Local angiotensin II generation in the rat heart: role of renin uptake

RAS inhibition in resident fibroblast biology

Angiotensin II (Ang II) is a primary mediator of profibrotic signaling in the heart and more specifically, the cardiac fibroblast. Ang II-mediated cardiomyocyte hypertrophy in combination with cardiac fibroblast proliferation, activation, and extracellular matrix production compromise cardiac function and increase mortality in humans. Profibrotic actions of Ang II are mediated by increasing production of fibrogenic mediators (e.g. transforming growth factor beta, scleraxis, osteopontin, and periostin), recruitment of immune cells, and via increased reactive oxygen species generation. Drugs that inhibit Ang II production or action, collectively referred to as renin angiotensin system (RAS) inhibitors, are first line therapeutics for heart failure. Moreover, transient RAS inhibition has been found to persistently alter hypertensive cardiac fibroblast responses to injury providing a useful tool to identify novel therapeutic targets. This review summarizes the profibrotic actions of Ang II and the known impact of RAS inhibition on cardiac fibroblast phenotype and cardiac remodeling.

Plasma renin activity, response to aliskiren, and clinical outcomes in patients hospitalized for heart failure: the ASTRONAUT trial

AIMS

The direct renin inhibitor, aliskiren, is known to reduce plasma renin activity (PRA), but whether the efficacy of aliskiren varies based on an individual's baseline PRA in patients hospitalized for heart failure (HF) is presently unknown. We characterized the prognostic value of PRA and determined if this risk is modifiable with use of aliskiren.

METHODS AND RESULTS

This pre-specified neurohormonal substudy of ASTRONAUT analysed all patients hospitalized for HF with ejection fraction (EF) ≤40% with available baseline PRA data (n = 1306, 80.9%). Risk associated with baseline PRA and short-term changes in PRA from baseline to 1 month was modelled with respect to 12-month clinical events. Median baseline PRA was 3.0 (interquartile range 0.6-16.4) ng/mL/h. Aliskiren significantly reduced PRA early after treatment initiation through 12-month follow-up compared with placebo (P < 0.001). The lowest baseline PRA quartile (<0.6 ng/mL/h) was independently predictive of lower all-cause mortality [adjusted hazard ratio (HR) 0.50, 95% confidence interval (CI) 0.31-0.81] and the composite of cardiovascular mortality and HF hospitalization (adjusted HR 0.57, 95% CI 0.40-0.79). Delta log-normalized PRA (from baseline to 1 month) was not predictive of either primary endpoint at 12 months (P ≥ 0.43). The prognostic value of baseline PRA and short-term changes in PRA did not vary by randomization to aliskiren or placebo (interaction P ≥ 0.13).

CONCLUSIONS

Plasma renin activity is reduced early and durably by aliskiren, but this did not translate into improved clinical outcomes in ASTRONAUT. Baseline PRA or short-term reduction in PRA do not identify a subgroup who may preferentially benefit from direct renin inhibition. Clinical Trial Registration ClinicalTrials.gov Unique Identifier: NCT00894387.

Intracellular renin increases the inward calcium current in smooth muscle cells of mesenteric artery of SHR. Implications for hypertension and vascular remodeling

The influence of intracellular renin on the inward calcium current in isolated smooth muscle cells from SHR mesenteric arteries was investigated. Measurements of calcium current were performed using the whole cell configuration of pCLAMP. The results indicated that: 1) renin (100nM) dialyzed into smooth muscle cells, increased the inward calcium current; 2) verapamil (10-9M) administered to the bath inhibited the effect of renin on the inward calcium current; 3) concurrently with the increase of calcium current a depolarization of 6.8+/-2.1mV (n=16)(P<0.05) was found in cells dialyzed with renin; 4) intracellular dialysis of renin (100nM) into smooth muscle cells isolated from mesenteric arteries of normal Wystar Kyoto rats showed no significant change on calcium current; 5) aliskiren (10-9M) dialyzed into the cell together with renin (100nM) abolished the effect of the enzyme on the calcium current in SHR; 6) Ang II (100nM) dialyzed into the smooth muscle cell from mesenteric artery of SHR in absence of renin, decreased the calcium current-an effect greatly reduced by valsartan (10-9M) added to the cytosol; 7) administration of renin (100nM) plus angiotensinogen (100nM) into the cytosol of muscles cells from SHR rats reduced the inward calcium current; 8) extracellular administration of Ang II (100nM) increased the inward calcium current in mesenteric arteries of SHR.

CONCLUSIONS

intracellular renin in vascular resistance vessels from SHR due to internalization or expression, contributes to the regulation of vascular tone and control of peripheral resistance-an effect independently of Ang II. Implications for hypertension and vascular remodeling are discussed.

The Role of the (Pro)renin Receptor in Hypertensive Disease

Tissue angiotensin generation depends on the uptake of circulating (kidney-derived) renin and/or its precursor prorenin (together denoted as (pro)renin). Since tissue renin levels are usually higher than expected based upon the amount of (renin-containing) blood in tissue, an active uptake mechanism has been proposed. The (pro)renin receptor ((P)RR), discovered in 2002, appeared a promising candidate, although its nanomolar affinity for renin/prorenin is many orders of magnitude above their levels in blood. This review discusses (P)RR-related research since its discovery. First, encouraging in vitro findings supported detrimental effects of (pro)renin-(P)RR interaction, even resulting in angiotensin-independent signaling. Moreover, the putative (P)RR blocker "handle region peptide" (HRP) yielded beneficial effects in various cardiovascular animal models. Then doubt arose whether such interaction truly occurs in vivo, and (P)RR deletion unexpectedly turned out to be lethal. Moreover, HRP results could not be confirmed. Finally, it was discovered that the (P)RR actually is a component of vacuolar-type H(+)-ATPase, a multisubunit protein found in virtually every cell type which is essential for vesicle trafficking, protein degradation, and coupled transport. Nevertheless, selective (P)RR blockade in the brain with the putative antagonist PRO20 (corresponding with the first 20 amino acids of prorenin's prosegment) reduced blood pressure in the deoxycorticosteroneacetate (DOCA)-salt model, and (P)RR gene single nucleotide polymorphisms associate with hypertension. To what degree this relates to (pro)renin remains uncertain. The concept of (P)RR blockade in hypertension, if pursued, requires rigorous testing of any newly designed antagonist, and may not hold promise given the early death of tissue-specific (P)RR knockout animals.

Long-term effects of early overnutrition in the heart of male adult rats: role of the renin-angiotensin system

To analyze the long-term effects of early overfeeding on the heart and coronary circulation, the effect of ischemia-reperfusion (I/R) and the role of the renin-angiotensin system (RAS) was studied in isolated hearts from control and overfed rats during lactation. On the day of birth litters were adjusted to twelve pups per mother (controls) or to three pups per mother (overfed). At 5 months of age, the rats from reduced litters showed higher body weight and body fat than the controls. The hearts from these rats were perfused in a Langendorff system and subjected to 30 min of ischemia followed by 15 min of reperfusion (I/R). The myocardial contractility (dP/dt) and the coronary vasoconstriction to angiotensin II were lower, and the expression of the apoptotic marker was higher, in the hearts from overfed rats compared to controls. I/R reduced the myocardial contractily, the coronary vasoconstriction to angiotensin II and the vasodilatation to bradykinin, and increased the expression of (pro)renin receptor and of apoptotic and antiapoptotic markers, in both experimental groups. I/R also increased the expression of angiotensinogen in control but not in overfed rats. In summary, the results of this study suggest that early overnutrition induces reduced activity of the RAS and impairment of myocardial and coronary function in adult life, due to increased apoptosis. Ischemia-reperfusion produced myocardial and coronary impairment and apoptosis, which may be related to activation of RAS in control but not in overfed rats, and there may be protective mechanisms in both experimental groups.

Exercise induces renin-angiotensin system unbalance and high collagen expression in the heart of Mas-deficient mice

The renin-angiotensin system (RAS) is involved in the cardiac and vascular remodeling associated with cardiovascular diseases. Angiotensin (Ang) II/AT(1) axis is known to promote cardiac hypertrophy and collagen deposition. In contrast, Ang-(1-7)/Mas axis opposes Ang II effects in the heart producing anti-trophic and anti-fibrotic effects. Exercise training is known to induce cardiac remodeling with physiological hypertrophy without fibrosis. We hypothesize that cardiac remodeling induced by chronic exercise depends on the action of Ang-(1-7)/Mas axis. Thus, we evaluated the effect of exercise training on collagen deposition and RAS components in the heart of FVB/N mice lacking Mas receptor (Mas-KO). Male wild-type and Mas-KO mice were subjected to a moderate-intense swimming exercise training for 6 weeks. The left ventricle (LV) of the animals was sectioned and submitted to qRT-PCR and histological analysis. Circulating and tissue angiotensin peptides were measured by RIA. Sedentary Mas-KO presented a higher circulating Ang II/Ang-(1-7) ratio and an increased ACE2 expression in the LV. Physical training induced in Mas-KO and WT a similar cardiac hypertrophy accompanied by a pronounced increase in collagen I and III mRNA expression. Trained Mas-KO and trained WT presented increased Ang-(1-7) in the blood. However, only in trained-WT there was an increase in Ang-(1-7) in the LV. In summary, we showed that deletion of Mas in FVB/N mice produced an unbalance in RAS equilibrium increasing Ang II/AT(1) arm and inducing deleterious cardiac effects as deposition of extracellular matrix proteins. These data indicate that Ang-(1-7)/Mas axis is an important counter-regulatory mechanism in physical training mediate cardiac adaptations.

The intracrine renin-angiotensin system

The RAS (renin-angiotensin system) is one of the earliest and most extensively studied hormonal systems. The RAS is an atypical hormonal system in several ways. The major bioactive peptide of the system, AngII (angiotensin II), is neither synthesized in nor targets one specific organ. New research has identified additional peptides with important physiological and pathological roles. More peptides also mean newer enzymatic cascades that generate these peptides and more receptors that mediate their function. In addition, completely different roles of components that constitute the RAS have been uncovered, such as that for prorenin via the prorenin receptor. Complexity of the RAS is enhanced further by the presence of sub-systems in tissues, which act in an autocrine/paracrine manner independent of the endocrine system. The RAS seems relevant at the cellular level, wherein individual cells have a complete system, termed the intracellular RAS. Thus, from cells to tissues to the entire organism, the RAS exhibits continuity while maintaining independent control at different levels. The intracellular RAS is a relatively new concept for the RAS. The present review provides a synopsis of the literature on this system in different tissues.

Emerging role of G protein-coupled receptors in microvascular myogenic tone

Blood flow autoregulation results from the ability of resistance arteries to reduce or increase their diameters in response to changes in intravascular pressure. The mechanism by which arteries maintain a constant blood flow to organs over a range of pressures relies on this myogenic response, which defines the intrinsic property of the smooth muscle to contract in response to stretch. The resistance to flow created by myogenic tone (MT) prevents tissue damage and allows the maintenance of a constant perfusion, despite fluctuations in arterial pressure. Interventions targeting MT may provide a more rational therapeutic approach in vascular disorders, such as hypertension, vasospasm, chronic heart failure, or diabetes. Despite its early description by Bayliss in 1902, the cellular and molecular mechanisms underlying MT remain poorly understood. We now appreciate that MT requires a complex mechanotransduction converting a physical stimulus (pressure) into a biological response (change in vessel diameter). Although smooth muscle cell depolarization and a rise in intracellular calcium concentration are recognized as cornerstones of the myogenic response, the role of wall strain-induced formation of vasoactive mediators is less well established. The vascular system expresses a large variety of Class 1 G protein-coupled receptors (GPCR) activated by an eclectic range of chemical entities, including peptides, lipids, nucleotides, and amines. These messengers can function in blood vessels as vasoconstrictors. This review focuses on locally generated GPCR agonists and their proposed contributions to MT. Their interplay with pivotal G(q-11) and G(12-13) protein signalling is also discussed.

Targeting cardiac mast cells: pharmacological modulation of the local renin-angiotensin system

Enhanced production of angiotensin II and excessive release of norepinephrine in the ischemic heart are major causes of arrhythmias and sudden cardiac death. Mast cell-dependent mechanisms are pivotal in the local formation of angiotensin II and modulation of norepinephrine release in cardiac pathophysiology. Cardiac mast cells increase in number in myocardial ischemia and are located in close proximity to sympathetic neurons expressing angiotensin AT1- and histamine H3-receptors. Once activated, cardiac mast cells release a host of potent pro-inflammatory and pro-fibrotic cytokines, chemokines, preformed mediators (e.g., histamine) and proteases (e.g., renin). In myocardial ischemia, angiotensin II (formed locally from mast cell-derived renin) and histamine (also released from local mast cells) respectively activate AT1- and H3-receptors on sympathetic nerve endings. Stimulation of angiotensin AT1-receptors is arrhythmogenic whereas H3-receptor activation is cardioprotective. It is likely that in ischemia/reperfusion the balance may be tipped toward the deleterious effects of mast cell renin, as demonstrated in mast cell-deficient mice, lacking mast cell renin and histamine in the heart. In these mice, no ventricular fibrillation occurs at reperfusion following ischemia, as opposed to wild-type hearts which all fibrillate. Preventing mast cell degranulation in the heart and inhibiting the activation of a local renin-angiotensin system, hence abolishing its detrimental effects on cardiac rhythmicity, appears to be more significant than the loss of histamine-induced cardioprotection. This suggests that therapeutic targets in the treatment of myocardial ischemia, and potentially congestive heart failure and hypertension, should include prevention of mast cell degranulation, mast cell renin inhibition, local ACE inhibition, ANG II antagonism and H3-receptor activation.

(Pro)renin and its receptors: pathophysiological implications

Tissue angiotensin generation depends on the uptake of circulating (kidney-derived) renin and/or its precursor prorenin [together denoted as (pro)renin]. Since tissue renin levels are usually somewhat higher than expected based upon the amount of (renin-containing) blood in tissue, an active uptake mechanism has been proposed. Several candidates have been evaluated in the past three decades, including a renin-binding protein, the mannose 6-phosphate/insulin-like growth factor II receptor and the (pro)renin receptor. Although the latter seemed the most promising, its nanomolar affinity for renin and prorenin is several orders of magnitude above their actual (picomolar) levels in blood, raising doubt on whether (pro)renin–(pro)renin receptor interaction will ever occur in vivo. A wide range of in vitro studies have now demonstrated (pro)renin-receptor-induced effects at nanomolar renin and prorenin concentrations, resulting in a profibrotic phenotype. In addition, beneficial in vivo effects of the putative (pro)renin receptor blocker HRP (handle region peptide) have been observed, particularly in diabetic animal models. Despite these encouraging results, many other studies have reported either no or even contrasting effects of HRP, and (pro)renin-receptor-knockout studies revealed lethal consequences that are (pro)renin-independent, most probably due to the fact that the (pro)renin receptor co-localizes with vacuolar H+-ATPase and possibly determines the stability of this vital enzyme. The present review summarizes all of the recent findings on the (pro)renin receptor and its blockade, and critically compares it with the other candidates that have been proposed to mediate (pro)renin uptake from blood. It ends with the conclusion that the (pro)renin–(pro)renin receptor interaction, if it occurs in vivo, is limited to (pro)renin-synthesizing organs such as the kidney.

Targeting the ACE2-Ang-(1-7) pathway in cardiac fibroblasts to treat cardiac remodeling and heart failure

Fibroblasts play a pivotal role in cardiac remodeling and the development of heart failure through the deposition of extra-cellular matrix (ECM) proteins and also by affecting cardiomyocyte growth and function. The renin-angiotensin system (RAS) is a key regulator of the cardiovascular system in health and disease and many of its effects involve cardiac fibroblasts. Levels of angiotensin II (Ang II), the main effector molecule of the RAS, are elevated in the failing heart and there is a substantial body of evidence indicating that this peptide contributes to changes in cardiac structure and function which ultimately lead to progressive worsening in heart failure. A pathway involving angiotensin converting enzyme 2 (ACE2) has the capacity to break down Ang II while generating angiotensin-(1-7) (Ang-(1-7)), a heptapeptide, which in contrast to Ang II, has cardioprotective and anti-remodeling effects. Many Ang-(1-7) actions involve cardiac fibroblasts and there is information indicating that it reduces collagen production and also may protect against cardiac hypertrophy. This report describes the effects of ACE2 and Ang-(1-7) that appear to be relevant in cardiac remodeling and heart failure and explores potential therapeutic strategies designed to increase ACE2 activity and Ang-(1-7) levels to treat these conditions. This article is part of a special issue entitled ''Key Signaling Molecules in Hypertrophy and Heart Failure.''

Cardiac chymase converts rat proAngiotensin-12 (PA12) to angiotensin II: effects of PA12 upon cardiac haemodynamics

AIMS

The aim of this study was to observe the direct physiological and biochemical cardiac effects in response to a newly identified putative component of the renin-angiotensin system, proangiotensin-12 (PA12); and investigate whether PA12 can serve as a substrate for Angiotensin II (AngII) generation.

METHODS AND RESULTS

The direct cardiac actions of PA12 and its role as a substrate for chymase-dependent AngII generation were investigated in Sprague-Dawley rats using an isolated heart model of cardiac ischaemia-reperfusion injury. PA12 potently constricted coronary arteries with no significant effect on left-ventricular contractility. PA12 impaired recovery from global ischaemia, maintaining coronary constriction and markedly increasing release of creatine kinase and troponin I (TnI), indicating greater myocardial injury. Analysis of perfusate collected after transcardiac passage revealed a marked increase in AngII production from hearts infused with PA12. Cardiac AngII production was not blocked by angiotensin-converting enzyme inhibitors, whereas inhibition of chymase with chymostatin significantly reduced AngII production and attenuated PA12-induced vasoconstriction and myocardial damage following ischaemia. Furthermore, Angiotensin II type 1 receptor (AT(1)R) blockade abolished PA12 activity. In vitro, PA12 was efficiently and precisely converted to AngII as assessed on reverse phase-high performance liquid chromatography coupled to tandem mass spectrometry. This conversion was blocked by chymostatin.

CONCLUSION

PA12 may act as a circulating substrate for cardiac chymase-mediated AngII production, in contrast to ACE-mediated AngII production from AngI.

AT2 receptors: functional relevance in cardiovascular disease

The renin angiotensin system (RAS) is intricately involved in normal cardiovascular homeostasis. Excessive stimulation by the octapeptide angiotensin II contributes to a range of cardiovascular pathologies and diseases via angiotensin type 1 receptor (AT1R) activation. On the other hand, tElsevier Inc.he angiotensin type 2 receptor (AT2R) is thought to counter-regulate AT1R function. In this review, we describe the enhanced expression and function of AT2R in various cardiovascular disease settings. In addition, we illustrate that the RAS consists of a family of angiotensin peptides that exert cardiovascular effects that are often distinct from those of Ang II. During cardiovascular disease, there is likely to be an increased functional importance of AT2R, stimulated by Ang II, or even shorter angiotensin peptide fragments, to limit AT1R-mediated overactivity and cardiovascular pathologies.

Cardiovascular effects of prorenin blockade in genetically spontaneously hypertensive rats on normal and high-salt diet

Recent reports have demonstrated a potential role of tissue prorenin in the pathogenesis of cardiovascular and renal damage. This study was designed to examine the role of prorenin in the pathogenesis of target organ damage in spontaneously hypertensive rats (SHRs), the best naturally occurring experimental model of essential hypertension. To this end, we studied 20-wk-old male SHRs receiving a normal diet and 8-wk-old male SHRs given food with 8% NaCl. One-half the rats in each group were given prorenin inhibitor (PRAM-1, 0.1 mg.kg(-1).day(-1)) via osmotic minipumps; the other half served as controls. Arterial pressure, left ventricular function, cardiovascular mass indexes, cardiac fibrosis, and renal function were examined at the end of the experiment. Arterial pressure was unaffected by PRAM-1 in rats on either regular or salt-excess diets. In those rats receiving a normal diet, the blockade of prorenin activation consistently reduced left ventricular mass but affected no other variable. Salt-loaded rats given PRAM-1 for 8 wk demonstrated (1) reduced serum creatinine level, (2) decreased left ventricular mass, (3) improved left ventricular function, and (4) reduced left ventricular fibrosis. These data demonstrated that the blockade of nonproteolytic activation of prorenin exerted significant cardiovascular and renal benefit in SHRs with cardiovascular damage produced by salt excess and suggested that the activation of cardiovascular or renal prorenin may be a major mechanism that mediates cardiac and renal damage in this form of accelerated hypertension.

The putative (pro)renin receptor blocker HRP fails to prevent (pro)renin signaling

The prorenin/renin receptor is a recently discovered component of the renin-angiotensin system. The effects of aliskiren, a direct inhibitor of human renin, were compared with the handle region decoy peptide (HRP), which blocks the prorenin/renin receptor, in double-transgenic rats overexpressing the human renin and angiotensinogen genes. After 7 wk, all aliskiren-treated rats were alive, whereas mortality was 40% in vehicle-treated and 58% in HRP-treated rats. Aliskiren but not the HRP reduced BP and normalized albuminuria, cystatin C, and neutrophil gelatinase-associated lipocalin, a marker of renal tubular damage, to the levels of nontransgenic controls. In vitro, human renin and prorenin induced extracellular signal-regulated kinase 1/2 phosphorylation, independent of angiotensin II (AngII), in vascular smooth muscle cells. Preincubation with the HRP or aliskiren did not prevent renin- and prorenin-induced extracellular signal-regulated kinase 1/2 phosphorylation, whereas the MAP kinase kinase (MEK1/2) inhibitor PD98059 prevented both. In conclusion, renin inhibition but not treatment with the HRP protects against AngII-induced renal damage in double-transgenic rats. In addition, the in vitro data do not support the use of the HRP to block AngII-independent prorenin- or renin-mediated effects.

Angiotensin II Signaling in Vascular Physiology and Pathophysiology

Initially recognized as a physiologic regulator of blood pressure and body fluid homeostasis, angiotensin (Ang) II has now been shown in innumerable experiments and clinical studies to contribute to the development and maintenance of cardiovascular disease. Dissection of its signaling mechanisms over the past decades has led to the discovery of several novel concepts, such as tissue-specific metabolism of Ang peptides. Identification and cloning of the various receptors through which Ang II acts on almost all tissues has led to the development of specific pharmacologic inhibitors with proven clinical benefit in patients with cardiovascular disorders. Work on the G-protein-coupled Ang II Type 1 receptor has demonstrated that different receptors interact through oligomerization, compartmentalization, and transactivation, and may explain how Ang II can activate G-protein-independent pathways. Unraveling the downstream effects of Ang II in specific cell types corroborates the importance of the cellular redox state on certain signaling pathways. Finally, the effects of Ang II on cell function and phenotype, such as the expression of inflammatory cytokines and receptors promoting the recruitment of inflammatory cells into vascular tissues, have indicated its role in local inflammation as a general pathogenetic basis of cardiovascular disease. The recognition of Ang II as a contributor to such fundamental pathophysiologic mechanisms, which are believed to be a common pathway for diverse cardiovascular risk factors like hypertension and diabetes, has greatly advanced our knowledge of pathologic signaling in vascular tissues and may help to eventually define novel targets for pharmacologic interventions.

Increased hypothalamic angiotensin-(1-7) levels in rats with aortic coarctation-induced hypertension

Since angiotensin (Ang) (1-7) injected into the brain blocked Ang II pressor actions in rats made hypertensive by aortic coarctation (CH), we examined systemic and tissue angiotensin peptide levels, specifically concentrating on the hypothalamic Ang-(1-7) levels. Plasma, heart and kidney isolated from CH rats showed increased levels of Ang I, Ang II and Ang-(1-7) compared with the normotensive group, with Ang II being the predominant peptide in heart and kidney. In the hypothalamus, equimolar amounts of Ang II and Ang-(1-7) were found in the sham group, whereas only Ang-(1-7) levels increased in CH rats. We conclude that aortic coarctation activates systemic and tissue renin-angiotensin system. The increased central levels of Ang-(1-7) in the CH rats suggest a potential mitigating role of this peptide in central control of the hypertensive process.

The intracellular renin-angiotensin system: a new paradigm

More than a century after its discovery, the physiological implications of the renin-angiotensin system (RAS) continue to expand, with the identification of new components, functions and subsystems. These advancements have led to better management and understanding of a broad range of cardiovascular and metabolic disorders. The RAS has traditionally been viewed as a circulatory system, involved in the short-term regulation of volume and blood pressure homeostasis. Recently, local RASs have been described as regulators of chronic tissue effects. Most recently, studies have provided evidence of a complete, functional RAS within cells, described as an 'intracrine' or intracellular system. A more comprehensive understanding of the intracellular RAS provides for new strategies in system regulation and a more efficacious approach to the management of RAS-related diseases.

Functional renin receptors in renal mesangial cells

Activation of the renin-angiotensin system (RAS) and generation of angiotensin II (Ang II) play a crucial role in fibrotic renal disease beyond this system's hemodynamic actions. Ang II blockade was a great therapeutic breakthrough for renal and cardiovascular diseases; however, this slows, but does not stop, disease progression. These limitations leave other molecules unopposed to sustain disease progression. One is renin, which is markedly elevated by Ang II blockade. Recently, a new renin receptor was cloned in renal mesangial cells. This receptor acts as a renin/prorenin cofactor on the cell surface, enhancing efficiency of angiotensinogen cleavage by renin and unmasking prorenin catalytic activity. Unexpectedly, the receptor induces angiotensin-independent cellular effects in renal mesangial cells, suggesting that renin has novel receptor-mediated actions that could play a role in renal fibrosis. Proof of this could lead to a pharmacological compound blocking renin/prorenin binding and activity as an alternative or adjunct to classical inhibitors of the RAS.

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.

Antioxidant effects and the therapeutic mode of action of calcium channel blockers in hypertension and atherosclerosis

Drugs currently known as calcium channel blockers (CCB) were initially called calcium antagonists because of their ability to inhibit calcium-evoked contractions in depolarized smooth muscles. Blocking the entry of calcium reduces the active tone of vascular smooth muscle and produces vasodilatation. This pharmacological property has been the basis for the use of CCBs in the management of hypertension and coronary heart disease. A major question is whether drugs reducing blood pressure have other effects that help prevent the main complications of hypertension, such as atherosclerosis, stroke, peripheral arterial disease, heart failure and end-state renal disease. Experimental studies that focus on this question are reviewed in the present paper.

Renin/prorenin receptors

The existence of a tissue renin-angiotensin (RAS) system independent of the circulating RAS has prompted the search for cellular binding sites for angiotensinogen and for renin in order to explain their tissue uptake. Two receptors that bind with similar affinity mature renin and prorenin were identified, the mannose-6-phosphate receptor (M6P-R) and a specific receptor. The M6P-R is a clearance receptor that binds exclusively the glycosylated forms of renin and prorenin. Binding of renin and prorenin to the M6P-R is followed by internalization and degradation, and the intracellular proteolysis of prorenin in mature renin did not provoke any generation of intracellular angiotensins. In contrast to the M6P-R, (pro)renin bound to the specific receptor was not degraded. Instead, receptor-bound renin showed increased catalytic activity, and receptor-bound prorenin exhibited full catalytic activity. This 'gain of activity' was explained by a conformational change of the (pro)renin molecule upon binding. Furthermore, (pro)renin binding provoked a rapid activation of the mitogen-activated protein kinases p44/p42, indicating that the receptor has mediated specific, angiotensin II-independent effects of (pro)renin. This receptor represents an elegant concept to explain the existence of active prorenin in vivo, and it provides a pathological role for prorenin in situations with paradoxical low renin and high prorenin concentrations such as in diabetes. Experimental models of rats overexpressing the receptor either in vascular smooth muscle cells and developing high blood pressure or with ubiquitous expression associated with glomerulosclerosis and proteinuria confirm a role for the receptor in cardiovascular and renal diseases.

Genomic and nongenomic effects of aldosterone in the rat heart: why is spironolactone cardioprotective?

1. Mineralocorticoid receptor (MR) antagonism with spironolactone reduces mortality in heart failure on top of ACE inhibition. To investigate the underlying mechanism, we compared the actions of both aldosterone and spironolactone to those of angiotensin (Ang) II in the rat heart. 2. Hearts of male Wistar rats were perfused according to Langendorff. Ang II and aldosterone increased left ventricular pressure (LVP) by maximally 11+/-4 and 9+/-2%, and decreased coronary flow (CF) by maximally 36+/-7 and 20+/-4%, respectively. Spironolactone did not significantly affect LVP or CF. 3. In hearts that were exposed to a 45-min coronary artery occlusion and 3 h of reperfusion, a 15-min exposure to spironolactone prior to occlusion reduced infarct size (% of risk area) from 68+/-2 to 45+/-3%, similar to the reduction (34+/-2%) observed following 'preconditioning' (15 min occlusion followed by 10 min reperfusion) prior to the 45-min occlusion. Aldosterone exposure did not affect infarct size (71+/-5%). 4. In cardiomyocytes, aldosterone decreased [(3)H]thymidine incorporation maximally by 73+/-3%, whereas in cardiac fibroblasts it decreased [(3)H]proline incorporation by 33+/-7%. Spironolactone inhibited both effects. Ang II increased DNA and collagen synthesis, and these effects were reversed by aldosterone. 5. In conclusion, aldosterone induces positive inotropic and vasoconstrictor effects in a nongenomic manner, and these effects are comparable to those of Ang II. Aldosterone reduces DNA and collagen synthesis via MR activation, and counteracts the Ang II-induced increases in these parameters. MR blockade reduces infarct size and increases LVP recovery following coronary artery occlusion. The MR-related phenomena may underlie, at least in part, the beneficial actions of spironolactone in heart failure.

Calcium channel blocker inhibits Western-type diet-evoked atherosclerosis development in ApoE-deficient mice

Calcium channel blockers slow the progression of atherosclerosis. The purpose of the present experiments was to examine the action of lacidipine in a condition that accelerates the development of atherosclerosis in order to test the hypothesis that the protective action of lacidipine in atherosclerosis is unrelated to the reduction of blood pressure. Male ApoE-deficient mice (6 weeks old) were exposed either to normal chow (ND) or to a Western-type diet (WD, adjusted calorie diet containing 42% from fat) for 8 weeks. Western-type diet induced a reduction of nitric oxide (NO)-mediated endothelium-dependent relaxation to acetylcholine (Max relaxation % = 55.8 +/- 2 for ND and 46.6 +/- 2 for WD, n = 8, p < 0.05). Dose-relaxation curves to S-nitroso-N-acetylpenicillamine (SNAP) NO donor were also significantly rightward-shifted (n = 7, ANOVA, p < 0.01) in WD compared with ND arteries. Chronic treatment of WD mice with lacidipine (1 and 3 mg/kg/day) increased significantly the acetylcholine-evoked relaxation (to 76.6 +/- 3.5%, n = 6, ANOVA, p < 0.001) and prevented the loss of responsiveness to SNAP in mice exposed to WD. Plasma renin activity and endothelin-1 plasma levels as well as thiobarbituric acid-reactive substance levels in kidneys were significantly lower in WD mice treated with lacidipine than in untreated ones. In mice exposed to WD lacidipine reduced extension of atherosclerotic lesions, renal injury and increase in blood pressure. Experimental data indicate that inhibition of Western-type diet-evoked alterations is related to both antioxidant and vasoactive properties of lacidipine.

Raised prorenin and renin concentrations in pre-eclamptic placentae when measured after acid activation

AIM

The aim of the study was to ascertain if there was any difference in the levels of prorenin and active renin between pre-eclamptic and normotensive feto-placental tissues.

METHODS

Supernatants of homogenates from fresh, vaginally delivered placentae from 15 normotensive and 15 pre-eclamptic women were measured for renin concentration (RC), prorenin concentration and renin activity (RA). RA and RC were measured in the absence and presence of nephrectomised sheep plasma, respectively. Prorenin was estimated as the difference between renin concentration in the sample before and after acid activation. All concentrations are expressed as rate of angiotensin generation (ng/ml/h). Angiotensin I was measured by radioimmunoassay. Statistical analysis was performed using Student's 't' test for unpaired samples. All results are presented as mean+/-SEM.

RESULTS

The concentrations of renin and prorenin were highest in the chorion laeve when compared to amnion and placenta (p < 0.01) in both the groups. Furthermore, the concentrations of renin and prorenin were significantly higher in all the tissues from women with pre-eclampsia (p < 0.01).

CONCLUSION

Renin and prorenin levels are raised in the placental tissues from women with pre-eclampsia. With recent evidence suggesting that both prorenin and renin may have cellular effects independent of angiotensin II generation, there is a need for further study into its role in placentation.

Combined renin-angiotensin system blockade and dietary sodium restriction impairs cardiomyocyte contractility

INTRODUCTION

Blockade of the renin-angiotensin system (RAS) by combined angiotensin-converting enzyme inhibitor and angiotensin type 1 receptor (AT(1)) antagonist treatment with reduced dietary sodium intake produces suppression of cardiac growth and regression of cardiac hypertrophy. The purpose of this study was to investigate whether cardiac growth suppression by combined RAS blockade under conditions of dietary sodium restriction is associated with cardiomyocyte atrophy and contractile dysfunction and whether this intervention modifies cardiomyocyte inotropic responsiveness to angiotensin II (Ang II).

METHODS

Rats were fed a high (4% w/w) or low (0.2% w/w) NaCl diet for six days. Both groups were then given a combined intraperitoneal injection of perindopril (6 mg/kg/day) and losartan (10 mg/kg/day) with maintained dietary treatment for another seven days. At the end of the treatment period, animals were anaesthetised and their hearts were removed and weighed. Left ventricular cardiomyocytes were isolated by enzymatic dissociation and cell dimensions were evaluated. A line scan camera and digital imaging technique were used to assess cardiomyocyte contraction and inotropic responses to exogenous Ang II (10 to 10(-8) M).

RESULTS

Dietary treatment alone had no effect on body growth, whereas combined RAS blockade suppressed somatic growth in the low sodium (LS) group, compared with the high sodium (HS) group. This growth suppression in the LS group was also evident in the heart at the organ and cellular level. Studies of cardiomyocyte contraction showed that myocytes from the LS group exhibited contractile instability and depression of contractile performance. Compared with the HS group, myocytes from the LS group showed a significant reduction in maximum cell shortening (6.40+0.17 vs. 7.32+0.16% resting length, p<0.05), and maximum rate of shortening (3.85+0.14 vs. 4.29+0.11 cell length/ms, p<0.05). Myocytes of the HS group exhibited negative inotropic responses to Ang II at all concentrations tested, with a significant reduction in maximum cell shortening by 11 16% after 12 minutes peptide exposure (p<0.05 vs. non-treated control). In comparison, Ang II elicited both positive and negative responses in myocytes from the LS group, with a predominant negative inotropic effect.

CONCLUSIONS

This study provides evidence that combined RAS blockade treatment under restricted sodium intake conditions can impair cardiomyocyte contractile function in association with cardiomyocyte growth suppression. Chronic RAS blockade qualitatively alters the intrinsic inotropic status and responsiveness of ventricular cardiomyocytes, and this shift is further modulated by dietary sodium intake conditions.

Specific MAP-kinase blockade protects against renal damage in homozygous TGR(mRen2)27 rats

Angiotensin II (AngII) plays an important role in renal damage by acting on hemodynamics, cell-growth, proliferation, and fibrosis, mainly by effects on the AngII type 1 (AT(1)) receptor. The AT(1) receptor activates several intracellular signaling molecules such as mitogen-activated protein kinases extracellular signal-regulated kinase (ERK) and p38, but their role in AngII-mediated renal damage is not well characterized. We therefore investigated whether pharmacologic blockade of ERK and p38 could prevent renal damage in high-renin homozygous transgenic rats (Ren2), with the effects of an AT(1) receptor antagonist (AT(1)-RA) as a reference. Seven-week-old homozygous Ren2 rats were treated with low-dose AT(1)-RA candesartan, ERK inhibitor tyrphostin, or p38 inhibitor SB239063 for 4 weeks. Untreated Ren2 and SD rats served as controls. Blood pressure was measured at 7 and 11 weeks. At 11 weeks, plasma renin activity (PRA) and serum aldosterone were determined, and the animals were killed. Kidney sections were scored for glomerular and interstitial smooth muscle actin and glomerular desmin expression as early markers for renal damage. Mesangial matrix expansion was determined as a marker for structural damage. PRA and aldosterone levels were elevated in untreated Ren2 rats in comparison to SD controls. AT(1)-RA further increased PRA but decreased aldosterone. All parameters of renal damage were elevated in untreated Ren2 rats. Blood pressure was not elevated at week 7 in Ren2 and not affected by either treatment. Mild signs of hypertensive damage were found in untreated Ren2 rats. All interventions significantly diminished damage to glomerular epithelium and interstitium. In addition, AT(1) receptor and p38 blockade reduced mesangial matrix expansion. In homozygous Ren2 rats, renal damage was ameliorated by a nonhypotensive dose of an AT(1)-RA and, similarly, by blockade of ERK or p38. This suggests that ERK and p38 are involved in AngII-mediated renal damage.

Local renin-angiotensin systems: the unanswered questions

The concept of local renin-angiotensin systems has been introduced almost 20 years ago to explain the beneficial blood pressure-independent effects of ACE inhibitors and AT(1) receptor antagonists in cardiovascular diseases. In the past decade, research has focussed on the local effects of angiotensin II rather than on the mechanism(s) of its local generation. This review addresses several of the unanswered questions with regard to tissue angiotensin II generation, focussing in particular on the heart and vascular wall: (1) what is the origin of the renin that is required to generate angiotensin II locally, (2) where does tissue angiotensin generation occur (intra- versus extracellular), (3) what is the importance of alternative (non-renin, non-ACE) angiotensin-generating enzymes, (4) do ACE inhibitors and AT(1) receptor antagonists exert local effects that are renin-angiotensin system independent (thereby incorrectly leading to the conclusion that they interfere with the local generation or effects of angiotensin II), and (5) to what degree do differences in tissue angiotensin generation underlie the association between cardiovascular diseases and renin-angiotensin system gene polymorphisms?

Enalaprilat, losartan and LU 135252 in coronary blood flow regulation

BACKGROUND

High plasma levels of angiotensin II are found in several pathologies such as hypertension, heart failure and myocardial infarction. The effect of high concentrations of angiotensin II on coronary circulation is not well defined. The aim of the present study was to assess coronary blood flow regulation during tachycardia in the presence of elevated coronary plasma levels of angiotensin II, and the changes induced by ACE inhibition and blockade of angiotensin II and endothelin-A receptors.

DESIGN

Left anterior coronary artery was catheterized in 38 pigs to infuse the study drugs. Saline was infused for 15 min. Then, the first atrial pacing was performed. The pigs were distributed to: Group 1 (n = 7) angiotensin II; Group 2 (n = 7) enalaprilat + angiotensin II; Group 3 (n = 9) the bradykinin B2 antagonist HOE 140 + enalaprilat + angiotensin II; Group 4 (n = 7) losartan + angiotensin II; and Group 5 (n = 8) endothelin-A receptor antagonist LU 135252 + angiotensin II. After giving these infusions, a second pacing was repeated.

RESULTS

The increase in coronary blood flow induced by pacing with angiotensin II was reduced from 181 +/- 21% to 116 +/- 37% (P = 0.006 vs. saline). Enalaprilat, losartan and LU 135252 restored the capacity of coronary blood flow to increase during pacing (151 +/- 39%, 162 +/- 35% and 161 +/- 16%, respectively; P = NS, vs. saline), while HOE 140 abolished the effect of enalaprilat.

CONCLUSIONS

Moderately elevated coronary concentrations of angiotensin II reduced coronary blood flow during pacing. Enalaprilat, losartan and LU 135252 restored the hyperaemic coronary flow to similar values observed with saline. The beneficial effect of ACE inhibition is mediated through an increase in bradykinin.

The renin receptor: the facts, the promise and the hope

PURPOSE OF REVIEW

The renin-angiotensin system plays a major role in the control of blood pressure and of salt balance, but it is also involved in physiological and pathological processes, development, inflammation and cardiac hypertrophy. A concept has emerged suggesting that these effects are due to a local activation of the renin-angiotensin system. The search for a receptor of renin was based on the idea that tissue (pro)renin is taken up from the circulation and on data suggesting that renin has cellular effects independent of angiotensin II.

RECENT FINDINGS

Endothelial cells and cardiac myocytes bind (pro)renin via the mannose-6-phosphate receptor, mainly a clearance receptor as no cellular effect has been specifically attributed to prorenin binding. A functional receptor was cloned recently. It mediates intracellular signalling by activating the mitogen activated protein kinases, extracellular signal regulated kinases 1 and 2, and acts as a co-factor by increasing the efficiency of angiotensinogen cleavage by receptor-bound (pro)renin. The receptor is abundantly expressed in heart, brain, placenta and eye, compared with a lower expression in liver and kidney. In normal human kidney and heart, it is localized in the mesangium and in the coronary and kidney artery, associated with smooth-muscle cells and co-localized with renin.

SUMMARY

This receptor provides a functional role for prorenin and may help to understand the physiological and pathological role of elevated levels of prorenin and of local activation of the renin-angiotensin system. From a practical point of view, it questions the need for a pharmacological compound blocking (pro)renin binding and activity as an alternative to the classical inhibitors of the renin-angiotensin system.

Role of the local renin-angiotensin system in cardiac damage: a minireview focussing on transgenic animal models

The local generation of all components of the renin-angiotensin system (RAS) in the heart has been the basis for the postulation of a tissue RAS in this organ. Since angiotensin II is involved in the induction of cardiac hypertrophy and fibrosis the local generation of this peptide may be of highest clinical importance. Several transgenic animal models have been generated to evaluate the functional importance of the cardiac RAS. We have established a new hypertensive mouse model lacking local angiotensinogen expression in the heart. In these animals, cardiac weight and collagen synthesis are increased compared to normotensive control mice but to a lesser extent than in mice with equally enhanced blood pressure but intact cardiac angiotensinogen generation. Thus, we have shown that local synthesis of this protein is involved but not essential in the development of cardiac hypertrophy and fibrosis.

Prorenin uptake in the heart: a prerequisite for local angiotensin generation?

Interference with locally generated angiotensin II most likely underlies the beneficial effects of renin-angiotensin system blockers in cardiac disorders. Since renin is not synthesized in the heart, this enzyme must be sequestered from the circulation in order to allow angiotensin generation at cardiac tissue sites. This review addresses the various ways through which circulating (i.e., kidney-derived) renin may reach cardiac tissue sites, considering in particular the possibility that prorenin, the inactive precursor of renin, is involved in cardiac angiotensin generation, as the plasma concentrations of prorenin are tenfold higher than those of renin. Renin and prorenin diffuse into the cardiac interstitial space and bind to cardiac (pro)renin receptors/renin-binding proteins. One of these receptors is the mannose 6-phosphate/insulin-like growth factor II receptor. This receptor not only binds mannose 6-phosphate-containing ligands like renin and prorenin, it also internalizes these enzymes, and activates prorenin intracellularly. This process possibly represents (pro)renin clearance, since intracellular angiotensin generation could not be demonstrated following (pro)renin uptake by cardiomyocytes. Angiotensin II-mediated myocyte proliferation did occur when incubating cardiomyocytes with prorenin plus angiotensionogen. The effects of prorenin plus angiotensinogen were comparable to those of 100nmol/l angiotensin II, although the angiotensin II levels in the medium during exposure of the cells to prorenin plus angiotensinogen were <1nmol/l. This suggests that cardiac angiotensin II generation by circulating renin occurs predominantly on the cell surface. The presence of ACE and/or renin on the cell membrane, in the microenvironment of angiotensin receptors, would allow maximal efficiency of local angiotensin II generation, i.e., immediate binding of angiotensin II to its receptors with minimal loss into the extracellular space.

Transgenic study of the function of chymase in heart remodeling

OBJECTIVES

To study the function of chymase on heart remodeling by overexpression of human chymase in the heart of transgenic mice.

METHODS

Transgenic mice were produced by microinjection. The chymase mRNA levels in the heart and other tissues were assessed by competitive reverse transcriptase-polymerase chain reaction (RT-PCR). The expression of collagen I/III genes was analyzed by Northern blot hybridization. Chymase and angiotensin-converting enzyme (ACE) activities, and angiotensin II (Ang II) content in the heart were determined by radioimmunoassay (RIA). The matrix metalloprotease-9 (MMP-9) in protein and activity levels were measured by Western blot and zymogram, respectively.

RESULTS

A model of transgenic mice with selective overexpression of a rat myosin light chain 2 promoter-human heart chymase (MLC(2-)-hChymase) fusion gene was produced. In MLC(2)-hChymase transgenic mice (the F(6) line), the human heart chymase gene was expressed at a high level in heart and at lower levels in skeletal muscle and kidney, while no expression was detected in the liver or lung. The heart chymase activity increased markedly in the F(6) transgenic mice versus non-transgenic mice (0.274 +/- 0.071 U/mg versus 0.152 +/- 0.021 U/mg) ( P < 0.05), with no difference in ACE activity. Heart Ang II level in the F(6) transgenic mice increased nearly threefold (1984 +/- 184 versus 568 +/- 88 pg/g protein) ( P < 0.05) but was unchanged in plasma. MMP-9 activity increased significantly in the cardiac tissue of F(6) transgenic mice ( P < 0.05), while both collagen I and the ratio of collagen I : III mRNA levels decreased significantly (both P < 0.05). The F(6) transgenic mice showed no significant changes in cardiac parameters.

CONCLUSIONS

We have demonstrated selective overexpression of human chymase gene in the heart of transgenic mice, and the results support the hypothesis of a dual Ang II-forming pathway from chymase and ACE in the cardiac tissue in vivo. The results also suggest that chymase may play a role in heart remodeling by increasing Ang II formation and activating MMP-9, and the regulation of collagen I gene expression.

Transendothelial transport of renin-angiotensin system components

BACKGROUND

Vascular (interstitial) angiotensin (ANG) II production depends on circulating renin-angiotensin system (RAS) components. Mannose 6-phosphate (man-6-P) receptors and angiotensin II type 1 (AT(1)) receptors, via binding and internalization of (pro)renin and ANG II, respectively, could contribute to the transportation of these components across the endothelium.

OBJECTIVE

To investigate the mechanism(s) contributing to transendothelial RAS component transport.

METHODS

Human umbilical vein endothelial cells were cultured on transwell polycarbonate filters, and incubated with RAS components in the absence or presence of man-6-P, eprosartan or PD123319, to block man-6-P, AT(1) and angiotensin II type 2 (AT(2)) receptors, respectively.

RESULTS

Apically applied (pro)renin and angiotensinogen slowly entered the basolateral compartment, in a similar manner as horseradish peroxidase, a molecule of comparable size that reaches the interstitium via diffusion only. Prorenin transport was unaffected by man-6-P. Apical ANG I and ANG II rapidly reached the basolateral fluid independent of AT(1) and AT(2) receptors. Basolateral ANG II during apical ANG I application was as high as apical ANG II, whereas during apical ANG II application it was lower. During basolateral ANG I application, ANG II generation occurred basolaterally only, in an angiotensin-converting enzyme (ACE)-dependent manner.

CONCLUSIONS

Circulating (pro)renin, angiotensinogen, ANG I and ANG II enter the interstitium via diffusion, and interstitial ANG II generation is mediated, at least in part, by basolaterally located endothelial ACE.

Angiotensin II receptor blockers in the treatment of heart failure

Heart failure treatment centers on antagonism of the renin-angiotensin-aldosterone system and adrenergic nervous system. Angiotensin-converting enzyme (ACE) inhibitors have been shown to benefit patients with left ventricular systolic dysfunction irrespective of symptoms. Despite ACE inhibitor use, left ventricular dysfunction continues to progress in most patients. In addition, ACE inhibitors are substantially underused in patients who would benefit, in large part due to physician concern over potential adverse effects. Angiotensin receptor blockers (ARBs) have been proposed as either potential substitutes for ACE inhibitors or as additive therapy for heart failure patients. The authors will review the importance of the renin-angiotensin-aldosterone system in the progression of heart failure, as well as the mechanisms by which ACE inhibitors and ARBs counteract this effect. The clinical evidence to date supporting the use of ARBs in heart failure also will be reviewed. Based on current trials, ARBs are suitable substitutes for ACE inhibitors in patients who have true ACE inhibitor intolerance, but ACE inhibitors should still be considered first-line therapy in the treatment of left ventricular systolic dysfunction and heart failure. ARBs are a reasonable additive therapy in patients on maximal ACE inhibitor therapy who remain symptomatic, especially in patients unable to tolerate beta blockade.

Renal interstitial fluid angiotensin I and angiotensin II concentrations during local angiotensin-converting enzyme inhibition

It was recently demonstrated that angiotensin II (AngII) concentrations in the renal interstitial fluid (RIF) of anesthetized rats were in the nanomolar range and were not reduced by intra-arterial infusion of an angiotensin-converting enzyme (ACE) inhibitor (enalaprilat). This study was performed to determine changes in RIF AngI and AngII concentrations during interstitial administration of ACE inhibitors (enalaprilat and perindoprilat). Studies were also performed to determine the effects of enalaprilat on the de novo formation of RIF AngII elicited by interstitial infusion of AngI. Microdialysis probes (cut-off point, 30,000 D) were implanted in the renal cortex of anesthetized rats and were perfused at 2 micro l/min. The effluent dialysate concentrations of AngI and AngII were measured by RIA, and reported values were corrected for the equilibrium rates at this perfusion rate. Basal RIF AngI (0.74 +/- 0.05 nM) and AngII (3.30 +/- 0.17 nM) concentrations were much higher than plasma AngI and AngII concentrations (0.15 +/- 0.01 and 0.14 +/- 0.01 nM, respectively; n = 27). Interstitial infusion of enalaprilat through the microdialysis probe (1 or 10 mM in the perfusate; n = 5 and 8, respectively) significantly increased RIF AngI concentrations but did not significantly alter AngII concentrations. However, perindoprilat (10 mM in the perfusate, n = 7) significantly decreased RIF AngII concentrations by 22 +/- 4% and increased RIF AngI concentrations. Interstitial infusion of AngI (100 nM in the perfusate, n = 7) significantly increased the RIF AngII concentration to 8.26 +/- 0.75 nM, whereas plasma AngI and AngII levels were not affected (0.15 +/- 0.02 and 0.14 +/- 0.02 nM, respectively). Addition of enalaprilat to the perfusate (10 mM) prevented the conversion of exogenously added AngI. These results indicate that addition of AngI in the interstitial compartment leads to low but significant conversion to AngII via ACE activity (blocked by enalaprilat). However, the addition of ACE inhibitors directly into the renal interstitium, via the microdialysis probe, either did not reduce RIF AngII levels or reduced levels by a small fraction of the total basal level, suggesting that much of the RIF AngII is formed at sites not readily accessible to ACE inhibition or is formed via non-ACE-dependent pathways.

Ischemia promotes renin activation and angiotensin formation in sympathetic nerve terminals isolated from the human heart: contribution to carrier-mediated norepinephrine release

We recently reported that in the ischemic human heart, locally formed angiotensin II activates angiotensin II type 1 (AT(1)) receptors on sympathetic nerve terminals, promoting reversal of the norepinephrine transporter in an outward direction (i.e., carrier-mediated norepinephrine release). The purpose of this study was to assess whether cardiac sympathetic nerve endings contribute to local angiotensin II formation, in addition to being a target of angiotensin II. To this end, we isolated sympathetic nerve endings (cardiac synaptosomes) from surgical specimens of human right atrium and incubated them in ischemic conditions (95% N(2,) sodium dithionite, and no glucose for 70 min). These synaptosomes released large amounts of endogenous norepinephrine via a carrier-mediated mechanism, as evidenced by the inhibitory effect of desipramine on this process. Norepinephrine release was further enhanced by preincubation of synaptosomes with angiotensinogen and was prevented by two renin inhibitors, pepstatin-A and BILA 2157BS, as well as by the angiotensin-converting enzyme inhibitor enalaprilat and the AT(1) receptor antagonist EXP 3174 [2-N-butyl-4-chloro-1-[2'-(1H-tetrazol-5-yl)biphenyl-4-yl] methyl]imidazole-5-carboxylic acid]. Western blot analysis revealed the presence of renin in cardiac sympathetic nerve terminals; renin abundance increased ~3-fold during ischemia. Thus, renin is rapidly activated during ischemia in cardiac sympathetic nerve terminals, and this process eventually culminates in angiotensin II formation, stimulation of AT(1) receptors, and carrier-mediated norepinephrine release. Our findings uncover a novel autocrine/paracrine mechanism whereby angiotensin II, formed at adrenergic nerve endings in myocardial ischemia, elicits carrier-mediated norepinephrine release by activating adjacent AT(1) receptors.

Effects of amlodipine and lacidipine on cardiac remodelling and renin production in salt-loaded stroke-prone hypertensive rats

1. Calcium channel blockers (CCBs) are anti-hypertensive drugs that are usually considered to act mainly as vasodilators. We investigated the relation between the reduction of blood pressure evoked by two long-acting CCBs and their protective effect against cardiac and renal damage in salt-loaded stroke-prone spontaneously hypertensive rats (SHRSP). 2. SHRSP were exposed to high dietary salt intake (1% NaCl in drinking solution) from 8 to 14 weeks of age, with or without amlodipine or lacidipine at three dosage regimens producing similar effects on blood pressure. 3. The lowest dosages of both drugs had non-significant effects on blood pressure but inhibited the paradoxical increases in plasma renin activity (PRA) and in renin mRNA in kidney that were found in salt-loaded SHRSP. The lowest dosage of lacidipine (but not of amlodipine) restored the physiological downregulation of renin production by high salt and reduced left ventricular hypertrophy and mRNA levels of atrial natriuretic factor and transforming growth factor-beta1. 4. The intermediate dosages reduced blood pressure and PRA in a comparable manner, but cardiac hypertrophy was more reduced by lacidipine than by amlodipine. 5. Although the highest doses exhibited a further action on blood pressure, they had no additional effect on cardiac hypertrophy, and they increased PRA and kidney levels of renin mRNA even more than in the absence of drug treatment. 6. We conclude that reduction of blood pressure is not the sole mechanism involved in the prevention of cardiac remodelling by CCBs, and that protection against kidney damage and excessive renin production by low and intermediate dosages of these drugs contributes to their beneficial cardiovascular effects.

The relevance of tissue angiotensin-converting enzyme: manifestations in mechanistic and endpoint data

Angiotensin-converting enzyme (ACE) is primarily localized (>90%) in various tissues and organs, most notably on the endothelium but also within parenchyma and inflammatory cells. Tissue ACE is now recognized as a key factor in cardiovascular and renal diseases. Endothelial dysfunction, in response to a number of risk factors or injury such as hypertension, diabetes mellitus, hypercholesteremia, and cigarette smoking, disrupts the balance of vasodilation and vasoconstriction, vascular smooth muscle cell growth, the inflammatory and oxidative state of the vessel wall, and is associated with activation of tissue ACE. Pathologic activation of local ACE can have deleterious effects on the heart, vasculature, and the kidneys. The imbalance resulting from increased local formation of angiotensin II and increased bradykinin degradation favors cardiovascular disease. Indeed, ACE inhibitors effectively reduce high blood pressure and exert cardio- and renoprotective actions. Recent evidence suggests that a principal target of ACE inhibitor action is at the tissue sites. Pharmacokinetic properties of various ACE inhibitors indicate that there are differences in their binding characteristics for tissue ACE. Clinical studies comparing the effects of antihypertensives (especially ACE inhibitors) on endothelial function suggest differences. More comparative experimental and clinical studies should address the significance of these drug differences and their impact on clinical events.

Effects of combined administration of ACE inhibitor and angiotensin II receptor antagonist are prevented by a high NaCl intake

BACKGROUND

To prevent the action of angiotensin II by blockade with either an angiotensin converting enzyme inhibitor (ACE I) or an angiotensin receptor antagonist (ARA) is difficult due to the physiological compensations. Combined therapy with both drugs may enable complete blockade, and in rats in high doses this has produced a syndrome that results in death.

OBJECTIVE

To determine the effect of combined blockade using losartan (10 mg/kg per day) and perindopril (6 mg/kg per day) on blood pressure, cardiac growth, renal function and behaviour, and to determine how this is influenced by different salt intakes in normotensive Sprague Dawley rats.

METHODS

Rats were fed an 0.2 or 4% NaCl diet and received the above drugs intraperitoneally. Blood pressure was measured by telemetry. Cardiac weight was measured after 10 days of therapy. Renal function was assessed by plasma creatinine and electrolytes, plasma renin and angiotensinogen concentrations were measured.

RESULTS

On 0.2% NaCl intake, combined blockade lowered blood pressure progressively; at day 7, rats on 0.2% NaCl developed a syndrome of listlessness and failure to eat which led to loss of weight and death. Cardiac size was dramatically reduced. Plasma creatinine was elevated to 50% above normal. There was a polyuria. The syndrome was reversed by adding NaCl to the drinking water or prevented in rats on a 4% NaCl intake. In rats on 0.2% NaCl plasma renin rose dramatically with medication and angiotensinogen became depleted. Haematocrit in all groups of rats did not differ.

CONCLUSION

Combined blockade of the renin-angiotensin system can cause death in rats on a reduced NaCl intake. This was prevented by a high salt intake. The syndrome may result from depletion of angiotensinogen and the failure to synthesize sufficient angiotensin II that may be critical for normal cardiac growth and function and critical for survival.

Cellular and molecular aspects of myocardial dysfunction

Disruption of any one of a large number of balanced systems that maintain cardiomyocyte structure and function can cause myocardial dysfunction. Such disruption can occur either in response to acute stresses such as cardiac surgery with cardiopulmonary bypass and cross-clamping of the aorta or because of more chronic stresses resulting from factors such as genetic abnormalities, infection, or chronic ischemia. Several currently available therapies such as beta-adrenergic receptor agonists and antagonists, phosphodiesterase inhibitors, angiotensin-converting enzyme inhibitors, angiotensin receptor blockers, and other agents affect cardiomyocytes in ways that are more far reaching than initially appreciated when these agents were first introduced into clinical practice. As our knowledge and understanding of myocardial dysfunction increases, particularly in the neonatal and pediatric patient, we will be able to further target interventions to highly specific perturbations of cellular function and individual genetic variability.

Activation of cardiac renin-angiotensin system in unstable angina

OBJECTIVES

The aim of this study was to investigate the activity of the cardiac renin-angiotensin system (RAS) in unstable angina (UA).

BACKGROUND

Angiotensin (Ang) II locally produced by continuously operating cardiac RAS may affect the pathophysiology of UA.

METHODS

In 35 patients with UA, 32 with stable effort angina (SA) and 21 with atypical chest pain (controls), cardiac RAS was investigated during coronary angiography after five days of Holter monitoring by combining the measurement of aorta-coronary sinus gradient for Ang I and Ang II with the kinetics study of 125I-Ang I. Messenger RNAs (mRNA) for all the components of RAS were also quantified with the reverse transcriptase-polymerase chain reaction (RT-PCR) and localized by in situ hybridization in myocardial biopsy specimens from patients who underwent aorta-coronary bypass surgery.

RESULTS

Cardiac Ang II generation was higher in patients with UA than it was in patients with SA or in controls (p < 0.001) due to increased de novo cardiac Ang I formation and its enhanced fractional conversion rate to Ang II. Messenger RNA levels for angiotensinogen (AGTN), angiotensin-converting enzyme (ACE) and Ang II type 1 (AT1) subtype receptors were higher in patients with UA (p < 0.01) than they were in patients with SA or in control hearts. Messenger RNAs for AGTN and ACE were almost exclusively expressed on endothelial and interstitial cells. Angiotensin II formation was correlated with ischemia burden (p < 0.001). However, the amount of Ang II formed and the expression levels of mRNAs for AGTN, ACE and AT1 were not related to the time that had elapsed since the last anginal attack.

CONCLUSIONS

In patients with UA, cardiac RAS is activated, resulting in increased Ang II formation. Myocardial ischemia is essential for RAS activation, but it is unlikely to be a direct and immediate cause of RAS activation.

Angiotensin converting enzyme is the main contributor to angiotensin I-II conversion in the interstitium of the isolated perfused rat heart

OBJECTIVES

Recent studies in homogenized hearts suggest that chymase rather than angiotensin converting enzyme (ACE) is responsible for cardiac angiotensin I to angiotensin II conversion. We investigated in intact rat hearts whether (i) enzymes other than ACE contribute to angiotensin I to angiotensin II conversion and (ii) the localization (endothelial/extra-endothelial) of converting enzymes.

DESIGN AND METHODS

We used a modified version of the rat Langendorff heart, allowing separate collection of coronary effluent and interstitial fluid. Hearts were perfused with angiotensin I (arterial concentration 5-10 pmol/ml) under control conditions, in the presence of captopril (1 micromol/l) or after endothelium removal with 0.2% triton X-100. Endothelium removal was verified as the absence of a coronary vasodilator response to 10 nmol bradykinin. Angiotensin I and angiotensin II were measured in coronary effluent and interstitial fluid with sensitive radioimmunoassays.

RESULTS

In control hearts, 45% of arterial angiotensin I was metabolized during coronary passage, partly through conversion to angiotensin II. At steady-state, the angiotensin I concentration in interstitial fluid was three to four-fold lower than in coronary effluent, while the angiotensin II concentrations in both fluids were similar. Captopril and endothelium removal did not affect coronary angiotensin I extraction, but increased the interstitial fluid levels of angiotensin I two- and three-fold, respectively, thereby demonstrating that metabolism (by ACE) as well as the physical presence of the endothelium normally prevent arterial angiotensin I from reaching similar levels in coronary effluent and interstitial fluid. Captopril, but not endothelium removal, greatly reduced the angiotensin II levels in coronary effluent and interstitial fluid. With the ACE inhibitor, the angiotensin II/I ratios in coronary effluent and interstitial fluid were 83 and 93% lower, while after endothelium removal, the ratios were 33 and 71% lower.

CONCLUSIONS

In the intact rat heart, ACE is the main contributor to angiotensin I to angiotensin II conversion, both in the coronary vascular bed and the interstitium. Cardiac ACE is not limited to the coronary vascular endothelium.

A novel vascular smooth muscle chymase is upregulated in hypertensive rats

While greater than 80% of angiotensin II (Ang II) formation in the human heart and greater than 60% in arteries appears to result from chymase activity, no cardiovascular cell-expressed chymase has been previously reported. We now describe the cloning of a full-length cDNA encoding a novel chymase from rat vascular smooth muscle cells. The cDNA encompasses 953 nucleotides, encodes 247 amino acids, and exhibits 74% and 80% homology in amino acid sequence to rat mast cell chymase I and II, respectively. Southern blot analysis indicates that the rat vascular chymase is encoded by a separate gene. This chymase was induced in hypertrophied rat pulmonary arteries, with 11-fold and 8-fold higher chymase mRNA levels in aortic and pulmonary artery smooth muscle cells from spontaneously hypertensive than in corresponding tissues from normotensive rats. We assayed the activity of the endogenous enzyme and of a recombinant, epitope-tagged chymase in transfected smooth muscle cells and showed that Ang II production from Ang I can be inhibited with chymostatin, but not EDTA or captopril. Spontaneously hypertensive rats show elevated chymase expression and increased chymostatin-inhibitable angiotensin-converting activity, suggesting a possible role for this novel enzyme in the pathophysiology of hypertension.

Effects of high sodium intake on cardiovascular aldosterone synthesis in stroke-prone spontaneously hypertensive rats

OBJECTIVES

Aldosterone is synthesized in extra-adrenal tissues such as the vasculature, heart and brain. The mechanisms underlying the effect of high salt intake on the development and acceleration of vascular injury and cardiac hypertrophy in the stroke-prone spontaneously hypertensive rats (SHRSP) are still not clear. The goal of this study was to determine whether high salt intake increases cardiovascular aldosterone synthesis in SHRSP.

METHODS

Four-week-old SHRSP were given tap water or 0.9% NaCl solution for hydration for 4 weeks in addition to a normal salt diet. Isolated rat mesenteric arteries and hearts were perfused for 2 h, and the perfusate was analysed by high-performance liquid chromatography. The concentrations of aldosterone synthase gene (CYP11B2) mRNA and angiotensin II receptor (AT1R) mRNA were determined by competitive polymerase chain reaction.

RESULTS

Salt-loaded SHRSP had higher blood pressures than SHRSP with normal salt intake. Plasma aldosterone concentrations and plasma renin activity were decreased by high salt intake. Aldosterone production, the expression of CYP11B2 mRNA and AT1R mRNA in mesenteric arteries and hearts were significantly increased by high salt intake.

CONCLUSIONS

These results suggest that high salt intake increases aldosterone production and expression of the AT1R mRNA in the cardiovascular tissue in SHRSP, which may contribute to the development of malignant hypertension in salt-loaded SHRSP.

Renin-angiotensin system gene polymorphisms: potential mechanisms for their association with cardiovascular diseases

Since the first description of the angiotensin-converting enzyme insertion/deletion polymorphism more than a decade ago, many hundreds of investigations have reported associations between this polymorphism and cardiovascular diseases. Subsequently, similar studies were performed in relationship with several other renin-angiotensin system gene polymorphisms, most notably the angiotensinogen M235T polymorphism and the angiotensin AT(1) receptor A1166C polymorphism. Surprisingly however, especially in view of the many contradictory results that have been obtained, very little attention has been paid to the mechanism(s) that may link these genetic variants and respective diseases. Here, we review the limited evidence that is currently available on the functional consequences (including compensatory mechanisms) of the above three renin-angiotensin system gene polymorphisms, in order to provide an explanation for the reported associations (or lack thereof) between these polymorphisms and cardiovascular diseases.

Angiotensin II type 1 receptor blockade abolishes specific K(ATP)channel gene expression in rats with myocardial ischemia

The cardiac ATP-sensitive potassium (K(ATP)) channel is potentially composed of an inward rectifier potassium channel (Kir6.1 and/or Kir6.2) subunit and the cardiac type of sulfonylurea receptor (SUR2A). We reported that cardiac Kir6.1 mRNA and protein are specifically upregulated in the non-ischemic as well as the ischemic regions in rats with myocardial ischemia, suggesting that humoral and/or hemodynamic factors are responsible for this regulation. In the present study, pretreatment with TCV-116, an angiotensin (Ang) II type 1 receptor antagonist, completely inhibited the upregulation of Kir6.1 mRNA and protein expression in both regions of rat hearts subjected to 60 min of coronary artery occlusion followed by 24 h of reperfusion; whereas pretreatment with lisinopril, an Ang converting enzyme (ACE) inhibitor, partly inhibited this upregulation. Except for rats pretreated with TCV-116, Kir6.1 mRNA levels were positively correlated with those for brain natriuretic peptide (BNP), a molecular indicator of regional wall stress, in both the non-ischemic and the ischemic regions. Plasma Ang II levels were not elevated in rats with control myocardial ischemia compared with sham rats. Thus, the stress-related induction of cardiac Kir6.1 mRNA and protein expression under myocardial ischemia is inhibited by pretreatment with an AT1 antagonist, but also in part by an ACE inhibitor, suggesting that activation of local renin-angiotensin system may play a role.