Regional distribution and molecular forms of rat islet amyloid polypeptide

Molecular studies of CGRP and the CGRP family of peptides in the central nervous system

BACKGROUND

Calcitonin gene-related peptide is an important target for migraine and other painful neurovascular conditions. Understanding the normal biological functions of calcitonin gene-related peptide is critical to understand the mechanisms of calcitonin gene-related peptide-blocking therapies as well as engineering improvements to these medications. Calcitonin gene-related peptide is closely related to other peptides in the calcitonin gene-related peptide family of peptides, including amylin. Relatedness in peptide sequence and in receptor biology makes it difficult to tease apart the contributions that each peptide and receptor makes to physiological processes and to disorders.

SUMMARY

The focus of this review is the expression of calcitonin gene-related peptide, related peptides and their receptors in the central nervous system. Calcitonin gene-related peptide is expressed throughout the nervous system, whereas amylin and adrenomedullin have only limited expression at discrete sites in the brain. The components of two receptors that respond to calcitonin gene-related peptide, the calcitonin gene-related peptide receptor (calcitonin receptor-like receptor with receptor activity-modifying protein 1) and the AMY₁ receptor (calcitonin receptor with receptor activity-modifying protein 1), are expressed throughout the nervous system. Understanding expression of the peptides and their receptors lays the foundation for more deeply understanding their physiology, pathophysiology and therapeutic use.

Amylin-mediated control of glycemia, energy balance, and cognition

Amylin, a peptide hormone produced in the pancreas and in the brain, has well-established physiological roles in glycemic regulation and energy balance control. It improves postprandial blood glucose levels by suppressing gastric emptying and glucagon secretion; these beneficial effects have led to the FDA-approved use of the amylin analog pramlintide in the treatment of diabetes mellitus. Amylin also acts centrally as a satiation signal, reducing food intake and body weight. The ability of amylin to promote negative energy balance, along with its unique capacity to cooperatively facilitate or enhance the intake- and body weight-suppressive effects of other neuroendocrine signals like leptin, have made amylin a leading target for the development of novel pharmacotherapies for the treatment of obesity. In addition to these more widely studied effects, a growing body of literature suggests that amylin may play a role in processes related to cognition, including the neurodegeneration and cognitive deficits associated with Alzheimer's disease (AD). Although the function of amylin in AD is still unclear, intriguing recent reports indicate that amylin may improve cognitive ability and reduce hallmarks of neurodegeneration in the brain. The frequent comorbidity of diabetes mellitus and obesity, as well as the increased risk for and occurrence of AD associated with these metabolic diseases, suggests that amylin-based pharmaceutical strategies may provide multiple therapeutic benefits. This review will discuss the known effects of amylin on glycemic regulation, energy balance control, and cognitive/motivational processes. Particular focus will be devoted to the current and/or potential future clinical use of amylin pharmacotherapies for the treatment of diseases in each of these realms.

Amylin prevents TRAIL-mediated apoptotic effects of reserpine in the rat gastric mucosa

We have previously shown that amylin has a protective effect upon the damaged rat gastric mucosa via a cytokine-mediated mechanism. Here, the effects of amylin on the proapoptotic cytokine TNF-related-apoptosis-inducing-ligand (TRAIL) were tested in the rat gastric mucosa damaged by reserpine administration in vivo. Intraperitoneal administration of reserpine in adult male Sprague-Dawley rats resulted in increased TRAIL expression in the gastric mucosa. Immunohistochemistry showed that the TRAIL death-receptor 5 (DR5) was constitutively expressed by the mucosa cells. Western blot showed that pretreatment of reserpine-treated rats with amylin was associated with attenuated expression of TRAIL. In the same samples, we also investigated about TRAIL-related signaling and observed that activation of caspases-8 and -3 occurs in parallel to increased TRAIL expression in rats treated with reserpine. Similarly to the latter, activation of caspases was attenuated in rats pretreated with amylin. Treatment with reserpine was associated with increased expression of the proapoptotic protein Bax, whereas that of the antiapoptotic protein Bcl-2 was significantly decreased. Amylin prevented the effects of reserpine on these genes. Reserpine sets into motion mechanisms of apoptosis in the rat gastric mucosa, which appear mediated, at least in part, by TRAIL. In addition, TRAIL downstream signaling is activated along with subversion of gene expression related to apoptosis. Amylin was able to prevent detrimental effects of reserpine. Finally, amylin and related molecules may be envisioned as protective agent in gastric mucosa damage.

Tissue Expression and Secretion of Amylin

Amylin and insulin are co-localized within the same secretory granules of pancreatic beta-cells. Acutely, the secreted ratio of amylin:insulin is comparatively invariant, but long-standing hyperglycemia may favor induction of amylin synthesis and secretion over that of insulin. Amylin is also found in much lesser quantities in the gut and other tissues. In humans, both type 1 diabetes mellitus and the later stages of type 2 diabetes mellitus are characterized by deficiency of both insulin and amylin secretion. The severity of amylin deficiency appears to correlate with the severity of insulin deficiency. This concordance of deficiencies in amylin and insulin secretion observed with the progression of diabetes mellitus is consistent with their co-localization in pancreatic beta-cells. Amylin is cleared mainly by proteolytic degradation at the kidney. The terminal t1/2 for rat amylin in rats is approximately 13 min, and that for pramlintide in humans is approximately 20-45 min.

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.

In vitro autoradiographic localization of calcitonin and amylin binding sites in monkey brain

Calcitonin (CT) and amylin are related peptides with potent central actions, including suppression of appetite and gastric acid secretion. Little is known about the distribution and binding characteristics of amylin receptors in species other than rat; therefore, in this study, by using in vitro autoradiography, we have mapped the distribution of 125I-rat amylin binding sites in the monkey brain and compared this distribution to that of binding sites for 125I-salmon CT (125I-sCT). Highest densities of 125I-amylin binding were in the hypothalamus, including the arcuate nucleus and parts of the ventromedial hypothalamic nuclei, and the solitary nucleus. Rostrally, moderate to high density binding was present in parts of the preoptic area, bed nucleus of the stria terminalis, amygdala and accumbens nucleus (Acb). Caudally, binding of amylin was more restricted, with moderate to high density binding present only in dorsal raphe, and area postrema. The primary visual cortex displayed strong and periodic CT binding in layer 4. The subcortical pattern of distribution of amylin and CT receptors in the monkey was similar to that seen previously in the rat, although the relative densities of binding to different brain structures were not always conserved. As with rat, monkey amylin receptors were a subset of the sites labeled with 125I-sCT. Analysis of receptor specificity indicated a greater relative potency of CT peptides in competing for 125I-amylin binding in monkey, when compared to rat, while, there was a decrease in the relative potency of CT gene-related peptides, potentially due to differences the level of receptor activity modifying proteins (RAMPs) in monkey versus rat brain. Amylin receptors in primates are likely to perform a similar role to those in rats; however, the interaction of the receptors with related peptides may differ.

Vascular Actions of Calcitonin Gene-Related Peptide and Adrenomedullin

This review summarizes the receptor-mediated vascular activities of calcitonin gene-related peptide (CGRP) and the structurally related peptide adrenomedullin (AM). CGRP is a 37-amino acid neuropeptide, primarily released from sensory nerves, whilst AM is produced by stimulated vascular cells, and amylin is secreted from the pancreas. They share vasodilator activity, albeit to varying extents depending on species and tissue. In particular, CGRP has potent activity in the cerebral circulation, which is possibly relevant to the pathology of migraine, whilst vascular sources of AM contribute to dysfunction in cardiovascular disease. Both peptides exhibit potent activity in microvascular beds. All three peptides can act on a family of CGRP receptors that consist of calcitonin receptor-like receptor (CL) linked to one of three receptor activity-modifying proteins (RAMPs) that are essential for functional activity. The association of CL with RAMP1 produces a CGRP receptor, with RAMP2 an AM receptor and with RAMP3 a CGRP/AM receptor. Evidence for the selective activity of the first nonpeptide CGRP antagonist BIBN4096BS for the CGRP receptor is presented. The cardiovascular activity of these peptides in a range of species and in human clinical conditions is detailed, and potential therapeutic applications based on use of antagonists and gene targeting of agonists are discussed.

Gastric amylin expression. Cellular identity and lack of requirement for the homeobox protein PDX-1. A study in normal and PDX-1-deficient animals with a cautionary note on antiserum evaluation

The gene encoding amylin is implicated in the generation of amyloid in the islets of Langerhans of diabetics and is believed to be regulated by the homeodomain transcription factor PDX-1. Although gastric mucosa also produces amylin, studies on its cellular site of production have yielded highly divergent results, localizing this peptide to either gastrin, serotonin, or somatostatin cells or to combinations thereof. Using region-specific amylin antisera in combination with reverse transcriptase-polymerase chain reaction, we now document that the majority of cells expressing amylin correspond to somatostatin cells. Only a small subpopulation of gastrin cells contained immunoreactive amylin. Studies of PDX-1-deficient mice, which fail to develop gastrin cells while possessing normal numbers of somatostatin cells, revealed no detectable change in gastric amylin expression. These data show that neither normal gastrin cell development nor PDX-1 expression is needed for gastric amylin expression.

Sufficiency of postprandial plasma levels of islet amyloid polypeptide for suppression of feeding in rats

Our objective was to study whether islet amyloid polypeptide (IAPP) produces satiety by an endocrine mechanism. We used a rat model to determine whether postprandial plasma levels of IAPP are comparable to those required to inhibit feeding when IAPP is administered by continuous intravenous infusion. Food intake in rats with aortic catheters increased plasma IAPP levels from a fasting level of 10.8 +/- 0.5 pM to a peak level of 19.0 +/- 1.0 pM at 2.2 +/- 0.5 h. In rats with jugular vein and aortic catheters, the threshold intravenous dose for IAPP suppression of feeding was between 1 and 3 pmol . kg-1 . min-1. The 3 pmol . kg-1 . min-1 dose decreased 4-h intake by approximately 25% by decreasing meal frequency rather than meal size. This dose increased plasma IAPP by approximately 24 pM. These results suggest that postprandial plasma levels of IAPP are not quite sufficient to independently produce satiety.

Amylin and gastrointestinal activity

1. Amylin is a new pancreatic islet peptide with a role in the maintenance of glucose homeostasis. 2. Amylin is predominantly present in the beta cells of the pancreas and to a lesser extent in the gastrointestinal tract and in the nervous system, where amylin mRNA is also present along with specific binding sites. 3. Amylin given peripherally or centrally inhibits acid gastric secretion in a dose-dependent manner and has a protective effect against indomethacin- or ethanol-induced ulcers only when injected centrally. 4. Subcutaneous or central injection of amylin produces a dose-dependent inhibition of gastric emptying, which may contribute to the activity of amylin in the regulation of carbohydrate absorption. In addition amylin inhibits food intake both when injected peripherally or centrally. 5. Amylin may thus be considered a novel brain-gut peptide taking part in the rapid endocrine response during digestion to maintain euglycemia.

Neuroanatomical localization, pharmacological characterization and functions of CGRP, related peptides and their receptors

Calcitonin generelated peptide (CGRP) is a neuropeptide discovered by a molecular approach over 10 years ago. More recently, islet amyloid polypeptide or amylin, and adrenomedullin were isolated from human insulinoma and pheochromocytoma respectively, and revealed between 25 and 50% sequence homology with CGRP. This review discusses findings on the anatomical distributions of CGRP mRNA, CGRP-like immunoreactivity and receptors in the central nervous system, as well as the potential physiological roles for CGRP. The anatomical distribution and biological activities of amylin and adrenomedullin are also presented. Based upon the differential biological activity of various CGRP analogs, the CGRP receptors have been classified in two major classes, namely the CGRP1 and CGRP2 subtypes. A third subtype has also been proposed (e.g. in the nucleus accumbens) as it does not share the pharmacological properties of the other two classes. The anatomical distribution and the pharmacological characteristics of amylin binding sites in the rat brain are different from those reported for CGRP but share several similarities with the salmon calcitonin receptors. The receptors identified thus far for CGRP and related peptides belong to the G protein-coupled receptor superfamily. Indeed, modulation of adenylate cyclase activity following receptor activation has been reported for CGRP, amylin and adrenomedullin. Furthermore, the binding affinity of CGRP and related peptides is modulated by nucleotides such as GTP. The cloning of various calcitonin and most recently of CGRP1 and adrenomedullin receptors was reported and revealed structural similarities but also significant differences to other members of the G protein-coupled receptors. They may thus form a new subfamily. The cloning of the amylin receptor(s) as well as of the other putative CGRP receptor subtype(s) are still awaited. Finally, a broad variety of biological activities has been described for CGRP-like peptides. These include vasodilation, nociception, glucose uptake and the stimulation of glycolysis in skeletal muscles. These effects may thus suggest their potential role and therapeutic applications in migraine, subarachnoid haemorrhage, diabetes and pain-related mechanisms, among other disorders.

Effect of amylin in various experimental models of gastric ulcer

Subcutaneous administration of amylin (20-40 micrograms/kg) prevented, in a dose-dependent manner, reserpine- and serotonin-induced gastric damage, but the anti-ulcer effect was not present when lesions were induced by pylorus ligation. The protective effect of amylin was inhibited by pretreatment with capsicin as well as CGRP-(8-37), a calcitonin gene-related peptide (CGRP) and amylin receptor antagonist, and was significantly reduced by domperidone, a dopamine D2 receptor antagonist, or neostigmine, an inhibitor of acetylcholinesterase. Our data suggest that the gastroprotective activity of amylin in some experimental models of gastric ulcers involves capsaicin-sensitive fibers and CGRP receptors. Moreover, the peptide interferes, at least in part, with the dopaminergic and parasympathetic systems.

Insulin-like growth factor I protects and rescues hippocampal neurons against beta-amyloid- and human amylin-induced toxicity

Insulin-like growth factors (IGF-I and IGF-II) are well known trophic factors and their specific receptors are uniquely distributed throughout the brain, being especially concentrated in the hippocampal formation. IGFs possess neurotrophic activities in the hippocampus, an area severely affected in Alzheimer disease. These data, together with the evidence that beta-amyloid (Abeta)-derived peptides likely play an important role in the neurodegenerative process observed in Alzheimer disease, led us to investigate if IGFs could be neuroprotective to hippocampal neurons against toxicity induced by amyloidogenic derivatives. Exposure of rat primary hippocampal neurons to different concentrations of Abeta25-35, Abeta1-40, Abeta1-42, and human amylin produced marked toxicity, while similar concentrations of two control Abeta peptides-reverse (Abeta40-1) and scrambled sequence (Abeta25-35)-and rat amylin failed to exhibit any significant effect on neuronal survival. IGF-I (10-100 nM) significantly protected hippocampal neurons against neurotoxicity induced by Abeta derivatives and human amylin. The homolog IGF-II was also effective although less potent than IGF-I suggesting the involvement of a typical IGF-I receptor in the observed neuroprotective effect. Most interestingly, IGF-I (10-100 nM) was even able to rescue neurons pre-exposed (up to 4 days) to amyloidogenic peptides. Other neurotrophic factors are reported to lack such rescuing abilities. These results suggest that IGF-I may have unique properties as a potent neuroprotective and neurorescuing agent against amyloid-related neurotoxicity.

Expression of the calcitonin gene family in medullary thyroid carcinoma

The regulatory peptide calcitonin was discovered in 1962. During the last decade it has been demonstrated to be part of a gene family. Calcitonin is synthesized in the parafollicular cells (C cells) of the thyroid gland. These cells give rise to an endocrine tumor, medullary thyroid carcinoma (MTC), which is found in a sporadic and an inherited form. Calcitonin is used as a tumor marker for MTC. The calcitonin gene was demonstrated in 1981 to give rise to an alternative peptide product, alpha-CGRP, and a second gene encoding a very similar peptide, beta-CGRP, has also been identified. A third CGRP-like peptide, amylin, was identified in 1986. This article summarizes the present knowledge about gene structure, regulation of gene expression, and expression of the calcitonin gene family in MTC and in MTC-derived cell lines. The methods employed for detection of gene expression and for measurement and characterized of peptide products are described, and finally the relevance of biochemical tumor markers is discussed in relation to the new diagnostic methods for inherited MTC based on molecular biological techniques.

Amylin given by central or peripheral routes decreases gastric emptying and intestinal transit in the rat

The effect of rat amylin on gastric emptying and intestinal transit in the rat was examined. Amylin administered intracerebroventricularly (1, 2, 2.5 or 4 micrograms/rat) produced the maximal decrease in gastric emptying and intestinal transit at the dose of 2.5 micrograms/rat. Higher doses produced a lower effect. Peripheral administration (25, 50 or 100 micrograms/kg) produced dose-dependent effects. Pre-treatment with neostigmine blocked the effect of amylin when it was centrally injected, while the effect of amylin given peripherally was partially reduced. Pre-treatment with domperidone decreased the inhibitory effect of peripherally injected amylin, but no effect was observed when the peptide was centrally injected.

The effects of amylin on motivated behavior in rats

The effects of administration of amylin into the lateral brain ventricle on active and passive avoidance behavior and open field activity were studied in rats. Amylin increased the latency of passive avoidance behavior in a dose-dependent manner between 250 ng and 1 microgram, but 50 and 100 ng and 2 micrograms were ineffective when given immediately after passive avoidance conditioning. As concerns active avoidance behavior, the extinction was facilitated in a dose-dependent manner. The locomotion in an open field was inhibited at 3 h at 500 ng and 1 microgram doses and at 6 h at 250 and 500 ng and 1 microgram doses. However, the action was still present 24 h after administration of a large dose (1 microgram). The rearing activity was increased 3 and 6 h after administration of 1 microgram. Smaller doses (50, 100, 250, and 500 ng) were ineffective. Twenty-four hours following administration, none of the doses used exhibited any effect. The grooming activity was increased 3, 6, and 24 h after administration of 1 microgram amylin. Other doses were ineffective. The results suggest that amylin influences the behavioral reaction in these paradigms mainly by acting on locomotion, and not by modifying learning and memory processes.

Behavioral effects of amylin injected intracerebroventricularly in the rat

Amylin is a peptide of pancreatic origin that has been reported to possess high-affinity binding sites in the brain and to affect central dopaminergic and serotonergic neurotransmission. Administered ICV the peptide induced a dose-dependent decrease of locomotor activity without affecting grooming and sniffing. At a dose of 5 micrograms/ rat, it antagonized the hypermotility and stereotypies induced by s.c. injection of amphetamine (2 mg/kg) or of the dopamine receptor agonist, apomorphine (250 mg/kg). Amylin did not change significantly the effect of haloperidol (0.5 mg/kg, s.c.) on locomotor activity, grooming, and sniffing. Moreover, the peptide did not modify the locomotor behavior of animals injected with the 5-HT2 antagonist, ritanserin (2 mg/kg, s.c.). These results suggest that amylin may exert motor effects, probably by interfering with central dopaminergic neurotransmission.

Comparative immunohistochemical distribution of amylin-like and calcitonin gene related peptide like immunoreactivity in the rat central nervous system

Using the indirect immunofluorescence method with a polyclonal antiserum raised in rabbits and directed against amylin (AMY), we have investigated the distribution of AMY-like immunoreactivity (-ir) throughout the central nervous system of the rat. The widespread distribution of AMY-ir was much more abundant than that previously reported for calcitonin gene related peptide (CGRP) immunoreactivity. In most brain areas there was no overlap between AMY- and CGRP-ir cell body groupings, with the exception of the motor nuclei of the hindbrain and spinal cord, which were found to contain large numbers of AMY- and CGRP-immunoreactive cell bodies. Areas with a moderate to dense appearance of AMY-ir were the rhinencephalon, the nucleus of the diagonal band, the magnocellular, dorso- and ventro-medial and mammillary nuclei of the hypothalamus, the habenula, the compact part of the substantia nigra, the ruber and pontine nuclei, and the inferior olive and the cerebellar nuclei. The widespread immunohistochemical distribution of AMY-ir in the rat brain is in partial agreement with the distribution of AMY-binding sites.

In vitro autoradiographic localization of amylin binding sites in rat brain

Amylin is a recently discovered 37 amino acid peptide which is co-secreted from the pancreas with insulin and acts to modulate carbohydrate metabolism. Recently, high-affinity binding sites for [125I]rat amylin have been identified in the rat central nervous system. These sites also have high affinity for the structurally related peptides calcitonin gene-related peptide and salmon calcitonin. In the present study we have used in vitro autoradiography to map the distribution of these [125I]rat amylin binding sites in rat brain. High to moderate levels of binding were present in mid-caudal accumbens nucleus, fundus striati and parts of the bed nucleus of the stria terminalis and substantia inominata. This binding extended caudally into parts of the amygdalostriatal transition zone and the central and medial amygdaloid nuclei. High to moderate levels of binding also occurred in much of the hypothalamus including the medial preoptic, dorsomedial hypothalamic and medial tuberal nuclei as well as the ventrolateral subnucleus of the ventromedial hypothalamic nucleus. Other regions of high level binding included the subfornical organ, the vascular organ of the lamina terminalis, area postrema, locus coeruleus, dorsal raphe and caudal parts of the nucleus of the solitary tract. The subfornical organ, vascular organ of the lamina terminalis and area postrema, which display some of the highest binding densities, lack a patent blood-brain barrier and thus could be responsive to blood-borne amylin. In conclusion we have mapped, in detail, the distribution of amylin binding sites in rat brain. The location of binding is consistent with potential roles for these sites in appetite, fluid and electrolyte homeostasis, autonomic function and regulation of mood.

Natural history of B-cell dysfunction in spontaneously diabetic Chinese hamsters

To elucidate the pathogenesis of diabetes in spontaneously diabetic Chinese hamsters (CHAD strain), a longitudinal study from just after weaning to overt diabetic state was performed. Fasting and non-fasting plasma glucose, non-fasting plasma insulin and pancreatic hormone contents (insulin, glucagon and amylin) were measured, and light microscopic examination of pancreatic islets by immunohistochemical technique and pancreas perfusion study were performed. No insulitis was found in the islets of the CHAD strain. In animals aged 1 month, there was no significant difference in the percentage of B-cell area to islet area between the CHAD strain and the control. At this stage, hyperinsulinemia was observed despite normal plasma glucose levels both in fasting and non-fasting states. In the animals of the CHAD strain aged 2-4 months, insulin secretion from the pancreas, pancreatic insulin content and non-fasting plasma insulin level decreased in proportion to the decrease of B-cell mass. In animals aged about ten months, severe hyperglycemia and hypoinsulinemia were observed. We demonstrated the existence of amylin-like immunoreactivity in the B-cells of Chinese hamsters. However, no amyloid deposit was observed in the islets of the CHAD strain. After the onset of diabetes, amylin secretion from the pancreas and pancreatic amylin content in the CHAD strain were significantly lower than those in the control. We demonstrated the natural history of B-cell dysfunction in the CHAD strain. It could mean the process of B-cell exhaustion. The profile of the CHAD strain is similar to some types of human NIDDM. Therefore, the CHAD strain is a useful diabetic model in the study of NIDDM.

Direct plasma radioimmunoassay for rat amylin-(1-37): concentrations with acquired and genetic obesity

Amylin (islet-associated polypeptide) is a 37-amino acid peptide that is cosecreted with insulin from the pancreatic beta-cell. Accurate measurement of its plasma levels is important for delineating the physiological range over which amylin acts. We describe a reproducible, highly specific, and sensitive radioimmunoassay for direct measurement of plasma amylin-(1-37). We measured changes in portal and systemic plasma amylin and insulin in three groups of anesthetized rats: lean young adult and old adult Wistar rats with acquired obesity, and Wistar fatty [WDF/TaFa (fa/fa)] rats, a model of genetic obesity and insulin resistance derived from the Wistar strain. Changes in response to fasting, feeding, and intravenous stimulation with glucose plus arginine were assessed. We find that the amylin-to-insulin ratio is constant in fasted or fed young and old rats because of proportionate increases in both entities with aging. In genetically obese Wistar rats, amylin and insulin levels are three- to tenfold higher than in lean young or obese old normal controls. Islet stimulation by feeding or intravenous glucose plus arginine resulted in a decreased amylin-to-insulin molar ratio in all groups. When normalized for the degree of islet stimulation, amylin-to-insulin ratios were significantly elevated in genetically obese vs. normal rats, both in the portal and systemic circulation. These results demonstrate that aging-related weight gain in normal rats is associated with moderate and proportional increases in amylin and insulin, whereas genetic obesity is characterized by elevated amylin and an increased amylin-to-insulin ratio. Implications for the pathogenesis of insulin resistance and obesity are discussed.

Isolation and sequence determination of two N-terminal fragments of islet amyloid polypeptide in rat pancreas

Using a highly sensitive and specific radioimmunoassay (RIA) for the N-terminal hexadecapeptide of islet amyloid polypeptide (IAPP), we isolated two N-terminal fragments of IAPP from rat pancreas. They were identified as IAPP(1-16) and IAPP(1-17) by amino acid sequencing. The two fragments were also found in rat plasma. IAPP(1-37) was the major molecular form of rat IAPP, IAPP(1-16) and IAPP(1-17) accounting for 6.0% and 32.3% of the immunoreactivity for the N-terminal region of the peptide in pancreata of normally fed rats. In human pancreas, the N-terminal fragments of IAPP were not present, indicating that the processing of IAPP in the pancreas differs between human and rat. Food deprivation increased the molar ratios of IAPP(1-16) and IAPP(1-17) to IAPP(1-37) in comparison to values for fed rats. Identification of novel fragments of IAPP, in addition to IAPP(1-37), offers a promise for the elucidation of the physiological function of IAPP and the identification of factors that regulate the biosynthesis and catabolism of the peptide.

Molecular physiology of the islet amyloid polypeptide (IAPP)/amylin gene in man, rat, and transgenic mice

Islet amyloid polypeptide ("amylin") is the major protein component of amyloid deposits in pancreatic islets of type 2 (non-insulin-dependent) diabetic patients. Islet amyloid polypeptide consists of 37 amino acids, is co-produced and co-secreted with insulin from islet beta-cells, can act as a hormone in regulation of carbohydrate metabolism, and is implicated in the pathogenesis of islet amyloid formation and of type 2 diabetes mellitus. Rat islet amyloid polypeptide differs from human islet amyloid polypeptide particularly in the region of amino acids 25-28, which is important for amyloid fibril formation. In rat and mouse, diabetes-associated islet amyloid does not develop. To study the genetic organization and biosynthesis of islet amyloid polypeptide, we have isolated and analyzed the human and rat islet amyloid polypeptide gene and corresponding cDNAs. Both genes contain 3 exons, encoding precursor proteins of 89 amino acids and 93 amino acids, respectively. Apart from a putative signal sequence, these precursors contain amino- and carboxy-terminal flanking peptides in addition to the mature islet amyloid polypeptide. To understand regulation of islet amyloid polypeptide gene expression, we have identified several potential cis-acting transcriptional control elements that influence beta-cell-specific islet amyloid polypeptide gene expression. Using antisera raised against synthetic human islet amyloid polypeptide we developed a specific and sensitive radioimmunoassay to measure levels of islet amyloid polypeptide in plasma and tissue extracts. Also antisera raised against the flanking peptides will be used in studying human islet amyloid polypeptide biosynthesis. Elevated plasma islet amyloid polypeptide levels have been demonstrated in some diabetic, glucose-intolerant, and obese individuals, as well as in rodent models of diabetes and obesity. To examine the potential role of islet amyloid polypeptide overproduction in the pathogenesis of islet amyloid formation and type 2 diabetes, we generated transgenic mice that overproduce either the amyloidogenic human islet amyloid polypeptide or the nonamyloidogenic rat islet amyloid polypeptide in their islet beta-cells. Despite moderately to highly (up to 15-fold) elevated plasma islet amyloid polypeptide levels, no marked hyperglycemia, hyperinsulinemia or obesity was observed. This suggests that chronic overproduction of islet amyloid polypeptide "per se" does not cause insulin resistance. No islet amyloid deposits were detected in mice up to 63 weeks of age, but in every mouse producing human islet amyloid polypeptide (as in man), accumulation of islet amyloid polypeptide was observed in beta-cell lysosomal bodies. This may represent an initial phase in intracellular amyloid fibril formation.(ABSTRACT TRUNCATED AT 400 WORDS)

Islet amyloid polypeptide gene expression in the endocrine pancreas of the rat: a combined in situ hybridization and immunocytochemical study

The expression of the islet amyloid polypeptide (IAPP) gene within the endocrine pancreas and its correlation with insular neuroendocrine peptide localization were investigated in the rat. In situ hybridization with a 35S-labelled IAPP-mRNA specific oligonucleotide probe was combined with immunocytochemistry. In situ hybridization alone showed strong autoradiographic labelling of the pancreatic islets. In situ hybridization combined with immunocytochemistry for IAPP, revealed labelling of the IAPP-immunoreactive cells. However, when in situ hybridization was combined with immunocytochemistry for proinsulin, we noted a lack of proinsulin immunoreactivity in some peripherally located autoradiographically labelled islet cells. Furthermore, combination of in situ hybridization and immunocytochemistry for somatostatin showed autoradiographic labelling of somatostatin cells to a varying degree. This was further confirmed by showing cellular co-localization of IAPP and somatostatin by immunocytochemical double staining. We conclude that IAPP is mainly synthesized in insulin cells. Additionally, a subpopulation of the somatostatin cells is capable of IAPP synthesis. This may account for the relatively small reduction in the content of IAPP-mRNA in islets compared to the marked reduction of insulin mRNA after streptozotocin-induced diabetes in rats as previously reported.

Islet amyloid polypeptide: a review of its biology and potential roles in the pathogenesis of diabetes mellitus

Islet amyloidosis (IA) is the principal lesion in the endocrine pancreas of human beings with non-insulin-dependent diabetes mellitus (NIDDM) and in the similar forms of diabetes mellitus in domestic cats and macaques. As such, the delineation of the pathogenesis of this form of amyloidosis may be crucial to the understanding of the development and progression of NIDDM. Islet amyloid polypeptide (IAPP) is a recently discovered polypeptide that is the principal constituent of IA in human beings, cats, and macaques. IAPP is produced by the pancreatic beta-cells and is co-packaged with insulin in the beta-cell secretory vesicles. Immunohistochemical and physiologic evidence supports the notion that the beta-cells are heterogenous with respect to their relative contents of insulin and IAPP. Therefore, although IAPP is co-secreted with insulin in response to a variety of well-known insulin secretagogues, the molar ratio of these two proteins that is released from the islets may vary, depending upon the glucose concentration and prevailing metabolic milieu. IAPP is highly conserved among mammalian species and has about 45% homology to another neuropeptide, calcitonin gene-related peptide. IAPP is encoded by a single-copy gene located, in the human being, on chromosome 12. IAPP is expressed as a 93 (murine)-89 (human)-amino acid prepropolypeptide that is processed enzymatically, resulting in the removal of amino- and carboxy-terminal propeptide segments. The 20-29 region of the IAPP molecule is most important in the ability of IAPP to form amyloid fibrils. The role of IAPP and IA in the pathogenesis of human NIDDM and similar forms of diabetes mellitus in cats and macaques may involve several possible mechanisms, including 1) direct physical/chemical damage to beta-cells, resulting in necrosis and loss of functional islet tissue, 2) biologic activities of IAPP that oppose those of insulin or abnormally suppress insulin secretion, and 3) interference by IA deposits of passage of insulin out of beta-cells and/or entrance of glucose and other secretogogues into the islet. The roles of each of these possible mechanisms have yet to be demonstrated. In addition, the physiological significance of the apparent IAPP deficiency in both insulin-dependent diabetes mellitus and NIDDM is currently unknown.

Cosecretion of islet amyloid polypeptide (IAPP) and insulin from isolated rat pancreatic islets following stimulation or inhibition of beta-cell function

The aim of this work was to simultaneously study the secretion of islet amyloid polypeptide (IAPP) and insulin from isolated rat pancreatic islets in vitro. For examination of stimulated beta-cells, nutrient secretagogues (16.7 mM glucose, 10 mM leucine + 2 mM glutamine), phosphodiesterase inhibition (5 mM theophylline), a sulphonylurea (0.5 microgram/ml glipizide), a non-nutrient amino acid (10 mM arginine), cholinergic stimulation (0.1 mM carbamylcholine) and insulinotropic peptides (0.1 microM vasoactive intestinal polypeptide and 0.1 microM glucagon), were used. For beta-cell suppression glucose phosphorylation inhibition (10 mM mannoheptulose), depletion of extracellular calcium, activation of the ATP-regulated K(+)-channel (0.5 mM diazoxide), adrenoreceptor stimulation (3 microM adrenaline), paracrine modulation (0.1 microM somatostatin), short-term treatment with a selective beta-cytotoxin (1.1 and 2.2 mM streptozotocin) and long-term treatment with a cytokine (25 U/ml interleukin-1 beta), were studied. The compounds with known effects on insulin secretion exerted their expected actions and this was paralleled by similar relative changes, with a possible exception for glucagon, in the IAPP secretion. The ratio of IAPP/insulin released did not change significantly under any of the tested experimental conditions, except for a slight increase following carbamylcholine stimulation. On a molar basis approx. 1% of IAPP was released when compared with insulin. These results are consistent with the hypothesis that the regulation of IAPP secretion from beta-cells of isolated rat pancreatic islets is essentially regulated by the same mechanisms as insulin secretion.

Anorexia following the systemic injection of amylin

The intravenous injection of 100 micrograms/kg of rat amylin reduced food intake in schedule-fed rats for 1 h of an 8 h measurement period. Associated with this brief anorexia was a hyperglycemic response, observed 30 min after a subsequent amylin administration. Determination of neurochemical alterations revealed increased concentration of serotonin in the hypothalamus and decreased level of the dopamine metabolite, 3-methoxytyramine, in the corpus striatum. Since similar neurochemical alterations were observed following the systemic injection of glucose, both the neurochemical changes and anorexia following intravenous amylin treatment may be secondary to hyperglycemia.

Islet amyloid polypeptide in the rabbit and European hare: studies on its relationship to amyloidogenesis

In this study, we determined the cDNA-predicted amino acid sequence of positions 9-31 of islet amyloid polypeptide from the rabbit and European hare. A synthetic rabbit/hare islet amyloid polypeptide 20-29 peptide was subsequently shown to be strongly fibrillogenic in vitro even though the putative amyloidogenic AILS sequence at positions 25-28 of human and cat islet amyloid polypeptide is modified in the rabbit and hare by a substitution of phenylalanine for leucine at position 27 (i.e. AIFS). Although islet amyloid polypeptide of both the rabbit and hare has an amyloidogenic sequence and is in fact amyloidogenic in vitro, the apparent lack of in vivo islet amyloidosis in rabbits and hares may be related to relatively low levels of islet amyloid polypeptide production by the islet beta cells in these species. This was supported by our findings that there is no substantial immunoreactivity in either rabbit or hare islets, and no measurable amount either in extracts of rabbit pancreases, or in rabbit plasma. This study supports the need for at least two prerequisites for the development of islet amyloidosis in vivo: an inherent fibrillogenic sequence within the islet amyloid polypeptide molecule and an adequate local concentration of islet amyloid polypeptide to promote self aggregation and formation of islet amyloid.

Islet amyloid polypeptide--hen or egg in type 2 diabetes pathogenesis?

Islet amyloid polypeptide (IAPP or amylin) was first identified as the major peptide constituent of amyloid deposited in the islets of Langerhans in patients with type-2 diabetes mellitus or in insulinomas. It was subsequently shown that IAPP is produced by the pancreatic beta-cells, co-stored and co-released with insulin. IAPP is homologous with the neuropeptide calcitonin gene-related peptide (CGRP) and has therefore been assumed to have a function as an endocrine, paracrine or autocrine hormone. This has prompted the search for its physiological function as well as a putative pathogenic role in type 2 diabetes mellitus.

Amylin increases transport of tyrosine and tryptophan into the brain

Injection of amylin (diabetes-associated peptide) into the hypothalamus induces anorexia, increases brain metabolism of dopamine and serotonin and elevates brain level of tryptophan. When male Sprague-Dawley rats were treated with 50 mg/kg L-tryptophan and L-tyrosine ethyl ester 30 min prior to the intrahypothalamic injection of 2 micrograms amylin, brain tryptophan and tyrosine levels were selectively increased as compared to rats treated with amylin alone. Hypothalamic and striatal serotonin metabolism also appeared to be increased following the amino acid-amylin treatment combination. These results suggest that amylin may increase transport of tyrosine and tryptophan into the brain, and that the increased availability of tryptophan may contribute to increased serotonin turnover observed following intrahypothalamic amylin treatment.

Tests of adipsia and conditioned taste aversion following the intrahypothalamic injection of amylin

Intrahypothalamic injection of amylin (AMY) was shown to reduce the intake of rat chow and water for 8 and 4 h, respectively, in schedule-fed rats. Amylin also reduced water intake to a much lesser degree in 24-h water-deprived rats. A test of the ability of AMY to form a conditioned taste aversion yielded no change in saccharin preference, as compared to controls treated with vehicle. These results suggest that although AMY has adipsic effects, the reduction in water is not of sufficient magnitude to cause the anorexia. In addition, the failure of AMY to support a conditioned taste aversion suggests that AMY does not cause anorexia by inducing malaise. Therefore, in addition to other metabolic effects, AMY may be involved in the control of food and water intake.

Establishment of hypersensitive radioimmunoassay for islet amyloid polypeptide using antiserum specific for its N-terminal region

Using a synthetic N-terminal hexadecapeptide of islet amyloid polypeptide (IAPP), we prepared an antiserum specific for IAPP[1-16] and established an extremely sensitive radioimmunoassay (RIA) for the peptide with a minimum detection level of 0.26 fmol/tube. Since the N-terminal sequence of IAPP is 100% conserved in many mammalian species, the RIA is widely applicable in quantifying their IAPP. Analyses of pancreatic extracts of human and hamster using reverse-phase high performance liquid chromatography coupled with the RIA revealed that almost all pancreatic IAPP consisted of IAPP[1-37]. On the other hand, rat and mouse pancreata contained substantial amounts of IAPP[1-16] and IAPP[1-17] in addition to IAPP[1-37] as a major molecular form. In human plasma, IAPP[1-37] is the major molecular form secreted into the circulation in response to glucose administration. The RIA established in this study is promising in elucidating the physiological functions and the pathophysiological significance of IAPP in diabetes mellitus.

The physiology of calcitonin gene-related peptide in the islet compared with that of islet amyloid polypeptide (amylin)

Following the discovery of a second gene containing a CGRP-like sequence, we demonstrated that "beta-CGRP" was indeed translated as a 37-amino acid peptide in vivo and was the predominant form of CGRP produced by the enteric nervous system. The presence of CGRP in the islet has been reported by several groups. We now show that beta-CGRP is again the major form. Another 37-amino acid peptide was recently isolated from islet amyloid deposits and found to have approximately 50% amino acid sequence homology with CGRP. Islet amyloid polypeptide, or amylin, is co-localized with insulin to the beta-cell secretory granule and is synthesized and released in parallel with insulin in response to a range of physiological and pharmacological stimuli. IAPP was subsequently shown, like CGRP, to inhibit the release of insulin pharmacologically. Interestingly, it was also shown to decrease the uptake of glucose by striated muscle, though it was considerably less potent than CGRP. This led to the suggestion that IAPP might be a circulating hormone regulating peripheral insulin sensitivity. Infusion of IAPP in human volunteers to produce plasma concentrations more than 100-fold higher than those seen physiologically, however, failed to alter peripheral glucose disposal. We conclude that beta-CGRP and IAPP are likely to play a role in local paracrine control of the islet.

Islet amyloid polypeptide plasma concentrations in individuals at increased risk of developing type 2 (non-insulin-dependent) diabetes mellitus

To study whether abnormal secretion of islet amyloid polypeptide is involved in the development of insulin resistance and impaired insulin secretion in Type 2 (non-insulin-dependent) diabetes mellitus, we measured islet amyloid polypeptide concentrations in 56 first-degree relatives of Type 2 diabetic subjects and in 10 healthy control subjects. Fasting islet amyloid polypeptide concentrations were similar in control subjects, glucose-tolerant and glucose-intolerant relatives (8 +/- 1, 9 +/- 1 and 11 +/- 2 fmol/ml; p = NS). The area under the islet amyloid polypeptide curve measured during an oral glucose load was larger in glucose-intolerant relatives (115 +/- 13 fmol/ml) compared to glucose tolerant relatives and control subjects (88 +/- 3 and 79 +/- 12 fmol/ml; p less than 0.05). The insulin response during the oral glucose load was inversely correlated with the rate of glucose disposal measured during a euglycaemic hyperinsulinaemic clamp (r = -0.725; p less than 0.01), while no significant correlation was observed between the corresponding values for islet amyloid polypeptide and glucose disposal (r = -0.380; p = NS). Hypersecretion of islet amyloid polypeptide is observed in glucose-intolerant first-degree relatives of patients with Type 2 diabetes. Since these patients are characterized by insulin resistance and abnormal first-phase insulin secretion, the putative role of islet amyloid polypeptide in the development of these abnormalities remains to be established. It is however, unlikely that islet amyloid polypeptide is involved in the development of insulin resistance as insulin-resistant relatives with normal glucose-tolerance showed normal islet amyloid polypeptide concentrations.

Molecular forms of islet amyloid polypeptide (IAPP/amylin) in four mammals

Using reverse-phase high performance liquid chromatography combined with radioimmunoassays for human and rat/mouse islet amyloid polypeptide (IAPP), we identified molecular forms of IAPPs in pancreata of four mammals including species in which islet amyloid deposition occurs (human and cat) and those in which amyloid deposition does not occur (rat and mouse). In human pancreas, IAPP (1-37) was the major molecular form, and IAPP (17-37), IAPP (24-37) and four IAPP-immunoreactive peptides were detected as minor components. In rat, mouse and cat pancreata, IAPP (1-37) and IAPP (19-37) were identified with the latter being the major molecular form. Major processing takes place at a single arginine residue at position 18 of rat/mouse and cat IAPPs, but not at the histidine at position 18 of human IAPP, indicating that arginine could yield different processing of IAPP between the 3 species and human. Different processing of IAPP by species suggests that processing of IAPP in pancreas is not responsible for islet amyloid formation. Identification of molecular forms of IAPP is helpful in elucidating the physiological function of the IAPP molecule and in determining the type of system regulating biosynthesis and catabolism of the peptide.

Amylin

Amylin is a 37 amino-acid peptide which is secreted from the pancreatic islets of Langerhans. It has major sequence homology with calcitonin gene related peptide. Amylin can precipitate out in these cells to form amyloid. Amylin is secreted by similar stimuli to those that secrete insulin. Amylin has a number of effects that may counteract the effect of secreted insulin, i.e., decreased second phase insulin secretion, increased hepatic glucose output, and inhibition of insulin effects on skeletal muscle. It must, however, be recognized that in many cases the doses necessary to produce these effects appear to be supraphysiological. The putative role of amylin in the hyperglycemia of aging and Type II diabetes mellitus therefore remains controversial. Amylin has a number of other effects including inhibition of osteoclastic activity, vasodilatation, anorectic effects and enhanced memory retention. This review postulates a role for amylin in the pathogenesis of a number of age-related changes.

Islet amyloid polypeptide (IAPP) and pancreatic islet amyloid deposition in diabetic and non-diabetic patients

Twenty pancreata of non-diabetic patients and 17 pancreata of diabetic patients, including two patients with insulin-dependent diabetes mellitus, were immunohistochemically studied using antiserum against human islet amyloid polypeptide (IAPP). The islet beta cells in non-diabetic patients were immunoreactive for both IAPP and insulin. Amyloid deposition immunoreactive for IAPP was detected in six of 20 pancreata of non-diabetic patients. The plasma glucose level of three of these six patients was elevated to more than 200 mg/dl, and that of the other three ranged from 143 to 162 mg/dl; all six were receiving intravenous hyper-alimentation and had no history of diabetes prior to treatment. Amyloid deposition was present in all patients with non-insulin-dependent diabetes mellitus (NIDDM). The deposition was absent in the pancreata of two secondary diabetic patients, one of whom had received steroid hormone for bronchial asthma and the other of whom had liver cirrhosis with hepatocellular carcinoma; deposition was also absent in the pancreas of a patient with impaired glucose tolerance diagnosed on a 75-g oral glucose load. Heterogeneous expression of immunoreactivities of beta cells for insulin and for IAPP was present, suggesting independently regulated production and secretion of the peptides. Immunoreactivity of beta cells was more sensitively decreased for IAPP than for insulin in the islets of NIDDM patients. The decreased immunoreactivity for IAPP suggested an initial stage of disturbed beta-cell function, even if the immunoreactivity for insulin was apparently intact or the amyloid deposition in the islets was insignificant. The degree of amyloid deposition immunoreactivity for IAPP did not necessarily reflect the severity of diabetes mellitus. Amyloid deposits were seen at the narrow spaces beneath the insular capsule of connective tissues and the perivascular region or, in some cases, occupying the whole of the islet. The diabetogenic role of IAPP is unclear, but the deposition might be an accelerating factor which disturbs beta-cell function.

Identification and characterization of islet amyloid polypeptide in mammalian gastrointestinal tract

We identified and determined the content and molecular form of islet amyloid polypeptide (IAPP/amylin) in the gastrointestinal (GI) tract of human, rat, mouse and cat. IAPP was isolated by anti- IAPP- IgG immunoaffinity chromatography and reverse-phase high performance liquid chromatography coupled with radioimmunoassays for human and rat/mouse IAPPs. Human IAPP [1-37], [17-37] and [24-37] were identified in human stomach with IAPP [1-37] being the major molecular form. In the GI tract of rat, mouse and cat, IAPP [1-37] and IAPP [19-37] were identified with the latter being the major molecular form. IAPP is present from stomach to colon with the highest concentration being observed in pyloric antrum of stomach. IAPP content in rat antrum fell to 69% of control after 4 days of fasting, with the molar ratio of IAPP [19-37] to IAPP [1-37] increasing from 1.4 in controls to 2.9 in fasted rats. Identification of IAPP and characteristic morphology of IAPP- cells in the GI tract indicate a possible biological function of IAPP as a gastrointestinal peptide.

Islet amyloid polypeptide in insulinoma and in the islets of the pancreas of non-diabetic and diabetic subjects

Amyloid deposition is a common pathological feature in insulinoma and in the islets of the pancreas in type-2 diabetic patients. The present immunohistochemical study revealed that normal B-cells, insulinoma, and amyloid deposits in insulinoma and diabetic pancreatic islets were commonly immunoreactive with antiserum to C-terminal synthetic tetradecapeptide of human islet amyloid polypeptide (IAPP) (24-37). Amyloid fibrils in insulinoma were also positive to IAPP by immunoelectron microscopy. A high level of IAPP was detected in the plasma and tissue of a insulinoma patient by radioimmunoassay suggesting that amyloid deposition in insulinoma is due to overproduction of IAPP. Amyloid deposits immunoreactive to IAPP were also seen in all diabetic pancreatic islets, but in no non-diabetic islets. There was much amyloid deposition in the islets of severe diabetics, whose B-cells demonstrated decreased immunoreactivities for IAPP and insulin. The IAPP content of the pancreas was 649.0 and 847.7 pg/mg wet weight in each of two diabetic patients, and 1034.6 and 1447.7 pg/mg wet weight in two non-diabetic patients. The present study revealed that IAPP is a bioactive peptide secreted from islet B-cells and are amyloidogenic peptide concerned in diabetogenensis and/or the progression of type-2 diabetes mellitus.

Syntheses, structures and anorectic effects of human and rat amylin

Amylin, a 37-residue polypeptide with a single disulfide bond originally isolated from the pancreas of type-II diabetic patients, has been shown to cause peripheral insulin resistance and to attenuate the inhibition of hepatic glucose output by insulin. We have also shown that amylin is present in the rat hypothalamus and that it inhibits food intake by rats. In order to further investigate the anorectic properties we synthesized both human and rat amylin by the solid phase method and purified to homogeneity in an overall yield of 10-20%. Structural analyses indicated that human amylin exhibited predominantly a beta-sheet structure at both acidic and alkaline pH, whereas no ordered structure was evident in the case of rat amylin. Intrahypothalamic injection of rat amylin resulted in a potent dose-dependent inhibitory effect on the food intake by rats adapted to eat their daily ration of food in an eight-hour period. Human amylin was less effective as an anorectic agent. Furthermore, rat amylin completely blocked the potent orexigenic effect of neuropeptide Y (NPY). These investigations show that there is a fundamental difference in the secondary structures of human and rat amylin and that rat amylin is a potent inhibitor of both basal and NPY-induced feeding by rats.

Islet amyloid polypeptide (IAPP). A short review

The islet amyloid polypeptide (IAPP) was originally identified by chemical analysis of the amyloid component in a human pancreatic islet cell tumor. It consists of 37 amino acids and displays about 50% homology with the neuropeptide calcitonin gene-related peptide (CGRP). In the pancreatic islets the IAPP is confined to the beta-cells, co-stored with insulin in the secretory granules and apparently co-secreted with insulin on glucose stimulation. In beta-cell depletion states such as streptozotocin diabetes in animals and in human type I diabetes mellitus both the IAPP and the insulin levels display reduction or are even absent. Within the mature IAPP molecule the amino acid sequence 23-29 shows considerable amino acid heterogenicity among various mammalian species. The amino acid composition of human IAPP in this specific region promotes the development of pancreatic islet amyloidosis, a phenomenon related to the ability to develop type II diabetes in that particular species. However, as type II diabetes is an inherited disease affecting a subpopulation of humans, not only the gene coding mature IAPP, but also one or several other hereditary factors of unknown origin are needed for the disease to develop. We have established a radioimmunoassay for plasma measurements of IAPP. During screening investigations of a large material of endocrine tumors we found a patient with extremely elevated plasma levels of IAPP, about 20,000 pmol/l. Immunohistochemical investigations confirmed the IAPP content and also revealed amyloid deposits. While performing an oral glucose tolerance test insulin levels remained unchanged whereas there was an increase in the glucose and IAPP levels. It is thus concluded that IAPP can be used as a tumor marker in pancreatic islet cell tumors and that high plasma levels of IAPP can inhibit glucose stimulated insulin secretion.

Isolation and identification of islet amyloid polypeptide in normal human pancreas

To identify islet amyloid polypeptide (IAPP) present in normal human pancreas, we isolated the peptide from a soluble peptide fraction of amyloid deposit-free pancreata of two non-diabetic patients by using reverse-phase high performance liquid chromatography coupled with a radioimmunoassay specific for human IAPP. IAPP(1-37) and IAPP(17-37) were isolated and their complete amino acid sequences were determined up to the C-terminus. Identification of IAPP in normal human pancreas suggests the possible biological function of IAPP as a novel pancreatic hormone in humans.