MYOCARDIAL ACTIONS OF ANGIOTENSIN

Tissue renin-angiotensin-aldosterone systems: Targets for pharmacological therapy

The renin-angiotensin-aldosterone system is one of the most important systems in cardiovascular control and in the pathogenesis of cardiovascular diseases. Therefore, it is already a very successful drug target for the therapy of these diseases. However, angiotensins are generated not only in the plasma but also locally in tissues from precursors and substrates either locally expressed or imported from the circulation. In most areas of the brain, only locally generated angiotensins can exert effects on their receptors owing to the blood-brain barrier. Other tissue renin-angiotensin-aldosterone systems are found in cardiovascular organs such as kidney, heart, and vessels and play important roles in the function of these organs and in the deleterious actions of hypertension and diabetes on these tissues. Novel components with mostly opposite actions to the classical renin-angiotensin-aldosterone systems have been described and need functional characterization to evaluate their suitability as novel drug targets.

Physiology of Local Renin-Angiotensin Systems

Since the first identification of renin by Tigerstedt and Bergmann in 1898, the renin-angiotensin system (RAS) has been extensively studied. The current view of the system is characterized by an increased complexity, as evidenced by the discovery of new functional components and pathways of the RAS. In recent years, the pathophysiological implications of the system have been the main focus of attention, and inhibitors of the RAS such as angiotensin-converting enzyme (ACE) inhibitors and angiotensin (ANG) II receptor blockers have become important clinical tools in the treatment of cardiovascular and renal diseases such as hypertension, heart failure, and diabetic nephropathy. Nevertheless, the tissue RAS also plays an important role in mediating diverse physiological functions. These focus not only on the classical actions of ANG on the cardiovascular system, namely, the maintenance of cardiovascular homeostasis, but also on other functions. Recently, the research efforts studying these noncardiovascular effects of the RAS have intensified, and a large body of data are now available to support the existence of numerous organ-based RAS exerting diverse physiological effects. ANG II has direct effects at the cellular level and can influence, for example, cell growth and differentiation, but also may play a role as a mediator of apoptosis. These universal paracrine and autocrine actions may be important in many organ systems and can mediate important physiological stimuli. Transgenic overexpression and knock-out strategies of RAS genes in animals have also shown a central functional role of the RAS in prenatal development. Taken together, these findings may become increasingly important in the study of organ physiology but also for a fresh look at the implications of these findings for organ pathophysiology.

Electrical activity of the heart and angiotensin-converting enzyme inhibitors on the hyperpolarising action of enalapril

This paper describes the effect of angiotensin- converting enzyme (ACE) inhibitors on the electrical properties of the heart, particularly the effect of these drugs on heart cell coupling and impulse propagation and the possible implications for the generation of malignant ventricular arrhythmias. The hyperpolarising action of enalapril is described and evidence is presented that the activation of the sodium-potassium pump is involved in the increment in membrane potential elicited by the ACE inhibitor.

Gene expression of brain natriuretic peptide in isolated atrial and ventricular human myocardium: influence of angiotensin II and diastolic fiber length

BACKGROUND

We studied the effects of angiotensin II (Ang II) and diastolic overstretch on the induction of cardiac growth in isometrically contracting muscle preparations from human right atria and left ventricles. We used the gene expression of brain natriuretic peptide (BNP) as a molecular marker of cardiac hypertrophy.

METHODS AND RESULTS

Northern blot analysis was performed in human atrial muscle preparations, which were either incubated in 10(-6) mol/L Ang II for 45 minutes or diastolically stretched to 120% of optimum muscle length. Similar experiments were performed with human left ventricular muscle preparations. Results were as follows: (1) BNP gene expression increased in human atrial myocardium 4-fold when stimulated by Ang II (n=7, P<0.001). (2) Diastolic overstretch increased BNP expression in a time-dependent manner. The linear regression equations for the BNP/GAPDH ratio as a function of time (hours) were y=1.21+0.62x (P:<0.001) for overstretched preparations and y=1.07-0.01x (P:=NS) for atrial preparations kept at physiological muscle length. (3) In left ventricular human muscle preparations, diastolic overstretch and Ang II increased BNP gene expression as well. (4) In addition, the Ang II subtype 1 receptor blocker losartan was able to block the effects of Ang II and diastolic overstretch.

CONCLUSIONS

Cardiac hypertrophy can be induced in isolated human atrial and left ventricular intact myocardium by Ang II and diastolic overstretch but not by isometric afterload. The fact that the induction of cardiac growth is inhibited by the blockade of Ang II subtype 1 receptors is of scientific and clinical importance.

Myocardial angiotensin II receptor expression and ischemia-reperfusion injury

The renin-angiotensin system plays an important role in myocardial ischemia-reperfusion injury. Angiotensin II (Ang II) contributes to the evolution of ischemic coronary events through its hemodynamic, hemostatic and mitogenic effects. Angiotensin-converting enzyme (ACE) inhibitors and Ang II receptor antagonists have been shown to be cardioprotective in experimental animal models, with ischemia-reperfusion injury and in patients with congestive heart failure. Ang II receptors include at least two different subtypes, AT1 and AT2. Both AT1 and AT2 are expressed in the rat heart. Myocardial AT1 receptor density increases in association with ACE expression, and AT1 receptor activation is related to collagen formation following myocardial infarction in rats. Studies from the authors' laboratory have shown significant myocardial dysfunction in association with a concurrent increase in AT1 receptor expression in the rat myocardium immediately following a brief period of ischemia and reperfusion. Application of antisense oligodeoxynucleotides (AS-ODN) directed at AT1 receptor messenger RNA and AT1 receptor antagonist, losartan, significantly attenuates myocardial dysfunction induced by ischemia-reperfusion in the isolated rat heart. These observations suggest that myocardial AT1 receptor expression is involved in myocardial dysfunction following ischemia-reperfusion. Unlike losartan, which upregulates the plasma Ang II level, administration of AS-ODN does not affect plasma Ang II level. Although the reason for this is not clear, the difference in plasma Ang II levels implies that AS-ODN may be, at least theoretically, more beneficial than losartan in limiting ischemia-reperfusion-induced cardiac dysfunction. Apoptosis, or programmed cell death, also contributes to the outcome of myocardial ischemia-reperfusion injury. Recent studies from the authors' laboratory have demonstrated that Ang II induces apoptosis in cultured human coronary artery endothelial cells via activation of AT1 receptors and this can be blocked by losartan. These observations collectively underscore the importance of myocardial AT1 receptor expression in ischemia-reperfusion injury.

Effects of enalapril on vasoactive intestinal peptide metabolism and tissue levels

Angiotensin converting enzyme inhibitor therapy results in an increase in cardiac output without an increase in heart rate suggesting a positive inotropic effect. This cannot be explained by changes in angiotensin II and bradykinin concentrations. Angiotensin converting enzyme may also metabolise vasoactive intestinal peptide (VIP), a vasodilator and positive inotrope whose concentration in the heart declines in heart failure. We sought to determine whether changes in plasma VIP or its metabolism might explain the positive inotropic effect of angiotensin converting enzyme inhibitors. We also measured VIP in the heart to determine whether a local increase in VIP might explain this effect. Male Sprague-Dawley rats were randomised to control and enalapril groups (2 mg kg(-1) day(-1)). After 7 days, rats were anaesthetised and underwent metabolic clearance studies for VIP or had hearts, lungs and kidneys removed and snap frozen. VIP concentrations in plasma, infusate and tissue extracts were measured by radioimmunoassay. Plasma concentrations of VIP were unchanged by treatment with enalapril (control: 7.7 +/- 0.8 pmol l(-1); enalapril: 7.9 +/- 0.8 pmol l(-1) ), while the metabolic clearance rate of) VIP increased significantly (control: 10.4 +/- 1.4 ml min(-1) 100 g(-1); enalapril: 17.3 +/- 1.6 ml min(-1) 100 g(-1); p < 0.005). Secretion rate) also increased in enalapril treated rats (139.1 +/- 25.0 pmol min(-1) 100 g(-1) compared with controls (96.3 +/- 13.4 pmol min (-1) 100 g(-1); P< 0.01). VIP in the heart increased after enalapril (control: 208.4 +/- 39.0 pmol g (-1); enalapril: 928.9 +/- 123.6 fmol g(-1); P < 0.0005). Angiotensin converting enzyme inhibition increases the metabolism of VIP. However, the significant increase in the myocardial concentration of VIP may contribute to the beneficial haemodynamic inotrope effects of angiotensin converting enzyme inhibitors.

Vasoactive peptides in the heart of Champsocephalus gunnari

The presence of vasoactive peptides known to control cardiovascular functions in mammals and sub-mammalian vertebrates was investigated in the sub-Antarctic icefish Champsocephalus gunnari. Western immuno blotting was used to demonstrate immunoreactive atrial natriuretic peptide (ANP), angiotensin II (Ang II), bradykinin (BK) and endothelin-1 (ET-1) in heart homogenates. Immunohistochemistry was used to investigate the distribution of ANP, Ang II, BK and ET-1 in the cardiocytes of the three chambers of the heart (atrium, ventricle and the very short conus arteriosus).

Effects of the AT1-selective angiotensin II antagonist, telmisartan, on hemodynamics and ventricular function after cardiopulmonary resuscitation in pigs

The purpose of this study was to investigate the effects of the angiotensin II (ANG II) antagonist, telmisartan, on hemodynamics, myocardial function and myocardial blood flow during the postresuscitation phase in a porcine model of CPR and to compare these to saline. After 4 min of ventricular fibrillation and 5 min of closed-chest CPR, defibrillation was performed in 16 domestic pigs to restore spontaneous circulation (ROSC). Ten minutes after ROSC, animals were allocated to receive either the ANG II antagonist, telmisartan, at a dose of 1 mg/kg (n = 8) or saline (n = 8). Hemodynamics, myocardial function and myocardial blood flow were measured prearrest and at 5, 30, 90 and 240 min after ROSC. Using a Swan-Ganz catheter with a fast responding-thermistor and a micromanometer tipped catheter, right ventricular end-diastolic and end-systolic volume, right ventricular ejection fraction, left ventricular contractility were 67 +/- 6 ml (mean +/- S.E.M.), 42 +/- 4 ml, 38 +/- 2%, 2036 +/- 77 mmHg/s in the telmisartan group and 82 +/- 2 ml (P < 0.05), 59 +/- 3 ml (P < 0.01), 28 +/- 2% (P < 0.01), 1596 +/- 82 mmHg/s (P < 0.01) in the control group, at 240 min after ROSC. No significant differences in mean aortic and pulmonary artery pressure, cardiac index or myocardial blood flow between the two groups were found. We conclude that the ANG II antagonist telmisartan administered during the postresuscitation phase in pigs increases myocardial contractility without changing cardiac index, systemic vascular resistance, pulmonary vascular resistance, or myocardial perfusion.

Angiotensin II and myocyte growth: role of fibroblasts

Angiotensin II (Ang II) has been implicated in stimulating myocyte growth in vitro, but the mechanism for such stimulation is still an open question. To understand the role of Ang II, we studied its effect on protein synthesis in rat neonatal and adult myocytes. Ang II (10(-8) mol/L) stimulated protein synthesis in neonatal myocytes by 43+/-3.5% over control. To prevent the proliferation of fibroblasts, bromodeoxyuridine was added, and protein synthesis in neonatal myocytes was reduced to 21+/-2.2% over control. In adult myocytes (cultured without bromodeoxyuridine), Ang II stimulated [3H]leucine incorporation by 24+/-2.3% over control; with bromodeoxyuridine, that stimulation was reduced significantly (13+/-0.93% over control). These data suggest that the presence of fibroblasts in the cultures may control myocyte growth. When supernatant from pure fibroblast culture was added to myocyte preparations, a significant increase (49.8+/-3.5% over control) in protein synthesis occurred. Pretreatment of these fibroblasts with Ang II (10(-3) mol/L) further stimulated protein synthesis, suggesting that Ang II directly stimulates the production of a factor from fibroblasts. The stimulatory effect of Ang II on the release of the factor can be completely blocked by pretreatment with losartan, an Ang II receptor (AT1) blocker. Our data are the first to demonstrate a paracrine effect of a fibroblast-derived factor that modulates myocyte growth. Fibroblast-derived factor loses its biological activity by (1) tryptic digestion, (2) exposure to pH below 4.0 and above 9.0, and (3) heating to 95 degrees C. The molecular weight of the factor is approximately 65 kD. The antibodies against fibroblast growth factor (both acidic and basic) could not inhibit this factor's stimulatory effect. Furthermore, this factor is heart specific and is produced at least up to the 16th passage of neonatal rat heart fibroblasts. Skin fibroblasts, aortic endothelial cells, and aortic smooth muscle cells do not produce this protein. Our data suggest that the observed myocyte growth by Ang II comes about via fibroblast-derived factor, which is increased by Ang II. Cross talk between fibroblasts and myocytes is an important factor in stimulating myocyte growth by Ang II.

Activation of protein kinase C by angiotensin II decreases beta 1-adrenergic receptor responsiveness in the rat heart

Cardiac beta-adrenergic receptors are the primary driving force for the enhancement of contractility in response to sympathetic stimulation. Angiotensin II influences cardiac function by modulating sympathetic activity and by activating cardiac angiotensin II receptors. The aim of this study was to determine whether activation of cardiac angiotensin II receptors modulates the responsiveness of the heart to beta-adrenergic receptor activation. Male Sprague-Dawley rats were anesthetized and the hearts isolated and perfused with oxygenated Krebs-Henseleit buffer (KHB). Coronary artery perfusion pressure, left ventricular pressure (LVP), left ventricular dP/dtmax, and heart rate (HR) were measured. Bolus administration of the beta-adrenergic receptor agonists, isoproterenol, dobutamine, and salbutamol, produced dose-related increases in LVP, LV dP/dt(max), and HR. Addition of angiotensin-II (10-100 nM) to the KHB slightly increased coronary perfusion pressure but did not alter baseline LVP, LV dP/dt(max), or HR. Angiotensin II reduced the increase in LVP, LV dP/dt(max), and HR elicited by isoproterenol and dobutamine but did not affect responses to salbutamol. The inhibitory effect of angiotensin II was blocked by the AT1-receptor antagonist, losartan, and the protein kinase C inhibitor, calphostin C (50 nM). Activation of protein kinase C with phorbol-12, 13-dibutyrate (PDBu; 10 nM) reduced cardiac responses to all three agonists, although the effects were less on responses elicited by salbutamol. These data suggest that activation of protein kinase C by angiotensin II decreases the responsiveness of the rat heart to beta 1-adrenergic stimulation and that angiotensin II-mediated protein kinase C activation may differ from that activated by phorbol esters.

Angiotensin-converting enzyme inhibition with enalapril increases the cardiac concentration of vasoactive intestinal peptide

In patients with congestive cardiac failure, treatment with ACE inhibitors results in peripheral vasodilatation and an increase in cardiac output without an increase in heart rate, which suggests a positive inotropic effect. This cannot be explained by the changes in angiotensin II and bradykinin concentrations that occur. It has been suggested that ACE also metabolizes VIP, which is a positive inotrope. As VIP is synthetized by the heart and acts locally to increase cardiac output, we postulated that ACE inhibition would increase the myocardial concentration of VIP. Male Sprague-Dawley rats received enalapril (2 mg/kg/day) in the drinking water or no therapy for seven days. On day seven they were anaesthetized and blood sampled. The hearts and kidneys were then harvested and snap frozen by immersion in liquid nitrogen. Concentrations of VIP in plasma and tissue extracts were measured by radioimmunoassay. Plasma and renal concentrations of VIP did not change in the enalapril-treated rats. However, the myocardial concentration of VIP increased significantly in the rats receiving enalapril compared with control animals (p < 0.0005). We conclude that treatment with ACE inhibitors results in increased myocardial VIP concentrations and suggest that this may contribute to the improvement in cardiac function that occurs with these agents.

Role of vascular adrenergic mechanisms in the haemodynamic and PGI2 stimulating effects of angiotensin in diabetic dogs

In aware of the well-known altered vascular responsiveness in the diabetic vasculature, this study aimed to compare the haemodynamic and PGI2 releasing effects of angiotensin in metabolically healthy (12) and alloxan-(560 umol/kg) diabetic (12) dogs as well as to analyze the role of vascular adrenoceptors in this. In vivo the effect of intracoronarially administered angiotensin (63-125-250-500-1000 pmol/kg/min) on coronary blood flow, mean arterial blood pressure, myocardial contractile force and heart rate was investigated without and with pretreatment of 2 umol/kg phentolamine. In vitro PGI2 release by isolated coronary rings was induced by 50 nmol/l angiotensin before and after pretreatment with 5 umol/l phentolamine and measured by radioimmunoassay. Angiotensin enhances dose-dependently both the mean arterial blood pressure and coronary blood flow, while it provokes a considerable (p < 0.05) increase of PGI2 formation by isolated coronary arterial rings. These alterations could be prevented by phentolamine administration both in vivo and in vitro, while this drug did not affect the angiotensin-induced enhancement of diabetic coronary blood flow. On the other hand the increase of blood pressure by angiotensin was found to be more (p < 0.05) expressed in diabetes and it could be further potentiated by phentolamine. PGI2 synthesis by isolated diabetic coronary rings could not be modified either by angiotensin alone or in combination with phentolamine. On the basis of above data, the lack of stimulated vascular PGI2 formation mediated by alpha-adrenergic mechanisms is supposed to causatively contribute to the diminished sensitivity of diabetic coronary arteries to vasodilation.

Increases in the myocardial concentration of vasoactive intestinal peptide may explain the positive inotropic effect of angiotensin converting enzyme inhibitors

1. In patients with congestive cardiac failure, treatment with angiotensin converting enzyme (ACE) inhibitors results in peripheral vasodilatation and an increase in cardiac output without an increase in heart rate, which suggests a positive inotropic effect. This cannot be explained by changes in angiotensin II and bradykinin concentrations that occur. ACE has been suggested to also metabolise vasoactive intestinal peptide (VIP), which is a positive inotrope. As VIP is synthesized by the heart and acts locally to increase cardiac output, we postulated that ACE inhibition would increase the myocardial concentration of VIP. 2. Male Sprague-Dawley rats received enalapril (2 mg/kg per day) in their drinking water or no therapy for 7 days. On day 7 the rats were anaesthetized and blood was sampled. Hearts and kidneys were then harvested and snap frozen by immersion in liquid nitrogen. Concentrations of VIP in plasma and tissue extracts were measured by radioimmunoassay. 3. Plasma and renal concentrations of VIP did not change in enalapril-treated rats. However, the myocardial concentration of VIP increased significantly in rats receiving enalapril compared with control animals (P < 0.0005). 4. We conclude that treatment with ACE inhibitors results in increased myocardial VIP concentrations and suggest that this may contribute to the improvement in cardiac function that occurs with these agents.

Influence of forebrain lesions and isoflurane anaesthesia, respectively, on responses to the intracarotid infusion of angiotensin II in sheep

In conscious sheep, lesions involving the anterior wall of the third cerebral ventricle extinguished the dipsogenic and cortisol releasing effects of 10 min intracarotid (i.c.) infusions of angiotensin II (Ang II) (20 pmol kg-1 min-1). The sustained hypertension obtained in response to i.c. Ang II in the intact animal was not significantly changed by the forebrain lesions. Pronounced, but short-lasting tachycardia immediately developed in response to i.c. Ang II in control animals. After a 1 min return to initial values the heart rate (HR) rose to about 20 beats min-1 above pre-infusion level during the remaining infusion period. After a brief post-infusion fall, HR retook that level. It then gradually declined but did not return to initial level until about 25 min later. During isoflurane anaesthesia in the latter animals carotid blood pressure (cBP) was reduced by approximately 30% while the HR was more than doubled. The cBP rise in response to i.c. Ang II infusion was < 50% of that seen in awake animals and the pressure returned to initial level within approximately 5 min after the end of the infusion. The infusion did not influence the HR. This study provides support for the idea that blood borne Ang II, bound at cerebral sites posterior to the hypothalamic region, continues to have cardiovascular effects for approximately 20 min after the concentration in the blood has returned to pre-infusion level.

Effects of angiotensin II on intracellular Ca2+ and pH in isolated beating rabbit hearts and myocytes loaded with the indicator indo-1

1. Angiotensin II increases myocardial contractility in several species, including the rabbit and man. However, it is controversial whether the predominant mechanism is an increase in free cytosolic [Ca2+]i or a change in myofilament Ca2+ sensitivity. To address this question, we infused angiotensin II in isolated perfused rabbit hearts loaded with the Ca2+ indicator indo-1 AM and measured changes in beat-to-beat surface transients of the Ca2+i-sensitive 400:500 nm ratio and left ventricular contractility. The effects of angiotensin II were compared with the response to a Ca(2+)-dependent increase in the inotropic state produced by a change in the perfusate [Ca2+] from 0.9 to 3.6 nM. 2. In the isolated beating heart, an increase in perfusate [Ca2+] caused an increase in left ventricular pressure +dP/dt in association with an increase in peak systolic [Ca2+]i. Angiotensin II perfusion caused a similar increase in left ventricular +dP/dt in the absence of any increase in peak systolic [Ca2+]i. 3. To exclude any contribution of non-myocyte sources of Ca(2+)-sensitive fluorescence which may be present in the intact heart, we also compared the effects of angiotensin II and a change in superfusate [Ca2+] in collagenase-dissociated paced adult rabbit ventricular myocytes loaded with indo-1 AM. In the isolated rabbit myocytes a change in perfusate [Ca2+] from 0.9 to 3.6 mM caused an increase in peak systolic cell shortening coincident with an increase in peak systolic [Ca2+]i. In contrast, angiotensin II caused a similar increase in peak systolic cell shortening whereas there was no increase in peak systolic [Ca2+]i. There was also no change in inward Ca2+ current (ICa) in response to angiotensin II. 4. To investigate further the mechanism of the positive inotropic action of angiotensin II, its effects on intracellular pH were studied in isolated rabbit myocytes loaded with the fluorescent H+ probe SNARF 1. These experiments demonstrated that angiotensin II induced a 0.2 pH unit increase coincident with the development of a positive inotropic effect in isolated rabbit myocytes. 5. In summary, angiotensin II has a direct positive inotropic effect in beating rabbit hearts and in isolated paced rabbit myocytes. These experiments provide support for the hypothesis that the predominant mechanism is not an increase in free cytosolic Ca2+ but is due in part to an increase in myofilament Ca2+ sensitivity due to intracellular alkalosis.

Cardioprotective effect of the angiotensin II type 1 receptor antagonist TCV-116 on ischemia-reperfusion injury

We investigated the protective effect of angiotensin II (Ang II) type 1 receptor antagonist on myocardial ischemia-reperfusion injury and the role of exogenous Ang II to this injury in perfused hearts. We orally administered TCV-116 (Ang II type 1 receptor antagonist) and delapril (angiotensin converting enzyme inhibitor) to Wistar rats for 1 week and measured the immunoreactive cardiac Ang II. Immunoreactive cardiac Ang II (pg/gm tissue) was 14.3 +/- 2.0 in control group, 11.8 +/- 0.8 in TCV-116-treated group, and 7.3 +/- 0.6 in delapril-treated group (p < 0.05 compared to TCV-116-treated group; p < 0.01 compared to control group). The 15 hearts (five rats in each group) were perfused by a langendorff method and global ischemia was maintained for 30 min. Both TCV-116 and delapril were found to improve postischemic cardiac function and decrease reperfusion creatine kinase (CK) release. Ang II injection before ischemia worsened postischemic cardiac function and increased reperfusion CK release. Only TCV-116 prevented this injury. These data indicated that TCV-116 Ang II type 1 receptor antagonist was effective against myocardial ischemia-reperfusion injury, and exogenous Ang II accelerated this injury through Ang II type 1 receptor.

Angiotensin I and II exert inotropic effects in atrial but not in ventricular human myocardium. An in vitro study under physiological experimental conditions

BACKGROUND

The renin-angiotensin system with its renal-humoral and local myocardial components plays an important role in the development and progression of chronic heart failure. Whereas angiotensin receptors have been found in atrial and ventricular myocardium of different species including humans, its influence on myocardial contractility is not yet defined in human failing myocardium and especially in human nonfailing myocardium.

METHODS AND RESULTS

We measured force development of right atrial and right and left ventricular myocardial preparations of patients with a variety of cardiac diseases. To evaluate the physiological effects of angiotensin, experimental temperature and stimulation rates were 37 degrees C and 60 beats per minute, respectively. Angiotensin I and II increased peak developed force in atrial myocardial preparations obtained from patients without heart failure in a concentration-dependent manner. At optimal concentrations, peak developed force is increased from 10.2 +/- 1.8 to 12.3 +/- 1.9 mN/mm2 by angiotensin I (P < .05) and from 15.4 +/- 2.1 to 20.5 +/- 3.3 mN/mm2 by angiotensin II (P < .05). This effect was not influenced by pretreatment with propranolol (10(-6) mol/L) and prazosin (10(-5) mol/L) but was completely blocked by saralasin (10(-6) mol/L). The positive inotropic effect of angiotensin I could be blocked by enalaprilate (10(-5) mol/L). Neither angiotensin I nor angiotensin II had any effect in preparations of the left ventricle from patients with idiopathic dilated cardiomyopathy, mitral valve stenosis, and incompetence or in patients with no significant heart disease. Additionally, no effect could be seen when angiotensin II was applied to right ventricular preparations from infants undergoing reconstructive heart surgery for tetralogy of Fallot.

CONCLUSIONS

Angiotensin I and II exert positive inotropic effects via angiotensin receptors in atrial preparations but not in right or left ventricular preparations. Furthermore, the existence of a local myocardial angiotensin converting enzyme with functional importance is shown.

Lisinopril increases the recovery during reoxygenation and resistance to oxidative damage in cardiomyocytes

The action of the angiotensin-converting enzyme (ACE) inhibitor lisinopril on the consequences of myocardial reoxygenation and oxidative damage was assessed in cultured chick embryonic ventricular cardiomyocytes. Lisinopril, 10(-8) M to 10(-6) M, produced a significant (P < 0.05) dose-dependent enhancement of the restoration of contractile frequency occurring during myocardial reoxygenation but did not alter the depression in contractile frequency during hypoxia. Lisinopril significantly (P < 0.05) shifted the dose-response relationship of ammonium persulfate-induced reduction in cardiac contractile frequency. Lisinopril significantly (P < 0.05) reduced the effect of another oxidative agent, tertbutylhydroperoxide which produced a time-dependent reduction in cardiac contractile frequency. Lisinopril did not alter cardiac contractile frequency in the absence of hypoxia or ammonium persulfate or tertbutylhydroperoxide. The viability of cardiomyocytes, assessed by trypan blue exclusion, paralleled the changes in cardiac contractile frequency. Lisinopril significantly (P < 0.05) improved viability of cardiomyocytes exposed to either ammonium persulfate or tertbutylhydroperoxide. Lisinopril did not display any antioxidant properties against the free radical alpha,alpha-diphenyl-beta-picrylhydrazyl. These data suggest that lisinopril accelerates the recovery of cardiomyocytes during reoxygenation and blunts the effects of oxidative agents through mechanisms involving the endogenous renin angiotensin system and/or a direct cellular action.

ACE-inhibitors in coronary artery disease?

Angiotensin converting enzyme (ACE)-inhibitors are established in the treatment of arterial hypertension and heart failure. In recent years ACE-inhibitors have also been used in the treatment of patients with coronary artery disease (CAD), since from experimental data an antiischemic action of these agents is suggested. Antiischemic effects of ACE-inhibitors may be exerted through a reduction of myocardial oxygen demand, by a reduction of angiotensin-mediated coronary vasoconstriction, by an interaction with bradykinin and the prostaglandin system, by a modulation of endothelial control of vascular tone, and by an interaction with the sympathetic nervous system. However, clinical findings on potential beneficial effects of ACE-inhibitors in patients with CAD are inconsistent and controversial. While in hypertensive patients with CAD ACE-inhibitors generally seem to attenuate myocardial ischemia at rest and during exercise, a significant fraction of about 30% of normotensive patients with CAD does not benefit or even deteriorates. Lowering of coronary perfusion pressure and alteration of transmural blood flow distribution may be responsible for this. In patients with left ventricular dysfunction (SOLVD) or congestive heart failure (CONSENSUS, SOLVD) ACE-inhibitors have been proven to prevent progressive deterioration in left ventricular function and to reduce mortality. In patients with asymptomatic left ventricular dysfunction after myocardial infarction (SAVE), long-term administration of captropril was associated with an improvement in survival and reduced morbidity and mortality due to major cardiovascular events. Therefore, from a prognostic viewpoint patients with CAD and left ventricular dysfunction or congestive heart failure should be treated with ACE-inhibitors, although the clinical use of ACE-inhibitors in patients with ongoing angina pectoris may be limited by an aggravation of angina, presumably due to critically lowering coronary perfusion pressure. Finally, ACE-inhibitors failed to prevent restenosis after successful PTCA. In conclusion, from a prognostic viewpoint patients with CAD and congestive heart failure or left ventricular dysfunction should be treated with ACE-inhibitors. In hypertensive patients ACE-inhibitors generally seem to attenuate myocardial ischemia. In normotensive patients with CAD and angina pectoris but without left ventricular dysfunction ACE-inhibitors cannot generally be recommended at present, unless the patients, which may have benefit from ACE-inhibitor treatment can be better defined.

Local expression and pathophysiological role of renin-angiotensin in the blood vessels and heart

While the circulating renin-angiotensin system (RAS) plays an important role in short-term maintenance of cardiovascular homeostasis, recent studies point to a role in long-term cardiovascular regulation for endogenous RAS in target tissues. This article focuses on the multiple effects of tissue angiotensin enzyme (ACE) and angiotensin II (Ang II), its active peptide product. Ang II has been shown to be a potent growth factor in vascular smooth muscle cells. Depending on the local conditions, the vascular response may be either hypertrophy or hyperplasia. The molecular mechanisms involved in the interactions of Ang II with endothelium- and smooth muscle-derived cell products may play important roles in the modulation of vascular structure in hypertension and vascular injury. Evidence also points to a role for Ang II in the development of left ventricular hypertrophy in hypertension. In addition, cardiac RAS may contribute to the pathophysiology of heart failure. Experimental and clinical studies with ACE inhibitors point to a role for tissue ACE activity in the development of atherosclerosis, as well as cardiac hypertrophy and remodeling.

Effects of angiotensin-converting enzyme inhibitors on tissue renin-angiotensin systems

The renin-angiotensin system (RAS) plays a major role in the control of blood pressure and cardiovascular homeostasis and is involved in the pathogenesis of a number of cardiovascular disorders. The efficacy of angiotensin-converting enzyme (ACE) inhibitors in the treatment of hypertension and congestive heart failure has led to the widespread clinical use of ACE inhibitors in primary or secondary prevention of heart disease. The demonstration of the expression of the components of the RAS in several extrarenal tissues, as well as local generation of angiotensin II, has confirmed the existence of a tissue RAS that may serve organ-specific functions and act independently from the plasma RAS. The concept of paracrine/autocrine functions of the local RAS has changed our understanding of the functions of the RAS and suggests that tissue ACE inhibition may be of greater importance than inhibition of circulating ACE in the treatment of congestive heart failure and other cardiovascular disorders. Whereas the circulating endocrine RAS appears to be responsible for mediation of acute effects, the tissue RAS seems to be involved in more chronic situations, such as secondary structural changes of the cardiovascular system, and therefore could contribute to the pathogenesis of hypertension as well as other cardiovascular disorders, such as cardiac hypertrophy, coronary artery disease, and atherosclerosis. Several experimental and clinical findings suggest that reversal of cardiovascular structural changes secondary to cardiovascular disease and enhancement of renal sodium excretion by ACE inhibitors are important long-term antihypertensive actions possibly mediated by inhibition of the tissue RAS.

Exacerbation of left ventricular ischemic diastolic dysfunction by pressure-overload hypertrophy. Modification by specific inhibition of cardiac angiotensin converting enzyme

Hearts with compensatory pressure-overload hypertrophy show an increased intracardiac activation of angiotensin II that may contribute to ischemic diastolic dysfunction. We studied whether pressure-overload hypertrophy in response to aortic banding would result in exaggerated diastolic dysfunction during low-flow ischemia and whether the specific inhibition of the cardiac angiotensin converting enzyme by enalaprilat would modify systolic and diastolic function during ischemia and reperfusion in either hypertrophied or nonhypertrophied hearts. Isolated, red blood cell-perfused isovolumic nonhypertrophied and hypertrophied rat hearts were subjected to enalaprilat (2.5 x 10(-7) M final concentration) infusion during 20 minutes of baseline perfusion and during 30 minutes of low-flow ischemia and 30 minutes of reperfusion. Coronary flow per gram was similar in nonhypertrophied and hypertrophied hearts during baseline perfusion, ischemia, and reperfusion. At baseline, left ventricular developed pressure was higher in hypertrophied than nonhypertrophied hearts in untreated groups (224 +/- 8 versus 150 +/- 9 mm Hg; p less than 0.01) and in enalaprilat-treated groups (223 +/- 9 versus 145 +/- 8 mm Hg; p less than 0.01). During low-flow ischemia, left ventricular developed pressure was depressed but similar in all groups. All groups showed deterioration of diastolic function; however, left ventricular end-diastolic pressure increased to a significantly higher level in untreated hypertrophied than in nonhypertrophied hearts (65 +/- 7 versus 33 +/- 3 mm Hg; p less than 0.001). Enalaprilat had no effect in nonhypertrophied hearts, but it significantly attenuated the greater increase in left ventricular end-diastolic pressure in hypertrophied hearts treated with enalaprilat compared with no drug (65 +/- 7 versus 50 +/- 5 mm Hg; p less than 0.01). The beneficial effect could not be explained by differences in coronary blood flow per gram left ventricular weight, glycolytic flux as reported by lactate production, myocardial water content, oxygen consumption, and tissue levels of glycogen and high energy phosphate compounds. During reperfusion, all hearts showed a partial recovery of developed pressure to 70-74% of initial values. No effect of enalaprilat could be detected during reperfusion on systolic and diastolic function or restoration of tissue levels of high energy compounds. In conclusion, our experiments show that hypertrophied red blood cell-perfused hearts manifest a severe impairment of left ventricular diastolic relaxation in response to low-flow ischemia in comparison with control hearts. Further, our experiments support the hypothesis that the enhanced conversion of angiotensin I to angiotensin II in rats with pressure-overload hypertrophy contributes to the enhanced sensitivity of hypertrophied hearts to diastolic dysfunction during low-flow ischemia.

The myocardial renin-angiotensin system: existence, importance, and clinical implications

Major components of the renin-angiotensin system have been localized to cardiac tissue. Cardiac-derived angiotensin II may benefit myocardial contractility but may promote detrimental myocardial hypertrophy, coronary vasoconstriction, and arrhythmias. The benefits of ACE inhibition probably extend beyond the classic circulating RAS to include the heart directly.

Tissue-specific activation of cardiac angiotensin converting enzyme in experimental heart failure

In addition to the circulating renin-angiotensin system, recent data demonstrate the existence of tissue renin-angiotensin systems that may be important in cardiovascular homeostasis. However, the relative activities of the circulating and tissue renin-angiotensin systems have not been examined previously in pathophysiological states, such as congestive heart failure. The present study was performed to examine the status of plasma and tissue angiotensin converting enzyme (ACE) activities in compensated experimental heart failure induced by coronary artery ligation in the rat. Three groups of male Sprague-Dawley rats were examined: 1) nonoperated rats (NO, n = 5), 2) sham-operated rats (SO, n = 5), and 3) heart failure rats (HF, n = 11). Rats were studied an averaged of 85 days postoperatively. In HF animals, plasma renin concentration and serum ACE activities were not different compared with NO and SO control animals. Cardiac ACE activity was 50% greater in the right ventricle than the interventricular septum in NO and SO rats. Both right ventricular and interventricular septal ACE activity increased approximately twofold in HF animals as compared with NO and SO groups (p less than 0.05). In contrast, pulmonary, aortic, and renal ACE activities were not altered in HF rats compared with control animals. A positive correlation existed between the histopathological size of myocardial infarction and the level of right ventricular ACE activity (r = 0.75, p less than or equal to 0.05). Such a relation between infarct size and either serum or noncardiac tissue ACE activities was not observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of angiotensin II on cytosolic free calcium in neonatal rat cardiomyocytes

The effect of angiotensin II (ANG II) on cytosolic free Ca2+ concentration ([Ca2+]i) was studied in cultured neonatal rat ventricular myocytes. [Ca2+]i was estimated in groups of one to three cells by dual-wavelength microfluorometry or in cell populations using conventional fluorometry. ANG II (10(-8) M) produced an acute short-lived increase over the control basal diastolic [Ca2+]i and increased the frequency of the [Ca2+]i transients. The amplitude of the [Ca2+]i transients was decreased to 64.4% of basal values. The effect of ANG II on [Ca2+]i was blocked by the selective AT1 receptor subtype antagonist Du Pont 753 but not by the AT2 antagonist PD 123319. Removal of extracellular Ca2+ or blockade of voltage-gated Ca2+ channels in cells cultured for 5-7 days abolished the [Ca2+]i transients, but only partially diminished the effect of ANG II on [Ca2+]i. Thapsigargin, an inhibitor of sarcoplasmic reticulum Ca(2+)-Mg(2+)-ATPase, reduced or abolished the [Ca2+]i response to ANG II. Phorbol 12-myristate 13-acetate (PMA), 10(-6) and 10(-7) M, also decreased the amplitude of the Ca2+ transients similar to ANG II. Pretreatment with 10(-6) M PMA or 10(-6) M 1-oleoyl-2-acetyl-glycerol (OAG) inhibited the initial rise in [Ca2+]i and the Ca2+ transients. Thus ANG II produces an acute rise in [Ca2+]i which is derived predominantly from sarcoplasmic reticulum intracellular stores. This acute effect is followed by a significant reduction in the amplitude for the Ca2+ transient and may be mediated by activation of protein kinase C.

Influence of angiotensin converting enzyme inhibition on pump function and cardiac contractility in patients with chronic congestive heart failure

Eleven patients with coronary artery disease and chronic heart failure were studied before and three months after the angiotensin converting enzyme inhibitor enalapril was added to their frusemide medication. The following were measured: left ventricular pressure and volume with transient occlusion of the inferior vena cava, radionuclide angiography, and hormone concentrations in plasma. As in other reported studies, the clinical condition of the patients improved and their exercise tolerance increased moderately. Addition of enalapril reduced end diastolic and systolic pressure, reduced ventricular volume, and concomitantly increased the ejection fraction. The end systolic pressure-volume relation shifted to the left as it did in a similar animal study. In the animal study unloading by a vasodilator did not induce a leftward shift, so it can be inferred that in the present study unloading combined with a decrease in the angiotensin concentration was instrumental in remodelling the heart. Though unloading was expected to have a beneficial effect on the oxygen supply/demand ratio of the heart, the patients still showed the same drop in the ejection fraction during exercise as they did before treatment with enalapril, and early diastolic filling did not improve. Normally, regression of cardiac dilatation is only found if pump function improves; the present study showed that unloading in combination with angiotensin converting enzyme inhibition reshapes the ventricle without improving intrinsic pump function.

Effect of a pressor infusion of angiotensin II on sympathetic activity and heart rate in normal humans

We tested the hypothesis that pressor infusions of angiotensin II (AII) could stimulate the sympathetic nervous system as reflected by norepinephrine (NE) spillover in humans. AII was infused at 5 ng/kg/min in six healthy volunteers, with vehicle and phenylephrine infusions as controls, on 3 separate days. Heart rate, mean arterial pressure, plasma NE, NE clearance, and NE spillover were assessed before and after 30-minute infusions of AII, vehicle, or phenylephrine in the supine position and then after 15 minutes of head-up and 15 minutes of head-down tilt. Both AII and phenylephrine raised mean arterial pressure (88 +/- 9.6 to 103 +/- 14 mm Hg, p less than 0.001, and 91 +/- 7.6 to 104 +/- 9.2 mm Hg, p less than 0.001, respectively), whereas heart rate fell only with phenylephrine (60 +/- 6 to 51 +/- 6.3 beats/min, p less than 0.001). Neither plasma NE nor NE spillover was affected by either infusion, and NE clearance declined slightly with both. No changes occurred in any variable during vehicle infusions in the supine position. During upright tilt, NE spillover increases were attenuated by both AII and phenylephrine while NE clearance changes were slightly greater, leaving plasma NE increases similar on each day. During head-down tilt, NE and NE spillover declined comparably on each study day.(ABSTRACT TRUNCATED AT 250 WORDS)

Increased rat cardiac angiotensin converting enzyme activity and mRNA expression in pressure overload left ventricular hypertrophy. Effects on coronary resistance, contractility, and relaxation

We compared the activity and physiologic effects of cardiac angiotensin converting enzyme (ACE) using isovolumic hearts from male Wistar rats with left ventricular hypertrophy due to chronic experimental aortic stenosis and from control rats. In response to the infusion of 3.5 X 10(-8) M angiotensin I in the isolated buffer perfused beating hearts, the intracardiac fractional conversion to angiotensin II was higher in the hypertrophied hearts compared with the controls (17.3 +/- 4.1% vs 6.8 +/- 1.3%, P less than 0.01). ACE activity was also significantly increased in the free wall, septum, and apex of the hypertrophied left ventricle, whereas ACE activity from the nonhypertrophied right ventricle of the aortic stenosis rats was not different from that of the control rats. Northern blot analyses of poly(A)+ purified RNA demonstrated the expression of ACE mRNA, which was increased fourfold in left ventricular tissue obtained from the hearts with left ventricular hypertrophy compared with the controls. In both groups, the intracardiac conversion of angiotensin I to angiotensin II caused a comparable dose-dependent increase in coronary resistance. In the control hearts, angiotensin II activation had no significant effect on systolic or diastolic function; however, it was associated with a dose-dependent depression of left ventricular diastolic relaxation in the hypertrophied hearts. These novel observations suggest that cardiac ACE is induced in hearts with left ventricular hypertrophy, and that the resultant intracardiac activation of angiotensin II may have differential effects on myocardial relaxation in hypertrophied hearts relative to controls.

Potential role of the tissue renin-angiotensin system in the pathophysiology of congestive heart failure

The circulating renin-angiotensin system (RAS) plays an important role in the maintenance of cardiovascular homeostasis. It has recently been demonstrated that endogenous RAS exist in target tissues that are important in cardiovascular regulation. This article reviews the multiple effects of angiotensin II in target tissues, the evidence for the presence of functional tissue RAS and the data that suggest a role for these tissue RAS in the pathophysiology of heart failure. Activation of circulating neurohormones is predictive of worsened survival in heart failure; however, cardiac and renal tissue RAS activities are also increased in the compensated stage of heart failure, when plasma renin-angiotensin activity is normal. It is hypothesized that the plasma RAS maintains circulatory homeostasis during acute cardiac decompensation, while changes in tissue RAS contribute to homeostatic responses during chronic sustained cardiac impairment. This concept of different functions of circulating and tissue RAS in the pathophysiology of heart failure may have important pharmacologic implications.

Cardiac angiotensinogen and its local activation in the isolated perfused beating heart

Increasing evidence suggests that the renin-angiotensin system modulates cardiovascular homeostasis both via its circulating, plasma-borne components and through locally present, tissue-resident systems with site-specific activity. The existence of such a system in the heart has been proposed, based on biochemical studies as well as on the demonstration of renin and angiotensinogen messenger RNA in cardiac tissue. We conducted the present study to determine whether biologically active angiotensin peptides may be cleaved within the heart from locally present angiotensinogen. Isolated, perfused rat hearts were exposed to infusions of purified hog renin; the coronary sinus effluent was collected and subsequently assayed for angiotensin I (Ang I) and angiotensin II (Ang II) by high-pressure liquid chromatography and specific radioimmunoassay. Both Ang I and II were undetectable under control conditions but appeared promptly after the addition of renin. Dose-dependent peak values for Ang I release ranged from 2.42 +/- 0.65 fmol/min to 1.38 +/- 0.18 pmol/min during renin infusions at concentrations between 10 microunits/ml and 5 milliunits/ml. Ang II levels measured in the perfusate reflected a mean fractional intracardiac conversion of Ang I to Ang II of 7.18 +/- 1.09%. Generation of Ang I and Ang II was inhibited in the presence of specific inhibitors of renin and converting enzyme, respectively. To investigate the source of angiotensinogen, we measured spontaneous angiotensinogen release from isolated perfused hearts. In the absence of renin in the perfusate, angiotensinogen was initially released in high, but rapidly declining, concentrations and subsequently at a low, but stable, rate. Prior perfusion with angiotensinogen-rich plasma resulted in enhanced early angiotensinogen release but did not alter the second, delayed phase, suggesting that, in addition to plasma-derived substrate, locally produced angiotensinogen may also participate in the intracardiac formation of angiotensin. Supporting this interpretation, hearts from animals pretreated with dexamethasone showed increased angiotensinogen messenger RNA concentrations as well as increased rates of angiotensinogen release not only during the early but also during the late phase. Our study newly demonstrates that Ang I and II may be formed within the isolated heart from locally present substrate, which appears to be derived in part from the circulating pool and in part from endogenous synthesis. These findings add support to the concept of a functionally active and locally integrated cardiac renin-angiotensin system and emphasize its potential physiological and pathological relevance.

Tissue renin-angiotensin systems in the heart and vasculature: possible involvement in the cardiovascular actions of converting enzyme inhibitors

The existence of independently functioning local renin-angiotensin systems in a number of tissues has been firmly established by biochemical and functional evidence and, most recently, by the demonstration of genetic messages for components of the renin-angiotensin systems, such as renin and angiotensinogen, in several organs. In this review, local renin-angiotensin systems in the heart and vascular walls are described and the contribution of a local inhibition of converting enzymes to the cardiovascular actions of converting enzyme inhibitors is discussed. Most of the studies cited support the hypothesis that an inhibition of cardiac converting enzyme may be involved in the beneficial hemodynamic and metabolic actions of converting enzyme inhibitors in cardiovascular disease, such as hypertension and congestive heart failure, independent of the circulating renin-angiotensin system. Local effects on cardiac converting enzyme may contribute to the ability of converting enzyme inhibitors to reduce cardiac hypertrophy. Similarly, local converting enzyme inhibition in the vascular wall may not only constitute a mechanism involved in the antihypertensive effects of converting enzyme inhibitors, but may also contribute to the regression of hypertension-induced vascular hypertrophy. In addition to reduced local angiotensin II generation, converting enzyme inhibition may engender a potentiation of the local effects of kinins. This mechanism may be more important to the cardiovascular actions of converting enzyme inhibitors than initially thought.

Positive chronotropic activity of angiotensin II in the pithed normotensive rat is primarily due to activation of cardiac beta 1-adrenoceptors

In the pithed normotensive rat the intravenous administration of increasing doses of angiotensin II caused an increase in heart rate by a maximum of 93 +/- 6 beats/min. This increase in heart rate was diminished dose-dependently by the concomitant infusion of the angiotensin II analogue saralasin. It was not affected by acute bilateral adrenalectomy but strongly reduced by pretreatment with reserpine. By using the selective antagonists of alpha 1-, alpha 2-, beta 1- or beta 2-adrenoceptors prazosin, yohimbine, atenolol and ICI 118,551, respectively, it was shown that this tachycardia was mediated by stimulation of cardiac beta 1-adrenoceptors. Moreover, the high endogenous angiotensin II level following pithing contributed to the basal heart rate in the pithed rat model. From our experiments it may be concluded that angiotensin II induces tachycardia in the pithed rat primarily by stimulating the sympathetic ganglia leading to the release of noradrenaline, which subsequently activates cardiac beta 1-adrenoceptors.

Identification and characterization of guinea pig angiotensin II ventricular and atrial receptors: coupling to inositol phosphate production

Angiotensin II elicits a receptor-mediated positive inotropic response in cardiac tissue from most mammalian species by activating voltage-sensitive slow Ca2+ channels. In the guinea pig, we confirm there is no in vitro contractile force development in bioassay systems using isolated hearts or left atrial tissue in response to angiotensin peptides. However, 125I-angiotensin II binding sites that have the characteristics of a membrane receptor were identified in ventricular (myocardial) and atrial membrane preparations from guinea pigs. In ventricles, saturation-binding data yielded an optimal fit to a two-site model with a high-affinity site Kd1 = 3.6 +/- 0.7 nM and a low-affinity site Kd2 = 433 +/- 126 nM and binding capacities of 66 +/- 10 and 821 +/- 49 fmol/mg protein, respectively. In atria, saturation binding data yielded an optimal fit to a two-site model with a high-affinity site Kd1 = 1.6 nM and a low-affinity site Kd2 = 300 nM and capacities of 145 and 752 fmol/mg protein, respectively. The ventricular binding of 125I-angiotensin II was stimulated approximately twofold in the presence of the divalent cations calcium and magnesium (10 mM). Nonhydrolyzable analogues of guanosine triphosphate increased the dissociation rate of the bound 125I-angiotensin II and decreased hormone binding to the receptor at equilibrium. Competition for 125I-angiotensin II binding by an agonist-antagonist analogue series correlated with previous studies obtained in the rabbit, a mammal in which inotropic responses to angiotensin II were demonstrated. The data indicate the presence of angiotensin II myocardial and atrial receptors and a G-type coupling protein in guinea pig. Although this species lacks an inotropic response to angiotensin peptides, there is a dose-dependent increase in inositol-1-phosphate production in response to angiotensin II, and this response is blocked by the selective angiotensin II antagonist [Sar1,Ile8]angiotensin II.(ABSTRACT TRUNCATED AT 250 WORDS)

Endogenous tissue renin-angiotensin systems. From molecular biology to therapy

Expression of the genes for renin and angiotensinogen has been documented in the heart and brain of several species, including rodents and primates. In the same tissues, local generation of angiotensin II has also been demonstrated. Neuropeptidergic brain angiotensin and local cardiac angiotensin participate in cardiovascular regulation. Inhibition of cardiac angiotensin II protects against deleterious arrythmogenic and metabolic effects of transient regional myocardial ischemia, and blockade of brain angiotensin II effectively lowers blood pressure in spontaneously hypertensive rats. It is surmised therefore that the therapeutic effects of converting enzyme inhibitors are, in part, brought about by inhibition of local tissue angiotensin II generation in addition to their interference with the hormonal plasma renin-angiotensin system. This would help to explain their therapeutic efficacy in pathophysiologic conditions in which hypertension is associated with low plasma renin activity.

Direct myocardial and coronary effects of enalaprilat in patients with dilated cardiomyopathy: assessment by a bilateral intracoronary infusion technique

Angiotensin II elicits contractile responses in the coronary arteries and myocardial tissue, which suggests that blockade of the renin-angiotensin system by specific agents should lead to both coronary vasodilation and an alteration of left ventricular inotropism. The present work was designed to delineate--independently from its systemic effects--the intrinsic actions of an angiotensin converting-enzyme inhibitor on the coronary circulation and left ventricular function. To minimize peripheral effects, a bilateral intracoronary infusion of enalaprilat (0.05 mg.min-1, 1 ml.min-1 in each coronary artery) was performed in 16 patients with dilated cardiomyopathy. All patients had normal coronary arteriograms. In 12 patients (group I) the intracoronary infusion of enalaprilat resulted in minimal peripheral changes, with a 5% reduction in the mean aortic pressure (p less than .05) and no significant alteration in indexes of preload, i.e., left ventricular end-diastolic pressure and volume, or of afterload, i.e., left ventricular end-systolic stress and systemic resistances. Myocardial oxygen consumption was also unaffected by the intracoronary infusion of enalaprilat. Coronary vasodilation was demonstrated by a significant elevation of coronary sinus blood flow (+19%, from 181 +/- 73 to 214 +/- 79 ml.min-1, p less than .001) and a reduction of coronary resistance (-18%, from 0.51 +/- 0.17 to 0.41 +/- 0.15 mm Hg.ml-1.min, p less than .001), with a parallel increase in coronary sinus oxygen content and pressure (both p less than .05). Oxygen extraction by the myocardium was reduced (p less than .01).(ABSTRACT TRUNCATED AT 250 WORDS)

Inconsistency of the slope and the volume intercept of the end-systolic pressure-volume relationship as individual indexes of inotropic state in conscious dogs: presentation of an index combining both variables

We tested the ability of the slope (Emax) and the volume intercept (Vo) of the end-systolic pressure-volume relationship (ESPVR) to indicate contractility changes in conscious dogs instrumented with sonomicrometers measuring left ventricular diameter in three orthogonal axes and a left ventricular pressure microtransducer. ESPVRs were generated by inferior vena caval occlusion under control conditions (C1 and C2) and during enhanced (I+) and depressed (I-) inotropic states achieved by infusion of dobutamine and injection of propranolol, respectively. No significant difference between the first control (C1) and I+ or between the second control (C2) and I- were found for either Emax (C1, 5.31 +/- 1.68 mm Hg/ml, mean +/- SD; I+, 5.37 +/- 1.44; C2, 5.20 +/- 1.62; I-, 4.18 +/- 1.32) or Vo (C1, 10.3 +/- 9.6 ml; I+, 7.3 +/- 9.1; C2, 9.9 +/- 9.0; I-, 12.7 +/- 12.5), despite significant changes in other indexes of contractility. Comparison of changes in Emax in individual animals in response to I+ and I- revealed that 63% were nonsignificant, 28% were significant and expected, and 9% were significant and paradoxical. Within defined volume limits and irrespective of individual changes in Emax and Vo, in all animals I+ shifted the ESPVR above and to the left of C1 and I- shifted the ESPVR below and to the right of C2. We thus integrated the changes in Emax and Vo by measuring the area beneath each ESPVR between defined limits of end-systolic volume. The values for area were: C1, 612 +/- 150 mm Hg.ml; I+, 745 +/- 191 (p less than .001); C2, 520 +/- 198; I-, 420 +/- 139 (p less than .001). We conclude that (1) neither Emax nor Vo are individually reliable indexes of changed contractility, and (2) the area beneath the ESPVR between defined end-systolic volume limits is a consistent indicator of variations in inotropic state.

Defining the physiologic and pathophysiologic roles of renin: the role of specific inhibitors

Although renin was identified as playing a role in cardiovascular homeostasis by the experiments of Goldblatt in the 1930's, neither its physiologic role in organs other than the kidney nor its contribution to the genesis of essential hypertension has been defined as yet. It is difficult to interpret studies with converting enzyme inhibitors because of their multiple pharmacologic effects. Specific inhibitors of renin appropriate for clinical investigation would help resolve many questions. Four classes of compounds have been demonstrated to be renin inhibitors of high potency: specific antibody, general peptide inhibitors of acid proteases, analogs of angiotensinogens, and peptides that are related to the amino-terminal sequence of prorenin. Of these, it is likely that angiotensinogen analogs will be the first applied in human studies. The minimal substrate for renin has the sequence: His-Pro-Phe-His-Leu-Leu-Val-Tyr. Variants of this sequence have yielded competitive inhibitors. Recently, remarkably active compounds have been synthesized by reducing the peptide bond that is cleaved by renin, or by incorporating the amino acid statine, found in pepstatin. These compounds have been shown now to be effective in dogs, rats, and monkeys, and most recently, preliminary studies have reported their efficacy in humans. Recent studies with one of these inhibitors, RIP, raise questions concerning both its specificity and site of action.

Evaluation of relationship between myocardial contractile state and left ventricular function in patients with aortic regurgitation

We studied the relationship between myocardial contractile state and left ventricular functional response to exercise in 14 asymptomatic patients with isolated moderate-to-severe aortic regurgitation and six control subjects. The slope of the systolic blood pressure-left ventricular end-systolic volume (pressure-volume) relationship determined by radionuclide ventriculography during angiotensin infusion was used as an indirect measure of myocardial contractility and was compared with left ventricular ejection fraction at rest and during both isometric handgrip and dynamic bicycle exercise. The slope of the pressure-volume relationship was significantly lower in patients with aortic regurgitation than in the control subjects (1.75 +/- 0.57 vs 2.78 +/- 0.42, p less than 0.01). The slope correlated exponentially with resting ejection fraction and was linearly related to changes in left ventricular ejection fraction during both handgrip and bicycle exercise. In patients with aortic regurgitation, resting ejection fraction may overestimate myocardial function. The slope of the pressure-volume relationship measured during afterload stress and left ventricular ejection fraction response to exercise intervention more reliably reflect the degree of left ventricular dysfunction.

Effects of locally generated angiotensin II on noradrenergic transmission in guinea-pig isolated atria

The extent to which cardiac neuroeffector function may be modulated by angiotensin II (AII) generated locally from angiotensin I (AI) was investigated in guinea-pig spontaneously beating isolated atria radiolabelled with [3H]noradrenaline. AI and AII were equipotent in increasing the rate and force of atrial contractions with a threshold concentration between 0.1 and 1.0 nM. In contrast, AI was approximately twenty times less potent than AII in enhancing the stimulation-induced (S-I) efflux of radioactivity. The actions of AI and AII on rate and force of atrial contractions, as well as those on S-I efflux of radioactivity, were substantially blocked by the AII receptor antagonist saralasin. On the other hand, the converting enzyme inhibitors captopril and MK-422 selectively blocked the actions of AI without affecting those of AII. These results suggest that significant local conversion of AI to AII occurs in guinea-pig isolated atria. This locally generated AII can act on myocardial AII receptors to increase rate and force of atrial contractions, and on AII receptors located on sympathetic nerve terminals to facilitate sympathetic neurotransmission. The lower potency of AI compared to AII in modulating transmitter release, in contrast to the equal potency of the peptides in mediating chronotropic and inotropic responses, may be due to conversion of AI to AII at sites remote from the neuroeffector junction.

Identification and characterization of the rabbit angiotensin II myocardial receptor

The octapeptide, angiotensin II, elicits a positive inotropic response in myocardial tissue by activating slow calcium channels. Pharmacological studies suggest that the inotropic action of angiotensin II is receptor mediated. The current investigation was performed to characterize the binding of 125I-angiotensin II to the putative receptor in a plasma membrane-sarcoplasmic reticulum preparation of the rabbit left ventricle. In experiments performed at 18 degrees C, steady state binding occurred at 45 minutes and saturable binding was 80-85% of the total binding. Analysis of the binding data indicated that the 125I-angiotensin II interacted with a single class of sites with a Kd = 4.5 +/- 0.8 nM and exhibited a binding capacity of 53.5 +/- 8 fmol/mg protein. The potency order for the competitive binding of analogues and antagonists of angiotensin II paralleled that observed for in vitro contractile force development in bioassay systems utilizing left atrial tissue. The binding of 125I-angiotensin II was stimulated 2-fold in the presence of the divalent cations of calcium and magnesium (10 mM). Guanine nucleotides modified the binding of 125I-angiotensin II to the rabbit myocardial particulate fraction. Guanine triphosphate and nonhydrolyzable analogues of guanine triphosphate increased the dissociation rate of the bound 125I-angiotensin II and decreased hormone binding to the receptor at equilibrium. In the absence of magnesium, guanine nucleotides had no effect on the dissociation rate of 125I-angiotensin II. 125I-Angiotensin II binding to a rabbit myocardial particulate fraction was found to have high affinity, to be saturable, reversible, specific, and modulated by guanine nucleotides.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of angiotensin II on the cardiac responses to sympathetic nerve stimulation in dogs

In anesthetized dogs with the left cardiac sympathetic nerves and both vagal nerves intact, angiotensin II (AII) induced a substantial, dose-dependent increase in arterial blood pressure and small increments in cardiac cycle length and ventricular contractile force. In dogs in which the cardiac sympathetic and vagal nerves had been interrupted, AII produced similar increases in blood pressure and larger increases in contractile force, but it decreased the cardiac cycle length. In both groups of dogs, AII augmented substantially the positive inotropic responses to sympathetic nerve stimulation, but it enhanced the positive chronotropic responses only slightly. However, AII did not appreciably prolong the cardiac responses to sympathetic nerve stimulation, nor did it alter significantly the cardiac responses to norepinephrine infusions. Hence, at the dosage levels used, AII probably did not inhibit the neuronal uptake of norepinephrine appreciably nor did it enhance the responsiveness of the cardiac effector sites to norepinephrine. Therefore, the potentiation of the cardiac responses to sympathetic nerve stimulation by AII in these experiments was probably achieved principally by facilitating norepinephrine release from the adrenergic nerve terminals in the heart.

Evidence for a prostacyclin-mediated chronotropic effect of angiotensin II in the isolated cat right atria

The positive chronotropic effect of angiotensin II was studied on the isolated spontaneously beating cat right atria. Angiotensin II and prostacyclin produced similar positive chronotropic and inotropic effects which were not affected by beta-adrenoceptor and histamine H2-receptor blockers. [Sar1,Ile8] angiotensin II, however, inhibited the positive chronotropic and inotropic responses to angiotensin II without altering that to prostacyclin, noradrenaline and histamine. Nicotine completely abolished the positive chronotropic action of angiontensin II without altering its positive inotropic effect. Nicotine failed to abolish the effects of prostacyclin, noradrenaline and histamine on the rate and contractility. These results were taken as evidence that the angiotensin II-induced increase in heart rate is probably mediated through the increase of prostacyclin biosynthesis in the isolated cat right atria.

Chronotropic effects of angiotensin I, angiotensin II, bradykinin and vasopressin in guinea pig atria

Chronotropic responses to angiotensin I and angiotensin II, vasopressin and bradykinin were measured in guinea pig isolated right atria. Angiotensin II (100-30,000 pg/ml) was slightly more potent than angiotensin I and caused a maximum tachycardia of 30-40 b/min; only 20% of the maximum response to (--)-noradrenaline. Propranolol (1 micro M) or reserpine pretreatment (1 mg/kg i.p., 24 h) did not alter the response to angiotensin II or bradykinin. Converting enzyme inhibition by captopril (10 micrograms/ml) did not affect resting rate nor the response to angiotensin II but shifted the location of the angiotensin I curve by 40 fold to the right. Bradykinin (5-500 ng/ml) caused small increases in rate while vasopressin 1-100 ng/ml was completely without effect. These results suggest that angiotensin II has a small positive chronotropic effect that is not dependent on tissue noradrenaline release or beta-adrenoceptors and that tissue converting enzyme is active in right atria. Relatively high concentrations of angiotensin and bradykinin were required to directly stimulate the sino-atrial node compared with plasma levels measured during physiological stimuli. Therefore these effects on atria are probably of little physiological significance for peptide concentrations in plasma but may be important in relation to local tissue generation of angiotensin II.