Adrenomedullin, amylin, calcitonin gene-related peptide and their fragments are potent inhibitors of gastric acid secretion in rats

Structural Analysis of Amylin and Amyloid β Peptide Signaling in Alzheimer's Disease

Amylin and amyloid β belong to the same protein family and activate the same receptors. Amyloid β levels are elevated in Alzheimer's disease. Recent studies have demonstrated that amylin-based peptides can reduce the symptoms of Alzheimer's disease in animal models. Replica exchange molecular dynamics simulation machine learning, as well as other computational analyses, were applied to improve the understanding of the amino acid residues in these amylin-based peptides. Comparisons were made between amylin, amylin-based peptides, and amyloid β. These studies converged on amylin residues 10Q, 28S, 29S, 30T, 31N, 32V, 33G, 34S, and 35N (residues 10 and 28-35) being ranked highest, meaning that they were the most likely to be involved in activating the same targets as amyloid β. Surprisingly, the amyloid β signaling domain most closely matched amylin residues 29-35 in the simulated structures. These findings suggest important residues that are structurally similar between amylin and amyloid β and are thus implicated in the activation of the amylin receptor.

Adrenomedullin: Not Just Another Gastrointestinal Peptide

Adrenomedullin (AM) and proadrenomedullin N-terminal 20 peptide (PAMP) are two bioactive peptides derived from the same precursor with several biological functions including vasodilation, angiogenesis, or anti-inflammation, among others. AM and PAMP are widely expressed throughout the gastrointestinal (GI) tract where they behave as GI hormones, regulating numerous physiological processes such as gastric emptying, gastric acid release, insulin secretion, bowel movements, or intestinal barrier function. Furthermore, it has been recently demonstrated that AM/PAMP have an impact on gut microbiome composition, inhibiting the growth of bacteria related with disease and increasing the number of beneficial bacteria such as Lactobacillus or Bifidobacterium. Due to their wide functions in the GI tract, AM and PAMP are involved in several digestive pathologies such as peptic ulcer, diabetes, colon cancer, or inflammatory bowel disease (IBD). AM is a key protective factor in IBD onset and development, as it regulates cytokine production in the intestinal mucosa, improves vascular and lymphatic regeneration and function and mucosal epithelial repair, and promotes a beneficial gut microbiome composition. AM and PAMP are relevant GI hormones that can be targeted to develop novel therapeutic agents for IBD, other GI disorders, or microbiome-related pathologies.

Oral Amylin Treatment Reduces the Pathological Cascade of Alzheimer's Disease in a Mouse Model

Intraperitoneal injection of amylin or its analog reduces Alzheimer's disease (AD) pathology in the brains. However, self-injecting amylin analogs is difficult for patients due to cognitive deficits. This work aims to study the effects of amylin on the brain could be achieved by oral delivery as some study reported that amylin receptor may be present in the gastrointestinal tract. A 6-week course of oral amylin treatment reduced components of AD pathology, including the levels of amyloid-β, phosphorylated tau, and ionized calcium binding adaptor molecule 1. The treatment reduced active forms of cyclin-dependent kinase 5. Oral amylin treatment led to improvements in social deficit in AD mouse. Using immunofluorescence, we observed the amylin receptor complexed with the calcitonin receptor and receptor activity-modifying proteins in the enteric neurons. The study suggests the potential of the oral delivery of amylin analogs for the treatment of AD and other neurodegenerative diseases through enteric neurons.

Adrenomedullin as a Protein with Multifunctional Behavior and Effects in Various Organs and Tissues

In literature, it has been reported that adrenomedullin, which is generally thought to have vasodilator, natriuretic and diuretic effects, is synthesized in almost all body, especially CNS, vascular muscles and endothelium, heart, liver, lung, kidney, gastric mocosa, intestinal endothelium and various blood cells. It has been found that the possible effects of adrenomedullin can be demonstrated directly or indirectly by means of active mediators, neuropeptides, enzymes and hormones. It is also suggested that it regulates the endocrine system by affecting the hypothalamic-pituitary axis. It increases in heart failure, acute coronary syndromes, hypertensive conditions, cerebrovascular accessory, chronic renal failure and periodontitis and decreases in peptic ulcer and intestinal diseases. However, it is still not clear whether increase/decrease in adrenomedullin level is a cause of a disease or is a result of damage due to an illness. This peptide, which could be thought to multifunctional, should be considered as a molecule with genetic coding that may have different effects on different tissues and conditions. For all these reasons, we aimed to review the multifonctional behavior of adrenomedullin in the light of the current literature to pioneer new hypotheses and discuss possible mechanisms.

Noncompetitive affinity assays of glucagon and amylin using mirror-image aptamers as affinity probes

The ability to detect picomolar concentrations of glucagon and amylin using fluorescently labeled mirror-image aptamers, so-called Spiegelmers, is demonstrated. Spiegelmers rival the specificity of antibodies and overcome the problem of biostability of natural aptamers in a biological matrix. Using Spiegelmers as affinity probes, noncompetitive capillary electrophoresis affinity assays of glucagon and murine amylin were developed and optimized. The detection limit for glucagon was 6 pM and for amylin was 40 pM. Glucagon-like peptide-1 and -2 did not interfere with the glucagon assay, while the amylin assay showed cross-reactivity to calcitonin gene related peptide. The developed assays were combined with a competitive immunoassay for insulin to measure glucagon, amylin, and insulin secretion from batches of islets after incubation with different glucose concentrations. The development of these assays is an important step towards incorporation into an online measurement system for monitoring dynamic secretion from single islets.

Adrenomedullin regulates intestinal physiology and pathophysiology

Adrenomedullin (AM) and proadrenomedullin N-terminal 20 peptide (PAMP) are 2 biologically active peptides produced by the same gene, ADM, with ubiquitous distribution and many physiological functions. Adrenomedullin is composed of 52 amino acids, has an internal molecular ring composed by 6 amino acids and a disulfide bond, and shares structural similarities with calcitonin gene-related peptide, amylin, and intermedin. The AM receptor consists of a 7-transmembrane domain protein called calcitonin receptor-like receptor in combination with a single transmembrane domain protein known as receptor activity-modifying protein. Using morphologic techniques, it has been shown that AM and PAMP are expressed throughout the gastrointestinal tract, being specially abundant in the neuroendocrine cells of the gastrointestinal mucosa; in the enterochromaffin-like and chief cells of the gastric fundus; and in the submucosa of the duodenum, ileum, and colon. This wide distribution in the gastrointestinal tract suggests that AM and PAMP may act as gut hormones regulating many physiological and pathologic conditions. To date, it has been proven that AM and PAMP act as autocrine/paracrine growth factors in the gastrointestinal epithelium, play key roles in the protection of gastric mucosa from various kinds of injury, and accelerate healing in diseases such as gastric ulcer and inflammatory bowel diseases. In addition, both peptides are potent inhibitors of gastric acid secretion and gastric emptying; they regulate the active transport of sugars in the intestine, regulate water and ion transport in the colon, modulate colonic bowel movements and small-intestine motility, improve endothelial barrier function, and stabilize circulatory function during gastrointestinal inflammation. Furthermore, AM and PAMP are antimicrobial peptides, and they contribute to the mucosal host defense system by regulating gut microbiota. To get a formal demonstration of the effects that endogenous AM and PAMP may have in gut microbiota, we developed an inducible knockout of the ADM gene. Using this model, we have shown, for the first time, that lack of AM/PAMP leads to changes in gut microbiota composition in mice. Further studies are needed to investigate whether this lack of AM/PAMP may have an impact in the development and/or progression of intestinal diseases through their effect on microbiota composition.

Amylin structure-function relationships and receptor pharmacology: implications for amylin mimetic drug development

Amylin is an important, but poorly understood, 37 amino acid glucoregulatory hormone with great potential to target metabolic diseases. A working example that the amylin system is one worth developing is the FDA-approved drug used in insulin-requiring diabetic patients, pramlintide. However, certain characteristics of pramlintide pharmacokinetics and formulation leave considerable room for further development of amylin-mimetic compounds. Given that amylin-mimetic drug design and development is an active area of research, surprisingly little is known about the structure/function relationships of amylin. This is largely due to the unfavourable aggregative and solubility properties of the native peptide sequence, which are further complicated by the composition of amylin receptors. These are complexes of the calcitonin receptor with receptor activity-modifying proteins. This review explores what is known of the structure-function relationships of amylin and provides insights that can be drawn from the closely related peptide, CGRP. We also describe how this information is aiding the development of more potent and stable amylin mimetics, including peptide hybrids.

Intermedin in rat uterus: changes in gene expression and peptide levels across the estrous cycle and its effects on uterine contraction

BACKGROUND

The present study demonstrates the expression of intermedin (IMD) and its receptor components in the uterus of the female rat during the estrous cycle and its effect on uterine contraction.

METHODS

The gene expression level of intermedin and its receptor components and the peptide level of intermedin were studied by real-time RT-PCR and enzyme immunoassay (EIA) respectively. The separation of precursor and mature IMD was studied by gel filtration chromatography and EIA. The localization of IMD in the uterus was investigated by immunohistochemistry. The effect of IMD on in vitro uterine contraction was studied by organ bath technique.

RESULTS

Uterine mRNAs of Imd and its receptor components and IMD levels displayed cyclic changes across the estrous cycle. Imd mRNA level was the highest at proestrus while the IMD level was the highest at diestrus. IMD was found in the luminal and glandular epithelia and IMD treatment significantly reduced the amplitude and frequency of uterine contraction but not the basal tone. Both calcitonin gene-related peptide (CGRP) receptor antagonist hCGRP8-37 and adrenomedullin (ADM) receptor antagonist hADM22-52 partially abolished the inhibitory effect of IMD on uterine contraction while the specific IMD receptor antagonist hIMD17-47 completely blocked the actions. The enzyme inhibitors of NO (L-NAME) and PI3K (Wortmannin) pathways diminished the IMD effects on uterine contraction while the cAMP/PKA blocker, KT5720, had no effect, indicating an involvement of NO and PI3K/Akt but not PKA.

CONCLUSIONS

IMD and the gene expression of its receptor components are differentially regulated in the uterus during the estrous cycle and IMD inhibits uterine contraction by decreasing the amplitude and frequency.

Amylin(1-8) is devoid of anabolic activity in bone

Amylin(1-8), a cyclic peptide consisting of the eight N-terminal amino acids of the 37-amino acid peptide amylin, has been shown to induce proliferation of primary osteoblasts and to induce bone formation in healthy male mice, whereas no data on efficacy in bone disease-related models have been reported. Therefore, we evaluated any effects of amylin(1-8) in ovariectomized rats with established osteopenia, a model for postmenopausal osteoporosis. At doses up to 100 nmol/kg/day, a dose highly effective in healthy mice, amylin(1-8) was unable to increase bone mineral density in ovariectomized rats during an 8-week treatment period. Histomorphometric analysis of the tibia indicated that amylin(1-8) did not change bone histomorphometric parameters. In an attempt to verify any potential biological effects of amylin(1-8), we investigated the efficacy of this peptide in various in vitro assays. Experiments designed to confirm previously published results on the proliferative effects of amylin(1-8) on primary osteoblasts failed to show any response. Amylin(1-8) was able to partially displace (125)I-rat amylin(1-37) from amylin receptors composed of the calcitonin receptor and RAMP1, indicating specific interaction of the peptide with the amylin binding site. However, in vitro efficacy assays with amylin(1-8) in calcitonin receptor-RAMP-positive HEK293T and MCF7 cells failed to reveal any agonist activity of amylin(1-8), whereas amylin(1-37) showed the expected agonist activity. In conclusion, our results indicate that amylin(1-8) does not show agonist activity on amylin receptors, does not affect osteoblast proliferation, and is devoid of anabolic activity in bone.

Adrenomedullin reduces antioxidant defense system and enhances kidney tissue damage in cadmium and lead exposed rats

Adrenomedullin (AdM) is synthesized and secreted by a number of cells and tissue. AdM is a potent vasodilator but it is also considered a neuromodulator, an angiogenic factor, and a hormone regulator. AdM possess antiapoptotic, antioxidant, and antimicrobial properties. Heavy metals such as cadmium and lead are found widely in the environment and they have important biological functions. Lead (Pb) and cadmium (Cd) can accumulate in the lungs, liver, bone, and kidneys and cause serious organ damage. In the present study, we investigated the effect of AdM, Pb + AdM, and Cd + AdM treatments on superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GSH-Px) activities as well as the level of malondialdehyde (MDA) in the kidney. Heavy metal accumulation was determined in kidney with and without AdM infusion and kidney damage was evaluated by light and electron microscopy. Increased heavy metal accumulation was observed in the heavy metal and AdM treated groups. SOD, CAT, GSH-Px activities, and MDA levels were significantly different in the treatment groups when compared with the control group. Tubular degeneration, necrosis, cell swelling, mononuclear cell infiltration, and degenerated organelles were observed in the kidney following treatment. Therefore, AdM infusion has no beneficial and/or compensatory role in cadmium and lead toxicity in the kidney. We conclude that heavy metal accumulation in the kidney in conjunction with AdM infusion is cytotoxic despite the known beneficial effects of adrenomedullin.

Effect of calcitonin gene-related peptide (CGRP), adrenomedullin and adrenomedullin-2/intermedin on food intake in goldfish (Carassius auratus)

The purpose of the present study was to elucidate the possible role of calcitonin gene-related peptide (CGRP), adrenomedullin (AM) and adrenomedullin-2/intermedin (IMD) on food intake regulation in goldfish (Carassius auratus). We examined the effects of intracerebroventricular (ICV) administration of these related hormones on food intake. Food-deprived goldfish were subjected to ICV injections of CGRP, AM and IMD and their food intake were quantified. CGRP at 10ng/g body weight (bw) significantly decreased food intake as compared to saline-treated fish. IMD at 10 and 50ng/g bw both significantly decreased food intake as compared to saline group. No significant differences were observed after AM administration. Our results suggest, for the first time in fish, a role for both CGRP and IMD in the central regulation of feeding in fish.

Molecular characterization of calcitonin gene-related peptide (CGRP) related peptides (CGRP, amylin, adrenomedullin and adrenomedullin-2/intermedin) in goldfish (Carassius auratus): cloning and distribution

To further characterize the structure and function of calcitonin gene-related peptide (CGRP) related peptides in fish, we have cloned cDNA sequences for CGRP, amylin, adrenomedullin (AM) and adrenomedullin-2/intermedin (IMD) in goldfish (Carassius auratus) and examined their tissue distribution. CGRP, amylin, AM and IMD cDNAs were isolated by reverse transcription (RT) and rapid amplification of cDNA ends (RACE). The cloned sequences contain the complete four mature peptides, which present a high degree of identity with mature peptide sequences from other fish. Phylogenetic analyses show that goldfish AM and IMD form a sub-family within the CGRP-related peptides that is distinct from the CGRP/amylin sub-family. The distribution of goldfish CGRP-like peptides mRNA expression in different tissues and within the brain was studied by RT-PCR. CGRP, IMD and AM are detected throughout the brain, in pituitary and in most peripheral tissues examined. Amylin mRNA is mostly expressed in the brain, in particular posterior brain, optic tectum and hypothalamus, but is also present in pituitary, gonad, kidney and muscle. Our results suggest that goldfish CGRP, amylin, AM and IMD are conserved peptides that show the typical structure characteristics present in their mammalian counterparts. The widespread distributions of CGRP, AM and IMD suggest that these peptides could be involved in the regulation of many diverse physiological functions in fish. Amylin mRNA distribution suggests possible new roles for this peptide in teleosts, including the control of reproduction.

Protective roles of alpha-calcitonin and beta-calcitonin gene-related peptide in spontaneous and experimentally induced colitis

Calcitonin gene-related peptide (CGRP) is thought to be involved in the regulation of gastric and mesenteric blood flow, in the control of gastric acid secretion and in the modulation of intestinal motility, yet the precise physiological roles of CGRP remain to be elucidated. To further examine the role(s) of CGRP in gastrointestinal function, we examined mutant mice lacking alphaCGRP or betaCGRP expression. Mutant mice did not demonstrate any overt phenotypic changes, yet exhibited a spontaneous, adult-onset colitis and increased colonic damage using a dextran sulfate sodium model of experimental colitis. Surprisingly, mice lacking betaCGRP show no obvious alterations in CGRP immunoreactivity in the gut, accompanied by an increase in alphaCGRP messenger RNA expression, suggesting an adaptive mechanism to compensate for the lack of betaCGRP. These data demonstrate that both alphaCGRP and betaCGRP play a protective role in the generation of spontaneous colitis, supporting a role for both extrinsic and intrinsic CGRP-containing neurons.

Differential regulation of adrenomedullin gene expression in the fundic and pyloric regions of the rat stomach during acute and chronic starvation

Adrenomedullin (AM) has been shown to be present in the stomach but the role of gastric AM is obscure. To investigate the effects of starvation on AM in the stomach, we studied the changes in gene expression of preproadrenomedullin (preproAM) and AM receptors by reverse transcription-polymerase chain reaction (RT-PCR), and tissue AM concentrations by radioimmunoassay (RIA) in the fundus and pylorus of the stomach of rats subjected to either acute (1-day) or chronic (4-day) starvation. An up-regulation of preproAM gene expression was observed in the fundus after acute starvation, and in the pylorus after chronic starvation. Immunoreactive AM (ir-AM) levels were increased in both fundus and pylorus after chronic starvation. In addition, marked reductions in the gene expression of fundic calcitonin receptor-like receptor (CRLR) and receptor activity-modifying protein (RAMP) 3 as well as the pyloric CRLR and RAMP2 were observed in the chronically starved rats. The present study suggests that the gene expression of preproadrenomedullin mRNA is differentially regulated by starvation in the different parts of the stomach.

Investigation of the Mechanisms Underlying the Gastroprotective Effect of Nicorandil

AIM

This study investigated possible mechanisms underlying the gastroprotective effect of nicorandil on experimentally-induced gastric lesions in rats.

METHODS

Rats were randomly assigned to vehicle-, nicorandil (10 mg/kg)-, glibenclamide (6 mg/kg)-, nicorandil + glibenclamide- and cimetidine-pretreated groups, in addition to non-stressed control group, to demonstrate whether the K(ATP )channel opening contributed to nicorandil's gastroprotection. Lesions were induced by water immersion-restraint stress (WIRS) and ulcer indices were determined. Gastric juice parameters (pH, acid output, pepsin and mucin concentrations) were determined. Another set of rats was divided into control, saline-pretreated and nicorandil (10 mg/kg)-pretreated groups. Rats underwent WIRS and their stomachs were used for determination of gastric mucosal lipid peroxides, histamine, PGE(2), and total nitrites levels.

RESULTS

Nicorandil displayed significant protection against gastric lesions formation, abolished by concomitant administration of glibenclamide. Nicorandil significantly reduced gastric acid and pepsin secretion, but upon coadministration with glibenclamide, these effects were blocked. Additionally, nicorandil significantly reduced gastric mucosal lipid peroxides and total nitrites, but did not affect PGE(2) and histamine levels.

CONCLUSION

Results confirm a gastroprotective effect for nicorandil, the mechanism of which comprises K(ATP) channel opening, free radical scavenging, decrease of pepsin and acid secretion and prevention of the detrimental rise in nitric oxide during WIRS.

Peripheral adrenomedullin inhibits gastric emptying through CGRP8-37 -sensitive receptors and prostaglandins pathways in rats

The effects of intravenous (iv) adrenomedullin (AM) on gastric emptying were investigated in conscious rats. AM induced a maximal 50% inhibition of gastric emptying at a dose of 1.2 nmol/kg. AM was about two-fold less potent than alpha-calcitonin gene-related peptide (alpha-CGRP), which induced a similar 50% maximal inhibition of gastric emptying at 0.6 nmol/kg. Delayed gastric emptying induced by i.v. AM and alpha-CGRP was prevented by peripheral injection of the selective CGRP1 antagonist, CGRP8-37, and by pretreatment with indomethacin, while not altered by blockade of the sympathetic nervous system with propranolol. These data indicate that peripheral AM inhibits gastric emptying through the interaction with CGRP8-37 -sensitive receptors, likely CGRP1 receptors, and the recruitment of prostaglandin-dependent mechanisms.

Adrenomedullin: a new target for the design of small molecule modulators with promising pharmacological activities

Adrenomedullin (AM) is a 52-amino acid peptide with a pluripotential activity. AM is expressed in many tissues throughout the body, and plays a critical role in several diseases such as cancer, diabetes, cardiovascular and renal disorders, among others. While AM is a protective agent against cardiovascular disorders, it behaves as a stimulating factor in other pathologies such as cancer and diabetes. Therefore, AM is a new and promising target for the development of molecules which, through their ability to regulate AM levels, could be used in the treatment of these pathologies.

Intermedin is a calcitonin/calcitonin gene-related peptide family peptide acting through the calcitonin receptor-like receptor/receptor activity-modifying protein receptor complexes

Calcitonin, calcitonin gene-related peptide (CGRP), adrenomedullin (ADM), and amylin belong to a unique group of peptide hormones important for homeostasis in diverse tissues. Calcitonin is essential for calcium balance, whereas CGRP and ADM are important for neurotransmission and cardiovascular and respiratory regulation. Based on phylogenetic analysis, we identified intermedin as a novel member of the calcitonin/CGRP peptide family. Analysis of intermedin expression indicated that intermedin is expressed primarily in the pituitary and gastrointestinal tract. Intermedin increased cAMP production in SK-N-MC and L6 cells expressing endogenous CGRP receptors and competed with labeled CGRP for binding to its receptors in these cells. In addition, treatment of 293T cells expressing recombinant calcitonin receptor-like receptor (CRLR) and one of the three receptor activity-modifying proteins (RAMPs) showed that a CRLR/RAMP receptor complex is required for intermedin signaling. In contrast to CGRP and ADM, which exhibited a preferential stimulation of CRLR when co-expressed with RAMP1 and RAMP2 or RAMP3, respectively, intermedin represents a nonselective agonist for the RAMP coreceptors. In vivo studies demonstrated that intermedin treatment led to blood pressure reduction in both normal and spontaneously hypertensive rats via interactions with the CRLR/RAMP receptor complexes. Furthermore, in vivo treatment in mice with intermedin led to suppression of gastric emptying activity and food intake. Thus, identification of intermedin as a novel member of the calcitonin/CGRP peptide family capable of signaling through CRLR/RAMP receptor complexes provides an additional player in the regulation of peripheral tissues by CRLR and will allow development of new therapeutic agents for pathologies associated with diverse vascular and gastrointestinal disorders.

Regulation of amylin release from cultured rabbit gastric fundic mucosal cells

Background

Amylin (islet amyloid polypeptide) is a hormone with suggested roles in the regulation of glucose homeostasis, gastric motor and secretory function and gastroprotection. In the gastric mucosa amylin is found co-localised with somatostatin in D-cells. The factors regulating gastric amylin release are unknown. In this study we have investigated the regulation of amylin release from gastric mucosal cells in primary culture. Rabbit fundic mucosal cells enriched for D-cells by counterflow elutriation were cultured for 40 hours. Amylin and somatostatin release over 2 hours in response to agonists were assessed.

Results

Amylin release was significantly enhanced by activation of protein kinase C with phorbol-12-myristate-13-acetate, adenylate cyclase with forskolin and elevation of intracellular calcium with A23187. Cholecystokinin (CCK), epinephrine and glucagon-like peptide-1 (GLP-1) each stimulated amylin release in a dose-dependent manner. Maximal CCK-stimulated release was greater than either epinephrine or GLP-1, even when the effects of the latter two were enhanced by isobutylmethylxanthine. Stimulated amylin release was significantly inhibited by carbachol (by 51–59%) and octreotide (by 33–42%). Somatostatin release paralleled that of amylin.

Conclusions

The cultured D-cell model provides a means of studying amylin release. Amylin secretion is stimulated by receptor-dependent and -independent activation of Ca²⁺/protein kinase C and adenylate cyclase pathways. Inhibition involves activation of muscarinic receptors and auto-regulation by somatostatin.

Secretory and vascular effects of adrenomedullin in gastric ulcer: role of CGRP- and adrenomedullin-receptors

Adrenomedullin prevents damage of gastric mucosa in either reserpine-treated or pylorus-ligated rats. Pre-treatment with CGRP(8-37) resulted in a decrease of the gastro-protective effect of adrenomedullin in both models and reversed the inhibitory effect of adrenomedullin on gastric acid output in the pylorus-ligated rats. These adrenomedullin actions were less effectively modified by pre-treatment with adrenomedullin(22-52). These data suggest that the anti-ulcer effect of adrenomedullin is mainly related to its anti-secretory action, presumably mediated through CGRP-receptors.

Cell and molecular biology of the multifunctional peptide, adrenomedullin

Adrenomedullin (AM) is a recently discovered regulatory peptide involved in many functions including vasodilatation, electrolyte balance, neurotransmission, growth, and hormone secretion regulation, among others. This 52-amino acid peptide is expressed by specific cell types in many organs throughout the body. A complex receptor system has been described for AM; it requires at least the presence of a seven-transmembrane-domain G-protein-coupled receptor, a single-transmembrane-domain receptor activity modifying protein, and a receptor component protein needed to establish the connection with the downstream signal transduction pathway, which usually involves cyclicAMP. In addition, a serum-binding protein regulates the biological actions of AM, frequently by increasing AM functional attributes. Changes in levels of circulating AM correlate with several critical diseases, including cardiovascular and renal disorders, sepsis, cancer, and diabetes. Whether AM is a causal agent, a protective reaction, or just a marker for these diseases is currently under investigation. New technologies seeking to elevate and/or reduce AM levels are being investigated as potential therapeutic avenues.

Adrenomedullin influences dissociated rat area postrema neurons

The area postrema (AP) is one of a specialized group of central nervous system (CNS) structures devoid of a significant blood-brain barrier (BBB), collectively known as the circumventricular organs (CVO). While peptides are normally excluded from access to most regions of the CNS, the AP contains neurons with a high density of receptors for many circulating peptides, very likely including those for adrenomedullin (AM). In this study, whole-cell patch-clamp recordings were obtained from 114 dissociated rat AP neurons. The mean resting membrane potential (RMP) of these neurons (n=79) was -54.3+/-0.8 mV, the mean input resistance (IR) was 3.1+/-0.2 GOmega and the spike amplitude of neurons included in this study was always greater than 90 mV. Current-clamp studies showed that bath application of AM depolarized 39.2% (31 of 79) and hyperpolarized 45.6% (36 of 79) of neurons tested. Both effects were found to be concentration dependent from 10(-12) to 10(-7) M. These data support the idea that specific populations of CNS neurons within the AP are directly influenced by AM and support the concept that AM may act at AP to influence central autonomic control. We also examined the roles of specific ion channels in regulating the AM-induced excitability of AP neurons through voltage-clamp studies. These experiments suggest potential actions of AM in modulating voltage gated calcium channels, effects which have the additional consequence of inhibiting calcium activated potassium conductances (I(K(Ca))). These data demonstrate direct effects of AM on dissociated AP neurons and identify ion channels, the modulation of which, may underlie these effects.

Adrenomedullin stimulates somatostatin and thus inhibits histamine and acid secretion in the fundus of the stomach

Adrenomedullin has recently been localized to enterochromaffin-like (ECL) and chief cells in the gastric fundus. It has been proposed that adrenomedullin may play a role in gastric mucosal defense and repair. In the present study, we have used the isolated, luminally perfused mouse stomach and superfused rat fundic segments to examine the effect of adrenomedullin on exocrine and endocrine secretion in this region of the stomach. Addition of adrenomedullin (1 pM to 1 microM) to the isolated mouse stomach caused a concentration-dependent decrease in acid secretion. The EC(50) value was 1.4 x 10(-9) and maximal inhibition of acid secretion was obtained at a concentration of 1 microM (31+/-4% below basal level, P<0.001). In rat fundic segments, superfusion with adrenomedullin (0.1 pM to 0.1 microM) caused a concentration-dependent increase in somatostatin secretion (EC(50), 1 x 10(-10)) that was accompanied by a reciprocal decrease in histamine secretion (EC(50), 1.2 x 10(-11)). Maximal stimulation of somatostatin secretion (60+/-5% above basal level, P<0.001) and inhibition of histamine secretion (50+/-5% below basal level, P<0.01) was obtained at a concentration of 0.1 microM. Changes in acid and histamine secretion induced by adrenomedullin reflected changes in somatostatin secretion and could be abolished by addition of somatostatin antibody. The axonal blocker, tetrodotoxin, also abolished the somatostatin and, consequently, the acid and histamine responses to adrenomedullin, implying that the effect of adrenomedullin on somatostatin secretion was mediated via activation of intramural neurons. We conclude that adrenomedullin, acting via intramural fundic neurons, stimulates somatostatin and thus inhibits histamine and acid secretion. This represents one mechanism by which adrenomedullin might enhance mucosal defense and repair.

Specific N-terminal CGRP fragments mitigate chronic hypoxic pulmonary hypertension in rats

Chronic hypoxic pulmonary hypertension (HPH) is characterized by elevated pulmonary arterial pressure (P(PA)), right ventricular hypertrophy (RVH), pulmonary vascular remodeling, pulmonary edema and polycythemia. Currently, there is no safe and effective treatment for HPH. Calcitonin gene-related peptide (CGRP) is the most potent peptide vasodilator discovered thus far. We previously demonstrated that exogenous CGRP reversed HPH in rats. However, the CGRP1 receptor antagonist CGRP(8-37) and smaller inhibitory C-terminal CGRP fragments that can be formed by enzymatic cleavage in vivo may compromise the beneficial effects of endogenous or exogenous CGRP. We here examine the agonistic efficacy of N-terminal rat alpha-CGRP peptides containing the disulfide bridge (Cys(2)-Cys(7)) with amidated C-terminal in prevention of HPH. Chronic infusion of CGRP(1-8), CGRP(1-13), or CGRP(1-14) at 7 nmol/h/rat via the right jugular vein during 14 days of hypobaric hypoxia (10% inspired O(2)) significantly decreased the P(PA), RVH and pulmonary arterial medial thickness in comparison with controls, suggesting that these CGRP sequences can mitigate chronic HPH in rats. Systemic pressure was unchanged by infused peptides indicating no carry-over effect. In conclusion, N-terminal CGRP fragments (CGRP(1-8), CGRP(1-13) and CGRP(1-14)) may have a protective role in hypoxic pulmonary hypertension.

Effects of adrenomedullin on the contraction of gastric arteries during reserpine-induced gastric ulcer

Adrenomedullin (100 ng/kg, s.c.) prevents reserpine-induced damage of gastric mucosa. In the model of in vitro gastric arteries from reserpine-treated rats, adrenomedullin pre-treatment resulted in a decrease of the vasoconstriction in response to 5-hydroxytryptamine. In contrast, adrenomedullin pre-treatment of rat with intact gastric mucosa did not affect the vasoconstriction to 5-hydroxytryptamine. In the presence of the NOS inhibitor N(G)-nitro-L-arginine, the responsiveness to 5-hydroxytryptamine in gastric arteries from rats treated with reserpine + adrenomedullin was enhanced to the same level of rats treated with reserpine alone. The anti-ulcer effect of adrenomedullin could therefore be related, at least in part, to an increase of blood flow at the gastric mucosa, by a mechanism involving nitric oxide.

Amylin, released from the gastric fundus, stimulates somatostatin and thus inhibits histamine and acid secretion in mice

BACKGROUND & AIMS

Amylin, a peptide that displays 50% homology with calcitonin gene-related peptide (CGRP), is colocalized with somatostatin in endocrine cells of the gastric fundus. The present study was designed to determine the mechanism of action of amylin on gastric exocrine and endocrine secretion.

METHODS

Acid secretion was measured in the isolated mouse stomach by titration. Somatostatin and histamine secretion were measured in rat fundic segments by radioimmunoassay.

RESULTS

In isolated mouse stomach, amylin caused a concentration-dependent decrease in acid secretion. In rat fundic segments, amylin and CGRP each caused a concentration-dependent increase in somatostatin and a decrease in histamine secretion. Changes in histamine secretion induced by amylin reflected changes in somatostatin secretion and could be abolished by addition of somatostatin antibody. Both the somatostatin and the histamine responses to amylin were abolished by the selective amylin antagonist AC187 but were unaffected by the CGRP antagonist CGRP8-37. In contrast, the responses to CGRP were abolished by CGRP8-37 but were unaffected by AC187. AC187 alone decreased somatostatin and increased histamine in fundic segments and increased acid secretion in isolated stomach, indicating that endogenous amylin participates in the regulation of gastric endocrine (somatostatin and histamine) and exocrine (acid) secretion.

CONCLUSIONS

In gastric fundus, release of amylin from somatostatin cells interacts with distinct amylin receptors to enhance somatostatin secretion via an autocrine pathway that leads to inhibition of histamine and acid secretion.

Gastroprotective effect of adrenomedullin administered subcutaneously in the rat

Subcutaneous injections of adrenomedullin prevented reserpine-induced gastric mucosal damage in a dose-dependent manner (1-1000 ng/kg), but did not interfere with the lesions produced by ethanol administration. In pylorus-ligated rats adrenomedullin significantly reduced gastric volume, total and free acid output as well as ulcer formation. The gastroprotective activity of adrenomedullin was not present in rats pretreated with cysteamine. These results suggest that adrenomedullin exerts its antiulcer effect, when it is administered subcutaneously (s.c.), probably by a mechanism which involves somatostatin related transmission.

Adrenomedullin (11-26): a novel endogenous hypertensive peptide isolated from bovine adrenal medulla

Adrenomedullin (AM) is a potent hypotensive peptide originally isolated from pheochromocytoma tissue. Both the ring structure and the C-terminal amide structure of AM are essential for its hypotensive activity. We have developed an RIA which recognizes the ring structure of human AM. Using this RIA, we have characterized the molecular form of AM in bovine adrenal medulla. Gel filtration chromatography revealed that three major peaks of immunoreactive AM existed in the adrenal medulla. The peptide corresponding to Mr 1500 Da was further purified to homogeneity. The peptide was determined to be AM (11-26) which has one intramolecular disulfide bond. Amino acid sequences of bovine AM and its precursor were deduced from the analyses of cDNA encoding bovine AM precursor. The synthetic AM (11-26) produced dose-dependent strong pressor responses in unanesthetized rats in vivo. The hypertensive activity lasted about one minute, and a dose dependent increase in heart rate was also observed. The present data indicate that AM (11-26) is a major component of immunoreactive AM in bovine adrenal medulla and shows pressor activity.

Comparative effects of amylin and cholecystokinin on food intake and gastric emptying in rats

CCK is a physiological inhibitor of gastric emptying and food intake. The pancreatic peptide amylin exerts similar actions, yet its physiological importance is uncertain. Objectives were to compare the dose-dependent effects of intravenous infusion of amylin and CCK-8 on gastric emptying and food intake in rats, and to assess whether physiological doses of amylin are effective. Amylin and CCK-8 inhibited gastric emptying with mean effective doses (ED(50)s) of 3 and 35 pmol x kg(-1) x min(-1) and maximal inhibitions of 60 and 65%, respectively. Amylin and CCK-8 inhibited food intake with ED(50)s of 8 and 14 pmol x kg(-1) x min(-1) and maximal inhibitions of 78 and 69%, respectively. The minimal effective amylin dose for each effect was 1 pmol x kg(-1) x min(-1). Our previous work suggests that this dose increases plasma amylin by an amount comparable to that produced by a meal. These results support the hypothesis that amylin acts as a hormonal signal to the brain to inhibit gastric emptying and food intake and that amylin produces satiety in part through inhibition of gastric emptying.

Pharmacodynamic modeling of pantoprazole's irreversible effect on gastric acid secretion in humans and rats

The relationship between the pharmacokinetics of pantoprazole, an irreversible proton pump inhibitor, and its effect on gastric acid secretion was evaluated in humans and rats. Pantoprazole pharmacokinetics were studied in 6 rats (5 mg/kg, i.v.) and 22 healthy volunteers (10 to 80 mg, i.v. and oral). Gastric acid secretion under maximum pentagastrin stimulation was measured after i.v. administration of placebo or pantoprazole in 31 rats (0.12 to 1.15 mg/kg) for 4 hours and in 31 subjects (20 to 120 mg) for 24 hours. Pantoprazole has short half-lives of 0.5 hours in rats and 0.8 hours in humans. After administration of the highest dose, acid secretion was fully inhibited within 1 hour and for the whole observation period in both species. An irreversible pharmacodynamic response model was successfully developed and validated. The apparent reaction rate constants of pantoprazole with the proton pumps were 0.691 L/mg/h in rats and 0.751 L/mg/h in humans, and the apparent recovery rates of the pumps were 0.053 h-1 and 0.031 h-1, respectively. The maximum inhibition and the overall effect of pantoprazole are related to exposure, and the onset is related to initial pantoprazole concentrations. It was concluded that this irreversible response model accurately describes the effect of i.v. and oral pantoprazole on gastric secretion and may be used to predict effects under other dosage regimens.

The role of sensory neurons in the antiulcer effect of centrally injected amylin in rat

Central administration of amylin (2.2 microg/rat, i.c.v.) reduces (from a minimum of 67% to 83%) indomethacin (Indo, 20 mg Kg(-1), orally) induced ulcers in rats. The anti-ulcer effect of the peptide is not removed by the administration of prokinetic drugs like domperidone or neostigmine but it is reduced by 35% in rats treated with capsaicin or with the CGRP antagonist, CGRP(8-37). These data indicate that amylin gastroprotection involves capsaicin-sensitive nerve fiber leading to CGRP-dependent gastric vasodilatory effect. Additional mechanisms could involve noradrenergic alpha(2) receptors as the peptide gastroprotective activity is reduced from 67% to 20% by the alpha(2) antagonist yohimbine.

Adrenomedullin, a multifunctional regulatory peptide

Since the discovery of adrenomedullin in 1993 several hundred papers have been published regarding the regulation of its secretion and the multiplicity of its actions. It has been shown to be an almost ubiquitous peptide, with the number of tissues and cell types synthesizing adrenomedullin far exceeding those that do not. In Section II of this paper we give a comprehensive review both of tissues and cell lines secreting adrenomedullin and of the mechanisms regulating gene expression. The data on circulating adrenomedullin, obtained with the various assays available, are also reviewed, and the disease states in which plasma adrenomedullin is elevated are listed. In Section III the pharmacology and biochemistry of adrenomedullin binding sites, both specific sites and calcitonin gene-related peptide (CGRP) receptors, are discussed. In particular, the putative adrenomedullin receptor clones and signal transduction pathways are described. In Section IV the various actions of adrenomedullin are discussed: its actions on cellular growth, the cardiovascular system, the central nervous system, and the endocrine system are all considered. Finally, in Section V, we consider some unresolved issues and propose future areas for research.

Mice lacking alpha-calcitonin gene-related peptide exhibit normal cardiovascular regulation and neuromuscular development

alpha-Calcitonin gene-related peptide (alphaCGRP) is a pleiotropic peptide neuromodulator that is widely expressed throughout the Central and peripheral nervous systems. CGRP has been implicated in a variety of physiological processes including peripheral vasodilation, cardiac acceleration nicotinic acetylcholine receptor (AChR) synthesis and function, testicular descent, nociception, carbohydrate metabolism, gastrointestinal motility, neurogenic inflammation, and gastric acid secretion. To provide a better understanding of the physiological role(s) mediated by this peptide neurotransmitter, we have generated alphaCGRP-null mice by targeted modification in embryonic stem cells. Mice lacking alpha CGRP expression demonstrate no obvious phenotypic differences from their wild-type littermates. Detailed analysis of systemic cardiovascular function revealed no differences between control and mutant mice regarding heart rate and blood pressure under basal or exercise-induced conditions and subsequent to pharmacological manipulation. Characterization of neuromuscular junction in morphology including nicotinic receptor localization, terminal sprouting in response to denervation, developmental regulation of AChR subunit expression, and synapse elimination also revealed no differences in alphaCGRP-deficient animals. These results suggest that alphaCGRP is not required for the systemic regulation of cardiovascular hemodynamics or development of the neuromuscular junction.

Inhibition of stimulated amylase secretion by adrenomedullin in rat pancreatic acini

Adrenomedullin is a novel hypotensive peptide originally isolated from human pheochromocytoma and recently localized to PP cells of the pancreatic islets of Langerhans. Based on the pancreatic islet-acinar axis model, we investigated the effect of adrenomedullin on regulated exocytosis of exocrine pancreas. Using rat [125I]-adrenomedullin, specific binding sites were localized to rat pancreatic acini. We next examined the effect of adrenomedullin on 100 pM cholecystokinin (CCK)-stimulated amylase release from pancreatic acini. Adrenomedullin inhibited amylase secretion in a dose-dependent manner by approximately 50% at maximum, and the IC50 was 1.1 pM. However, adrenomedullin did not affect rat [125I]CCK binding to isolated acini or reduce the intracellular free Ca2+ concentration increased by CCK. Adrenomedullin also inhibited amylase secretion induced by 1 microM calcium ionophore A23187, suggesting that adrenomedullin inhibits stimulated amylase secretion by functioning at a step(s) distal to the ligand-receptor binding system and intracellular calcium mobilizing mechanism. In streptolysin-O permeabilized acini, 10 nM adrenomedullin shifted the calcium dose-response curve to the right, indicating that adrenomedullin inhibits calcium-induced amylase secretion by reducing calcium sensitivity of the pancreatic exocytotic machinery. In addition, pretreatment of pancreatic acini with pertussis toxin abolished the inhibitory effect of adrenomedullin on CCK-stimulated amylase secretion. These results indicate that adrenomedullin inhibits stimulated amylase secretion by reducing the calcium sensitivity of the exocytotic machinery of the pancreatic acini. A pertussis toxin-sensitive GTP-binding protein(s) is also involved in this mechanism.

Pituitary adenylate cyclase activating polypeptide (PACAP) is present in human and cat gastric glands

In the present work we have studied the occurrence of pituitary adenylate cyclase activating polypeptide (PACAP) in human and cat stomach mucosa using immunohistochemistry. As seen under a light microscope, there were many large rounded and ovoid cells that were PACAP immunopositive, mainly in the neck of the gastric glands of both species. The immunopositive material was predominant in the perinuclear area. The PACAP immunolabeling was specific because the preincubation of the antiserum with PACAP abolished the immunostaining. In human samples under electron microscope, the PACAP immunoreactive cells have shown the characteristics of parietal cells. In faintly stained cells, the localization of DAB reaction product was associated with the surface of the intracellular canaliculi. Cell labeling could not be observed besides parietal cells.

Effects of nicorandil on experimentally induced gastric ulcers in rats: a possible role of K(ATP) channels

The anti-ulcer effects of nicorandil [N-(2-hydroxyethyl)nicotinamide nitrate ester] were examined on water-immersion plus restraint stress-induced and aspirin-induced gastric ulcers in rats, compared with those of cimetidine. Nicorandil (3 and 10 mg/kg) given orally to rats dose-dependently inhibited the development of acid-related damage (water-immersion- and aspirin-induced gastric lesions) in the models. Cimetidine (50 mg/kg, p.o.) also had anti-ulcer effects in the same models. However, in the presence of glibenclamide (20 mg/kg, i.v.), an antagonist of K(ATP) channels, nicorandil did not inhibit the formation of gastric lesions. Nicorandil (10 mg/kg) given intraduodenally (i.d.), like cimetidine (50 mg/kg), significantly reduced the volume of the gastric content, total acidity and total acid output in the pylorus ligation model. Glibenclamide reversed the changes caused by i.d. nicorandil. I.v. infusion of nicorandil (20 microg/kg per min) significantly increased gastric mucosal blood flow, without affecting blood pressure and heart rate, but the increase in the blood flow was not observed after i.v. treatment with glibenclamide (20 mg/kg). These results indicate that nicorandil administered orally to rats produces the anti-ulcer effect by reducing the aggressive factors and by enhancing the defensive process in the mucosa through its K(ATP)-channel-opening property.

Examination of somatostatin involvement in the inhibitory action of GIP, GLP-1, amylin and adrenomedullin on gastric acid release using a new SRIF antagonist analogue

1. The effect of a new type 2 selective somatostatin (SRIF) receptor antagonist (DC-41-33) on somatostatin-induced inhibition of pentagastrin-stimulated gastric acid secretion in conscious, chronic gastric fistula equipped rats was studied. 2. Infused intravenously, DC-41-33 dose-dependently inhibits SRIF-induced inhibition of pentagastrin-stimulated gastric acid secretion with an IC50 of 31.6+/-1.2 nmol kg(-1) versus 10 nmol kg(-1) SRIF and blocks the inhibitory effects of SRIF when simultaneously co-infused. Its effectiveness provides additional evidence that SRIF-inhibition of gastric acid release is a SRIF type 2 receptor-mediated process. 3. DC-41-33 is able to completely reverse the inhibitory effect of glucose-dependent insulinotropic polypeptides, GIP and GIP-(1-30)NH2, and glucagon-like polypeptide, GLP-1(7-36)NH2, on pentagastrin-stimulated gastric acid secretion thus confirming that they exert these effects through stimulation of endogenous SRIF release. 4. DC-41-33 only partially blocks potent amylin and adrenomedullin-induced inhibition of gastric acid secretion, therefore suggesting that somatostatin may not function as a primary mediator in the action of these peptides. 5. Our results indicate that DC-41-33, is a potent in vivo inhibitor of exogenous and endogenous SRIF in rats. It represents a new class of SRIF analogues which should eventually provide excellent tools for further evaluating the many physiological roles of SRIF and its five receptor subtypes.