Angiotensin II contractile effects in mouse colon: role for pre- and post-junctional AT(1A) receptors

Systematic-Narrative Hybrid Literature Review: Crosstalk between Gastrointestinal Renin-Angiotensin and Dopaminergic Systems in the Regulation of Intestinal Permeability by Tight Junctions

In the first part of this article, the role of intestinal epithelial tight junctions (TJs), together with gastrointestinal dopaminergic and renin-angiotensin systems, are narratively reviewed to provide sufficient background. In the second part, the current experimental data on the interplay between gastrointestinal (GI) dopaminergic and renin-angiotensin systems in the regulation of intestinal epithelial permeability are reviewed in a systematic manner using the PRISMA methodology. Experimental data confirmed the copresence of DOPA decarboxylase (DDC) and angiotensin converting enzyme 2 (ACE2) in human and rodent enterocytes. The intestinal barrier structure and integrity can be altered by angiotensin (1-7) and dopamine (DA). Both renin-angiotensin and dopaminergic systems influence intestinal Na+/K+-ATPase activity, thus maintaining electrolyte and nutritional homeostasis. The colocalization of B0AT1 and ACE2 indicates the direct role of the renin-angiotensin system in amino acid absorption. Yet, more studies are needed to thoroughly define the structural and functional interaction between TJ-associated proteins and GI renin-angiotensin and dopaminergic systems.

The renin-angiotensin-aldosterone system (RAAS) signaling pathways and cancer: foes versus allies

The renin-angiotensin-aldosterone system (RAAS), is an old system with new fundamental roles in cancer biology which influences cell growth, migration, death, and metastasis. RAAS signaling enhances cell proliferation in malignancy directly and indirectly by affecting tumor cells and modulating angiogenesis. Cancer development may be influenced by the balance between the ACE/Ang II/AT1R and the ACE2/Ang 1-7/Mas receptor pathways. The interactions between Ang II/AT1R and Ang I/AT2R as well as Ang1-7/Mas and alamandine/MrgD receptors in the RAAS pathway can significantly impact the development of cancer. Ang I/AT2R, Ang1-7/Mas, and alamandine/MrgD interactions can have anticancer effects while Ang II/AT1R interactions can be involved in the development of cancer. Evidence suggests that inhibitors of the RAAS, which are conventionally used to treat cardiovascular diseases, may be beneficial in cancer therapies.Herein, we aim to provide a thorough description of the elements of RAAS and their molecular play in cancer. Alongside this, the role of RAAS components in sex-dependent cancers as well as GI cancers will be discussed with the hope of enlightening new venues for adjuvant cancer treatment.

ACE and ACE2 catalytic activity in the fecal content along the gut

BACKGROUND

Angiotensin-converting enzyme (ACE) and ACE2 are two major enzymes of the renin-angiotensin-aldosterone system (RAAS), which control the formation/degradation of angiotensin (Ang) II and Ang1-7, regulating their opposite effects. We aimed at evaluating the catalytic activity of ACE and ACE2 in the intestinal content and corresponding intestinal tissue along the gut of Wistar Han rats.

METHODS

Portions of the ileum, cecum, proximal colon, and distal colon, and the corresponding intestinal content were collected from Wistar Han rats. Enzyme activity was evaluated by fluorometric assays using different substrates: Hippuryl-His-Leu for ACE-C-domain, Z-Phe-His-Leu for ACE-N-domain, and Mca-APK(Dnp) for ACE2. ACE and ACE2 concentration was assessed by ELISA. Ratios concerning concentrations and activities were calculated to evaluate the balance of the RAAS. Statistical analysis was performed using Friedman test followed by Dunn's multiple comparisons test or Wilcoxon matched-pairs test whenever needed.

KEY RESULTS

ACE and ACE2 are catalytically active in the intestinal content along the rat gut. The ACE N-domain shows higher activity than the C-domain both in the intestinal content and in the intestinal tissue. ACE and ACE2 are globally more active in the intestinal content than in the corresponding intestinal tissue. There was a distal-to-proximal prevalence of ACE2 over ACE in the intestinal tissue.

CONCLUSIONS & INFERENCES

This work is the first to report the presence of catalytically active ACE and ACE2 in the rat intestinal content, supporting future research on the regulatory role of the intestinal RAAS on gut function and a putative link to the microbiome.

Histomorphometry Changes and Decreased Reactivity to Angiotensin II in the Ileum and Colon of Streptozotocin-Induced Diabetic Rats

Diabetes mellitus (DM) is a chronic progressive metabolic disorder associated with several gastrointestinal complications, affecting up to 75% of patients. Knowing that Angiotensin II (AngII) also regulates intestinal contraction, we decided to evaluate changes in ileum and colon histomorphometry and AngII reactivity in a rat model of DM. Streptozotocin (STZ, 55 mg/kg) was administered to induce DM to 24 adult male Wistar rats. Diabetic rats displayed all the characteristic signs of type 1 DM (T1DM) and fecal excretion increased about 4-fold over 14 days, while the excretion of controls remained unaltered. Compared to controls, diabetic ileum and colon presented an increase in both macroscopic (length, perimeter and weight) and microscopic (muscular wall thickness) parameters. Functionally, AngII-induced smooth muscle contraction was lower in diabetic rats, except in the distal colon. These differences in the contractile response to AngII may result from an imbalance between AngII type 1 (antagonized by candesartan, 10 nM) and type 2 receptors activation (antagonized by PD123319, 100 nM). Taken together, these results indicate that an early and refined STZ-induced T1DM rat model already shows structural remodelling of the gut wall and decreased contractile response to AngII, findings that may help to explain diabetic dysmotility.

Aging modifies receptor expression but not muscular contractile response to angiotensin II in rat jejunum

The involvement of renin-angiotensin system in the modulation of gut motility and age-related changes in mRNA expression of angiotensin (Ang II) receptors (ATR) are well accepted. We aimed to characterize, in vitro, the contractile responses induced by Ang II, in jejunum from young (3–6 weeks old) and old rats (≥ 1 year old), to evaluate possible functional differences associated to changes in receptor expression. Mechanical responses to Ang II were examined in vitro as changes in isometric tension. ATR expression was assessed by qRT-PCR. Ang II induced a contractile effect, antagonized by losartan, AT1R antagonist, and increased by PD123319, AT2R antagonist, as well by neural blocker ω-conotoxin and by nitric oxide (NO) synthase inhibitor. No difference in the response was observed between young and old groups. AT1 receptor-mediated contractile response was decreased by U-73122, phospholipase C (PLC) inhibitor; or 2-aminoethoxy-diphenylborate (2-APB), inositol triphosphate (IP₃) receptor inhibitor; or nifedipine, l-type calcium channel blocker. Age-related changes in the expression of both AT1 receptor subtypes, AT1a and AT1b, and of AT2 receptors were detected. In conclusion, Ang II modulates the spontaneous contractility of rat jejunum via postjunctional AT1 receptors, involving Ca²⁺ mobilization from intracellular stores, via PLC/IP₃ pathway, and Ca²⁺ influx from extracellular space, via l-type channels. Prejunctional AT2 receptors would counteract AT1 receptor effects, via NO synthesis. The observed age-related differences in the expression of all AT receptor subtypes are not reflected in the muscular contractile response to Ang II.

Interaction between the Renin-Angiotensin System and Enteric Neurotransmission Contributes to Colonic Dysmotility in the TNBS-Induced Model of Colitis

Angiotensin II (Ang II) regulates colon contraction, acting not only directly on smooth muscle but also indirectly, interfering with myenteric neuromodulation mediated by the activation of AT₁ /AT₂ receptors. In this article, we aimed to explore which mediators and cells were involved in Ang II-mediated colonic contraction in the TNBS-induced rat model of colitis. The contractile responses to Ang II were evaluated in distinct regions of the colon of control animals or animals with colitis in the absence and presence of different antagonists/inhibitors. Endogenous levels of Ang II in the colon were assessed by ELISA and the number of AT₁/AT₂ receptors by qPCR. Ang II caused AT₁ receptor-mediated colonic contraction that was markedly decreased along the colons of TNBS-induced rats, consistent with reduced AT₁ mRNA expression. However, the effect mediated by Ang II is much more intricate, involving (in addition to smooth muscle cells and nerve terminals) ICC and EGC, which communicate by releasing ACh and NO in a complex mechanism that changes colitis, unveiling new therapeutic targets.

Opposite effects of dopamine on the mechanical activity of circular and longitudinal muscle of human colon

BACKGROUND

Because dopamine (DA) has gained increasing evidence as modulator of gut motility, we aimed to characterize dopaminergic response in human colon, evaluating function and distribution of dopamine receptors in circular vs longitudinal muscle strips.

METHODS

Mechanical responses to DA and dopaminergic agonists on slow phasic contractions and on basal tone were examined in vitro as changes in isometric tension. RT-PCR was used to reveal the distribution of dopaminergic receptors.

KEY RESULTS

In spontaneous active circular muscle, DA induced an increase in the amplitude of slow phasic contractions and of the basal tone, via activation of D1-like receptors. DA contractile responses were insensitive to neural blockers or to atropine and inhibited by phospholipase C (PLC) pathway inhibitors. In precontracted circular muscle strips, DA, at the higher concentrations tested, caused a relaxant response via activation of D2-like receptors. In the longitudinal muscle, DA caused only muscular relaxation due to activation of D2-like receptors. DA relaxant responses were insensitive to neural blockers or to nitric oxide synthase inhibitor and reduced by a wide-spectrum K⁺ channel blockers. Transcripts encoding for all the dopaminergic receptor subtypes was observed in both circular and longitudinal preparations.

CONCLUSIONS AND INFERENCES

Dopamine is able to modulate contractile activity of the human colon. In the circular muscle layer, DA induces mainly muscular contraction activating non-neural D1-like receptors, coupled to PLC/IP3 pathway. In the longitudinal muscle layer, DA induces muscular relaxation acting on non-neural D2-like receptors leading to the increase in K⁺ conductance.

PD123319, angiotensin II type II receptor antagonist, inhibits oxidative stress and inflammation in 2, 4-dinitrobenzene sulfonic acid-induced colitis in rat and ameliorates colonic contractility

Angiotensin II, the main effector of renin angiotensin system, plays an important role in the inflammatory process and most of its effects are mediated through the AT1 receptor activation. However, the knowledge about the AT2 receptor involvement in this process is still evolving. We previously found that in an experimental model of colitis, AT2 receptor activation can contribute to the impairment of the muscle contractility in vitro in the course of inflammation. Here, we investigated the potential alleviating effects of the in vivo treatment of PD123319 (1-[[4-(Dimethylamino)-3-methylphenyl]methyl]-5-(diphenylacetyl)-4,5,6,7- tetrahydro-1H-imidazo[4,5-c]pyridine-6-carboxylic acid ditrifluoroacetate), AT2 receptor antagonist, in 2,4-dinitrobenzene sulfonic acid (DNBS)-induced rat model of colitis. The effects of i.p PD123319 (0.3, 3 and 10 mg/kg) administration to rats subjected to intra-rectal DNBS instillation were investigated. The study revealed that the colon injury and the inflammatory signs were ameliorated by PD123319 when visualized by the histopathological examination. The colon shortening, myeloperoxidase activity, and colonic expression of IL-1β, IL-6 and iNOS were downregulated in a dose-dependent manner in DNBS-induced colitis rats treated with PD123319 and the anti-oxidant defense machinery was also improved. The mechanism of these beneficial effects was found in the ability of PD123319 to inhibit NF-κB activation induced by DNBS. The colonic contractility in inflamed tissues was also improved by PD123319 treatment. In conclusion, our data have demonstrated previously that undescribed proinflammatory effects for the AT2 receptors in DNBS-induced colitis in rats in which they are mediated likely by NF-κB activation and reactive oxygen species generation. Moreover, when the inflammatory process is mitigated by the AT2 receptor antagonist treatment, the smooth muscle is able to recover its functionality.

[Angiotensin II Regulates Excitability and Contractile Functions of Myocardium and Smooth Muscles through Autonomic Nervous Transmission]

Angiotensin II (Ang II) is an intrinsic peptide having strong vasopressor effects, and thus, it plays an important role in the physiological regulation of blood pressure. The vasopressor effects of Ang II include direct contraction of myocardium and vascular smooth muscles (SMs) along with aldosterone-mediated sodium retention. In addition, indirect vascular contractions induced by noradrenaline (NA), the release of which is mediated through Ang II receptor type 1 (AT₁) existing at the sympathetic nerve terminals (SNTs), also contribute to the vasopressor effects of Ang II. Stimulation of NA release from SNTs by Ang II also occurs in the myocardium leading to an increase in heart rate and cardiac contraction. Furthermore, Ang II enhances the contractions of non-vascular SMs, such as vas deferens, through induction of NA release from the SNTs. We have found that Ang II attenuated vagus nerve stimulation-induced bradycardia in a losartan-sensitive manner. This suggests that Ang II attenuates vagus nerve stimulation-induced bradycardia by inhibiting acetylcholine (ACh) release from the parasympathetic nerve terminals (PNTs) through activation of the AT₁ receptor. Ang II was also reported to attenuate the release of ACh from the PNTs in SMs, such as stomach and airway, thus suppressing their contractile functions. There are, however, conflicting reports of the effects of Ang II on parasympathetic nerve-mediated contractile regulation of SMs. In this review, we have highlighted the relevant research articles including our experimental reports on the regulation of sympathetic and parasympathetic nerve-mediated excitation and contraction by Ang II along with the future prospects.

Angiotensin II type II receptors and colonic dysmotility in 2,4-dinitrofluorobenzenesulfonic acid-induced colitis in rats

BACKGROUND

Angiotensin II (Ang II), the main peptide of the renin-angiotensin system (RAS), has been suggested to be involved in inflammatory bowel diseases. Since RAS has emerged as gut motility regulator, and dysmotility is associated with intestinal inflammation, our objective was to investigate in rat 2,4-dinitrobenzenesulfonic acid (DNBS)-induced colitis the functionality of RAS and its contribution to colonic motor alterations.

METHODS

The effects of Ang II on the longitudinal colonic muscular contractility of control and DNBS-treated rats were characterized in vitro. Transcripts encoding for Ang II receptors were investigated by RT-PCR.

KEY RESULTS

Inflamed preparations showed a longitudinal muscle marked hypocontractility. Angiotensin II caused contractile effects in both preparations, but the responses in DNBS preparations were reduced compared to controls. In both preparations, Losartan, AT₁ receptor antagonist, reduced Ang II effects. PD123319, AT₂ receptor antagonist, enhanced Ang II responses only in DNBS rats, as well as N^ω -Nitro-L-arginine (L-NNA), nitric oxide (NO) synthase inhibitor, or tetrodotoxin (TTX), neural toxin. The co-administration of PD123319 and TTX or L-NNA produced no additive effects. PD123319 per se improved colonic contractility in inflamed tissues. The effect was reduced in the presence of L-NNA or TTX. All Ang II receptor subtypes were expressed in both preparations.

CONCLUSIONS & INFERENCES

AT₁ receptors mediate Ang II contractile responses in rat colon. During inflammation a recruitment of Ang II AT₂ receptors would counteract AT₁ -contractile activity. A tonic activation of AT₂ receptors would contribute to the general reduction in muscle contractility during experimental inflammation. A role for enteric neurons and NO is also suggested.

Effects of Antihypertensive Agents on Intestinal Contractility in the Spontaneously Hypertensive Rat: Angiotensin Receptor System Downregulation by Losartan

Hypertension is an inflammatory condition controlled by the renin angiotensin system and is linked to kidney disease, diabetes mellitus, and recently to dysfunction of the gut. The aim of this study was to determine what effect antihypertensive drug treatments may have on intestinal function of the spontaneously hypertensive rat (SHR). In the first experiment, SHRs were treated with enalapril, hydralazine, or with no treatment as a control. In the second experiment, SHRs were treated with losartan or with no treatment as a control. All drug treatments led to significant lowering of blood pressure after 16 weeks. At termination, intact tissue sections of the ileum and colon were induced to contract ex vivo by KCl; electrical stimulation; and agonists carbachol, angiotensin II, and prostaglandin E₂ (PGE₂). There were no differences in ileal or colonic contractility due to hydralazine or enalapril compared with no-treatment SHR control. However, for the ileum, the losartan group responded significantly more to KCl and carbachol while responding less to angiotensin II, with no difference for PGE₂ compared with the no-treatment SHR control. In contrast, the colon responded similarly to KCl, electrical stimulation, and PGE₂ but responded significantly less to angiotensin II. These results demonstrate that the ileum responds differently (with KCl and carbachol as agonists) to the colon after losartan treatment, whereas there is a reduced contractile response in both the ileum and colon following losartan treatment. Although there are few well documented major contraindications for angiotensin receptor blockers, the modulation of gut contractility by losartan may have wider implications for bowel health.

Activation of angiotensin II type 1 receptors and contractile activity in human sigmoid colon in vitro

AIM

To analyse the effects of angiotensin II (Ang II) on the contractility of human sigmoid colon, and to characterize the subtype(s) of receptor(s) involved and the related action mechanism.

METHODS

The contractility of sigmoid colon circular muscle strips was recorded isometrically. RT-PCR and immunohistochemistry were used to reveal the eventual existence of a local renin-angiotensin system (RAS) and the distribution of Ang II receptors.

RESULTS

Transcripts encoding for the Ang II type 1 (AT1 ) and the Ang II type 2 (AT2 ) receptor subtypes and for the angiotensin-converting enzyme in the whole-thickness muscular wall were observed. Ang II caused a concentration-dependent contractile response, which is antagonized by losartan, AT1 receptor antagonist, but not by PD123319, AT2 receptor antagonist. The joint application of losartan and PD123319 did not produce any additive effect. The contractile response to Ang II was partially reduced by tetrodotoxin, Na(+) voltage-gated neural channel blocker, and to some extent by SR48968, tachykinin NK2 receptor antagonist. However, hexamethonium, nicotinic receptor antagonist, atropine, cholinergic muscarinic receptor antagonist and SR140333, tachykinin NK1 receptor antagonist, were ineffective. Immunohistochemical analysis showed that AT1 receptors were expressed on the smooth muscle layers and myenteric plexus.

CONCLUSION

Ang II positively modulates the spontaneous contractile activity of human sigmoid colon via activation of post-junctional and pre-junctional AT1 receptors, the latter located on the enteric nerves that modulate the release of tachykinins. The presence of the components of RAS in the human colon suggests that Ang II can be also locally generated to control colonic motility.

Interactions of the renin-angiotensin system in colorectal cancer and metastasis

The novel function of the renin-angiotensin system (RAS) is cardiovascular homeostasis. While the major active mediator angiotensin II (ANG II) produces most of the physiologic responses via angiotensin II type I receptor (AT1R), recent insights have looked at the implications of ANG II and its impact on solid tumor formation. Preclinical studies have demonstrated the direct effect of ANG II on the stimulation of angiogenesis via VEGF and other proliferative mediators. RAS components have thus been identified in numerous malignant tissues. Inhibition of the AT1R via angiotensin-converting enzyme inhibitors (ACE-Is) has demonstrated a decrease in solid tumor development and metastasis. Numerous retrospective analyses have demonstrated a reduction in colorectal cancer incidence, polyp formation, and distant metastasis in patients taking inhibitors of the RAS. The use of commonly prescribed anti-hypertensive medications as a chemo-preventative medication may have a remarkable impact in the colorectal cancer community. Further investigation and prospective clinical trials may provide further insight into the potentially beneficial use of ACE-Is and their impact on colorectal cancer.

Angiotensin IV induced contractions in human jejunal wall musculature in vitro

Angiotensin II (AngII) has been reported to mediate contractile actions in rats and human jejunal wall musculature. However, except for one report showing the angiotensin IV (AngIV) contractile effects on the internal anal sphincter of rats, no data is available describing the action of AngIV on smooth muscle in human small intestine. The aim of this study was to investigate the expression and localization of the enzymes responsible to AngIV formation, as well as the receptor, and to elucidate the contractile function of AngIV in the muscular layer of human jejunum in vitro. Jejunal smooth muscle was taken from 23 patients undergoing Roux-en-Y gastric bypass surgery and was used to record isometric tension in vitro in response to AngIV alone and in the presence of losartan or PD123319. ELISA, western blot and immunohistochemistry were used to investigate the expression and localization of key components for AngIV formation: the enzymes aminopeptidases-A, B, M, and the AngIV receptor insulin-regulated aminopeptidase (IRAP). AngIV elicited concentration-dependent contraction in both longitudinal and circular smooth-muscle preparation. Presence of losartan abolished AngIV-induced contraction, but not PD123319. The main peptide AngII, as well as the enzymes aminopeptidases-A, B and M was detected in all muscle samples. Immunohistochemistry localized the enzymes and IRAP in the myenteric plexus between longitudinal and circular muscle layers. The present study indicates that all enzymes necessary for AngIV formation exist in human jejunal smooth muscle and that the contractile action elicited by AngIV is primarily mediated through the AngII type 1 receptor.