Review article: the pathophysiological roles of the renin-angiotensin system in the gastrointestinal tract.

BACKGROUND: The renin-angiotensin system (RAS) is a homeostatic pathway widely known to regulate cardiovascular and renal physiology; however, little is known about its influence in gastrointestinal tissues. AIM: To elicit the anatomical distribution and physiological significance of the components of the RAS in the gastrointestinal tract.
METHODS: An extensive online literature review including Pubmed and Medline.
RESULTS: There is evidence for RAS involvement in gastrointestinal physiology and pathophysiology, with all the components required for autonomous regulation identified throughout the gastrointestinal tract. The RAS is implicated in the regulation of glucose, amino acid, fluid and electrolyte absorption and secretion, motility, inflammation, blood flow and possibly malignant disease within the gastrointestinal tract. Animal studies investigating the effects of RAS blockade in a range of conditions including inflammatory bowel disease, functional gut disorders, gastrointestinal malignancy and even intestinal ischaemia have been encouraging to date. Given the ready availability of drugs that modify the RAS and their excellent safety profile, an opportunity exists for investigation of their possible therapeutic role in a variety of human gastrointestinal diseases.
CONCLUSIONS: The gastrointestinal renin-angiotensin system appears to be intricately involved in a number of physiological processes, and provides a possible target for novel investigative and therapeutic approaches.

Introduction

The renin‐angiotensin system (RAS) plays a central role in regulating cardiovascular and renal physiology. The contemporary view of the RAS has evolved from that of a simple linear pathway involving the conversion of angiotensinogen to angiotensin II (Ang II) via a two‐step process facilitated by renin and angiotensin converting enzyme (ACE), to a much more complex system involving homologues of ACE and multiple angiotensin peptides which play supplementary and counter‐regulatory roles (Figure 1). The RAS was, for many years, thought of as an endocrine system with enzymes and peptides released into the systemic circulation to act on target organs. More recently, it has been recognised that most organs including the brain, kidney, heart, liver, pancreas, reproductive organs, skin and the gastrointestinal tract constitutively express all the components required to allow autonomous function of a local intra‐organ RAS, where it performs both paracrine and autocrine functions.

Figure 1

The contemporary renin‐angiotensin system (RAS). ACE, angiotensin converting enzyme; NEP, neutral endopeptidase; Am, aminopeptidase; AT1R, angiotensin type 1 receptor; AT2R, angiotensin type 2 receptor; AT4R, angiotensin type 4 receptor; PRR, (pro)renin receptor.

Understanding the Components of the RAS ‐ Old and New

Table 1 summarises the current view of the RAS, the key components and their physiological and clinical effects. Essentially, the relative activity of two counterbalancing pathways determines the predominant tissue effect.

Table 1

RAS components and their molecular and physiological effects

RAS components	Mediating receptor	Molecular signals identified	Predominant physiological and clinical effects
Renin and prorenin	(Pro)renin receptor (PRR)	Activates extracellular signal‐regulated kinases (ERK) 1/2, TGFβ, collagens, fibronectin, COX‐22	Diabetic microvascular complications, possibly cardiac and renal fibrosis143
Classical RAS components: ACE, Ang II	Angiotensin type 1 receptor (AT1R)	Activation of phospholipase C, mitogen‐activated protein (MAP) kinase, initiation of NADPH oxidase, signal transducer and inhibitor of transcription (STAT) 1 activation, ubiquitination of IkB; leading to increased IL‐6, TNFα, TGFβ, fibroblast growth factor (FGF), vascular endothelial growth factor (VEGF), insulin like growth factor (IGF)‐1144, 145, 146, 147, 148	Vasoconstriction, salt and water retention, thirst response, cardiac hypertrophy, tissue inflammation and fibrosis
Alternative RAS components: ACE2, Ang (1‐7)	Mas receptor	Inhibition of ERK1/2, MAP kinase, stimulation of nitric oxide (NO) release through endothelial nitric oxide syntetase, may directly antagonise the AT1R through heterodimerisation149	Vasodilatation, antihypertensive, anti‐thrombotic, cardioprotective, anti‐inflammatory and anti‐fibrotic
Angiotensin III	Angiotensin type 1 receptor (AT1R)	Increases monocyte chemoattractant protein (MCP‐1), NFκB and activating protein‐1 (AP‐1) activity in renalmesangialcells150; aldosterone secretion from adrenalglands151	Proinflammatory, possibly renal fibrosis
Angiotensin IV	Angiotensin type II receptor (AT2R)	Inhibition of tyrosine kinase/STAT signalling pathwayand NFκB stimulates nitric oxide production, may directly antagonise the AT1R through heterodimerisation152, 153, 154, 155	Anti‐inflammatory, central nervous system effects (neuronal development, learning and memory)
	Angiotensin type 4 receptor (AT4R)
Chymase		May convert Ang I to Ang II, activates TGFβ and MMP‐9156	Cardiac and vascular fibrosis
Neural endopeptidase (NEP, neprilysin)		Converts Ang I to Ang (1‐7), inactivates atrial natriuretic peptideand kinins,9 may degrade amyloid β peptide10	Vasoconstriction, antidiuresis, hypertension11

ACE, angiotensin converting enzyme; Ang II, angiotensin II; RAS, renin‐angiotensin system.

The proinflammatory, profibrotic pathway includes the classical RAS components ACE and Ang II, and renin, prorenin, chymase and neutral endopeptidase (NEP, also known as neprilysin). Renin, a glycoprotein derived predominantly from the juxtaglomerular apparatus in the kidney, is an aspartyl protease that cleaves the liver‐derived angiotensinogen to angiotensin I. Both renin and its proenzyme prorenin, which was previously considered physiologically inactive,1 have now been demonstrated to have independent pro‐inflammatory and pro‐fibrotic effects via signalling through the pro(renin) receptor (PRR).2

The classical RAS comprising the zinc metalloproteinase ACE and Ang II induces vasoconstriction, salt and water retention, thirst response, cardiac hypertrophy, tissue inflammation and fibrosis through the G‐protein coupled seven‐transmembrane domain receptor angiotensin type I receptor (AT1R). Ang II also stimulates adrenal gland secretion of aldosterone resulting in renal sodium and water retention. Inhibition of this pathway with either ACE inhibitors or AT1R antagonists has beneficial effects in hypertension, cardiac failure, ischaemic heart disease, diabetic nephropathy and renal fibrosis.

Chymase expressed in the heart and vascular wall and secreted by activated mast cells, acts as an alternative enzyme to ACE to generate Ang II from Ang I.3, 4, 5, 6 NEP, a membrane bound zinc metalloproteinase with a structure distinct from ACE, was discovered in the 1970s as a key enzyme involved in the cleavage of bradykinin.7, 8 In recent years, it has been shown to also have a role in the formation of Ang (1‐7) from Ang I, as an inactivator of atrial natriuretic peptide9 and in the degradation of amyloid β peptide,10 a protein involved in the pathogenesis of Alzheimer's disease. The net effect of NEP inhibition is vasodilatation and natriuresis, a property encompassed by vasopeptidase inhibitors that target both ACE and NEP and may have additional anti‐hypertensive effects to ACE inhibitors.11

In contrast, the alternative RAS, comprising ACE2 and Ang (1‐7), acting via the G‐protein coupled seven‐transmembrane receptor mas, 12, 13 has vasodilatory, antihypertensive, anti‐thrombotic, cardioprotective, anti‐inflammatory and anti‐fibrotic effects in a variety of tissues.14, 15, 16, 17, 18 ACE2 is a zinc metalloproteinase and homologue of ACE, which cleaves a single amino acid from Ang II to form the heptapeptide Ang (1‐7). Indeed, part of the clinical benefit attributed to ACE inhibitors and AT1R blockers (ARBs) may be through the diversion of the classical RAS components towards Ang (1‐7) with subsequent mas receptor activation.18, 19, 20

The complexity of the RAS is further highlighted by recent findings regarding the actions of other angiotensin peptides including angiotensin III [Ang III, also denoted as Ang (2‐8)], angiotensin IV [Ang IV, also known as Ang (3‐8)] and the AT2 and AT4 receptors. Ang III is formed by cleavage of Ang II by aminopeptidase A, and Ang IV results from further conversion by aminopeptidase B or N (Figure 1).

The AT2 receptor (AT2R) has affinity for Ang II, Ang III, Ang IV and Ang (1‐7), and is also thought to have effects counteracting the AT1R and analogous to those of the mas receptor, with vasodilatory, anti‐inflammatory and anti‐proliferative downstream actions. Previously recognised largely for an important role in foetal development, more recently, the AT2R has been shown to be upregulated in atherosclerotic disease,21 cutaneous wounds22 and pancreatic fibrosis,23 and to stimulate neurite outgrowth, a marker of neuronal regeneration.24, 25

Ang III is believed to have actions analogous to Ang II. Ang IV appears to have opposing effects to Ang II, and acts predominantly via the AT2R and the AT4R, formerly known as insulin regulated aminopeptidase (IRAP).26 The greatest role of Ang IV is in the central nervous system (CNS), where it has a positive effect on neuronal development, learning and memory.27, 28

Concept of Local RAS

There is considerable evidence that most or all of the components of the RAS are present in a variety of organs, supporting the theory that local expression and modulation of the RAS play important roles in tissue homeostasis. These roles may be summarised as involving (1) fluid and electrolyte transport, (2) regional blood flow regulation and (3) promoting the wound healing response, including cell proliferation, inflammation and fibrosis. Some of the regional effects of the RAS are listed below:

In the kidney, local angiotensinogen is converted by renin to Ang I, which in turn is cleaved by tubular brush border ACE to Ang II to facilitate sodium and fluid absorption via luminal AT1R. This may influence blood pressure independent of systemic Ang II levels and vascular tone.29, 30

The heart expresses renin, PRR, ACE, chymase, angiotensinogen, AT1R and AT2R, and these components modulate myocyte proliferation and cardiac remodelling.6, 31, 32, 33, 34, 35, 36, 37

The brain has been shown to express renin, angiotensinogen, Ang II, Ang III, Ang IV, Ang (1‐7), AT1R, AT2R and AT4R, with these components regulating blood pressure, fluid and electrolyte balance, thirst, maintenance of the blood‐brain barrier and neuronal development including learning and memory processes.38, 39, 40, 41, 42, 43, 44

The liver expresses renin, angiotensinogen, Ang II, ACE, AT1R, Ang (1‐7), ACE2 and mas receptors, all of which are upregulated in the diseased liver.45, 46 Furthermore, ARBs and Ang (1‐7) have been demonstrated to reduce liver fibrosis in animal models.47, 48, 49

In the pancreas, Ang II has been shown to inhibit glucose stimulated insulin secretion, and via AT1R and AT2R, regulates exocrine enzyme secretion and the microcirculation.50, 51, 52

A local RAS has been identified and shown to be involved in tissue homeostasis in the reproductive organs, skin and even adipose tissue.53, 54, 55, 56, 57, 58 Detailed reviews of these and other local RAS effects are published elsewhere.12, 30, 50, 54, 59, 60, 61

Localisation and Functionality of the RAS in the Gastrointestinal Tract

Our understanding of the involvement of the RAS in the gastrointestinal tract has gradually evolved over the past five decades since the formulation of the hypothesis that Ang II had a direct effect on intestinal smooth muscle in addition to an indirect effect via myenteric plexus cholinergic neurons.62, 63, 64 Since then, many of the components of the RAS have been identified throughout the gastrointestinal tract. An overview of the current state of knowledge is illustrated in Figure 2. The regions of the gut are addressed separately here.

Figure 2

Distribution of components of the RAS in the gastrointestinal tract. Diagrammatic representation of RAS components in (a) small intestine, (b) colon, (c) stomach and (d) oesophagus.

Small intestine

Most attention has been paid to the small intestine (Figure 2a), as outlined below:

ACE, ACE2 and neutral endopeptidase: ACE has been shown in humans to be located in abundance on the brush border of epithelial cells and in the mesenteric microvascular endothelium.65 ACE2 mRNA and protein is present in large amounts in small intestinal epithelial brush border, muscularis mucosa and muscularis propria, as well as microvascular endothelium and vascular smooth muscle cells.66 Remarkably, the highest tissue concentrations in the human body of ACE and ACE2 mRNA are found in the terminal ileum, duodenum and colon.67, 68 Expression of NEP has been demonstrated in the rat intestinal wall, and is suppressible by administration of the combined ACE/NEP inhibitor omapatrilat.69

Angiotensin receptors: AT1R has been localised to the epithelial brush border.70 The circular and longitudinal muscle layers and the myenteric plexus also strongly express AT1R, but the AT2R appears to be largely restricted to the myenteric plexus.71, 72 Small vessels in the muscularis propria also express AT1R.71, 72 In early studies in the rat intestine, Ang II binding sites were reported to be confined to the muscularis,73 but subsequent reports have identified expression of AT1R and a lesser amount of AT2R in the muscularis mucosa and mucosa, including in epithelial cells.74, 75

Renin: mRNA for renin has been detected in the human small intestine.76

Angiotensin peptides: Ang II has been detected in the crypt and crypt‐villus junction epithelial cells.70 To date, the expression of angiotensinogen, Ang I or Ang (1‐7) in the human small intestine has not been reported. However, angiotensinogen has been widely localised in the rat brush border, epithelial cells, lamina propria, muscularis mucosa, submucosal blood vessels and muscularis propria.77 Angiotensinogen mRNA has been isolated in concentrations of over one‐third that of the liver in the rat mesentery,78 and a high level of proangiotensin‐12, a precursor of Ang I, has been located in the rat intestine.79

Thus, all of the required components for local production and action of Ang II appear to be present in small intestine.

There is now evidence of important roles for the RAS in a variety of intestinal processes:

Bicarbonate secretion: This is stimulated by Ang II via AT1R and AT2R in the duodenum.80

Sodium and water absorption: In the jejunum and ileum, this process appears to be modulated by Ang II in conjunction with the enteric sympathetic nervous system.81, 82, 83 When applied in low dose to rat jejunum, Ang II stimulates sodium and water absorption through AT2R, but in high dose, it unexpectedly inhibits absorption through AT1R.84 Both Ang II and Ang III may also increase sodium and water absorption via stimulation of release of noradrenaline from sympathetic neurons, which in turn may act through adrenergic receptors on the basal surface of epithelial cells.83, 85, 86

Glucose absorption: Ang II has also been shown to inhibit rat jejunal sodium‐dependent glucose transporter (SGLT1)‐mediated glucose uptake in vitro.77

Digestion and absorption of peptides: Both brush border ACE and ACE2 are thought to function as peptidases, allowing for mucosal digestion and absorption of peptides.87, 88 ACE2 increases the activity of the neutral amino acid transporter B0AT1, which is mutated in a rare amino acid deficiency disorder, Hartnup disorder, clinically manifested by cerebellar ataxia and pellagra‐like skin rash.89

Secretion: A role for ACE2 in active secretion has been suggested by the observation that ACE2 is the target for the coronavirus mediating severe acute respiratory syndrome, SARS‐CoV. Some patients with this infection suffer from watery diarrhoea, but the exact mechanism remains to be determined.66

Colon

There have been limited studies of RAS components in the colonic wall (Figure 2b). By a combination of RT‐PCR and immunohistochemistry, renin was found in the surface epithelium, lamina propria mesenchymal cells, microvascular walls and muscularis mucosa. AT1R was detected on surface epithelial cells and in crypt bases, lamina propria macrophages, myofibroblasts and mucosal vessel walls, and weak expression of AT2R has been found on surface epithelium, in crypts and in some mesenchymal cells.90 ACE was also weakly expressed in parts of the surface epithelium, and more prominently in mesenteric microvascular walls, lamina propria and submucosal mesenchymal cells.90 ACE2 appears to be localised to the mesenteric microvascular endothelium in the colon and is not present in the epithelium.66 Angiotensinogen mRNA has also been isolated in homogenised rat colon,78 but its expression has not been examined in the human colon.

A more limited range of functional roles has been attributed to components of the RAS in the colon than in small intestine. Ang II has been shown to increase sodium and water reabsorption in rats through NaCl coupled transport.91 The response of circular and longitudinal muscle contraction to Ang II also suggests a role in normal colonic motility.92 As detailed below, the RAS may also be involved in the inflammation associated with IBD, as mucosal levels of Ang I and Ang II are higher in patients with active Crohn's colitis compared with normal controls and patients with ulcerative colitis.93

Stomach

Components of the RAS are present in the mucosal biopsy specimens of gastric antrum and body from healthy adults (Figure 2c).94 Renin and angiotensinogen were both seen in lamina propria mesenchymal cells and vascular endothelial cells. AT1R and AT2R were both observed in gastric epithelium (mainly in the basal surface), lamina propria mesenchymal cells and vascular endothelium. AT1R were noted in a subgroup of endocrine cells in the base of antral mucosal glands, and ACE and NEP in vascular endothelial cells, but not in other parts of the mucosa. Other investigators, however, have noted ACE in fundic chief cells and mucin secreting cells of the antrum.95 Longitudinal and circular muscle of the stomach has been demonstrated to respond in vitro to Ang II, suggesting the presence of appropriate receptors on gastric myocytes.96

To date, few functional or pathogenic roles have been attributed to the RAS in the stomach. A role of local RAS in gastric inflammation has been suggested by higher expression of AT1R expression in Helicobacter pylori positive than H. pylori negative patients94 and the potentiation of ulceration in animal models by Ang II.97, 98

Oesophagus

Immunoreactive ACE, AT1R and AT2R have been found in the lamina propria microvascular walls, and AT1R and AT2R were identified in the superficial stratified epithelium99 and circular and longitudinal muscle of the oesophagus100 (Figure 2d). Ang II caused contraction of isolated oesophageal smooth muscle in vitro, and the AT1R antagonist candesartan inhibited swallow‐induced peristaltic contractions in the distal oesophagus.100 The expression of other RAS components has not been reported.

Potential Clinical Implications of RAS in Gastrointestinal Disease

The presence of the various components of the RAS in the gastrointestinal tract raise the possibility that modification of this system locally may be a potential therapeutic target in a myriad of gastrointestinal diseases where current strategies are suboptimal. These include inflammatory bowel disease (IBD), gastrointestinal cancer, gut motility disorders and mesenteric ischaemia. Although clinical data are sparse, results from animal models and pre‐clinical studies provide support for further investigation.

Inflammatory bowel disease

Information relevant to IBD has arisen from studies in Crohn's disease (CD) and ulcerative colitis (UC) and animal models of both IBD and other chronic inflammatory conditions.

Studies in patients with Crohn's disease and ulcerative colitis

Two components of the RAS have been studied. The first is ACE, which was subject to intense interest from the 1980's in its role in sarcoidosis and other granulomatous conditions. Studies of serum ACE concentrations yielded largely conflicting findings in IBD, with many studies showing reduced levels101, 102, 103, 104 and some finding no difference.105, 106, 107 Many of these studies have been limited by relatively small numbers of patients.

Serum ACE levels are associated with ACE gene insertion/deletion (I/D) polymorphisms, with higher levels seen with the DD polymorphism than ID or II.108 Matsuda et al. showed that the ACE gene polymorphism variation was similar in 39 patients with CD and 43 patients with UC to 341 controls, but that serum ACE levels were lower in patients with IBD after adjusting for polymorphisms.101 Furthermore, they demonstrated that ACE levels significantly increased in all of nine patients with active CD when they achieved clinical remission. A larger study involving 124 UC patients and 108 CD patients also found no difference in ACE gene polymorphisms when compared with normal controls, but a subgroup analysis revealed a higher proportion of DD genotype in 25 UC patients with extra‐intestinal manifestations, with an odds ratio (OR) of 4.08.109 Indeed, it is difficult to reconcile these findings into a unifying hypothesis regarding the role of ACE in IBD pathogenesis. It is possible that inflammatory cytokines such as TNF‐α and IL‐1 downregulate systemic endothelial ACE production,110 whereas Ang II produced via local intestinal ACE contributes to tissue inflammation. Tissue ACE levels have not been measured in active IBD.

The other major area of investigation has focussed on angiotensin peptide levels in CD. Genotype analyses have revealed a significant association of angiotensinogen‐6 AA genotype (OR 2.38) in a cohort of 235 patients with IBD. This genotype results in increased production of angiotensinogen via a substitution in its gene promoter.111 Indeed, mucosal levels of Ang I and Ang II are elevated in rectosigmoid biopsies in patients with Crohn's colitis compared with patients with UC or normal controls,93 and a significant correlation was noted between these levels and endoscopic grade of colitis. Although these findings demonstrate that components of the local tissue RAS probably play a part in inflammation, they do not prove any causal link in the pathogenesis of IBD. Nonetheless, they suggest that inhibition of the local RAS provides a potential avenue for targeting inflammation and fibrosis.

Studies in animal models

Angiotensin converting enzyme inhibition or angiotensin receptor antagonism have been shown to produce a number of beneficial anti‐inflammatory effects in rodent models of intestinal inflammation. Ang II and AT1a receptor expression are both upregulated in dextran sodium sulphate (DSS)‐induced colitis, a widely studied mouse model of colitis, and inflammation was significantly ameliorated in AT1a receptor‐deficient mice.112 The ACE inhibitor enalaprilat, given parenterally, reduced inflammation and TNF‐α,113 and topical enalaprilat reduced TGF‐β expression and fibrosis in mice with DSS‐colitis.114 Administration of the ARB, valsartan, significantly reduced macroscopic inflammation, TNF‐α, TGF‐β and IL‐18 in mice with trinitrobenzene sulphonic acid (TNBS) induced colitis, reduced microscopic inflammation and raised IL‐10 (an anti‐inflammatory cytokine secreted by regulatory T cells) in DSS‐colitis.115, 116 The ACE inhibitor captopril reduced macroscopic and microscopic inflammation, fibrosis and TGFβ mRNA expression in mice with TNBS‐induced colitis.117 Homozygous deficiency of angiotensinogen protects against TNBS‐induced colitis, with reduced IL1‐β, IFN‐γ and greater IL‐4 and IL‐10 production than wild‐type mice.116 Animal studies have not been restricted to the classical RAS pathway. NEP knockout mice have been shown to have more severe colitis in response to dinitrobenzene sulphonic acid than wild‐type mice, which is prevented by the administration of recombinant NEP.118

Other evidence to suggest that RAS dysfunction may potentiate immune‐based diseases such as IBD and provide a target for therapy comes from recent studies involving models for multiple sclerosis (MS). T helper type 1 (Th1) and type 17 (Th17) cells are strongly implicated in the pathogenesis of IBD, especially CD, and lisinopril and candesartan have been demonstrated to suppress Th1 and Th17 cytokine expression and induce Foxp3+ regulatory T cells in experimental autoimmune encephalitis (EAE), a mouse model of MS.119 Other groups have shown a crucial role for Ang II and AT1R in EAE59, 120 and murine autoimmune nephritis.121

Gastrointestinal cancers

The possible involvement of perturbed RAS components in solid organ malignancies represents a fascinating expansion of our insight into local tissue RAS. Early epidemiological studies demonstrated a reduced risk of incident and fatal cancer in patients on ACE inhibitors versus those on other anti‐hypertensive medication.122, 123 In contrast, a recent meta‐analysis reported a higher risk of lung cancer in patients taking ARBs124; however, two large meta‐analyses since then have reported no increased risk.125, 126 All these studies have been limited by their retrospective design.

Increasing in vitro and pre‐clinical data suggest a protective effect of AT1R inhibition against cancer cell proliferation, invasion and metastasis in a variety of solid organs.127 In gastric cancer, AT1R and Ang II are expressed to a greater extent than in adjacent normal tissue.128 The AT1R and ACE gene polymorphism D allele increase risk of nodal metastasis and tumour stage in patients with intestinal‐type gastric cancer.129 In gastric cancer cell cultures, Ang II stimulates MAP kinase, NFκB and survivin activation, increasing proliferation.128 The most likely mechanism by which the RAS may influence cancer biology is through increasing angiogenesis,130, 131 via increased expression of vascular endothelial growth factor (VEFG) signalling.130, 131, 132

In mouse models, ACE inhibition and ARBs reduced colorectal cancer liver metastases 133 and prolonged survival in peritoneal carcinomatosis.128 Indeed, ACE inhibitor use independently protected against distal metastasis in a single centre retrospective review of patients with colorectal carcinoma.134 Also, patients treated with ACE inhibitors had a non‐significant trend towards reduction in risk of oesophageal adenocarcinoma in a retrospective study of the UK general practice research database.135

Recently, there have been intriguing insights into the involvement of the RAS in Peutz‐Jeghers Syndrome (PJS). Shorning and colleagues have shown that in mice LKB1 gene deletion, which is associated with PJS, results in marked transcriptional upregulation of the renin gene Ren1, and also increased ACE expression and Ang II production.70 In human PJS tumour tissue, AT1R was noted to be increased in stromal tissue, but reduced in the epithelium.70

Gut motility disorders

The functional role of the RAS in smooth muscle contraction suggests that it might be a target in motility dysfunction. For example, the ability of Ang II blockade to inhibit contraction of oesophageal body and lower oesophageal sphincter (LES) smooth muscle, together with the demonstrated reduction in the amplitude of contraction of primary peristaltic oesophageal waves and LES on manometry in vivo,100 suggests a possible role in treatment of hypercontractile oesophageal disorders such as diffuse oesophageal spasm, nutcracker oesophagus and achalasia. Furthermore, selective AT1R‐mediated contraction of the LES may be an option for treatment of gastro‐oesophageal reflux disease, a condition that affects up to 15‐20% of the population.136

The contribution of AT1R to small and large intestinal muscle contractility also provides an opportunity to intervene in functional intestinal and motility disorders through AT1R agonism or blockade. Furthermore, the role of ACE and ACE2 in intestinal fluid and electrolyte absorption suggests a potential mechanism for modulating fluid shifts across the brush border, with subsequent effects on stool consistency and frequency.

Mesenteric ischaemia

The RAS plays an important role in regulation of the smooth muscle tone of the mesenteric vasculature. In acute hypovolaemia and systemic sepsis, splanchnic vasoconstriction occurs as a homeostatic response to preserve cerebral and renal blood flow, predisposing the gut to ischaemia. This splanchnic response has been shown to correlate with a markedly increased expression of Ang II.137 Furthermore, lower‐body negative pressure induction in normal human volunteers has also been shown to raise serum Ang II and reduce intestinal mucosal nitric oxide production.138

The administration of candesartan maintained jejunal and mucosal perfusion during severe hypovolaemia in pigs and reduced mortality.139, 140 A further porcine study reported improved mucosal oxygen delivery, but not an improvement in intestinal mucosal acidosis in pigs administered candesartan during endotoxic shock.141 These results were replicated by Tardos et al. in pigs with burn and endotoxin induced gut ischaemia. In this model, intestinal permeability and bacterial translocation were reduced with the administration of the Ang II inhibitor DuP753.142

The obvious limiting factor in applying Ang II blockade in humans at risk of mesenteric ischaemia is the potential for current ARBs and ACE inhibitors to cause further hypotension and kidney injury, although no adverse renal consequences were noted in one porcine study.139

Manipulation of Local Gastrointestinal Tract RAS ‐ Potential Targets and Limitations

As a ubiquitous system with a wide array of homeostatic roles, investigation into therapies that manipulate the RAS has been extensive. Apart from the well‐established roles of AT1 receptor blockade and ACE inhibition in the treatment of hypertension, cardiovascular and kidney disease, new drugs targeting the mas receptor, AT2 receptor, AT4R, renin and NEP, as well as a recombinant Ang (1‐7), are under trial for various applications (Table 2).

Table 2

Potential therapeutic targets of the RAS

Mechanism of action	Drug/compound	Stage of development
ACE Inhibitors	Captopril	Clinical use
	Enalapril	Clinical use
	Fosinipril	Clinical use
	Lisinopril	Clinical use
	Perindopril	Clinical use
	Quinapril	Clinical use
	Ramipril	Clinical use
	Trandalopril	Clinical use
AT1 receptor blockers (ARB)	Azilsartan	Clinical use
	Candesartan	Clinical use
	Eprosartan	Clinical use
	Irbesartan	Clinical use
	Losartan	Clinical use
	Olmesartan	Clinical use
	Telmisartan	Clinical use
	Valsartan	Clinical use
AT2 receptor agonists	CGP42112A	In vitro studies
	Compound 21	Animal studies21, 157
	Compound 22	Animal studies158
AT2 receptor antagonists	Saralasin	Animal studies159
	PD123319	In vitro studies24, 25
AT4 receptor antagonists	Divalinal	Animal studies160
Ang (1‐7)/Mas receptor agonists	Ang (1‐7)	Animal studies47, 161, 162
	AVE 0991	Animal studies47, 163, 164, 165, 166
Mas receptor antagonists	A‐779	Animal studies161
ACE2 analogues	Recombinant hACE2(APN01	Animal studies167, 168(Phase I Clinical trial
ACE2 inhibitors	C16	Animal studies 169
NEP inhibitors	Omapatrilat (dual NEP and ACE inhibitor)	Withdrawn after clinical trials170, 171, 172
	LCZ696 (dual NEP and AT1R antagonist)	Phase 3 clinical trial173
(Pro)renin receptor antagonists	Handle region peptide (HRP)	Animal studies174, 175, 176, 177, 178, 179, 180
Direct renin inhibitors	Aliskiren	Clinical use

ACE, angiotensin converting enzyme; Ang II, angiotensin II; NEP, neutral endopeptidase; RAS, renin‐angiotensin system.

There is limited knowledge of the effect of currently available ACE inhibitors and ARBs on gastrointestinal function at doses employed for cardiovascular and renal effect at a molecular level. Most of these drugs may result in adverse effects including nausea, diarrhoea or constipation in an idiosyncratic manner in between 1 and 3 per cent of patients, often noted in similar numbers of patients in the placebo arm in randomised controlled trials. It is likely that gastrointestinal tissue concentrations achieved by these medications at current doses are insufficient to note clinical effect. Furthermore, the effect of these drugs in pathological states, like functional gut disorders or IBD, has not been described.

For applicability to human gastrointestinal disease, drugs manipulating the RAS will need to target the relevant areas of the gut to obtain satisfactory tissue effect without systemic adverse reactions. Ideally, this will comprise delivery to the mucosa, absorption and binding of cellular receptors and sufficient first pass hepatic conversion to inactive metabolites to limit systemic effect.

Conclusions

There is now a significant body of literature demonstrating the existence and pathophysiological relevance of local tissue renin‐angiotensin systems. Given the current evidence of involvement of the RAS in gastrointestinal fluid and electrolyte homeostasis, smooth muscle control, inflammation and malignancy, it follows that manipulation of this system could be of benefit in a range of GI pathologies. Therapies targeting the gastrointestinal RAS are attractive, given their excellent safety and tolerability profile and confirmed benefits in other organs and diseases. It is hoped that the recent emergence of further experimental evidence supporting a role for the local RAS in intestinal disorders will provide greater impetus for the initiation of well‐conducted clinical trials in human disease.