systems.

Characterization of angiotensin converting enzyme from rat tissue by radio-inhibitor binding studies

Angiotensin converting enzyme (ACE) derived from rat lung, aorta, epididymus, brain, kidney and plasma was characterized by radio-inhibitor (125I-MK351A) binding studies. Under optimal binding conditions at equilibrium 125I-MK351A bound to ACE was displaced from ACE in a concentration related manner by unlabelled MK351A. MK351A binding site concentration for each tissue and equilibrium dissociation constant (KD) was estimated by Scatchard analysis of binding data. Binding sites/mg protein was greatest in lung and least in brain. The KD for kidney ACE was significantly higher than that of lung, aorta, epididymus or brain ACE (P less than 0.005; t-test, d.f. = 10). 125I-MK351A bound to ACE prepared from lung and kidney was displaced in a concentration dependent manner by SQ20881, SQ14225, MK422, and Ro31-3113-000. Concentration of ACE inhibitor required to displace 50% of bound 125I-MK351A (DD50) was consistently higher for kidney-derived ACE than lung-derived ACE. The differences in radio-inhibitor binding characteristics of ACE from different rat tissues suggests that the enzyme active site may not be identical in all organs.

Vascular renin-angiotensin system in two-kidney, one clip hypertensive rats

The possible role of the renin-angiotensin system in the maintenance of hypertension in two-kidney, one clip hypertensive rats was studied. Plasma renin activity rose rapidly and markedly in association with the elevation of blood pressure and then decreased gradually, although blood pressure remained high. Renin activity in the lung, aorta, and mesenteric artery also increased with the development of hypertension and then decreased in a way similar to that of plasma renin activity at the chronic stage of hypertension. Plasma angiotensin converting enzyme activity did not change significantly until 16 weeks after unilateral renal artery clipping, whereas vascular angiotensin converting enzyme activity significantly increased at the chronic, but not the acute, stage of hypertension. In chronically renal hypertensive rats, 1-sarcosine, 8-isoleucine angiotensin II or enalapril, an angiotensin converting enzyme inhibitor, lowered the blood pressure and enalapril also lowered the angiotensin converting enzyme activity of vascular tissues. The constrictor effect of angiotensin I was greater in isolated arteries from chronically hypertensive rats than in those from age-matched normotensive rats. These results suggest that the vascular renin-angiotensin system plays an important role in the maintenance of two-kidney, one clip hypertension. Elevated vascular angiotensin converting enzyme activity appears to increase local production of angiotensin II, which results in vasoconstriction by acting directly and indirectly through adrenergic nerves on vascular smooth muscle.

Inhibition of [3H]captopril binding by peptide analog angiotensin converting enzyme inhibitors

Ketomethylene containing peptide analogs, modeled after a snake venom pentapeptide, have been shown to be potent angiotensin converting enzyme inhibitors. Although the most potent compounds are up to five times more potent than captopril in inhibiting angiotensin converting enzyme activity, they are relatively weak inhibitors of [3H]captopril binding to membrane bound angiotensin converting enzyme. This indicates that inhibition of [3H]captopril binding and enzymatic activity is due to binding to distinct sites. These results suggest that the inhibitors bind to an additional site on the enzyme distinct from the captopril binding site.

Effect of captopril on bradykinin inactivation by human foetal placental circulation

The inactivation of bradykinin on passage across the human foetal placental circulation was investigated in six full-term human placentas. The placentas were perfused with a modified Krebs-Henseleit solution and placenta perfusion pressure was recorded. Samples collected at the arterial inflow and at the venous effluent were assayed on the isolated guinea-pig ileum as an estimation of bradykinin activity. Bradykinin (100 ng ml-1) was infused through the foetal placental vessels before and during captopril 4 X 10(-7) M. Bradykinin produced a transient increase in placental vascular resistance that was not potentiated by captopril. Bradykinin activity was completely abolished after passage through the foetal placental circulation, and the inactivation was blocked by captopril. These data suggest that angiotensin I converting enzyme (kininase II) might occur in the foetal placental vessels.

Mechanisms of captopril-induced potentiation of the depressor responses to arachidonic acid in rats

The mechanisms underlying potentiation by captopril of the depressor responses to arachidonic acid were studied in chloralose-anaesthetized rats. Captopril, in a dose (0.5 mg/kg, i.v.) which inhibited the pressor responses to angiotensin I (0.03-1 microgram/kg, i.v.), enhanced the depressor responses to bradykinin (3-300 micrograms/kg, i.v.) and potentiated the hypotensive action of arachidonic acid (3 mg/kg, intravenously). This phenomenon was observed not only when captopril and arachidonic acid were administered intravenously, but also when these compounds were injected directly into the aortic arch. The enhancement of arachidonic acid-induced hypotension by captopril was not significantly affected by pretreatment with a low dose of aprotinin (3 mg/kg, i.v.), but was abolished by bilateral nephrectomy or by pretreatment with a higher dose of aprotinin (6 mg/kg, i.v.). It is suggested that captopril augments the depressor responses to arachidonic acid by inhibiting angiotensin converting enzyme. This results in accumulation of bradykinin which in turn increases release of vasodilator prostaglandins, originating most probably, from the kidneys. The possibility that blockade of angiotensin II formation by captopril may leave the vasodilator action of prostaglandin unopposed cannot be excluded.

Serum angiotensin converting enzyme activity in cigarette smokers

Low activity of angiotensin converting enzyme (ACE) has been reported in patients with smoking related diseases, such as chronic bronchitis, emphysaema and carcinoma of the lung [1] but this has not been reported in healthy, chronic smokers. Serum ACE was measured in 40 healthy cigarette smokers and in 42 healthy non-smokers. The mean value was significantly lower in the smokers. Hence a non-smokers. The mean value was significantly lower in the smokers. Hence a patient's smoking habits should be taken into consideration when assessing the significance of his serum ACE levels.

Converting enzyme activity in sow oviducts at different stages of the sex cycle. Influence of inhibitors of prostaglandin synthesis and its possible role in the inotropic effects of bradykinin

The existence of angiotensin I converting enzyme (CE) activity in isolated sow Fallopian tubes and the "in vitro" influences of exogenous bradykinin (BK) on the spontaneous motility of sow oviducts, were explored. Independently of the stage of the sex cycle, the ampullary region of sow Fallopian tubes presented a higher basal CE activity than the isthmic one. On the other hand a greater CE activity was found in the isthmus at estrus and metestrus than at proestrus. Acetylsalicylic acid and mefenamic acid, two inhibitors of prostaglandin synthesis, reduced the CE activity only in ampullary segments obtained at metestrus. BK (0.06 microgram.ml-1) stimulates the spontaneous contractions of isthmic and ampullary tubal segments, triggering tonic responses. The positive inotropism had a longer duration in the isthmus than in the ampulla; presented at estrus a higher magnitude than at proestrus and was enhanced by captopril (50 microgram.ml-1). The data suggest that the low CE activity found in the isthmic tubal region could be subserving the positive inotropic action of BK in the isthmus, long lasting in comparison to that evoked in the ampulla. Moreover, at metestrus some tissue eicosanoid might be modulating the CE activity of ampullary tubal segments. Alternatively the foregoing results could be explained assuming the presence of a carboxy-peptidase different than CE in one region of the oviducts and not in the other, i.e., either one with different sensitivity towards sex hormones and prostaglandin synthesis blockers.

Angiotensin I converting enzyme in human intestine and kidney. Ultrastructural immunohistochemical localization

The localization of immunoreactive angiotensin I-converting enzyme (ACE) has been investigated at the optical and ultrastructural level with anti-human ACE antibodies in the human kidney and small intestine. In both tissues ACE was found in blood vessels and in extravascular situation in the absorptive epithelial cells of intestinal mucosa and renal proximal tubules. Ultrastructural immunohistochemistry showed that in intestinal and renal proximal tubular cells ACE was prominent in microvilli and brush borders. In the kidney ACE was also present on the basolateral part of the plasmalemmal membrane, where it may contribute to the regulation of angiotensin II-dependent absorption processes. Intracellular positivities were also observed inside the renal vascular endothelial and proximal tubular cell in endoplasmic reticulum and nuclear envelope reflecting the synthesis and the cellular processing of ACE. The intestinal microvascular endothelium was strongly labeled suggesting that the mesenteric circulation is an important site for the production of angiotensin II. Vascular endothelial ACE was also detected in the peritubular but not glomerular capillaries of the kidney.

Interrelation between the renin-angiotensin system and kallikrein-kinin system in patients with essential hypertension

In order to investigate the relationship between the kallikrein-kinin (K-K) system and renin-angiotensin (R-A) system, plasma kinin (pKIN), plasma angiotensin II (pAII), plasma renin activity (PRA) and angiotensin converting enzyme activity (ACEA) were determined in 19 essential hypertensives (EHT). pKIN and pATII measurements were performed by highly sensitive radioimmunoassay (RIA), both of which were established in our laboratory. The assay sensitivity of pKIN and pAII were 0.5 pg/tube and 0.1 pg/tube, respectively. In pKIN RIA, pKIN was extracted by ethanol from 0.8 ml of plasma obtained with a syringe containing kinin generating and destroying enzyme inhibitors. In pAII RIA, pAII was measured directly in small amounts of 50-100 microliter, unextracted plasma samples. The level in pAII was significantly higher (p less than 0.05) in the normal renin group (NRH: n = 13) as compared with low renin group of EHT (LRH: n = 6). However, no significant difference was found in pKIN and ACEA between these two groups. Although a significantly positive correlation was observed between pAII and PRA (p less than 0.001) in EHT, the ratio of pAII/PRA tended to be higher in LHR than in NRH. ACEA correlated positively with pAII (p less than 0.01) or PRA (p less than 0.02), respectively. On the other hand, a significant negative correlation was also found between pKIN and pAII (p less than 0.05). From these findings, it was assumed that there was a closed relationship between R-A and K-K systems, and that angiotensin converting enzyme (kininase II) might play some role in the interrelation between both systems.

Angiotensin I converting enzyme activity in hypertension. Relationship to blood pressure, renin-sodium profiles, and antihypertensive therapy

The activity of the angiotensin I converting enzyme was measured in 55 patients with untreated essential hypertension, 11 patients with untreated renovascular hypertension, five patients with untreated primary aldosteronism, and 23 normotensive subjects. Converting enzyme activity was significantly higher (p less than 0.025 or less) in essential hypertension (28 +/- 1 units/ml) and renovascular hypertension (28.5 +/- 3 units/ml) when compared with the activity in the normotensive subjects (21 +/- 1.5 units/ml). Seventeen (31 percent) of the patients with essential hypertension and three (27 percent) patients with renovascular hypertension had an elevated converting enzyme activity above the mean +2 standard deviations value of the normotensive subjects (32.8 units/ml), ranging from 33 to 55.8 units/ml. Converting enzyme activity was similar in black and white patients and in male and female patients, but it tended to decrease with increasing age in both the hypertensive and the normotensive subjects. In the untreated patients with essential hypertension (n = 55), converting enzyme activity was inversely related to mean arterial pressure and age (r = -0.34, p less than 0.01) and positively related to plasma renin activity (r = 0.31, p less than 0.05). Converting enzyme activity was always decreased during captopril therapy, and it was not affected by beta blockers, but it was increased by diuretics. These findings indicate that converting enzyme activity is elevated in patients with essential and renovascular hypertension.

Potentiation of the inhibitory effect of a thromboxane A2 antagonist (L-640,035) on arterial thrombosis formation in rabbit by the angiotensin converting enzyme inhibitor enalapril

The angiotensin converting enzyme inhibitor enalapril (0.5 mg/kg i.v.) potentiated significantly the inhibitory effect of the thromboxane A2 antagonist L-640,035 (1 mg/kg i.v.) on electrically induced platelet accumulation in the rabbit in vivo. Enalapril had no effect upon platelet accumulation when given alone. The hypotensive effects of enalapril did not account for the potentiation because a combination of hexamethonium (5 mg/kg i.v.) and hydralazine (1 mg/kg i.v.), which decreased blood pressure similarly to enalapril, did not augment the effect of L-640,035. Determination by radioimmunoassay of plasma levels of immunoreactive 6-keto-PGF1 alpha, suggested that increases in PGI2 levels after combined administration of enalapril and L-640,035 could explain the observed potentiation.

Inhibition of rat and human prolidases by captopril

We examined the effects of captopril on prolidase activity of crude homogenates of various tissues in the rat and in the human. It was found that captopril caused significant inhibition of prolidases from liver, kidney, and intestine in both species, whereas it showed minimal inhibitory effect on erythrocyte prolidase. In the rat, Ki for the inhibition of kidney and liver prolidases was in the range of 25-35 microM while, in the human, the value was considerably higher. The nature of inhibition was found to be competitive both in the rat and in the human. Oral administration of captopril (daily dose, 40 mg/kg) for 7 days in the rat resulted in increased urinary excretion of peptide-bound 4-hydroxy-L-proline compared to controls, indicating in vivo inhibition of tissue prolidases by captopril.

Cleavage of des-Arg9-bradykinin by angiotensin I-converting enzyme from pig kidney cortex

Fast and very slow hydrolyses of des-Arg9-bradykinin and angiotensin II by angiotensin I-converting enzyme were detected by high performance liquid chromatography. The Michaelis constants of the enzyme, Km values, for des-Arg9-bradykinin and bradykinin were found to be 0.24 mM and 4.4 microM, and the maximum velocities, Vmax values (mumol . min-1 . mg protein-1) for these compounds to be 3.24 and 0.34, respectively. The enzyme also hydrolyzed Z-Gly-Pro-Gly-Gly-Pro-Ala to a tripeptide that was identified as dansyl-Gly-Pro-Ala by TLC on polyamide. These observations show that the enzyme hydrolyzes the peptides at the bond before the prolyl residue in the penultimate position.

Preservation of ischemic myocardium by a new converting enzyme inhibitor, enalaprilic acid, in acute myocardial infarction

Enalaprilic acid (MK-422), the biologically active diacid of the converting enzyme inhibitor enalapril, was studied in myocardial ischemia (MI). Acute left coronary artery ligation was produced in 62 male Sprague-Dawley rats, and infarct size was determined by left ventricular free wall (LVFW) creatine kinase (CK) activity. Administration of enalaprilic acid (2 mg/kg) 2 minutes and 24 hours after MI significantly blunted the reduction in LVFW CK activity at 48 hours after ligation, when compared to the MI rats given vehicle (6.4 +/- 0.5 vs 4.7 +/- 0.2 IU/mg protein, respectively; p less than 0.01). The percentage of LVFW spared was significantly (p less than 0.01) increased from 28 +/- 2% to 45 +/- 5% by MK-422. MK-422 also significantly blunted the loss of LVFW CK activity 48 hours after a coronary ligation (10 minutes) followed by reperfusion, when compared to vehicle (10.1 +/- 0.6 vs 8.3 +/- 0.6 IU/mg protein, respectively; p less than 0.05). This represents a significant increase in the percentage of LVFW spared, 65 +/- 5% vs 85 +/- 6% (p less than 0.05). These data indicate a significant protective action afforded by MK-422 in two different protocols of ischemic damage to the myocardium and suggest a role for the renin-angiotensin system in the extension of ischemic damage.

The effects of the components of the renin-angiotensin system on the isolated perfused human placental cotyledon

Angiotensins I, II, and III, renin substrate, and des-Asp1-angiotensin I were injected as a bolus into either the maternal or fetal circulation of human placental cotyledons perfused in vitro. All drugs tested produced dose-related increments in fetal perfusion pressure when injected into the fetal circulation, with the order of potency being angiotensin I approximately equal to angiotensin II approximately equal to angiotensin III greater than or equal to des-Asp1-angiotensin I greater than or equal to renin substrate. The responses to all the drugs could be blocked by the competitive inhibitor of angiotensin II, (Sar1, Ala8)-angiotensin II, but only the actions of angiotensin I, renin substrate, and des-Asp1-angiotensin I could be blocked by angiotensin converting enzyme inhibitor. When the agents were injected into the maternal circulation, only angiotensins II and III caused dose-related increments in fetal perfusion pressure. Possibly, the placenta may be the main site of conversion of angiotensin I to angiotensin II in the fetoplacental unit, and angiotensin II produced by the placenta could act locally to control fetoplacental blood flow.

Antihypertensive effect of MK-421 in rats. Role of the renal kallikrein-kinin system

To study the hypotensive mechanism of the new oral converting-enzyme inhibitor, MK-421, we evaluated the antihypertensive effect of MK-421 in rats with hypertension induced by chronic administration of norepinephrine (NE) or vasopressin and measured urinary kallikrein and kinin excretions as indices of the renal kallikrein-kinin system. When 6 mg/kg/day of MK-421 was administered simultaneously with 1.8 mg/kg/day of NE, the systolic blood pressure of conscious rats rose on Day 1 to only 122.6 +/- 3.4 mm Hg compared with the rise to 146.3 +/- 1.6 mm Hg when NE alone was infused (p less than 0.001). Similarly, when the same dose of MK-421 was administered simultaneously with 7.2 U/kg/day of vasopressin, the systolic blood pressure of conscious rats rose on Day 1 to only 117.4 +/- 3.8 mm Hg compared with the rise to 141.6 +/- 3.4 mm Hg when vasopressin alone was infused (p less than 0.01). The antihypertensive effect of MK-421 was sustained for 6 days in rats infused with NE or vasopressin. Infusion of NE alone resulted in a small but significant increase in urinary kallikrein excretion and no change in urinary kinin excretion. The combined administration of NE with MK-421 induced additional increases in urinary kallikrein and kinin excretions. Vasopressin alone resulted in marked decreases in urinary kallikrein and kinin excretions. The combined administration of vasopressin with MK-421 induced no additional changes in urinary kallikrein and kinin excretion. These results indicate that the hypotensive effect of MK-421 may depend on a reduced sensitivity of the peripheral arteries to vasoconstrictor substances.(ABSTRACT TRUNCATED AT 250 WORDS)

Renal kallikrein-kinin system and the depressor effect of angiotensin converting enzyme inhibitors MK 421, SA 446, and captopril in rats

Responses in urinary kallikrein and kinin excretion and systolic blood pressure to MK 421, SA 446 or captopril were studied in normotensive rats fed on a regular or a low sodium diet to assess the role of renal kallikrein-kinin system in their hypotensive effect. MK 421, SA 446 or captopril were infused at a rate of 6 mg/kg/day by osmotic minipump implanted intraperitoneally for up to 6 days. The magnitude of fall in systolic blood pressure was greater on a low sodium diet when compared to on a regular diet, whereas the pattern of the fall was similar on both diets. The magnitude of falls in plasma angiotensin II and aldosterone concentration induced by MK 421, SA 446 and captopril was not significantly different between both regular and low sodium diets. Urinary kallikrein and kinin excretion and sodium excretion were increased during infusion of MK 421, SA 446 or captopril on a low sodium diet, however any significant changes were not found in each of them on a regular diet. The present results suggest that on a low sodium diet the augmented hypotensive response to angiotensin converting enzyme inhibitors in the rats might be due to the enhanced renal kallikrein-kinin system in addition to suppressed renin-angiotensin system.

Angiotensin-converting enzyme in cultured endothelial cells. Synthesis, degradation, and transfer to culture medium

Cultured endothelial cells from swine aorta possess the ecto-enzyme angiotensin-converting enzyme (E.C.3.4.15.1) in cell-associated form and also release it in soluble form into the culture medium. Using antibody to purified converting enzyme from swine kidney and incorporation of 3H-leucine, we examined the synthesis, degradation, and release of enzyme into the medium. 3H-leucine is incorporated into cellular converting enzyme, and later appears in the enzyme in the culture medium. The amount of cell-associated enzyme activity remains constant, and the amount of activity in the medium increases linearly during this period. These data show unequivocally that the appearance of enzyme activity in the cell culture medium is accompanied by synthesis of new enzyme protein. In pulse-chase experiments, the radioactivity disappeared from the cellular enzyme in two kinetic components with apparent half-lives of 1-2 hours and 20 hours, respectively. The appearance of the radiolabel in the medium enzyme corresponds very closely in rate and amount to the slow disappearance from the cells. There was no apparent uptake of labeled medium enzyme by the cells. The data suggest that the pool of active enzyme on the cell surface is the immediate precursor of the medium enzyme. The effects of culture conditions on the turnover of angiotensin-converting enzyme were also examined. The incorporation of 3H-leucine into both cell and medium enzyme was greater when cells were maintained in medium containing serum than when they were maintained in serum-free medium. The rate of degradation of the cellular enzyme was similar under the two culture conditions.

Acute changes in the renin-angiotensin system modify bradykinin and angiotensin reactivity and metabolism in conscious rats

We have shown that angiotensin I (AI) conversion as well as bradykinin (BK) inactivation and reactivity are altered in chronic renal hypertensive rats. In the present experiments we tested the possibility that acute renal hypertension or AI and AII infusion cause alterations in both systems. Pulmonary inactivation of BK was estimated by comparing intravenous and intraaortic equipressor doses (20 mm Hg), and the extent of AI conversion was assessed by determining the equipressor doses of AI and AII that produced a 20 mm Hg rise in mean arterial pressure (MAP). Acute renal hypertension was produced by unclamping the renal pedicle (URP) occluded for 5 hours in conscious rats. Before URP, the MAP was already increased (131 +/- 2 mm Hg) and captopril (10 mg/kg, i.v.) produced a fall of 27 +/- 8 mm Hg, suggesting that the renin-angiotensin system was overactive. After URP, MAP rose to 151 +/- 3 mm Hg, and captopril completely abolished the hypertension. Before URP, reactivity to BK was increased [doses 6 times smaller than control (C), 34 +/- 5 ng], and URP produced no further elevation. Pulmonary BK inactivation (97.5% +/- 4%) was the same before and after URP. Before URP, doses of AII 5 times greater than C (2 +/- 4 pmol) were necessary, and hyporeactivity to AII was markedly increased after URP (doses 300 times larger than C). After URP, the conversion was maximal (104% +/- 2% vs 49% +/- 3% in C), and it was already elevated before URP (82% +/- 10%) when six of the nine rats studied had maximal extent of conversion.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibitors of the angiotensin I converting enzyme as antihypertensive drugs

Inhibitors of the angiotensin I converting enzyme (captopril, enalapril) offer a new principle in the drug treatment of hypertension and congestive heart failure. The present survey deals with the mode of action of the converting enzyme inhibitors, including possible interference with the renin-angiotensin systems in the kidney, the vascular wall and the brain, with the kallikrein--bradykinin system and with the sympathetic nervous system, at both pre- and postjunctional sites. Furthermore, the haemodynamic pattern as well as the therapeutic applications are discussed, including the most important side-effects, contra-indications, interactions and clinical pharmacokinetic properties.

Attenuation by captopril of pressor responses to sympathetic stimuli: effects of procedures reducing activity of the renin-angiotensin system

Low doses (0.1-1.0 mg/kg) of the converting enzyme inhibitor captopril given intravenously to pithed rats were followed by long lasting falls in systolic and diastolic arterial blood pressures. Concomitantly pressor responses were reduced to either electrical stimulation of the thoraco-lumbar sympathetic outflow or intravenous injection of the ganglion stimulant McNeil-A-343. Positive chronotropic responses of the heart to cardiac nerve stimulation were unchanged after relatively large doses of the drug (3.0 mg/kg). The reductions in arterial blood pressure and pressor responses to McNeil-A-343 caused by captopril persisted following beta-adrenoreceptor blockade, renal sympathectomy or unilateral nephrectomy, but did not occur after acute bilateral nephrectomy nor during sodium and water retention due to unilateral nephrectomy plus subacute administration of desoxycorticosterone and saline. Pressor responses to noradrenaline were reduced after 1.0 mg/kg captopril i.v. whereas those to methoxamine or vasopressin were unaltered after 5.0 mg/kg. It is concluded that in the rat, after elimination of sympathetic tone by pithing, the level of the arterial blood pressure and the magnitude of pressor responses to peripheral sympathetic nerve activation depend on the basal activity of the renin-angiotensin system. This maintains sufficient angiotensin II production to ensure retention of some tone in resistance vessels together with presynaptic augmentation of noradrenaline output at a sympathetic postganglionic nerve endings. The latter effects are abolished after converting enzyme inhibition with captopril, consequent to reduced tissue levels of angiotensin II and perhaps potentiation of the actions of bradykinin.

Prostanoids and cardiac reflexes of sympathetic and vagal origin

Prostaglandins in concentrations too low to stimulate afferent nerve endings in the heart may sensitize them to chemical or mechanical stimuli that activate cardiac reflexes during myocardial ischemia. Bradykinin, which is released from the heart during ischemia, elicits sympathetically mediated reflex pressor effects and tachycardia when applied in low doses (0.1 to 1 microgram) to the epicardium of the left ventricle in open-chest, anesthetized dogs. The reflex pressor effects evoked by bradykinin are reduced after inhibition of prostaglandins biosynthesis with indomethacin and potentiated by concomitant topical application of low doses (0.1 to 0.3 microgram/min) of PGE1 or PGE2 and prostacyclin (PGI2). The pressor and tachycardic responses to bradykinin are also enhanced after temporary (10-minute) coronary occlusion; this potentiation is abolished by indomethacin treatment and can be restored by superfusing the ventricle with prostaglandins. Nicotine is known to excite mechanosensitive vagal receptors with afferent C fibers, which supply the left ventricle, and to elicit reflex hypotension and bradycardia. This depressor vagal reflex evoked by epicardial or intracoronary administration of nicotine (10 to 50 micrograms) was not affected by either indomethacin or by topical application of PGE1, PGE2, or PGI2. Also, intracoronary infusion of PGE2 (0.1 to 0.3 microgram/min), which enhanced the pressor reflex effects of bradykinin, was without effect on nicotine-induced depressor reflex. However, intracoronary infusion of PGI2 (0.1 to 0.3 microgram/min) significantly enhanced the hypotensive and bradycardic responses to nicotine and, at the same time, reduced sympathetically mediated reflex effects of bradykinin. The hypotensive effects induced by epicardial or intracoronary administration of nicotine were also significantly enhanced during intravenous infusion of subdepressor doses of PGI2 (5 to 20 ng/kg/min). Treatment with captopril, which enhances the endogenous production of prostaglandins, greatly enhanced the reflex depressor effects of nicotine; this potentiating effect of captopril was completely abolished by indomethacin treatment. An increase in the magnitude of nicotine-induced reflex depressor effects was also observed after intravenous injection (1 microgram/kg) or infusion (25 to 50 ng/kg/min) of prostaglandin D2. A working hypothesis is proposed to account for the role of prostanoids in activation of cardiac reflexes during myocardial ischemia.

[Angiotensin-converting enzyme and diabetes mellitus]

Elevation of serum angiotensin-converting enzyme (ACE) has been reported to occur in 25 p. 100 of diabetic patients. This finding would decrease the specificity of increased ACE level in sarcoidosis. We measured serum ACE in 90 diabetic patients with simultaneous analysis of various parameters including those appreciating the degree of metabolic control. Serum ACE values, observed in diabetics are not different from values measured in controls, respectively 22,3 +/- 6,4 and 21,1 +/- 6,7 U/ml (means +/- SD). Only 5,5 p. 100 of diabetic patients had (slightly) increased serum ACE levels, i.e.: levels above mean +/- 2 SD of values observed in controls. Serum ACE levels were higher in insulin-dependent diabetics: 24,7 +/- 6,7 (n = 34) than in non insulin-dependent diabetics: 20,8 +/- 5,9 (n = 56) (p = 0,01). In the diabetic group, there is a positive linear correlation between serum ACE level and 24h-glycosuria or glycosylated A1c hemoglobin. These are parameters appreciating middle or long-term metabolic control. In the other hand, no correlation was observed between serum ACE and glycemia, triglyceridemia, serum creatinin, blood pressure or the presence of diabetic retinopathy. These findings do not show an elevation of serum ACE in diabetic patients. However, they suggest that long-term metabolic control of diabetes influence serum ACE level.