New Aspects of the Renin Angiotensin System in Cardiovascular and Renal Diseases

Recently, the continuing and increasing interest in the renin angiotensin system (RAS) after discovery of new peptides and enzymes, has been stimulating us to review the new concepts described in the literature. The classical RAS is a system in which renin acts on angiotensinogen (Agt) to form angiotensin I (Ang I) that is cleaved to the active peptide angiotensin II (Ang II) by angiotensin converting enzyme (ACE). The Ang II pharmacological actions occur after their interaction with Ang II type 1 and type 2 receptors (AT1R; AT2R). In the last decade, a new concept of local tissue RAS systems has been described in different organs, and also intracellular RAS has been stated, allowing expansion of the concept of functions of this system as endocrine, paracrine, autocrine and intracrine. This first chapter provides brief overview on the history of the RAS components, circulating and tissue RAS, and outlines the physiological functions of the RAS major active substance, Ang II. Here, we also describe other bioactive angiotensins, such as Ang III, Ang IV, Ang (1-7), Ang (1-12), angiotensin A and alamandine. Moreover, we report the studies on ACE2 and chymase, enzymes identified during the last years. The recent advances in the understanding of the RAS will provide new opportunities to treat and prevent hypertension and cardiovascular diseases.

Knowledge of renin-angiotensin system (RAS) has evolved through the years from the classical endocrine system view, which explains the homeostasis and arterial blood pressure control, to a more complex system, including new components and independent local RAS acting intracellularly and within different organs. It is wellknown that the circulating RAS plays a physiological and important role in blood pressure regulation through direct effects on vascular smooth muscle, aldosterone secretion and sodium, potassium and water equilibrium. The potent vasopressor peptide Angiotensin (Ang) II is the key regulator of the system and the Ang 1-7 counterregulates Ang II actions. Components are generated in liver (angiotensinogen), kidneys [renin] and vascular endothelial cell membranes (angiotensin I -converting enzyme) and secreted to the circulation to generate systemic Ang II. Recently, the focus of interest in the RAS changed the role of tissue/local system in specific tissue. Ang II synthesis within tissues from angiotensinogen and enzymes is defined as local RAS. The activation of the circulating and/or local RAS plays a fundamental role in the pathogenesis of hypertension and chronic kidney disease. RAS blockade with angiotensin I-converting enzyme (ACE) inhibitors or Ang II receptor blockers is a major approach to treat cardiovascular and renal diseases. However, it is still unclear if a dual blockade exerts a better protection than single blockade or shows a higher risk for renal complications and hyperkalemia.

Since the discovery of renin at the end of 19th century and the identification of angiotensin I (Ang I) and Angiotensin II (Ang II) seven decades ago, reninangiotensin system (RAS) has been widely studied. The RAS is an important endocrine system that regulates blood pressure and fluid homeostasis. This system is composed of peptides, enzymes and receptors. The RAS plays a key role in the development and progression of cardiovascular diseases. In this sense, Ang II is an important peptide of this system, presenting vasoconstrictor and salt-retaining properties and, at cellular level, promoting proliferation, fibrosis and hypertrophy. This chapter focuses on RAS and cardiovascular physiology addressing to an overview about RAS and cardiovascular disease, RAS and its relation with elevated blood pressure, sympathetic overactivity, cardiac hypertrophy, endothelial and vascular dysfunctions, and immune response activation, as well as the physiological basis of RAS inhibitors and cardiovascular disease treatment.

As a hormonal system, the renin-angiotensin system (RAS) is known for endocrine and autocrine/paracrine physiological functions. An intracrine role of the RAS has been proposed long ago; however, evidence for this function has started accumulating only recently. Angiotensin (Ang) II is the major hormone of the RAS and is the focus of research for the intracrine nature of this system. The intracrine functionality is characterized by intracellular actions of the hormone within the cell of synthesis or following internalization. Intracellular synthesis and actions of Ang II have been demonstrated in several cell types and tissues, with a significant amount of work in the heart. This review focuses on the cardiac intracellular RAS, delineating differences from the extracellular RAS that further consists of the circulatory and local RASs. The pathophysiological significance of the intracellular cardiac RAS has been discussed.

The renin-angiotensin system (RAS) is recognized as one of the most important modulators of the heart function, working intensely on maintaining cardiac homeostasis. Recent advances pointed out that this system is divided into two distinct counter-regulatory axes. The classical axis is well characterized and involves the formation and actions of the octapeptide Angiotensin (Ang) II, while the second axis has emerged in the last decades and has the heptapeptide Ang-(1-7) as the main effector. Ang-(1-7) modulates several aspects of the heart hemostasis, such as cardiac rhythm, contractility, hypertrophy, fibrosis and coronary flow. In this chapter, we will summarize the current literature addressing the role of Ang-(1-7) and its receptor Mas in the heart function and structure, highlighting its beneficial activities under pathological conditions.

The renin angiotensin system (RAS) has crucial action in the kidney; it is able to modulate intrarenal hemodynamics, glomerular filtration, and fluid and electrolytes homeostasis. Currently, six components of this system mediate their action through receptor(s). Four peptides, Ang II, Ang-(1-7), Ang III and Ang IV; and two enzymes, renin and prorenin through the renin and prorenin receptor, respectively. Angiotensin II (Ang II), the main peptide of RAS, through its type 1 receptor (AT1R) alters intrarenal hemodynamics, glomerular filtration, and fluid and electrolytes homeostasis readjusting blood pressure and body fluid balance. In the later functions, direct action of Ang II on the sodium and water transport was observed and related to diuretic/anti-diuretic and natriuretic/anti-natriuretic action depending on Ang II concentration. Angiotensin-(1-7) also influences the glomerular filtration rate but without changing the blood pressure. This heptapeptide showed biphasic direct action on tubular transport of sodium and water, but there is no consensus which receptor translates its tubular effect. Reports showed that Angiotensin III and Angiotensin IV could present natriuretic action; the pressor effect of both peptides is unclear. Direct action on tubular transport via renin and prorenin receptor has not yet been reported.

There is increasing evidence regarding the pivotal role of the intrarenal renin angiotensin system (RAS) in the pathogenesis of hypertension and its independent role in the regulation of interstitial and intratubular fluids. The presence of high intrarenal angiotensin II (Ang II) levels and the localization of Ang II type 1 receptor (AT1R) in proximal and distal nephron segments suggested a physiological role for this receptor. In the collecting duct, luminal AT1R activation directly stimulates the activity of epithelial sodium channel (ENaC), contributing to enhance sodium reabsorption. Several reports have demonstrated the expression of angiotensinogen (AGT) and prorenin/renin in proximal tubules and collecting ducts, respectively. Both, tubular AGT and prorenin/renin are upregulated during Ang II-dependent hypertension despite the suppression of renin in juxtaglomerular cells. The (pro)renin receptor (PRR) is a new member of the RAS. PRR by binding renin or prorenin, enhances renin activity and activates the non active enzyme prorenin. The PRR and its soluble form are increased in the renal tissue and urine of Ang II hypertensive rats; however, the contribution of PRR to enhance renin activity in the distal nephron remains unclear. The presence of angiotensin converting enzyme (ACE), the enzyme that converts Ang I into Ang II in the nephron, along with the augmentation of intratubular AGT, renin and PRR may contribute to additional production of Ang II in the tubular lumen, which enhances sodium reabsorption and high blood pressure.

Tonin, a serinoproteinase, is capable of releasing angiotensin II by the hydrolysis of Phe8-His9 bond in the angiotensinogen, angiotensin I and synthetic peptides corresponding to the N-terminal portion of angiotensinogen. Tonin is able to hydrolyze beta-lipotropic hormone releasing an opiate-like segment. It is capable of degrading adrenocorticotropic hormone, substance P but not bradykinin. Hydrolysis of Phe.His bond depends on a minimum sequence involving residues Ile.His.Pro.Phe.His.Leu. Tonin activity is present in human and rat tissues. In the rat, tonin is present in various tissues including kidney, brain and heart, and the activity levels vary according to age and sex being higher in males. Tonin is released into bloodstream and saliva after beta-adrenergic stimulation. There is evidence that tonin is involved in blood pressure control and participates in the hydromineral balance. Intracerebroventricular injection of tonin induces salt appetite and water intake and increases urinary volume and blood pressure. TGM(rTon), a transgenic mouse that expresses rat tonin, presents increased blood pressure. The levels of angiotensin II in the plasma are increased in TGM(rTon) and the AT1 receptors desensitized when compared to the wild type. A significant increase in the plasmatic and a decrease in urinary sodium were observed in TGM(rTon), suggesting alterations in the renal function. Induction of cardiac hypertrophy by isoproterenol injection in rats showed that tonin may be involved in this process. TGM(rTon) presents resistance to develop isoproterenol-induced hypertrophy. The molecular basis for the hypertrophy resistance is to be determined. These results lead us to conclude that tonin, an angiotensin II liberating enzyme may represent an alternative pathway with effects on the renal and cardiovascular systems.

Complex phenotypes are those depending on a complex interaction between the genotype and environmental factors. Genetic studies on different human populations have shown a great variability in the genetic background due to the presence of genetic polymorphisms. The renin-angiotensin system (RAS) is closely involved in regulation of several physiological processes, such as body fluids homeostasis and blood pressure. Angiotensin-converting enzyme (ACE) plays a central role in the RAS functioning because this enzyme activates angiotensin II (Ang II) generation (vasoconstrictor) and inactivates bradykinin (vasodilator) simultaneously. ACE activity in plasma is variable in different families, and this variability is related with the genomic structure of the ACE gene dependent of a 287 base-pairs Insertion (presence) or Deletion (absence) in the DNA sequence. Variation in the number of copies of a single gene has been interpreted as representing different “dosage” of a specific gene product. Presence of D allele in ACE gene, mainly when in the DD polymorphism, has been associated with a higher generation of Ang II either in the systemic and in the local RAS leading to higher blood pressure levels and incidence of hypertension. These associations, however, have not been found in Caucasians, but are stronger in populations with Asiatic genetic background (where presence of the D allele is less frequent), such as the Amerindian populations. Individuals with DD polymorphism also have higher predisposition to development of accelerated atherosclerosis as compared with those presenting the DI and II ACE genes. However, these findings are not found in all studies because associations are generally weak and complex phenotypes, such as blood pressure levels and atherosclerosis development, depend on many genetic and environmental factors. Therefore, genetic polymorphisms may contribute to different phenotypes. Since the relationships between genes and the gene-environmental interactions are nonlinear, their practical use in medicine should be done with caution. However, they can give important insights in relation to different prevalence of diseases in populations with different genetic background.

The renin-angiotensin-system (RAS) constitutes a key hormonal system in both the acute and long-term maintenance of blood pressure. Indeed, inappropriate regulation of this system is a major contributor to various pathologies that impact kidney function and blockade of the RAS either through attenuation of angiotensin converting enzyme (ACE) activity or angiotensin type 1 receptor (AT1R)-dependent signaling has important therapeutic benefit. The RAS is no longer considered a monolithic peptidergic system whereby Ang II is the sole effector acting through the AT1R, but a diverse system that reflects multiple peptides with distinct actions that are mediated by multiple receptors. The ACE-Ang II-AT1R axis is considered the classic pathway of the RAS that upon activation contributes to a number of peripheral and central mechanisms to effectively regulate blood pressure. However, the dysregulation of the AT1R axis may lead to sustained hypertension, inflammation, and an imbalance in redox mechanisms, cellular fibrosis, and other pathological responses. The ACE2- Ang-(1-7)-AT7R axis is now defined as the non-classical pathway of the RAS that in many situations exhibits actions that are opposite those of the Ang II-AT1R axis. The cellular actions of the Ang-(1-7)-AT7R axis primarily reflect the stimulation of both nitric oxide and prostaglandin pathways that would contribute to lower blood pressure and attenuation of inflammation, fibrosis and cellular injury. Moreover, the progression of various pathologies attributed to a stimulated Ang II-AT1R axis may, in part, reflect a reduced Ang-(1-7) tone. The current review assesses the non-classical axis of the RAS regarding the cellular and intracellular pathways for the expression and metabolism of Ang-(1-7), as well as the influence of the peptide in fetal-programmed hypertension.

Local angiotensin generation depends on the uptake of circulating renin and/or its precursor prorenin. Such uptake may involve binding to a receptor. In the past 3 decades, three potential receptor candidates have been evaluated: a renin-binding protein, the mannose 6-phosphate/insulin-like growth factor II receptor, and the (pro)renin receptor. The most promising candidate seemed to be the (pro)renin receptor; however its affinity for renin and prorenin is several orders of magnitude above their actual levels in blood, raising doubt on whether this interaction truly occurs in vivo. In addition, conflicting in-vivo data have been reported regarding the putative (pro)renin receptor blocker, handle region peptide, while (pro)renin receptor knockout studies revealed lethal consequences that are (pro)renin-independent. The latter is most likely due to the fact that the (pro)renin receptor colocalizes with vacuolar H+-ATPase, and possibly determines the stability of this vital enzyme. This chapter briefly discusses the various receptors, and ends with the conclusion that (pro)renin-(pro)renin receptor interaction, if it occurs in vivo, is limited to (pro)renin-synthesizing organs like the kidney.

The renin-angiotensin system (RAS) affects both the innate and adaptive immune responses. Since hyperactive RAS has been associated with several diseases, the contribution of tissue RAS to the progression of immune and non-immune conditions has been considered in the recent years. It has a well-established role in fibrinogenesis, leukocyte infiltration, activity of T cells and has been shown to be chemotactic to macrophages, T cells, and natural killer cells. Nitric oxide (NO) is synthesized by many cell types involved in immunity and inflammation and plays an important role in hypotension and regulates the functional activity, growth and death of many immune and inflammatory cell types. Current evidences suggest that catecholamines (CAs) play a key role in activating and limiting inflammatory and immune reactions. In this chapter, we will discuss some aspects related to the role of these molecules in inflammatory process and immunologically mediated conditions.

Stress reaction aims to preserve body homeostasis. During stress, the reninangiotensin system (RAS) increases the effects of the sympathetic nervous system and the hypothalamic-pituitary- adrenal (HPA) axis so that the organism will adapt to stressors. However, during chronic stress, these responses are sustained, adaptation does not occur and pathologies might be developed. In this chapter, we address evidences towards the correlation between RAS and the negative effects that stress has on behavior and cardiovascular system. Both circulating and local RAS are involved in stress – related hypertension. Among the mechanisms involved, it has been demonstrated that RAS activates HPA axis, catecholamines upregulate the renin synthesis and RAS activity in the peripheral organs, and vascular effects of angiotensin II (Ang II) impair the balance between vascular relaxing and contracting agents. Considering cardiac hypertrophy, hyperactivity of RAS and SNS promotes growth factors increase, fibroblast proliferation and collagen synthesis in the myocardium. Ang II triggers the production of superoxide and acts as a pro-inflammatory molecule in the blood vessels, and, consequently, might initiate and aggravate the process of atherosclerosis. Considering emotional and cognitive effects of stress, there is a link between depression and dysfunction of stress response systems involving the activation of RAS. In treating hypertension, patients having depression might also benefit from RAS blockers as these agents are known to have antidepressant effects. Inhibitors of ACE have been reported to improve cognition and memory, via Ang II conversion to Ang IV in the central nervous system. Stress-induced brain RAS has also been suggested as having a role in Alzheimer’s disease.

The number of people affected by cardiovascular disease is increasing in the Western world, and it is partially explained by urbanization, industrialization, work condition and inadequate diet. Cardiovascular disease killed nearly 17 million people in 2011, and of these, 7 million died of ischaemic heart disease and 6.2 million from stroke [1]. This chapter will focus on the contribution of regular physical exercise for prevention and reversal of cardiovascular disease, and on the role of renin angiotensin system (RAS) in these processes. Experimental and clinical studies show that exercise training is efficient to block RAS overactivity, thus preventing and/or reversing cardiac dysfunction and deleterious remodeling of the heart in pathological conditions such as hypertension, myocardial infarction, heart failure and obesity. Indeed, studies show that the association between RAS inhibition and exercise training can bring major benefits to individuals with heart disease (especially those with mild, moderate or severe heart failure) and also cardiometabolic alterations, acting synergically as a successful combination to the recovery and maintenance of health of these patients.

Translational research has currently become the focus of many ongoing studies. The use of inbred animals represents an advantage to human studies to a certain point because of the elimination of several uncontrollable variables. However, we need to consider the limitation of such approach in the translational potential to humans. Within the field of hypertension and diabetes research, animal models are irreplaceable research tools providing insight into human diseases. These two diseases independently predispose to renal and cardiovascular complications but, more importantly, can aggravate each other. Although some of the best models for diabetes and hypertension are spontaneous, the use of transgenic models provides a better control of the pathological mechanisms to be studied and the combination of the available tools will most likely make a difference in understanding how the RAS is modulated in diabetes and hypertension. Although these animals add a few layers of complexity and are sometimes closer to the human pathological mechanism, there are still many challenges to overcome.

The renin angiotensin system (RAS) is an essential regulator of renal and cardiovascular function. The components of this system are present in the circulation, organs, tissues and various cell types; and their syntheses in the organs are independent of the circulation. Understanding how the circulatory and organ RAS interact in the maintenance of the body function is knowledge that is not only key, but absolutely essential in helping us to unravel the implications of the RAS in a disease process. In this regard, this chapter aims to verify whether or not there is a consensus in the literature for the reported quantities and concentrations of Angiotensin I (Ang I), Angiotensin II (Ang II) and Angiotensin 1-7 (Ang1-7) in normal physiological states in mice, rats and humans. Because of the various methods for quantification of angiotensins, there is not presently a clear consensus for the absolute quantities of these peptides in plasma and the different tissues.