Does bradykinin play a role in the cardiac antiischemic effect of the ACE-inhibitors?

Non-enzymatic proteins from snake venoms: a gold mine of pharmacological tools and drug leads

Non-enzymatic proteins from snake venoms play important roles in the immobilization of prey, and include some large and well-recognized families of toxins. The study of such proteins has expanded not only our understanding of venom toxicity, but also the knowledge of normal and disease states in human physiology. In many cases their characterization has led to the development of powerful research tools, diagnostic techniques, and pharmaceutical drugs. They have further yielded basic understanding of protein structure-function relationships. Therefore a number of studies on these non-enzymatic proteins had major impact on several life science and medical fields. They have led to life-saving therapeutics, the Nobel prize, and development of molecular scalpels for elucidation of ion channel function, vasoconstriction, complement system activity, platelet aggregation, blood coagulation, signal transduction, and blood pressure regulation. Here, we identify research papers that have had significant impact on the life sciences. We discuss how these findings have changed the course of science, and have also included the personal recollections of the original authors of these studies. We expect that this review will provide impetus for even further exciting research on novel toxins yet to be discovered.

The effects of Z13752A, a combined ACE/NEP inhibitor, on responses to coronary artery occlusion; a primary protective role for bradykinin

The effects on the responses to coronary artery occlusion of a combined ACE/NEP inhibitor (Z13752A) were examined in anaesthetized dogs. A 1 h infusion of Z13752A (128 microgram kg(-1) min(-1) intravenously) decreased arterial blood pressure (by 11+/-3%; P<0. 05) and increased coronary blood flow (by 12+/-4%, P<0.05). There were no other significant haemodynamic changes. Z13752A inhibited both NEP and ACE enzymes both in dog plasma and in tissue (lung ACE; kidney NEP). Pressor responses to angiotensin I in vivo were inhibited and systemic vasodilator responses to bradykinin were potentiated. When the left anterior descending coronary artery was occluded for 25 min, Z13752A markedly reduced the severity of the resultant ventricular arrhythmias. No ventricular fibrillation (VF) occurred (compared to 7/16 in the controls; P<0.05), and ventricular tachycardia (VT) was reduced (VT in 2/9 dogs treated with Z13752A cp. 16/16 of controls; episodes of VT 0.2+/-0.1 c.p. 10.7+/-3.3; P<0. 05). Reperfusion of the ischaemic myocardium led to VF in all control dogs but occurred less frequently in dogs given Z13752A (survival from the combined ischaemia-reperfusion insult 67% c.p. 0% in controls; P<0.05). Z13752A reduced two other indices of ischaemia severity; epicardial ST-segment elevation and inhomogeneity of electrical activation. These protective effects of Z13752A during ischaemia and reperfusion were abolished by the administration of icatibant (0.3 mg kg(-1), i.v.) a selective antagonist of bradykinin at B(2) receptors; the ischaemic changes in dogs given both icatibant and Z13752A were similar to those in the controls. We conclude that this ACE/NEP inhibitor is effective at reducing the consequences of coronary artery occlusion in this canine model and that this protection is primarily due to potentiation of released bradykinin. British Journal of Pharmacology (2000) 129, 671 - 680

Inhibition of myocardial lesions in the JCR:LA-corpulent rat by captopril

The JCR:LA-cp rat is a unique strain that, if homozygous for the autosomal recessive cp gene, is obese and exhibits the metabolic syndrome of insulin resistance, hyperinsulinemia, and hypertriglyceridemia. Obese male rats spontaneously develop advanced atherosclerosis and ischemic myocardial lesions. The angiotensin-converting enzyme inhibitor, captopril, was administered to obese rats at 30 mg/kg body weight from 6 to 39 weeks of age. There were no significant changes in food consumption or body weights of the treated animals. Insulin sensitivity was not improved. Plasma insulin levels were unaltered, but the volume density of the islets of Langerhans was halved, reflecting both reduced hyperplasia and a more normal islet structure. Triglyceride concentrations were not reduced, but unesterified cholesterol and cholesteryl esters decreased by 50% and 34%, respectively (p < 0.01). The impaired nitric oxide-mediated vascular relaxation of the obese rats was not improved, and the relaxant sensitivity to acetylcholine as indicated by the median effective concentration (EC50) was reduced. In vitro, captopril significantly reduced the basal tension of aortic rings from untreated rats, antagonized the contractile effects of norepinephrine, and induced complete relaxation of the contraction in response to 10(-7) M norepinephrine. The severity of spontaneous, raised atherosclerotic lesions of the aortic arch at age 39 weeks was not significantly decreased by captopril treatment. In contrast, the frequency of ischemic myocardial lesions was reduced by 78% (p < 0.01). The protective effects of captopril on the heart and pancreas in this animal model of type II diabetes and atherosclerosis are probably the result of its bradykinin-enhancing effects.

The involvement of plasma kinins in the cardiovascular effects of Leiurus quinquestriatus scorpion venom in anaesthetised rabbits

The present study was undertaken to investigate the involvement of kinins in the cardiovascular- and respiratory effects of LQQ venom. Blood pressure, heart rate, electrocardiogram (ECG) and respiration were studied in anaesthetised rabbits, in the presence and absence of aprotinin and icatibant, a B2 bradykinin antagonist. Plasma bradykinin concentrations were also measured following venom injection. LQQ venom caused a triphasic effect on blood pressure comprising an immediate fall, a pronounced rise and a progressive decline until death. Bradycardia, myocardial damage, arrhythmias, respiratory distress and pulmonary oedema were also exhibited. Pretreatment with aprotinin attenuated the venom-induced hypotension, bradycardia, ECG and respiratory changes and prolonged survival. Pretreatment of atropinized animals with icatibant gave similar protection. In animals treated with LQQ venom, plasma bradykinin was significantly higher than controls, although there was considerable inter-animal variation in plasma kinin concentrations and the elevation was seen relatively late after venom administration. The data provides some support for the hypothesis that kinins are involved in the cardiovascular and lethal effects of LQQ venom in rabbits.

Role of angiotensin II and bradykinin on aortic collagen following converting enzyme inhibition in spontaneously hypertensive rats

We previously showed that chronic angiotensin-converting enzyme (ACE) inhibition prevented the increase in aortic collagen in spontaneously hypertensive rats (SHRs) independently of blood pressure reduction. The aim of the present study was to determine whether the effects of ACE inhibition on aortic fibrosis were due to inhibition of angiotensin II formation, preservation of bradykinin, or a combination of both. Four week-old SHRs were treated for 4 months with the ACE inhibitor quinapril, quinapril with the bradykinin B2 receptor antagonist Hoe 140, or the angiotensin II AT1 receptor antagonist CI996. Control SHR and Wistar-Kyoto (WKY) rats received a placebo for the same period of time. At the end of the treatment, as compared to conscious SHR and WKY controls, quinapril completely prevented the development of hypertension, whereas quinapril-Hoe 140 and the AT1 receptor antagonist produced only a partial reduction of blood pressure. In relation with blood pressure changes, aortic hypertrophy was significantly prevented by quinapril but not by quinapril-Hoe 140 or CI996. In contrast, aortic collagen accumulation was completely prevented by all three treatments. The study provides evidence that in young live SHRs, the prevention of aortic collagen accumulation is independent of blood pressure changes and bradykinin preservation and involves exclusively angiotensin II inhibition through AT1 receptors.

Perfusion-independent effect of bradykinin and fosinoprilate on glucose transport in Langendorff rat hearts

Angiotensin-converting enzyme (ACE) inhibitor-stimulated glucose metabolism and perfusion in muscle tissue seem to be, at least in part, mediated by kinins. However, the relative contribution of direct metabolic or secondary hemodynamically induced effects is unclear. It was the aim of this study to characterize the effects of ACE inhibition and bradykinin on glucose transport while changes in cardiocoronary function that might influence glucose transport were minimized. Hearts from Wistar rats were perfused by a Langendorff preparation and a set of functional parameters were simultaneously measured. Bradykinin (10[-11] M) and fosinoprilate (10[-7] M) were administered at concentrations that did not affect coronary flow. Insulin was employed as reference at half-maximal concentration. The nonmetabolizable glucose analog 3-O-[14C]methyl-D-glucose and the nontransportable tracer L-[3H]glucose were coperfused for the calculation of glucose transport. Using a 2-compartment mathematical model we found that the glucose transport rate, which was doubled with insulin, was increased almost 3-fold by either bradykinin or fosinoprilate. In the presence of the B2 bradykinin receptor antagonist HOE 140 (D-Arg[Hyp3,Thi5,D-Tic7,Oic8]-bradykinin; icatibant), the effect of both agents was completely abolished. Both agents also induced minor changes in contractility/relaxation parameters that again were completely neutralized with icatibant. A perfusion-independent but B2-kinin receptor-dependent stimulating effect on glucose transport by either bradykinin or fosinoprilate is concluded. This effect could, in analogy to insulin be due to increased glucose transporter translocation, increased endothelium-derived nitric oxide formation, or--despite constant coronary flow conditions--secondary to altered cardiac function.

Enhancement of left ventricular relaxation in the isolated heart by an angiotensin-converting enzyme inhibitor

BACKGROUND

ACE inhibitors exert both acute and chronic beneficial effects on cardiac function (eg, remodeling, diastolic dysfunction) in experimental studies and in patients. They inhibit the formation of angiotensin II as well as the degradation of endogenous bradykinin. We recently reported that bradykinin induces selective left ventricular (LV) relaxant effects in isolated hearts via the release of nitric oxide. The present study examined the direct effects of interaction between the ACE inhibitor captopril and endogenous bradykinin on cardiac contractile function.

METHODS AND RESULTS

Isolated ejecting guinea pig hearts were studied under conditions of constant loading and heart rate. LV pressure was monitored by a 2F micromanometer-tipped catheter. Captopril (1 mumol/L, n = 9) caused a progressive acceleration of LV relaxation without significantly affecting early systolic parameters (eg, LV dP/dtmax) or coronary flow. These effects were inhibited by the nitric oxide scavenger hemoglobin (1 mumol/L, n = 5) or by the B2-kinin receptor antagonist HOE140 (10 nmol/L, n = 5). In the presence of captopril, bradykinin (0.1 nmol/L, n = 6) markedly accelerated LV relaxation (significantly more than captopril alone), whereas bradykinin alone (0.1 nmol/L, n = 6) had no effect.

CONCLUSIONS

These data indicate that the ACE inhibitor captopril causes an acute and selective enhancement of LV relaxation independent of changes in coronary flow, probably via an endogenous bradykinin/nitric oxide pathway.

Effect of angiotensin-converting enzyme inhibitors on ventricular remodeling and survival following myocardial infarction

OBJECTIVE

To discuss the effects of angiotensin-converting enzyme (ACE) inhibitors on ventricular remodeling and survival after acute myocardial infarction (AMI). An overview is provided of the pathophysiologic changes produced by AMI and the ventricular remodeling process. ACE inhibitors have been studied for their use in the prevention of ventricular remodeling and reduction in postinfarction mortality. Trials in humans and animals are reviewed, including study methods, results, and limitations.

DATA SOURCES

MEDLINE searches identified applicable literature, including experimental trials and review articles.

STUDY SELECTION

All clinical trials of ACE inhibitors following AMI were reviewed.

DATA EXTRACTION

Morbidity and mortality data evaluating the effect of postinfarction ventricular remodeling are rare. At the time of publication, all available clinical trials studying the effects of ACE inhibitors on postinfarction ventricular remodeling were included, regardless of whether morbidity and mortality were assessed. Data from the Survival and Ventricular Enlargement (SAVE) and Cooperative New Scandinavian Enalapril Survival Study II (CONSENSUS II) trials include almost 10,000 patients. Data were extracted by two independent observers. Data quality and validity were assessed based on sample size, stratification of study population, and statistical power of the studies.

DATA SYNTHESIS

ACE inhibitors may prevent the deleterious consequences of AMI, including ventricular remodeling and neurohumoral activation. Ventricular hypertrophy begins acutely following infarction, an early physiologic response to myocardial injury. Hemodynamic benefits from the initial phase of left ventricular hypertrophy include increased ventricular working capacity, normalized systolic wall stress, and maintenance of stroke volume. Although acute dilatation may delay hemodynamic deterioration for six to eight months, it also results in reduced coronary reserve, decreased ventricular compliance, and altered myocardial contractility. With chronic dilatation, the beneficial effects reach a plateau, stroke volume decreases, contractility is reduced, and cardiac failure may ensue. Ventricular hypertrophy is associated with worsened prognosis following infarction and may be the most important single determinant of late prognosis. Ventricular hypertrophy contributes to postinfarction heart failure, angina, and sudden death. Clinical trials show a beneficial effect of the ACE inhibitor captopril on the prevention of left ventricular dysfunction. Although captopril therapy significantly improved survival and myocardial function following AMI in the SAVE trial, these results cannot be generalized to all patient subpopulations. The CONSENSUS II trial demonstrated a decreased survival rate when enalapril was administered within 24 hours of AMI, indicating that timing of therapy may be an important consideration. Captopril therapy may positively affect outcome when initiated 3-16 days following infarction in patients with ejection fractions below 40 percent and who have no signs of ischemia or heart failure. Based on the CONSENSUS II results, enalapril therapy immediately following AMI cannot be recommended.

CONCLUSIONS

Clinical trials have demonstrated that ACE inhibitors can limit ventricular hypertrophy following AMI, resulting in clinical benefit and improved survival. These effects may be secondary to modulation of neurohumoral activation or the antiischemic effect of ACE inhibitors, which may also reduce the incidence of reinfarction. Early intervention with ACE inhibitors (within 3-16 days of infarction) can slow the progression of cardiovascular disease and improve the survival rate.

Ramiprilat increases bradykinin outflow from isolated hearts of rat

To establish that bradykinin is formed in the heart we measured bradykinin in the venous effluent from rat isolated hearts perfused with Krebs-Henseleit buffer. In addition, we examined the effect on bradykinin outflow of the angiotensin converting enzyme (ACE) inhibitor, ramiprilat. From rat isolated normoxic hearts a bradykinin outflow of 0.85 +/- 0.1 ng ml-1 perfusate g-1 wet weight was measured. Perfusion with ramiprilat increased the bradykinin concentration to 2.8 +/- 0.3 ng ml-1 perfusate g-1 wet weight. During ischaemia bradykinin outflow maximally increased 8.2 fold to 7.0 +/- 0.5 ng ml-1 perfusate g-1, and in ramiprilat-perfused hearts 5.8 fold to 16.0 +/- 1.8 ng ml-1 perfusate g-1. In the reperfusion period bradykinin outflow normalized to values measured in the respective pre-ischaemic period. The presents data show that bradykinin is continuously formed in the rat isolated heart. Ischaemia increases bradykinin outflow from the heart. Presumably by inhibiting degradation of kinins, ACE inhibition significantly increased the bradykinin concentration during normoxia, ischaemia and reperfusion.

"Cardioprotection" by ACE-inhibitors in acute myocardial ischemia and infarction?

Coronary artery occlusion results in the acute activation of the renin-angiotensin system and production of angiotensin II, a potent vasoconstrictor and positive inotropic agent. This has raised the possibility that angiotensin converting enzyme (ACE) inhibitors might be "cardioprotective" (that is, might attenuate myocardial injury, dysfunction and necrosis) in the setting of acute ischemia and infarction. Captopril, enalapril and ramipril have, in fact, been reported to acutely limit myocardial injury and necrosis in models of permanent coronary artery occlusion. The mechanisms responsible for this cardioprotection are complex, but include favorable alterations in myocardial oxygen supply/demand, and, in some instances, inhibition of bradykinin metabolism and/or increased prostaglandin synthesis. Other studies, however, have failed to document a reduction in infarct size with ACE inhibitor treatment. Results obtained in models of coronary occlusion/reperfusion have also been mixed. In models of brief transient ischemia not associated with necrosis, captopril and zofenopril have consistently been found to attenuate postischemic contractile dysfunction of the viable but "stunned" myocardium during the early hours following relief of ischemia. In contrast, there is no consensus on the effects of enalapril on the stunned myocardium: both positive and negative results have been obtained. Similar disparity has been reported in models of more prolonged ischemia/reperfusion resulting in subendocardial necrosis: some studies have reported myocardial salvage, while others have provided disturbing evidence of apparent exacerbation of myocardial necrosis with captopril and enalapril therapy. Thus, after a decade of investigative effort, the question of whether ACE inhibitors are "cardioprotective" in the setting of acute myocardial ischemia and infarction remains unresolved. Nonetheless, clinical protocols are in progress to assess the effects of early ACE inhibitor treatment in patients with acute myocardial infarction.

Effect of angiotensin-converting enzyme inhibitors on myocardial ischemia/reperfusion injury: an overview

There are multiple mechanisms whereby ACE inhibitors could be beneficial during myocardial ischemia and reperfusion, including: i) reduced formation of angiotensin II, ii) decreased metabolism of bradykinin, iii) antioxidant activity, and iv) possibly other unknown mechanisms. Reduced formation of angiotensin II should be beneficial because this peptide exerts several actions that are potentially detrimental to the ischemic/reperfused myocardium, including vasoconstriction, increased release of norepinephrine, stimulation of phospholipase C and/or A2, and increased afterload with an attendant increase in oxygen demands. Reduced metabolism of bradykinin could be beneficial by increasing myocardial glucose uptake, by causing vasodilation, and by stimulating production of endothelium-derived relaxing factor and prostacyclin. Although earlier studies suggested that sulfhydryl-containing ACE inhibitors scavenge superoxide anions, recent data have shown that these drugs scavenge hydroxyl radical and hypochlorous acid with no effect on superoxide anion. Studies in isolated hearts have demonstrated that ACE inhibitors attenuate the metabolic, arrhythmic, and contractile dearrangements associated with ischemia and reperfusion, and have suggested that such beneficial effects are mediated by potentiation of bradykinin and/or increased synthesis of prostacyclin. Studies in models of myocardial stunning after brief (15-min) ischemia in vivo (anesthetized dogs) suggest that ACE inhibitors enhance the recovery of contractile function after a single brief ischemic episode. No data are available regarding the effect of these drugs on myocardial stunning after a prolonged, partly reversible episode, after multiple consecutive brief ischemic episodes, and after global ischemia. The mechanism for the salutary effects of ACE inhibitors on stunning remains a mystery. It may involve an antioxidant action (in the case of thiol-containing molecules) or potentiation of prostaglandins (in the case of non-thiol-containing molecules). What is clear is that the enhanced recovery of function effected by these drugs is not due to hemodynamic effects, inhibition of the converting enzyme per se, or an "antischemic" action (since the drugs were effective when given at the time of reperfusion). The effects of ACE inhibitors on myocardial infarct size remain controversial. Further studies will be necessary to conclusively establish whether ACE inhibitors can protect against the detrimental effects of myocardial ischemia and reperfusion. Nevertheless, the evidence provided thus far is encouraging and warrants an in-depth assessment of the role of these drugs in attenuating myocardial ischemia/reperfusion injury.

Mechanisms of cardiac growth. The role of the renin-angiotensin system

Hypertension is associated with cardiac hypertrophy, which is a structural adaptation of the heart in order to attenuate the systolic stress on the left ventricle. As cardiac myocytes cannot divide, they increase in mass and volume, probably by activating second messengers and proto-oncogenes involved in cellular differentiation and proliferation. Various mechanisms, such as pressure overload and angiotensin II (Ang II), have been proposed to trigger cardiocyte growth and left-ventricular hypertrophy (LVH). In both cases, activation of second messenger routes which increase the intracellular calcium concentration, protooncogene expression, and protein synthesis have been demonstrated. Ang II also facilitates the action of another trophic agent for cardiocytes, which is noradrenaline (NA). In addition, the prevention and reversal of LVH by inhibitors of angiotensin-converting enzyme (ACE) suggests a key role for Ang II. However, no conclusive evidence has demonstrated the role of a single pathophysiologic factor in LVH. Therefore, it is more attractive to suggest a link between high blood pressure, renin-angiotensin and other vasoactive systems, such as the adrenergic system, which might together lead in a synergistic way to cardiac hypertrophy.