Amylin release from perfused rat pancreas in response to glucose and arginine

Macronutrient intake: Hormonal controls, pathological states, and methodological considerations

A plethora of studies to date has examined the roles of feeding-related peptides in the control of food intake. However, the influence of these peptides on the intake of particular macronutrient constituents of food - carbohydrate, fat, and protein - has not been as extensively addressed in the literature. Here, the roles of several feeding-related peptides in controlling macronutrient intake are reviewed. Next, the relationship between macronutrient intake and diseases including diabetes mellitus, obesity, and eating disorders are examined. Finally, some key considerations in macronutrient intake research are discussed. We hope that this review will shed light onto this underappreciated topic in ingestive behavior research and will help to guide further scientific investigation in this area.

Spontaneous diabetes mellitus in captive Mandrillus sphinx monkeys: a case report

Case history The two obese mandrills (Mandrillus sphinx) showed clinical signs of depression, anorexia, hyperglycemia, hypertriglyceridemia, glucosuria, proteinuria and ketonuria. Septic bed sore wounds were noted on both fore and hind limbs. Results Histopathological study revealed severe islet amyloidosis in both mandrills. Immunohistochemical study using polyclonal anti-cat amylin antibody confirmed derivation of the islet amyloid from islet amyloid polypeptide (IAPP). Cardiomyopathy and myocardial fibrosis were also evident. Conclusions The present study documents diabetes mellitus in two obese mandrills. Diabetes in these animals had features very similar type 2 diabetes mellitus of humans, including the development of severe, IAPP-derived islet amyloidosis. The mandrill may, therefore, serve as an animal model of human type 2 diabetes mellitus.

Circulating amylin in human essential hypertension: heritability and early increase in individuals at genetic risk

BACKGROUND

Human essential hypertension is a complex trait with poorly understood genetic determination. Insulin resistance is frequently associated with this trait.

OBJECTIVE

To determine whether a potentially pathogenic feature of the insulin-resistant state, circulating amylin (islet amyloid polypeptide, co-released with insulin from pancreatic islet beta-cells), is already increased in prehypertensive individuals (normotensive persons at genetic risk of hypertension because of family history), whether such individuals already differ in their amylin response to beta-cell stimulation, and whether plasma amylin concentration is heritable. Such features could establish increased circulating amylin as a hereditary 'intermediate phenotype' useful in genetic analyses of hypertension.

METHODS

Plasma amylin and insulin were measured in 283 medication-free individuals stratified by blood pressure status (82 hypertensive and 201 normotensive), and genetic risk (family history) of hypertension. Differences in means were tested by ANOVA, variances by F test, and frequency distributions by maximum likelihood analysis. Co-release of amylin and insulin was provoked by intravenous infusion of mixed amino acids. The effect of antihypertensive treatment was evaluated after monotherapy with either angiotensin converting enzyme inhibition or calcium-channel blockade in hypertension.

RESULTS

Plasma amylin was increased in hypertension (P= 0.027), and body mass index was a strong predictor of increased circulating amylin (P = 0.0001). Plasma amylin and plasma renin activity were not correlated (P = 0.395), and effective antihypertensive monotherapy with either angiotensin converting enzyme inhibition or calcium-channel blockade did not affect either amylin (P = 0.87-0.97) or insulin (P= 0.55-0.59). Among normotensive individuals, those at genetic risk of hypertension (with positive family history) already had increased concentrations of amylin (P< 0.001), despite exhibiting no difference in blood pressure or body mass index compared with the family-history-negative group; however, among normotensive individuals, both family history (P = 0.043) and body mass index (P= 0.0059) were significant predictors of increased concentrations of amylin. By maximum likelihood analysis, plasma amylin was distributed heterogeneously in the normotensive individuals, with two modes best explaining the distribution (chi2 = 77.4, P< 0.001), and family-history-positive individuals completely accounting for the upper mode (chi2 = 4.63, P = 0.031). Family-history-positive normotensive individuals showed greater plasma amylin concentrations both before and during beta-cell stimulation by amino acid infusion (P = 0.014). Black (n = 111) and white (n = 172) individuals did not differ in mean (P = 0.946) or variance (P = 0.172) of plasma amylin concentrations.

CONCLUSIONS

These results suggest that plasma amylin concentration is in part determined by heredity. Both basal and stimulated plasma amylin excess may identify a subgroup of individuals bearing an inherited predisposition to hypertension. Measurement of amylin might identify a useful 'intermediate phenotype' in the genetic analysis of essential hypertension and its relationship to insulin resistance.

Amylin evokes phosphorylation of P20 in rat skeletal muscle

To investigate the signal transduction events underlying amylin's actions, the amylin-evoked protein phosphorylation cascade was analysed using two-dimensional gel electrophoresis. We found that phosphorylation of three isoelectric variants of P20 (termed ARPP1, ARPP2 and ARPP3) was associated with amylin's actions in rat skeletal muscle. Amylin decreased phosphorylation of ARPP1 and increased phosphorylation of ARPP2 and ARPP3 in a dose-dependent manner. Insulin inhibited amylin-evoked phosphorylation of ARPP2 and ARPP3. The amylin-selective antagonist rat amylin-(8-37) completely reversed amylin's action on ARPP3 and partially decreased phosphorylation of ARPP2. By contrast, the CGRP-selective antagonist, human CGRP-(8-37) blocked phosphorylation of ARPP2 but had little effect on ARPP3. These results suggest that amylin modifies phosphorylation of P20 via two independent mechanisms, and that P20 might be a molecule mediating amylin's biological functions.

Gastroprotections of escins Ia, Ib, IIa, and IIb on ethanol-induced gastric mucosal lesions in rats

Effects of escins Ia, Ib, IIa, and IIb isolated from horse chestnuts on ethanol-induced gastric mucosal lesions and the roles of capsaicin-sensitive afferent neurons, endogenous nitric oxide (NO), sulfhydryls, prostaglandins, as well as gastric secretion and the sympathetic nervous system, were investigated in rats. Test samples were given orally to fasted rats 1 h before ethanol (1.5 ml/rat, p.o.) treatment or ligation of the pylorus. Escins Ia-IIb (10-50 mg/kg) potently inhibited ethanol-induced gastric mucosal lesions, whereas desacylescins I and II (50 mg/kg) showed no such effect. These active saponins (10 and 20 mg/kg) did not decrease the gastric secretion. The gastroprotections of escins Ia-IIb were attenuated by the pretreatment with capsaicin, N(G)-nitro-L-arginine methyl ester, and indomethacin, but not by N-ethylmaleimide. The effects of escins Ia-IIb were also attenuated in streptozotocin-induced diabetic rats, in which the activity of the sympathetic nervous system was abnormal. These results suggest that the gastroprotections of escins Ia-IIb on ethanol-induced gastric mucosal lesions are acid-independent, whereas endogenous prostaglandins, NO, capsaicin-sensitive afferent neurons, and the sympathetic nervous system participate.

Amylin-mediated inhibition of insulin-stimulated glucose transport in skeletal muscle

We examined the effects of amylin on 3-O-methyl-D-glucose (3-O-MG) transport in perfused rat hindlimb muscle under hyperinsulinemic (350 microU/ml, 2,100 pmol/l) conditions. Amylin at 100 nmol/l concentration inhibited 3-O-MG transport relative to control in all three basic muscle fiber types. Transport decreased in slow-twitch oxidative (from 5.65 +/- 1.13 to 3.46 +/- 0.71 micromol . g-1 . h-1), fast-twitch oxidative (from 6.84 +/- 0.90 to 4.84 +/- 0.76 micromol . g-1 . h-1), and fast-twitch glycolytic (from 1.27 +/- 0.20 to 0.60 +/- 0.05 micromol . g-1 . h-1) muscle. Amylin inhibition of insulin-stimulated glucose transport in skeletal muscle was accompanied by a 433 +/- 72% increase in intracellular glucose 6-phosphate (G-6-P) despite the absence of extracellular glucose. The source of hexose units for the formation and maintenance of G-6-P was likely glycogen. Amylin increased glycogenolysis, increased lactate formation, and decreased glycogen synthase activity. Furthermore, the kinetics of glycogen synthase suggest that this enzyme may control intracellular G-6-P concentration. Despite the large increase in G-6-P, no detectable increase in uridine diphosphate-N-acetylhexosamines occurred, suggesting that the proposed glucosamine pathway may not be involved in transport inhibition. However, decreases in uridine diphosphate hexoses were detected. Therefore, uridine or hexosamine-based metabolites may be involved in amylin action.

Amylin influences insulin-stimulated glucose metabolism by two independent mechanisms

The effects of amylin on fiber type-specific muscle glucose metabolism under hyperglycemic (10 mmol/l) and hyperinsulinemic (2.1 nmol/l) conditions were investigated using a rat hindlimb perfusion system. Amylin concentration ranged from 1 to 100 nM. Efficacy for inhibition of glucose uptake traced with 2-deoxyglucose by amylin was demonstrated in all three fiber types. The incorporation of 2-deoxy-[3H]glucose tracer decreased from control values by 41% in fast oxidative (FO), 36% in fast glycolytic (FG), and 37% in slow oxidative (SO) muscle with 100 nM amylin. Amylin increased intracellular glucose 6-phosphate (G-6-P), and G-6-P was negatively correlated with 2-deoxyglucose uptake in both FO (r = -0.65; P < 0.01) and FG (r = -0.53; P < 0.01) muscle. Muscle glycogen concentration increased under control conditions and decreased in the presence of 100 nM amylin. Lactate arteriovenous efflux across the hindlimb increased significantly above control with 100 nM amylin (5.03 +/- 0.81 to 11.28 +/- 0.94 mumol.g-1.h-1). Adenosine 3',5'-cyclic monophosphate (cAMP) increased in FO and FG muscle with amylin. Salmon calcitonin-(8-32), an amylin antagonist, ameliorated the effect of amylin on all responses other than 2-deoxyglucose uptake and G-6-P concentration. These results suggest that amylin may work through at least two independent mechanisms, a cAMP-mediated effect on glycogen metabolism and a non-cAMP-mediated inhibition of glycolysis.

High prevalence of pancreatic islet amyloid in patients with end-stage renal failure on dialysis treatment

Islet amyloid polypeptide (IAPP) is the main proteinaceous component of pancreatic islet amyloid, which is a characteristic feature of type 2 diabetes. The factors responsible for amyloid deposition are unclear. Patients with end-stage renal failure (ESRF) on dialysis treatment have increased insulin resistance which is associated with hypersecretion of beta-cell products. Furthermore, elevated concentrations of circulating IAPP are found in these patients due to reduced renal clearance of IAPP. To determine the prevalence of islet amyloid in this group of patients, pancreas was examined from 23 non-diabetic [aged 62 (29-79) years, median and range] and four type 2 diabetic [aged 67 (56-72) years] patients with ESRF on dialysis treatment. Pancreatic specimens from 30 non-diabetic control subjects [aged 67.5 (56-86) years] and 14 type 2 diabetic subjects without renal disease [aged 69 (48-86) years] were used as control groups. Islet amyloid was present in all type 2 diabetic patients with ESRF and in 12 out of 14 type 2 diabetic control subjects (86 per cent). Amyloid deposits were found in 8 out of 23 non-diabetic patients with ESRF (35 per cent), which was a higher prevalence than that found in non-diabetic control subjects (3 per cent) (P < 0.01). This may be related to undiagnosed (pre)diabetes. Elevated secretion rates of IAPP due to insulin resistance and high circulating IAPP concentrations as a result of severely reduced renal clearance of IAPP will cause high pericellular concentrations of IAPP. This condition is likely to enhance amyloid fibril formation in pancreatic islets similar to that observed in type 2 diabetes.

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.

Diabetes mellitus in Macaca mulatta monkeys is characterised by islet amyloidosis and reduction in beta-cell population

Diabetes mellitus in Macaca mulatta rhesus monkeys is preceded by phases of obesity and hyperinsulinaemia and is similar to Type 2 (non-insulin-dependent) diabetes mellitus in man. To relate the progression of the disease to quantitative changes in islet morphology, post-mortem pancreatic tissue from 26 monkeys was examined. Four groups of animals were studied: group I--young, lean and normal (n = 3); group II--older (> 10 years), lean and obese, normoglycaemic (n = 9); group III--normoglycaemic and hyperinsulinaemic (n = 6); group IV--diabetic (n = 8). Areas of islet amyloid, beta cells and islets were measured on stained histological sections. Islet size was larger in animals from groups III (p < 0.01) and IV (p < 0.0001) compared to groups I and II. The mean beta-cell area per islet in micron 2 was increased in group III (p < 0.05) and reduced in group IV (p < 0.001) compared to groups I and II. Mean beta-cell area per islet correlated with fasting plasma insulin (r = 0.76, p < 0.001) suggesting that hyper- and hypoinsulinaemia are related to the beta-cell population. Amyloid was absent in group I but small deposits were present in three of nine (group II) and in four of six (group III) animals, occupying between 0.03-45% of the islet space. Amyloid was present in eight of eight diabetic animals (group IV) occupying between 37-81% of the islet area. Every islet was affected in seven of eight diabetic monkeys. There was no correlation of degree of amyloidosis with age, body weight, body fat proportion or fasting insulin. Islet amyloid appears to precede the development of overt diabetes in Macaca mulatta and is likely to be a factor in the destruction of islet cells and onset of hyperglycaemia.

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.

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.

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.

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.

Human and rat amylin have no effects on insulin secretion in isolated rat pancreatic islets

Amylin, an islet amyloid peptide secreted by the pancreatic beta cell, has been proposed as a humoral regulator of islet insulin secretion. Four separate preparations of amylin were tested for effects on hormone secretion in both freshly isolated and cultured rat islets and in HIT-T15, hamster insulinoma cells. With all three experimental models, exposure to human amylin acid and human and rat amylin at concentrations as high as 100 nM had no significant effect on rates of insulin or glucagon secretion. These observations suggest that amylin, even at concentrations appreciably higher than those measured in peripheral plasma, is not a significant humoral regulator of islet hormone secretion.