Differential antagonism of amylin's metabolic and vascular actions with amylin receptor antagonists

Variable CGRP family peptide signaling durations and the structural determinants thereof

Calcitonin gene-related peptides alpha and beta (αCGRP, βCGRP), adrenomedullin (AM), and adrenomedullin 2/intermedin (AM2/IMD) function in pain signaling, neuroimmune communication, and regulation of the cardiovascular and lymphatic systems by activating either of two class B GPCRs, CLR and CTR, in complex with a RAMP1, -2, or -3 modulatory subunit. Inspired by our recent discovery that AM2/IMD(1-47) activation of CLR-RAMP3 elicits long duration cAMP signaling, here we used a live-cell cAMP biosensor assay to characterize the signaling kinetics of the two CGRP peptides and several bioactive AM and AM2/IMD fragments with variable N-terminal extensions. Remarkably, AM2/IMD(8-47) and AM2/IMD-53 exhibited even longer duration signaling than the 1-47 fragment. AM2/IMD(8-47) was a striking 8-fold longer acting than AM(13-52) at CLR-RAMP3. In contrast, the N-terminal extension of AM had no effect on signaling duration. AM(1-52) and (13-52) were equally short-acting. Analysis of AM2/IMD-AM mid-region chimeras and AM2/IMD R23 and R33 point mutants showed the importance of these residues for long-duration signaling and identified AM2/IMD peptides that exhibited up to 17-fold diminished signaling duration at CLR-RAMP3, while retaining near wildtype signaling potencies. βCGRP was ∼ 3-fold longer acting than αCGRP at the CGRP (CLR-RAMP1) and the amylin1 (CTR-RAMP1) receptors. Chimeric CGRP peptides showed that the single residue difference near the N-terminus, and the two differences in the mid-region, equally contributed to the longer duration of βCGRP signaling. This work uncovers key temporal differences in cAMP signaling among the CGRP family peptides, elucidates the structural bases thereof, and provides pharmacological tools for studying long-duration AM2/IMD signaling.

Targeting appetite and satiety in diabetes and obesity, via G protein-coupled receptors

Type 2 diabetes and obesity have reached pandemic proportions throughout the world, so much so that the World Health Organisation coined the term "Globesity" to help encapsulate the magnitude of the problem. G protein-coupled receptors (GPCRs) are highly tractable drug targets due to their wide involvement in all aspects of physiology and pathophysiology, indeed, GPCRs are the targets of approximately 30% of the currently approved drugs. GPCRs are also broadly involved in key physiologies that underlie type 2 diabetes and obesity including feeding reward, appetite and satiety, regulation of blood glucose levels, energy homeostasis and adipose function. Despite this, only two GPCRs are the target of approved pharmaceuticals for treatment of type 2 diabetes and obesity. In this review we discuss the role of these, and select other candidate GPCRs, involved in various facets of type 2 diabetic or obese pathophysiology, how they might be targeted and the potential reasons why pharmaceuticals against these targets have not progressed to clinical use. Finally, we provide a perspective on the current development pipeline of anti-obesity drugs that target GPCRs.

Calcitonin Gene-Related Peptide Protects Against Cardiovascular Dysfunction Independently of Nitric Oxide In Vivo

[Figure: see text].

An amylin analogue attenuates alcohol-related behaviours in various animal models of alcohol use disorder

Recent findings have identified salmon calcitonin (sCT), an amylin receptor agonist and analogue of endogenous amylin, as a potential regulator of alcohol-induced activation of the mesolimbic dopamine system and alcohol consumption. Providing that the role of amylin signalling in alcohol-related behaviours remains unknown, the present experiments investigate the effect of sCT on these behaviours and the mechanisms involved. We showed that repeated sCT administration decreased alcohol and food intake in outbred rats. Moreover, single administration of the potent amylin receptor antagonist, AC187, increased short-term alcohol intake in outbred alcohol-consuming rats, but did not affect food intake. Acute administration of sCT prevented relapse-like drinking in the "alcohol deprivation effect" model in outbred alcohol-experienced rats. Additionally, acute sCT administration reduced operant oral alcohol self-administration (under the fixed ratio 4 schedule of reinforcement) in selectively bred Sardinian alcohol-preferring rats, while it did not alter operant self-administration (under the progressive ratio schedule of reinforcement) of a highly palatable chocolate-flavoured beverage in outbred rats. Lastly, we identified differential amylin receptor expression in high compared to low alcohol-consuming rats, as reflected by decreased calcitonin receptor and increased receptor activity modifying protein 1 expression in the nucleus accumbens (NAc) of high consumers. Collectively, our data suggest that amylin signalling, especially in the NAc, may contribute to reduction of various alcohol-related behaviours.

Amylin Enhances Amyloid-β Peptide Brain to Blood Efflux Across the Blood-Brain Barrier

Findings from Alzheimer's disease (AD) mouse models showed that amylin treatment improved AD pathology and enhanced amyloid-β (Aβ) brain to blood clearance; however, the mechanism was not investigated. Using the Tg2576 AD mouse model, a single intraperitoneal injection of amylin significantly increased Aβ serum levels, and the effect was abolished by AC253, an amylin receptor antagonist, suggesting that amylin effect could be mediated by its receptor. Subsequent mechanistic studies showed amylin enhanced Aβ transport across a cell-based model of the blood-brain barrier (BBB), an effect that was abolished when the amylin receptor was inhibited by two amylin antagonists and by siRNA knockdown of amylin receptor Ramp3. To explain this finding, amylin effect on Aβ transport proteins expressed at the BBB was evaluated. Findings indicated that cells treated with amylin induced LRP1 expression, a major receptor involved in brain Aβ efflux, in plasma membrane fraction, suggesting intracellular translocation of LRP1 from the cytoplasmic pool. Increased LRP1 in membrane fraction could explain, at least in part, the enhanced uptake and transport of Aβ across the BBB. Collectively, our findings indicated that amylin induced Aβ brain to blood clearance through amylin receptor by inducing LRP1 subcellular translocation to the plasma membrane of the BBB endothelium.

Acetyl-[Asn30,Tyr32]-calcitonin fragment 8-32 forms channels in phospholipid planar lipid membranes

The N-terminally truncated derivative of salmon calcitonin (sCt) (acetyl-[Asn(30),Tyr(32)]-calcitonin fragment 8-32) (AC 187) lacks hormonal activity and is a potent and selective antagonist of the hormone and amylin receptor. It was investigated for its capability to interact and form channels in palmitoleoylphosphatidylcholine:dioleoylphosphatidylglycerol planar lipid membranes. Interestingly, AC 187 exhibits channel activity, whose parameters, i.e., central conductance (Lambda (c)), occurrence (number of channels/min), voltage-dependence and lifetime, are similar to those found for sCt although, in the same experimental conditions, it takes longer to incorporate into the membrane than sCt. This channel activity can be modulated by changing either the holding potential or the pH of the medium, or by adding picomolar concentrations of SDS. One evident difference between the two peptides is that sCt is unselective (1.03) while AC 187 displays a cationic selectivity (P (K) (+)/P (Cl) (-) = 2.7) at pH 7, increasing to 3.87 when the pH drops to 3.8. The present findings indicate that the 1-7 disulfide bridge is sufficient but not necessary for membrane interaction, in accordance with the observation reported on the interaction with membrane receptors. Furthermore, the remarkable pH dependence of the cationic channel could be taken into consideration for full biotechnological study.

Role of endogenous amylin in glucagon secretion and gastric emptying in rats demonstrated with the selective antagonist, AC187

Amylin is a 37-amino acid polypeptide co-secreted with insulin from the pancreatic beta-cells. It complements insulin's stimulation of the rate of glucose disappearance (Rd) by slowing the rate of glucose appearance (Ra) through several mechanisms, including an inhibition of mealtime glucagon secretion and a slowing of gastric emptying. To determine if endogenous amylin tonically inhibits these processes, we studied the effects of the amylin receptor blocker AC187 upon glucagon secretion during euglycemic, hyperinsulinemic clamps in Sprague-Dawley (HSD) rats, upon gastric emptying in HSD rats, and upon gastric emptying and plasma glucose profile in hyperamylinemic, and genetically obese, Lister Albany/NIH rats during a glucose challenge. Amylin blockade increased glucagon concentration, accelerated gastric emptying of liquids, and resulted in an exaggerated post-challenge glycemia. These data collectively indicate a physiologic role for amylin in glucose homeostasis via mechanisms that include regulation of glucagon secretion and gastric emptying.

Effects in skeletal muscle

The first biological action of amylin to be described was the inhibition of insulin-stimulated incorporation of radiolabeled glucose into glycogen in the isolated soleus muscle of the rat. This antagonism of insulin action in muscle was non-competitive, occurring with equal potency and efficacy at all insulin concentrations. Amylin inhibited activation of glycogen synthase, partially accounting for the inhibition of radiolabeled glucose incorporation. However, this did not account for a low rate of labeling at higher amylin concentrations, wherein the radioglycogen accumulation was even less than in incubations where insulin was absent. The principal action of amylin accounting for reduction of insulin-stimulated accumulation of glycogen was activation of glycogen phosphorylase via a cyclic AMP-, protein kinase C-dependent signaling pathway to cause glycogenolysis (glycogen breakdown). At physiological concentrations, amylin activated glycogen phosphorylase at its ED50, but because glycogen phosphorylase is present in such high activity, the resulting flux out of glycogen was estimated to be similar to insulin-mediated flux of glucosyl moieties into glycogen. Thus, in the rat, endogenous amylin secreted in response to meals appeared to mobilize carbon from skeletal muscle. Amylin-induced glycogenolysis resulted in intramuscular accumulation of glucose-6-phosphate and release of lactate from tissue beds that included muscle. When muscle glycogen was pre-labeled with tritium in the three position, amylin could be shown to evoke the release of free glucose. This is made possible by glucosyl moieties cleaved at the branch points in glycogen being released as free glucose, rather than being phosphorylated, as occurs with the bulk of the glycogen glucosyls. Free glucose is free to exit cells via facilitated transport, down a concentration gradient that might exist under such circumstances. When measured by a sensitive technique utilizing efflux of labeled glucose, amylin was reported to not affect muscle glucose transport. In most of the above respects, amylin behaved similarly to catecholamines in skeletal muscle. The pharmacology of amylin's effects on muscle glycogen metabolism was consistent with a classic amylin pharmacology in whole animals and in isolated soleus muscle. In one cell line, the pharmacology was CGRPergic. Amylin, like insulin, stimulated Na+/K+ ATPase activity and enhanced muscle contractility in vitro.

Effects on digestive secretions

Rat amylin subcutaneously injected into rats dose-dependently inhibits pentagastrin-stimulated gastric acid secretion and protects the stomach from ethanol-induced gastritis. The ED50s for these actions (0.050 and 0.036 microg, respectively) are the lowest for any dose-dependent effect of amylin thus far described, and their similar potencies are consistent with a mechanistic (causal) association. At higher amylin doses, inhibition of gastric acid secretion was almost complete (93.4%). Gastric injury (measured by a subjective analog scale) was inhibited by up to 67%. The observation that effective doses of amylin result in plasma concentrations of 7-10 pM (i.e., within the reported range; Pieber et al., 1994) supports the interpretation that inhibition of gastric acid secretion and maintenance of gastric mucosal integrity are physiological actions of endogenous amylin. The pharmacology of these responses fits with one mediated via amylin-like receptors. Rat amylin inhibited CCK-stimulated secretion of pancreatic enzymes,amylase, and lipase by up to approximately 60% without having significant effect in the absence of CCK. ED50s for the effect were in the 0.1-0.2 microg range, calculated to produce plasma amylin excursions within the physiological range. Effects of informative ligands are consistent with the concept of amylin receptor mediation. Amylin was effective in ameliorating the severity of pancreatitis in a rodent model. The amylin analog pramlintide inhibited gallbladder emptying in mice as measured by total weight of acutely excised gallbladders. Amylin inhibition of gastric acid secretion, pancreatic enzyme secretion, and bile secretion likely represents part of an orchestrated control of nutrient appearance. Modulation of digestive function fits with a general role of amylin in regulating nutrient uptake. Rate of ingestion, rate of release from the stomach, and rate of digestion of various food groups appear to be under coordinate control.

Inhibition of gastric emptying

In studies aimed at defining the role of amylin in glucose control, elevations of postprandial glucose concentration were blunted in subjects infused with the human amylin analog, pramlintide (Kolterman et al., 1995, 1996). An effect similar to blunt glucose excursions was observed by Brown and others during infusions of amylin in dogs trained to drink glucose (Brown et al., 1994). The effect of pramlintide in humans was present when glucose was administered orally, but not when administered intravenously, suggesting that the effect was due to a deceleration of glucose uptake from the meal, rather than an acceleration of its metabolism (Kolterman et al., 1995). Since amylin did not affect the rate of glucose transit across exteriorized gut loops (Young and Gedulin, 2000), it was proposed that blunting of postprandial glucose profiles could reflect effects on gastric emptying. Rates of gastric emptying have been determined using three different approaches: (1) by measurement of remnant dye found in acutely excised stomachs, (2) by the systemic appearance of labels that are not significantly absorbed until they leave the stomach (e.g., labeled glucose, acetaminophen, 13C-labeled volatiles), and (3) by following the passage of radiolabeled meal components scintigraphically, with a gamma-camera. Amylin and/or pramlintide were shown to potently inhibit gastric emptying by the first method in animals (Clementi et al., 1996; Young et al., 1995a, 1996b), by the second method in animals (Gedulin et al., 1995; Young et al., 1995a, 1996a) and in humans, including those with type 1 and type 2 diabetes (Burrell et al., 2003b; Hücking et al., 2000; Kong et al., 1998; Lee et al., 2000; Vella et al., 2002), and by scintigraphy in patients with type 1 diabetes (Kong et al., 1997, 1998) and in nondiabetic subjects (Samsom et al., 2000). Depending upon dose, responses ranged from a slowing of emptying rate (e.g., by approximately 50%) to a complete cessation. In rats, amylin was 15-fold more potent on a molar basis than glucagon-like peptide-1 (GLP-1) and 20-fold more potent than cholecystokinin octapeptide (CCK-8) for inhibition of gastric emptying (Young et al., 1996b). It was the most potent mammalian peptide of 21 tested for this action (Gedulin et al., 1996b). Amylin inhibition of gastric emptying appears to be mediated by a central mechanism (Clementi et al., 1996; Dilts et al., 1997; Young et al.,2000). An intact vagus nerve (Jodka et al., 1996) and an intact area postrema (Edwards et al., 1998) are required for the effect. In rats that underwent total subdiaphragmatic vagotomy or surgical ablation of the area postrema, amylin was no longer effective at inhibiting gastric emptying (Edwards et al.,1998). The effect of amylin and amylin agonists (including pramlintide) to inhibit gastric emptying was reversed by insulin-induced hypoglycemia (Gedulin and Young, 1998; Gedulin et al., 1997b,c,d; Young et al., 1996a). This suggests the existence of a glucose-sensitive "fail-safe" mechanism that safeguards against severe hypoglycemia; nutrients ingested in response to the hunger that accompanies hypoglycemia can pass rapidly through the stomach for immediate digestion and absorption, unimpaired by the physiological restraint of amylin that would normally prevail at normal glucose concentrations. It seems likely that amylinergic control of gastric emptying is mediated via neurons in the area postrema shown in brain slices to be activated by amylin, and inhibited by low glucose (Riediger et al., 1999). Such neurons have been proposed to mediate glucoprivic gut reflexes (Adachi et al., 1995).

Receptor pharmacology

Despite clear evidence for a distinct amylin pharmacology and localization of such pharmacology to sites such as the nucleus accumbens,efforts to clone an amylin receptor were fruitless for over a decade. This enigma led many to doubt the status of amylin as a bona fide hormone. Yet it became apparent during those cloning efforts that, whatever the amylin receptor was, it was somehow similar to a calcitonin receptor. The enigma of the amylin receptor was solved following the identification of receptor activity modifying proteins (RAMPs). These single transmembrane spanning molecules, when associated with a calcitonin receptor, altered its pharmacology from calcitonin-preferring to amylin-preferring. With at least two forms of the calcitonin receptor and three forms of RAMP, there is the potential for six subtypes of amylin receptors. Of these, two appear to predominate. The CTa (shorter form) calcitonin receptor, dimerized with RAMP1 [amylin 1 (a) receptor], appears to represent binding sites at the nucleus accumbens and the subfornical organ. Binding sites at area postrema appear to be composed of CTa + RAMP3 [amylin3 (a) receptors]. Thus far, RAMP proteins have been associated in vivo only with the CT/CLR receptor system. It is presently unknown whether RAMPs are more general modulators of receptor function, dynamically modifying responsivity with time or across other receptor classes. The largest and first identified amylin-binding field was in the nucleus accumbens. The function of these receptors is yet undetermined, but because the nucleus accumbens is within the blood-brain barrier, the cognate ligand is unlikely to be circulating amylin. Dense amylin binding is present at the circumventricular organs, including the subfornical organ, the organum vasculosum lateralis terminalis (OVLT), and the area postrema. There is no diffusional (blood-brain) barrier at these structures, so they most likely respond to circulating (beta-cell-derived) amylin. Despite pharmacological evidence of amylin sensitivity in several peripheral tissues, selective amylin binding outside of the brain is observed only in the renal cortex. The newly designated amylinomimetic drug class was defined on the basis of its unique pharmacology prior to the molecular characterization of amylin receptors. Currently, the class includes any agent that acts as antagonist at characterized amylin receptors. Several peptides, typically analogs of truncated salmon calcitonin, have been developed as potent and selective amylin antagonists and have been useful in identifying amylinergic responses. Of these, AC187 (30Asn32Tyr[8-32]sCT; Amylin Pharmaceuticals Inc.) is particularly selective and potent, and has been most often cited in studies using amylin antagonists. Antagonism of a response with an order of potency of AC187> AC66 > CGRP[8-37] is suggestive that it is mediated via amylin receptors. Activation of a response with salmon calcitonin (sCT) > amylin >calcitonin gene-related peptide (CGRP) > mammalian CT suggests activation via the amylinl (a) receptor, while sCT = amylin >> CGRP >mammalian CT suggests activation via amylin3 (a) receptors. Absence of response to other ligands (e.g., adrenomedullin) is useful for excluding certain pharmacologies.

GSK3 involvement in amylin signaling in isolated rat soleus muscle

Amylin can evoke insulin resistance by antagonizing insulin in a non-competitive manner. Here, we investigated the glycogenolytic effect of amylin in isolated skeletal muscle and compared it to the effects of a calcitonin gene-related peptide (CGRP). Amylin alone had no statistically significant effect on glucose transport. However, amylin decreased insulin-stimulated glucose transport by about 30%. The involvement of cAMP could not be detected at the concentrations shown to promote glycogenolysis. Previously, it has been shown that increased glycogen synthase kinase 3 (GSK3) activity plays a role in insulin resistance. Here, the ratio of GSK3 alpha:beta isoforms in rat soleus was found to be 1.2:1. We found that amylin increased GSK3alpha activity, which in turn led to increased phosphorylation of glycogen synthase and decreased glycogen synthesis de novo.

Antiulcer effects of amylin: a review

Amylin belongs to the calcitonin peptide family. Amylin is a peptide synthesized not only in the beta cells of pancreatic islets, but in small quantities also in other organs like in the intestinal and gastric mucosa, lungs and central nervous system. It is located in the same secretory granules as insulin. Amylin participates in the maintenance of glucose and calcium homeostasis. It also inhibits food intake and decreases body weight. Furthermore, amylin inhibits gastric acid secretion. It protects the gastric mucosa in ulcer models like stress, vagal stimulation, ethanol, acetic acid, reserpine and serotonine administration and pylorus ligation. This protective antiulcer is seen not only at pharmacological but also at near-physiological doses-0.5mkg/kg. Moreover amylin also exerts curative properties in the acetic acid and indomethacin ulcer models. Amylin decreases the aggressive factors like acid-pepsin secretion, increases mast cell stability and increases protective mechanisms like bicarbonate gastric secretion, dilates blood vessels, and it increases lymphatic mesenteric activity. Amylin seems to be a powerful protector of gastric mucosa in animals by increasing the stability of gastric mucosa. Further research remains, however, to be done.

Receptor autoradiography as mean to explore the possible functional relevance of neuropeptides: focus on new agonists and antagonists to study natriuretic peptides, neuropeptide Y and calcitonin gene-related peptides

Over the past 20 years, receptor autoradiography has proven most useful to provide clues as to the role of various families of peptides expressed in the brain. Early on, we used this method to investigate the possible roles of various brain peptides. Natriuretic peptide (NP), neuropeptide Y (NPY) and calcitonin (CT) peptide families are widely distributed in the peripheral and central nervous system and induced multiple biological effects by activating plasma membrane receptor proteins. The NP family includes atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP) and C-type natriuretic peptide (CNP). The NPY family is composed of at least three peptides NPY, peptide YY (PYY) and the pancreatic polypeptides (PPs). The CT family includes CT, calcitonin gene-related peptide (CGRP), amylin (AMY), adrenomedullin (AM) and two newly isolated peptides, intermedin and calcitonin receptor-stimulating peptide (CRSP). Using quantitative receptor autoradiography as well as selective agonists and antagonists for each peptide family, in vivo and in vitro assays revealed complex pharmacological responses and radioligand binding profile. The existence of heterogeneous populations of NP, NPY and CT/CGRP receptors has been confirmed by cloning. Three NP receptors have been cloned. One is a single-transmembrane clearance receptor (NPR-C) while the other two known as CG-A (or NPR-A) and CG-B (or NPR-B) are coupled to guanylate cyclase. Five NPY receptors have been cloned designated as Y(1), Y(2), Y(4), Y(5) and y(6). All NPY receptors belong to the seven-transmembrane G-protein coupled receptors family (GPCRs; subfamily type I). CGRP, AMY and AM receptors are complexes which include a GPCR (the CT receptor or CTR and calcitonin receptor-like receptor or CRLR) and a single-transmembrane domain protein known as receptor-activity-modifying-proteins (RAMPs) as well as an intracellular protein named receptor-component-protein (RCP). We review here tools that are currently available in order to target each NP, NPY and CT/CGRP receptor subtype and establish their respective pathophysiological relevance.

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.

Role of amylin in insulin secretion and action in humans: antagonist studies across the spectrum of insulin sensitivity

BACKGROUND

Amylin is a peptide co-secreted with insulin by pancreatic beta-cells. A role for amylin in the pathogenesis of type 2 diabetes mellitus (DM2) has been suggested by in vitro and in vivo studies indicating an effect of amylin to cause insulin resistance and/or inhibit insulin secretion.

METHODS

We have determined the effect of endogenous amylin on insulin secretion and insulin action in humans by performing 4-h hyperglycemic clamps during infusion of placebo or a specific amylin receptor antagonist (ARA) in paired, double-blinded, crossover studies. We studied nine healthy lean, ten healthy obese (BMI>27) and ten obesity-matched DM2 subjects.

RESULTS

Infusion of ARA alone had no effect on basal insulin, glucose or glucose turnover in any group. Under combined hyperglycemia and ARA infusion, lean subjects displayed a 32% augmentation in insulin levels [AUC 33,565+/-3556 (placebo) to 44,562+/-1379 (ARA) pmol/l/min, p<0.01]. The concomitant increase in glucose disposal rate (GDR) was proportionate, indicating no change in insulin sensitivity (ISI 27.7+/-2.7 vs 27.3+/-2.1, p=NS). In obese subjects, basal insulin and the rise in insulin during the clamp were greater (AUC I 44% increase from 82,054+/-15 407 to 117,922+/-27,085, p<0.01), and also accompanied by a proportionate rise in GDR reflecting an unchanged insulin sensitivity (ISI 12.1+/-2.9 vs 10.8+/-3.0, p=NS). In lean and obese subjects, the C-peptide response to hyperglycemia was also augmented by ARA (p=0.007). No effect of ARA on insulin secretion or action was observed in diabetic subjects.

CONCLUSIONS

The present data are consistent with an effect of endogenous amylin on the beta-cell to modulate and/or restrain insulin secretion, and indicate that endogenous amylin does not affect insulin action. These observations provide the first human evidence that amylin plays a role in the modulation of insulin secretion.

Calcitonin and calcitonin receptors: bone and beyond

Calcitonin (CT), a 32 amino acid peptide hormone produced primarily by the thyroid, and its receptor (CTR) are well known for their ability to regulate osteoclast mediated bone resorption and enhance Ca2+ excretion by the kidney. However, recent studies now suggest that CT and CTRs may play an important role in a variety of processes as wide ranging as embryonic/foetal development and sperm function/physiology. In this review article, CT and CTR gene transcription, signal transduction and function are addressed. The effects of CT on the physiology of a variety of organ systems are discussed and the relationship between polymorphisms in the CTR gene and bone mineral density (BMD)/osteoporosis is examined. Recent studies demonstrating the ability of receptor activity modifying proteins (RAMPs) to post-translationally modify the calcitonin receptor-like receptor (CRLR) are detailed and studies employing transgenic mouse technology to determine the temporal and tissue specific transcriptional activity of the CTR gene in vivo are discussed.

The molecular pharmacology of CGRP and related peptide receptor subtypes

Calcitonin gene-related peptides (alpha and beta isoforms), better known as CGRPalpha and CGRPbeta, were isolated twenty years ago. In fact, these were the first peptides to be characterized using a molecular cloning strategy, which is not the traditional approach of biochemical extraction and purification. Paradoxically, progress in the characterization of CGRP receptor subtypes has been extremely slow as a result of difficulties in their cloning and the lack of selective receptor subtype agonists and antagonists. However, exciting progress has been made overthe pasttwo years and is briefly reviewed here.

Non-competitive antagonism of amylin on CGRP(1)-receptors in rat coronary small arteries

We examined the interaction between rat-amylin and relaxations induced by rat-alphaCGRP and isoprenaline in rat isolated coronary small arteries. Amylin, 0.1 - 100 nM, had a concentration dependent non-competitive antagonistic effect on rat-alphaCGRP-induced responses with an EC(50) of approximately 1 nM. Amylin did not affect the relaxations induced by isoprenaline at a concentration of 10 nM. The apparent equilibrium dissociation constant, K(A), for CGRP(1)-receptors in the rat coronary small arteries was approximately 2 nM. Analysis of the relationship between receptor occupancy and response to rat-alphaCGRP indicates that the receptor reserve is small. Our results show that amylin in low concentrations acts as a selective non-competitive inhibitor at CGRP(1)-receptors in rat isolated coronary small arteries.

Amylin receptors mediate the anorectic action of salmon calcitonin (sCT)

The teleost salmon calcitonin (sCT), but not mammalian CT, shows similar biologic actions in the skeletal muscle as amylin and calcitonin gene-related peptide (CGRP). The peptides have also been shown to reduce food intake in rams. Because sCT, but not amylin, binds irreversibly to amylin binding sites, the aim of the present study was to compare the anorectic potency of both peptides. To determine whether sCT reduces food intake through interaction with amylin binding sites, we also tested whether appropriate antagonists (CORP 8-37, AC 187) attenuate the anorectic effect of sCT. Finally, we wanted to know whether rat calcitonin (rCT) and sCT reduce food intake to the same extent. Peptides were injected intraperitoneally at dark onset in 24 h food-deprived rats. At doses of 5 or 0.5 microg/kg, the anorectic effect of sCT was more potent and lasted much longer (e.g. 5 microg/kg: sCT > 10 h; amylin approx. 2 h) than that of amylin. Both CORP 8-37 and AC 187 (10 microg/kg) markedly reduced the anorectic action of sCT (0.5 microg/kg). In contrast to sCT, rCT (0.5 microg/kg) had no effect on food intake. It is concluded that sCT s anorectic effect is partly mediated by amylin receptors. Irreversible binding of sCT to amylin receptors may lead to a stronger and prolonged effect in comparison to amylin due to a sustained activation of the binding sites. Similar to other actions of CTs, the anorectic potency of sCT in rats was higher than that of mammalian (rat) CT. This agrees with binding profiles of amylin, sCT, and rCT at amylin binding sites as observed in in vitro studies.

Pharmacological characterisation of amylin-related peptides activating subfornical organ neurones

Amylin, calcitonin gene-related peptide (CGRP) and calcitonin are structurally related peptides with overlapping peripheral and central actions. Amylin and calcitonin excite the majority of neurones in the subfornical organ (SFO), where high densities of so-called C-type G-protein-coupled receptors have been detected. Subcutaneous injection of these hormones stimulates drinking similar to angiotensin II (ANGII), a dipsogen acting via the SFO. We now show that in addition to amylin and rat calcitonin (rCT), CGRP and salmon calcitonin (sCT) also excite SFO neurones. In extracellular recordings of an in vitro slice preparation of the SFO, 78% of all neurones (n=31) superfused with CGRP (10(-6) M) were excited. The excitatory effect was dose-dependent and reversible with an average threshold concentration of 5x10(-7) M, which is approximately 15-fold higher than reported for amylin-induced excitations. sCT (10(-7) M), which behaves as a non-competitive agonist at amylin as well as calcitonin receptors, caused irreversible excitatory responses in 96% of all recordings (n=26). Amylin-, CRGP- and rCT-induced excitations could be blocked by the selective amylin receptor antagonist AC187 (10(-5) to 10(-6) M), whereas sCT-induced excitations were not inhibited. The receptor antagonist human CGRP(8-37) (10(-6) M) partly caused agonistic responses, but did not block CGRP-induced excitations. The pharmacological profile observed in the present work, and in a recent publication using the same preparation, indicating (1) that CGRP is a weaker agonist in the SFO than amylin, (2) that sCT excites SFO neurones, and (3) that responses are blocked by AC187 but not by CGRP(8-37), is inconsistent with activation via CGRP receptors, but is instead consistent with involvement of amylin (C3) and calcitonin (C1) receptors, which are co-localized to a high degree on the same subset of SFO-neurones. We propose that it is unlikely that blood-borne CGRP has a significant effect on neurones in the SFO.

Interaction of amylin with calcitonin gene-related peptide receptors in the microvasculature of the hamster cheek pouch in vivo

1. This study used intravital microscopy to investigate the receptors stimulated by amylin which shares around 50% sequence homology with the vasodilator calcitonin gene-related peptide (CGRP) in the hamster cheek pouch microvasculature in vivo. 2. Receptor agonists dilated arterioles (diameters 20-40 microm). The -log of the concentrations (+/- s.e.mean; n = 8) causing 50% increase in arteriole diameter were: human betaCGRP (10.8 +/- 0.3), human alphaCGRP (10.8 +/- 0.4), rat alphaCGRP (10.4 +/- 0.3). Rat amylin and the CGRP2 receptor selective agonist [Cys(ACM2,7]-human alphaCGRP were 100 fold less potent (estimates were 8.5 +/- 0.4 and 8.2 +/- 0.3 respectively). 3. The GCRP1 receptor antagonist, CGRP8-37 (300 nmol kg(-1); i.v.) reversibly inhibited the increase in diameter evoked by human alphaCGRP (0.3 nM) from 178 +/- 22% to 59 +/- 12% (n = 8; P < 0.05) and by rat amylin (100 nM) from 138 +/- 23% to 68 +/- 24% (n = 6; P < 0.05). CGRP8-37 did not inhibit vasodilation evoked by substance P (10 nM; n = 4: P > 0.05). 4. The amylin receptor antagonist, amylin8-37 (300 nmol kg(-1); i.v.) did not significantly inhibit the increase in diameter evoked by human alphaCGRP (0.3 nM) which was 112 +/- 26% in the absence, and 90 +/- 29% in the presence of antagonist (n = 4; P < 0.05); nor that evoked by rat amylin (100 nM) which was 146 +/- 23% in the absence and 144 +/- 32% in the presence of antagonist (n = 4; P > 0.05). 5. The agonist profile for vasodilatation and the inhibition of this dilatation by CGRP8-37, although not the amylin8-37 indicates that amylin causes vasodilatation through interaction with CGRP1 receptors in the hamster cheek pouch.

Characterization of amylin and calcitonin receptor binding in the mouse alpha-thyroid-stimulating hormone thyrotroph cell line

Recently, a high affinity amylin binding site was identified in the mouse alpha-TSH thyrotroph cell line. In this study, we have characterized binding sites for 125I-salmon calcitonin (125I-sCT), 125I-rat alpha-calcitonin gene-related peptide (125I-CGRP), and 125I-rat amylin in alpha-TSH cells. Using 125I-CGRP or 125I-rat amylin, equilibrium was rapidly reached, and binding was fully reversible. Competition binding revealed the relative potency of peptides was sCT>amylin, CGRP>>rCT, which is similar to the specificity profile of amylin receptors characterized in rat brain. Furthermore, specific binding of 125I-rat amylin and 125I-CGRP to membrane preparations was reduced by 52% and 39%, respectively, in the presence of 20 microM GTP-gamma-s, indicating a requirement of G protein coupling for high affinity binding. In contrast, 125I-sCT binding reached equilibrium more slowly, was essentially irreversible, and was unaltered by GTP-gamma-s. Competition binding studies using 125I-sCT as radioligand demonstrated only weak interaction by CGRP or amylin, consistent with other described CT receptors. Assessment of ligand-induced cAMP accumulation and intracellular calcium signaling revealed a relative specificity profile of sCT>rCT with little or no second messenger signaling stimulated by amylin or CGRP, consistent with a C1-CT receptor phenotype. RT-PCR amplification of messenger RNA indicated that the predominant isoform was the C1a CT receptor. In cross-linking studies, 125I-rat amylin and 125I-CGRP specifically labeled a major band of relative molecular mass (Mr) approximately 80K, being approximately 10 kDa higher than the major 125I-sCT binding protein. Full deglycosylation of N-linked carbohydrates with endoglycosidase F reduced the Mr of each of the labeled proteins to approximately 50K. Cross-linked amylin or CT receptors were immunoprecipitated with C-terminally directed antimouse or antirat CT receptor antibodies but were not immunoprecipitated with nonimmune sera or antihuman CT receptor antibodies. The current data demonstrate expression of two biochemically distinct receptor phenotypes in mouse alpha-TSH cells, a CT receptor phenotype and an amylin receptor phenotype that have highly similar protein backbones.

Protection by amylin of gastric erosions induced by indomethacin or ethanol in rats

1. The effect of amylin on gastric ulcers induced by oral administration of indomethacin (Indo, 20 mg kg-1 at a dosing volume of 5 ml) or ethanol 50% (EtOH, 1 ml/rat) was investigated in conscious rats. 2. Amylin given intracerebroventricularly (0.22, 0.66 and 2.2 micrograms/rat, i.c.v.) demonstrated a dose-dependent cytoprotective effect against both Indo and EtOH-induced ulcers. In contrast, amylin, given subcutaneously at doses effective in inhibiting acid gastric secretion (2.5, 10 and 40 micrograms kg-1, s.c.), did not show any cytoprotective effect. 3. The interaction between amylin and endogenous nitric oxide (NO) in the maintenance of gastric mucosal integrity was investigated by pretreating the rats with a selective inhibitor of NO-synthesis, NG-nitro-L-arginine methyl ester (L-NAME, 25 and 70 mg kg-1, s.c.). Administration of L-NAME to rats did not significantly increase the degree of the Indo-induced ulcer index and was not able to remove the protective effect of amylin on Indo-induced ulcers, thus excluding a role for endogenous NO in mediating the protective effect of this peptide. 4. To determine whether the cytoprotective effect of amylin was mediated by endogenous prostaglandins, we studied the effect of amylin (2.2 micrograms/rat, i.c.v.) on EtOH- induced ulcers in rats pretreated with Indo (10 mg kg-1, s.c.) to inhibit prostanoid biosynthesis; Indo was injected 30 min before amylin and EtOH after a further 30 min. Pretreatment with Indo did not significantly increase the ulcer index induced by EtOH but counteracted the ability of amylin to prevent the ulcer formation. 5. These findings suggest that amylin exerts a gastroprotective activity that is not strictly related to inhibition of acid gastric secretion and can be partly explained through a prostaglandin-dependent mechanism mediated by receptors for the peptide in the brain. Amylin might be considered as a new brain-gut peptide.

Islet amyloid polypeptide in the gut and pancreas: localization, ontogeny and gut motility effects

The occurrence of islet amyloid polypeptide (IAPP) in the gut and pancreas of several species and during ontogeny of the rat, was studied using immunocytochemistry. Effects of IAPP on rat ileal smooth muscle were assessed in vitro. Islets of most, but not all, species examined, displayed IAPP in insulin cells and, in some species, also in somatostatin- and peptide YY (PYY)-containing cells. In the gut, expression of IAPP varied among species; when present, IAPP was most abundant in the proximal part and co-localized with somatostatin, PYY, gastrin/cholecystokinin, enteroglucagon or serotonin. IAPP was first demonstrated at embryonic day 12 and 16 in islet and gastrointestinal endocrine cells, respectively. IAPP relaxed gut muscle and reduced electrical field stimulation-evoked contractions, presumably by inhibiting acetylcholine release. Thus, IAPP expression in islets is consistent with an important role for IAPP in fuel metabolism; the gastrointestinal expression and motor effects of IAPP suggest that IAPP may modulate gastrointestinal function.

Comparison of the in vitro and in vivo pharmacology of adrenomedullin, calcitonin gene-related peptide and amylin in rats

Adrenomedullin has been reported to be structurally similar to a group of peptides that includes amylin, calcitonin and calcitonin gene-related peptide (CGRP). Human and rat adrenomedullin displaced [125I]CGRP from membranes of SK-N-MC cells (CGRP receptors) with affinities intermediate between those of rat amylin and rat CGRP alpha (Ki values 0.12 +/- 0.06, 0.017 +/- 0.007, 3.83 +/- 1.14 and 0.007 +/- 0.001 nM, respectively). In contrast Ki values for displacement of [125I]rat amylin from accumbens membranes (amylin receptors), and [125I]salmon calcitonin from T47D cells (calcitonin receptors) were lower than with rat amylin or rat CGRP alpha in these preparations (51 +/- 5, 34 +/- 2, 0.024 +/- 0.002, 0.31 +/- 0.07 nM, respectively, at amylin receptors; 33 +/- 5, 69 +/- 29, 2.7 +/- 1.5 and 13 +/- 3 nM, respectively, at calcitonin receptors). In anesthetized rats, the hypotensive potency of adrenomedullin was between that of amylin and CGRP alpha. In contrast, for amylin or calcitonin agonist actions (inhibition of [14C]glycogen formation in soleus muscle, hyperlactemia, hypocalcemia and inhibition of gastric emptying), human adrenomedullin was without measurable effect. Thus, in its binding behaviour and in its biological actions, adrenomedullin appeared to behave as a potent CGRP agonist, but as a poor amylin or calcitonin agonist.