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.     

Effects of islet amyloid polypeptide (IAPP) on insulin biosynthesis or secretion in rat islets and mouse beta TC3 cells. Biosynthesis of IAPP in mouse beta TC3 cells.

Effects of rat islet amyloid polypeptide (IAPP) on insulin biosynthesis and secretion were examined in isolated rat islets and mouse beta TC3 cells. Culture of islets or mouse beta TC3 cells for 24 h in the presence of 10(-6) M IAPP and 5.5 mM glucose had no effect on insulin mRNA levels. The rates of proinsulin biosynthesis were not altered in islets incubated in 10(-4)-10(-9) M IAPP. In beta TC3 cells, proinsulin biosynthesis was stimulated by glucose, though no effects of IAPP were shown. Addition of 10(-5) M IAPP to islets incubated in 11 mM glucose decreased the fractional insulin secretion rates; however, the secretion of insulin from beta TC3 cells was not affected by 10(-5) M IAPP. On the other hand, mouse beta TC3 cells expressed the elevated level of IAPP mRNA. Metabolic labeling of beta TC3 cells revealed the synthesis of both proIAPP and mature IAPP. In pulse chase experiments, proIAPP was processed to IAPP in a manner similar to proinsulin. These data indicate that IAPP is a possible polypeptide hormone synthesized in pancreatic beta cells though it is unlikely that IAPP is a physiologically relevant modulator of insulin biosynthesis or secretion.     

The glucose-induced synthesis of insulin in liver

Pancreatic β cells, stimulated by glucose, are known to synthesize and secrete insulin. As liver diseases are reported to cause diabetes mellitus, studies were conducted to determine the possibility of glucose-induced insulin synthesis in the liver cells. The glucose-induced insulin synthesis was determined by in vitro translation of mRNA from the hepatocytes. The cDNA from mRNA was prepared and sequence analysis was performed. Incubation of hepatocytes from the liver of adult mice (n=10) with glucose (0.02 M) resulted in the insulin synthesis [0.03 (mean)±0.006 (S.D.) μunits/mg/h] compared to the pancreatic β cells [0.04±0.004 μunits/mg/h]. Immunohistochemical study also demonstrated the glucose-induced synthesis of insulin in liver cells. Incubation of the mice hepatocytes with glucose resulted in the synthesis of insulin mRNA. The purified mRNA which was used to prepare cDNA resulted in the formation of proinsulin I and proinsulin II genes corresponding to 182 and 188 base pairs, respectively. Sequence analysis of the cDNA indicated that proinsulin I as well as proinsulin II gene could be involved in the synthesis of insulin by hepatocytes. These results suggested that insulin synthesis in both hepatic and pancreatic cells could be involved in the control of diabetes mellitus.     

Quantitative immunohistochemical analysis of islet amyloid polypeptide (IAPP) in normal, impaired glucose tolerant, and diabetic cats.

Islet amyloid polypeptide (IAPP, "amylin") has been proposed as having important roles in the pathogenesis of type 2 diabetes mellitus via its biological activity and by forming islet amyloid. The domestic cat develops a type of diabetes that closely resembles type 2 diabetes in humans, including the frequent formation of islet amyloid deposits in the impaired glucose tolerant (IGT) and diabetic state. With the aid of computerized image analysis and immunohistochemistry, we examined the IAPP and insulin content in pancreatic islets of normal, IGT and diabetic cats. IAPP immunoreactivity in beta cells from IGT cats was significantly stronger (p < 0.01) as compared with cells from normal cats, while the insulin labelling strength was unchanged. Overtly diabetic cats were usually almost devoid of beta cells. As in humans, cellular IAPP but not IAPP in islet amyloid deposits was labelled by the newly developed monoclonal antibody to IAPP 4A5, thus providing further evidence that IAPP is modified by a yet unknown mechanism during the amyloidogenic process. The study provides evidence that an increased beta cell storage of IAPP independent of insulin may be an important factor in the early phase of the development of islet amyloid in this form of diabetes.     

Translational control of glucose-induced islet amyloid polypeptide production in pancreatic islets

Dysfunctional islet amyloid polypeptide (IAPP) biosynthesis and/or processing are thought contribute to formation of islet amyloid in type 2 diabetes. However, it is unclear how normal pro-IAPP biosynthesis and processing are regulated to be able to define such dysfunction. Here, it was found that acute exposure to high glucose concentrations coordinately regulated the biosynthesis of pro-IAPP, proinsulin, and its proprotein convertase PC1/3 in normal isolated rat islets, without affecting their respective mRNA levels. Pro-7B2 biosynthesis, like that of pro-PC2, did not appreciably change, but this was likely due to a much higher expression in pancreatic α-cells masking glucose regulation of their biosynthesis in β-cells. Biosynthesis of pro-SAAS, the putative PC1/3 chaperone, was unaffected by glucose, consistent with its scarce expression in β-cells. We conclude that translational control of pro-IAPP biosynthesis, in parallel to the pro-PC1/3, pro-PC2, and pro-7B2 proprotein-processing endopeptidases/chaperones, is the predominate mechanism to produce IAPP in islet β-cells.     

Intra- and extracellular amyloid fibrils are formed in cultured pancreatic islets of transgenic mice expressing human islet amyloid polypeptide.

Islet amyloid polypeptide (IAPP) is the constituent peptide of amyloid deposits found in the islets of non-insulin-dependent diabetic patients. Formation of islet amyloid is associated with a progressive destruction of insulin-producing beta cells. Factors responsible for the conversion of IAPP into insoluble amyloid fibrils are unknown. Both the amino acid sequence of human IAPP (hIAPP) and hypersecretion of hIAPP have been implicated as factors for amyloid fibril formation in man. We have generated transgenic mice using rat insulin promoter-hIAPP or rat IAPP (rIAPP) gene constructs. No fibrillar islet amyloid was detectable in vivo in these normoglycemic mice, although small amorphous perivascular accumulations of IAPP were observed in hIAPP mice only. To determine the effects of glucose on IAPP secretion and fibrillogenesis, pancreatic islets from transgenic and control mice were examined in vitro. Islet IAPP secretion and content were increased in transgenic islets compared with control islets. IAPP-immunoreactive fibrils were formed at both intra- and extracellular sites in isolated hIAPP islets cultured with glucose at 11.1 and 28 mM for only 7 days. At 28 mM glucose, fibrils were present in deep invaginations of beta cells as observed in non-insulin-dependent diabetic patients. No fibrils were present at low glucose concentrations in hIAPP islets or at any glucose concentration in rIAPP or control islets. Thus, glucose-induced expression and secretion of hIAPP in transgenic mouse islets can lead to formation of amyloid fibrils similar to that found in non-insulin-dependent diabetes mellitus.     

Parallel changes of proinsulin and islet amyloid polypeptide in glucose intolerance

Elevated proinsulin secretion and islet amyloid deposition are both features of Type 2 diabetes but their relationship to beta-cell dysfunction is unknown. To determine if islet amyloid polypeptide (IAPP) secretion is disproportionate with other beta-cell products at any stage of glucose intolerance, 116 subjects were studied. Non-diabetic subjects with equivalent body mass index (BMI) were assigned to three groups, (i) normal fasting glucose, fpg<5.5 mmol l(-1); (ii) intermediate fasting glucose, fpg> or =5.5<6.15 mmol l(-1); (iii) impaired fasting glucose (IFG), fpg> or =6.1<7.0 mmol l(-1). Diabetic subjects were divided according to therapy (9 diet, 19 tablet, and 11 insulin). IAPP, C-peptide and proinsulin were measured fasting and at the end of a 1-h glucose infusion. Fasting C-peptide, IAPP and proinsulin were significantly elevated in the IFG group compared with the other non-diabetic groups (P<0.02); fasting IAPP/C-peptide and proinsulin/C-peptide were 1-2% in all non-diabetic groups. Fasting and 1-h proinsulin and proinsulin/C-peptide were higher in diabetic compared with non-diabetic subjects (P<0.01). IAPP and IAPP/C-peptide in diabetic groups were similar to that in non-diabetic subjects but reduced in the insulin-treated group (P<0.01). Proinsulin was disproportionately increased compared with C-peptide and IAPP in Type 2 diabetes particularly in severe beta-cell failure implying more than one concurrent beta-cell pathology.     

Islet amyloid: a complication of islet dysfunction or an aetiological factor in Type 2 diabetes?

The role of islet amyloidosis in the onset and progression of Type 2 diabetes remains obscure. Islet amyloid polypeptide is a 37 amino-acid, beta-cell peptide which is co-stored and co-released with insulin. Human islet amyloid polypeptide refolds to a -conformation and oligomerises to form insoluble fibrils; proline substitutions in rodent islet amyloid polypeptide prevent this molecular transition. Pro-islet amyloid polypeptide (67 amino acids in man) is processed in secretory granules. Refolding of islet amyloid polypeptide may be prevented by intragranular heterodimer formation with insulin (but not proinsulin). Diabetes-associated abnormal proinsulin processing could contribute to de-stabilisation of granular islet amyloid polypeptide. Increased pro-islet amyloid polypeptide secretion as a consequence of islet dysfunction could promote fibrillogenesis; the propeptide forms fibrils and binds to basement membrane glycosamino-glycans. Islet amyloid polypeptide gene polymorphisms are not universally associated with Type 2 diabetes. Transgenic mice expressing human islet amyloid polypeptide gene have increased islet amyloid polypeptide concentrations but develop islet amyloid only against a background of obesity and/or high fat diet. In transgenic mice, obese monkeys and cats, initially small perivascular deposits progressively increase to occupy 80% islet mass; the severity of amyloidosis in animal models is related to the onset of hyperglycaemia, suggesting that islet amyloid and the associated destruction of islet cells cause diabetes. In human diabetes, islet amyloid can affect less than 1% or up to 80% of islets indicating that islet amyloidosis largely results from diabetes-related pathologies and is not an aetiological factor for hyperglycaemia. However, the associated progressive beta-cell destruction leads to severe islet dysfunction and insulin requirement.Abbreviations IAPP islet amyloid polypeptide - hIAPP human islet amyloid polypeptide - T2DM Type 2 diabetes - TM transgenic mice expressing the human IAPP gene - UKPDS United Kingdom Prospective Diabetes Study - GAGs glycosamino glycans     

Islet amyloid polypeptide (IAPP) secretion from islet cells and its plasma concentration in patients with non-insulin-dependent diabetes mellitus.

Islet amyloid polypeptide (IAPP/Amylin) is a novel peptide which was extracted from islet amyloid deposits in patients with non-insulin-dependent diabetes mellitus (NIDDM). However, its pattern of secretions and plasma concentrations under various conditions has not yet been made clear enough. In this study, we examined IAPP secretion from islet beta-cells in vitro using cultured islet cells of neonatal rat pancreas and plasma IAPP responses under various conditions in vivo in normal control subjects and patients with glucose intolerance. Our data revealed that (1) IAPP is co-secreted with insulin from islet cells of the rat pancreas by glucose and non-glucose stimuli; (2) fasting plasma IAPP levels in normal control subjects are 24.9 +/- 2.0 pg/ml and the molar ratio of IAPP/insulin is approximately 1/7; (3) fasting IAPP levels are high in obese patients and low in insulin-dependent diabetic patients, and the molar ratio of IAPP/C-peptide in NIDDM patients is lower than that in normal control subjects, suggesting the basal hyposecretion of IAPP relative to insulin in NIDDM; and (4) the obese patients who had a hyperresponsiveness of insulin relative to C-peptide had the hyperresponsiveness of IAPP relative to C-peptide during an oral glucose load, suggesting that IAPP may have some physiological effect in glucose metabolism.     

Spontaneous diabetes mellitus in transgenic mice expressing human islet amyloid polypeptide.

The islet in non-insulin-dependent diabetes mellitus (NIDDM) is characterized by loss of beta cells and large local deposits of amyloid derived from the 37-amino acid protein, islet amyloid polypeptide (IAPP). We have hypothesized that IAPP amyloid forms intracellularly causing beta-cell destruction under conditions of high rates of expression. To test this we developed a homozygous transgenic mouse model with high rates of expression of human IAPP. Male transgenic mice spontaneously developed diabetes mellitus by 8 weeks of age, which was associated with selective beta-cell death and impaired insulin secretion. Small intra- and extracellular amorphous IAPP aggregates were present in islets of transgenic mice during the development of diabetes mellitus. However, IAPP derived amyloid deposits were found in only a minority of islets at approximately 20 weeks of age, notably after development of diabetes mellitus in male transgenic mice. Approximately 20% of female transgenic mice spontaneously developed diabetes mellitus at 30+ weeks of age, when beta-cell degeneration and both amorphous and amyloid deposits of IAPP were present. We conclude that overexpression of human IAPP causes beta-cell death, impaired insulin secretion, and diabetes mellitus. Large deposits of IAPP derived amyloid do not appear to be important in this cytotoxicity, but early, small amorphous intra- and extracellular aggregates of human IAPP were consistently present at the time of beta-cell death and therefore may be the most cytotoxic form of IAPP.     

Metabolic stress, IAPP and islet amyloid

Amyloid forms within pancreatic islets in type 2 diabetes from aggregates of the β-cell peptide islet amyloid polypeptide (IAPP). These aggregates are toxic to β-cells, inducing β-cell death and dysfunction, as well as inciting islet inflammation. The β-cell is subject to a number of other stressors, including insulin resistance and hyperglycaemia, that may contribute to amyloid formation by increasing IAPP production by the β-cell. β-Cell dysfunction, evident as impaired glucose-stimulated insulin secretion and defective prohormone processing and exacerbated by metabolic stress, is also a likely prerequisite for islet amyloid formation to occur in type 2 diabetes. Islet transplants in patients with type 1 diabetes face similar stressors, and are subject to rapid amyloid formation and impaired proinsulin processing associated with progressive loss of β-cell function and mass. Declining β-cell mass is predicted to increase metabolic demand on remaining β-cells, promoting a feed-forward cycle of β-cell decline.     

Non-parallelism of islet amyloid polypeptide (amylin) and insulin gene expression in rat islets following dexamethasone treatment

Summary Islet amyloid polypeptide (IAPP), a novel islet hormone candidate, has been reported to be over-expressed relative to insulin in rats following dexamethasone treatment. In order to investigate the expression of IAPP and insulin following dexamethasone treatment of rats for 12 days, we applied in situ hybridization and immunocytochemistry, allowing us to evaluate islet changes in gene expression and morphology. Tissue concentrations of IAPP and insulin were measured by radioimmunoassay. A low dose of dexamethasone (0.2 mg/kg daily) increased the islet levels of IAPP and insulin mRNA to 249±13% and 150±24% of controls, respectively (p<0.001 and p<0.01). A high dose of dexamethasone (2.0 mg/kg daily) increased the islet levels of IAPP and insulin mRNA to 490±13% and 203±9% of controls, respectively (p<0.001 and p<0.001). The pancreatic concentration of IAPP increased more than that of insulin (p<0.05). Morphometric analysis revealed that dexamethasone treatment induced both hyperplasia and hypertrophy of insulin cells. Changes in the cellular localization of IAPP and insulin mRNA were not observed. Thus, we conclude that the increased level of IAPP mRNA is due to both an increase at the cellular level as well as hyperplasia/hypertrophy of insulin cells. In contrast, the increased level of insulin mRNA appears to be due to hyperplasia/hypertrophy of insulin cells, since insulin gene expression decreased at the cellular level (p<0.001 vs controls). These observations provide further evidence that IAPP and insulin gene expression are regulated in a non-parallel fashion, which may be relevant to the pathogenesis of non-insulin-dependent diabetes mellitusAbbreviations IAPP islet amyloid polypeptide - NIDDM non-insulin-dependent diabetes mellitus - ISH in situ hybridization - SSC saline sodium citrate - CGRP calcitonin gene related peptide     

Pancreatic islet amyloid formation in patients with noninsulin-dependent diabetes mellitus Impliation for therapeutic strategy

Summary Islet amyloid polypeptide (IAPP or amylin) is the main component of pancreatic islet amyloid found in the vast majority of patients with noninsulin-dependent (Type-2) diabetes mellitus (NIDDM). IAPP may also act as a hormone that antagonizes the effects of insulin on peripheral tissues, but the results with IAPP overproducing transgenic mice and other recent findings indicate that IAPP overproduction is unlikely to induce peripheral insulin resistance in NIDDM. However, IAPP may contribute to the progression of NIDDM by impairing β-cell function via islet amyloid formation. This may be initiated by locally elevated IAPP concentrations, induced by insulin-resistance-associated β-cell hyperactivity. In order to improve therapeutic results, we propose strategies to inhibit IAPP production and islet amyloid formation during the pathogenesis of NIDDM.     

Protective role of human insulin against the cytotoxicity associated with human mutant S20G islet amyloid polypeptide

Aims/Introduction

Islet amyloid polypeptide (IAPP) is a main component of islet amyloid in type 2 diabetes and cosecreted from β‐cell with insulin. Clinical evidence from the patients with S20G mutation of the IAPP gene, as well as experimental evidence that insulin could inhibit amyloid formation of IAPP, suggests that a gradual reduction of insulin could be related to the cytotoxicity associated with S20G‐IAPP through long‐term deterioration of β‐cells in type 2 diabetes. Our objective was to show an effect of human insulin on S20G‐IAPP associated cytotoxicity.

Materials and Methods

We analyzed the cytotoxicity associated with S20G‐IAPP by controlling human insulin expression using adenovirus vectors with micro ribonucleic acid specifically against human insulin in endocrine AtT‐20ins cells, which express human insulin permanently. Additionally, we carried out a follow‐up study of circulating IAPP and insulin in type 2 diabetic patients.

Results

S20G‐IAPP expression was associated with a decrease in viability and an increase in terminal deoxynucleotidyl transferase‐mediated deoxyuridine triphosphate‐biotin nick end labeling‐positive cells in AtT‐20ins cells. Furthermore, downregulation of human insulin enhanced the cytotoxicity associated with S20G‐IAPP, and induced the cytotoxicity associated with wild‐type (WT)‐IAPP. Reduction of ubiquitin carboxy‐terminal hydrolase L1 activity enhanced cytotoxicity under the downregulation of human insulin expression in both S20G‐ and WT‐IAPP transduced cells. A 5‐year follow up of type 2 diabetic patients showed a disproportionate increase of serum fasting IAPP‐to‐insulin ratio from baseline.

Conclusions

Human insulin plays a protective role against the cytotoxicity associated with S20G‐IAPP, as well as WT‐IAPP. The findings could suggest long‐term deterioration of insulin secretion associates with IAPP linked cytotoxicity in type 2 diabetes.     

Islet amyloid polypeptide in the islets of Langerhans: friend or foe?

Islet amyloid polypeptide (IAPP), or amylin, was originally discovered as the constituent peptide in amyloid occurring in human insulinomas and in pancreatic islets in human subjects with Type II (non-insulin-dependent) diabetes mellitus. Its normal expression in beta cells and its co-secretion with insulin in response to nutrient stimuli, suggest a metabolic function for the peptide. Specifically, IAPP has most frequently been shown to inhibit insulin secretion, implying that IAPP has a role in the regulation of islet hormone homeostasis. The physiological significance of IAPP in islets has been difficult to assess; very high IAPP concentrations are required to alter insulin secretion. Moreover, until recently, IAPP receptors have not been characterised at the molecular level, thus leaving the actual target cells for IAPP unidentified. Furthermore, in experimental diabetes in rodents, the ratio of IAPP expression to that of insulin invariably is increased. In view of the pleiotropic effects attributed to IAPP, such regulation could be both adverse and beneficial in diabetes. Metabolic characterisation of mice carrying a null mutation in the IAPP gene or which overexpress IAPP in beta cells have recently confirmed that IAPP is a physiological inhibitor of insulin secretion. Based on experiments in which IAPP-deficient mice develop a more severe form of alloxan-induced diabetes, we argue that the action of IAPP in the islets normally is beneficial for beta-cell function and survival; thus, the established up regulation of IAPP expression compared with that of insulin in experimental rodent diabetes could serve to protect islets under metabolically challenging circumstances. [Diabetologia (2000) 43: 687–695]     

Islet amyloid polypeptide (amylin) and insulin are differentially expressed in chronic diabetes induced by streptozotocin in rats

Summary Islet amyloid polypeptide (IAPP) is over-expressed relative to insulin under several experimental conditions relevant to diabetes mellitus, including the immediate phase (7 days) following induction of streptozotocin diabetes. In the present study, IAPP and insulin gene expression were examined in chronic streptozotocin diabetes (3 weeks) in rats. Quantitative in situ hybridization, determining grain areas and optical densities of mRNA labelling, revealed that IAPP and insulin expression were reduced at the islet level at both low and high streptozotocin doses, partly due to reduced beta-cell mass. In contrast, the cellular levels of IAPP mRNA were either increased or unaffected at the low and high streptozotocin doses, respectively, whereas those of insulin mRNA were unaffected or reduced. When dexamethasone was administered to rats given the low streptozotocin dose, IAPP expression was increased, whereas that of insulin was markedly reduced. Immunocytochemistry revealed that IAPP predominantly occurred in insulin cells and to a lesser extent in somatostatin cells at all treatments examined. Our findings demonstrate that IAPP and insulin gene expression are differentially regulated; the over-expression of IAPP relative to insulin is augmented when the beta-cell insult is aggravated, in our experiments represented by massive beta-cell destruction (high streptozotocin dose) or a combination of moderate beta-cell damage and peripheral insulin resistance (low streptozotocin dose and dexamethasone). An over-expression of IAPP relative to insulin may therefore be involved in diabetes pathogenesis, contributing to its metabolic perturbations, possibly through the capacity of IAPP to restrain insulin release and action and to form islet amyloid.Abbreviations IAPP Islet amyloid polypeptide - IR immunoreactive - NIDDM non-insulin-dependent diabetes mellitus - OD optical density - FITC fluorescein isothiocyanate - TRITC tetramethyl rhodamine isothiocyanate - AMCA 7-amino-4-methyl coumarin-3-acetic acid     

Exendin 4 controls insulin production in rat islet beta cells predominantly by potentiation of glucose-stimulated proinsulin biosynthesis at the translational level

Aims/hypothesis Ideally, a therapeutic insulin secretagogue should coordinately increase insulin production and insulin secretion to maintain islet beta cell secretory capacity. We compared the incretin mimetic exendin 4 and the sulfonylurea glibenclamide (known as glyburide in the USA and Canada) for their effects in upholding a balance between (pro)insulin biosynthesis and insulin secretion in pancreatic islets.Methods Isolated rat islets were incubated for 1 or 16 h over a range of glucose concentrations (2.8–16.7 mmol/l) with or without exendin 4 (10 nmol/l) or glibenclamide (1 μmol/l). Islets were then analysed for preproinsulin mRNA expression by RNase protection and quantitative real-time RT-PCR assays. Proinsulin biosynthesis was analysed by metabolic pulse-radiolabelling, immunoprecipitation and PAGE. Insulin secretion and insulin content were analysed by radioimmunoassay.Results Neither exendin 4 nor glibenclamide affected islet preproinsulin mRNA expression. However, exendin 4 significantly increased glucose-induced proinsulin biosynthesis at the translational level within 1 h, in marked contrast to glibenclamide, which inhibited proinsulin biosynthesis, especially at basal and intermediate glucose concentrations. Exendin 4 potentiated insulin secretion in a glucose-dependent manner, whereas glibenclamide stimulated insulin secretion independently of glucose. Exendin 4 better maintained rat islet insulin content compared with glibenclamide, which depleted intracellular stores of insulin in islet beta cells by 40% within 16 h.Conclusions/interpretation Exendin 4 maintains insulin stores and beta cell secretory capacity primarily by translational control of proinsulin biosynthesis in parallel to insulin secretion. Glibenclamide does not regulate insulin production in coordination with stimulated insulin secretion, and consequently depletes islet insulin stores, compromising secretory capacity. Thus, at the level of the beta cell, incretin mimetics have an advantage over sulfonylureas for treatment of type 2 diabetes.     

Plasma Concentrations of Islet Amyloid Polypeptide After Glucagon Administration in Type 2 Diabetic Patients and Non-diabetic Subjects

Islet amyloid polypeptide (IAPP) is the main constituent of pancreatic islet amyloid, observed in the pancreases from patients with Type 2 diabetes mellitus. IAPP is synthesized by the pancreatic β-cells. In order to study the secretion characteristics of IAPP in Type 2 diabetes mellitus, plasma IAPP was measured during a provocation test with glucagon in 33 Type 2 diabetic patients and 18 non-diabetic subjects. The median fasting IAPP level was 5.7 (range 1.1–13.1) pmol l^?1 in the 27 patients treated with oral hypoglycaemic agents and 2.7 (1.9–5.9) in the 6 patients on insulin. In the non-diabetic group fasting IAPP was 5.7 (2.2–10.1). Six minutes after glucagon administration median IAPP rose to 9.4 (1.7–31.0) and 6.1 (5.1–10.2) in the respective diabetic groups, and to 16.8 (4.0–41.0) in the non-diabetic subjects (p 0.05). The correlation coefficient between change in IAPP and change in C-peptide was 0.68 in the diabetic group. We conclude that intravenous administration of glucagon stimulates IAPP release from the β-cell. This provocation test is easy to perform and can be used on a large scale in the study of IAPP secretion in Type 2 diabetes mellitus.     

Inhibitory action of islet amyloid polypeptide and calcitonin gene-related peptide on release of insulin from the isolated perfused rat pancreas

Islet (or insulinoma) amyloid polypeptide (IAPP) is a 37-residue peptide recently purified from amyloid deposits in the pancreas of patients with type 2 diabetes and from amyloid deposits of a human insulinoma. IAPP immunoreactivity has been identified in islet B cells of diabetic and nondiabetic humans. IAPP is structurally similar to calcitonin gene-related peptide (CGRP). The purpose of this study was to examine the effects of IAPP and CGRP on glucose- and carbachol-stimulated release of insulin and pancreatic polypeptide (PP) from the isolated perfused rat pancreas. IAPP and CGRP, at 10(-7) M, failed to inhibit glucose-stimulated (16.7 mM) release of insulin. At the same concentration, however, IAPP significantly (p less than 0.05) inhibited carbachol-stimulated (10(-7) M) release of insulin by 30%, and CGRP significantly inhibited carbachol-stimulated release of insulin by 33% when compared with the control group. IAPP also significantly decreased carbachol-stimulated release release of PP. IAPP and CGRP, at 10(-8) M, did not inhibit carbachol-stimulated release of insulin and PP. These results suggest that IAPP and CGRP may have roles in the regulation of secretion of insulin. IAPP may inhibit secretion of insulin, at least in part, by blocking cholinergic mechanisms.