Effects of ciglitazone on endogenous plasma islet amyloid polypeptide and insulin sensitivity in obese-diabetic viable yellow mice

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.

Amylin-induced suppression of ANP secretion through receptors for CGRP1 and salmon calcitonin

Amylin cosecretes with insulin from pancreatic beta-cells and shows high sequence homology with CGRP, adrenomedullin, and salmon calcitonin. This study aimed to investigate the effect of amylin on the atrial hemodynamics and ANP release from rat atria and to identify its receptor subtypes. Isolated perfused left atria from either control or streptozotocin-treated rats were paced at 1.3 Hz. The concentration of ANP was measured by radioimmunoassay and the translocation of ECF was measured by [3H]-inulin clearance. Rat amylin increased atrial contractility and suppressed the release of ANP. Rat CGRP showed similar effects but was approximately 300-fold more potent than amylin. Pretreatment with receptor antagonist for CGRP1 [rat alpha-CGRP (8-37)] or salmon calcitonin [acetyl-(Asn30, Tyr32)-calcitonin(8-32), (AC 187)] blocked the suppressive effect of ANP release and the positive inotropic effect by rat amylin. However, receptor antagonists for amylin [amylin (8-37), acetyl-amylin] did not block those effects. Amylin (8-37), acetyl-amylin, or rat alpha-CGRP (8-37) alone accentuated the release of ANP with no changes in atrial contractility. The effect of rat amylin and rat amylin (8-37) on the ANP release was attenuated in streptozotocin-treated rats. We suggest that amylin suppressed ANP release with increased atrial contractility through receptors for CGRP1 and salmon calcitonin and the attenuation of amylin and its antagonist on ANP release from streptozotocin-treated rat atria may be due to the downregulation of amylin receptor.

Agouti signaling protein stimulates islet amyloid polypeptide (amylin) secretion in pancreatic beta-cells

Ectopic overexpression of the murine agouti gene results in yellow coat color, obesity, hyperinsulinemia, and type II diabetes. We have shown the human homologue of agouti (agouti signaling protein; ASP) to regulate human adipocyte metabolism and lipid storage via a Ca(2+)-dependent mechanism. We have also demonstrated agouti expression in human pancreas, and that ASP stimulates insulin release via a similar Ca(2+)-dependent mechanism. Plasma amylin is also elevated in agouti mutant mice. Amylin is cosecreted with insulin from beta-cells, and overexpression of human amylin in beta-cells in yellow agouti mutant mice resulted in accelerated pancreatic amyloid deposition, severely impaired beta-cell function, and a diabetic phenotype. We report here that ASP stimulates amylin release in both the HIT-T15 beta-cell line and human pancreatic islets in the presence of a wide range of glucose concentrations (0-16.7 mmol/L), similar to its effect on insulin release; this effect was blocked by 30 mumol/L nitrendipine, confirming a Ca(2+)-dependent mechanism. Accordingly, ASP stimulation of amylin release may serve as a compensatory system to regulate blood glucose in yellow agouti mutants.

Evidence that amylin stimulates lipolysis in vivo: a possible mediator of induced insulin resistance

The present study investigated the role of amylin in lipid metabolism and its possible implications for insulin resistance. In 5- to 7-h-fasted conscious rats, infusion of rat amylin (5 nmol/h for 4 h) elevated plasma glucose, lactate, and insulin (P <0.05 vs. control, repeated-measures ANOVA) with peak values occurring within 60 min. Despite the insulin rise, plasma nonesterified fatty acids (NEFA) and glycerol were also elevated (P < 0.001 vs. control), and these elevations (80% above basal) were sustained over the 4-h infusion period. Although unaltered in plasma, triglyceride content in liver was increased by 28% (P < 0.001) with a similar tendency in muscle (18%, P = 0.1). Infusion of the rat amylin antagonist amylin-(8-37) (125 nmol/h) induced opposite basal plasma changes to amylin, i.e., lowered plasma NEFA, glycerol, glucose, and insulin levels (all P < 0.05 vs. control); additionally, amylin-(8-37) blocked amylin-induced elevations of these parameters (P < 0.01). Treatment with acipimox (10 mg/kg), an anti-lipolytic agent, before or after amylin infusion blocked amylin's effects on plasma NEFA, glycerol, and insulin but not on glucose and lactate. We conclude that amylin could exert a lipolytic-like action in vivo that is blocked by and is opposite to effects of its antagonist amylin-(8-37). Further studies are warranted to examine the physiological implications of lipid mobilization for amylin-induced insulin resistance.

Hyperamylinemia is associated with hyperinsulinemia in the glucose-tolerant, insulin-resistant offspring of two Mexican-American non-insulin-dependent diabetic parents

Several investigations have presented evidence that amylin inhibits insulin secretion and induces insulin resistance both in vitro and in vivo. However, basal and postmeal amylin concentrations proved similar in non-insulin-dependent diabetes mellitus (NIDDM) patients and controls. Since hyperglycemia may alter both amylin and insulin secretion, we examined basal and glucose-stimulated amylin secretion in eight glucose-tolerant, insulin-resistant Mexican-American subjects with both parents affected with NIDDM (offspring) and correlated the findings with the insulin sensitivity data acquired by an insulin clamp. Eight offspring and eight Mexican-Americans without any family history of diabetes (controls) underwent measurement of fat free mass (3H2O dilution method), 180-minutes, 75-g oral glucose tolerance test (OGTT), and 40-mU/m2, 180-minute euglycemic insulin clamp associated with 3H-glucose infusion and indirect calorimetry. Fasting amylin was significantly increased in offspring versus controls (11.5 +/- 1.4 v 7.0 +/- 0.8 pmol/L, P < .05). After glucose ingestion, both total (3,073 +/- 257 v 1,870 +/- 202 pmol.L-1.min-1, P < .01) and incremental (1,075 +/- 170 v 518 +/- 124 pmol.L-1.min-1, P < .05) areas under the curve (AUCs) of amylin concentration were significantly greater in offspring. The amylin to insulin molar ratio was similar in offspring and controls at all time points. Basal and postglucose insulin and C-peptide concentrations were significantly increased in the offspring. No correlation was found between fasting amylin, postglucose amylin AUC or IAUC, and any measured parameter of glucose metabolism during a euglycemic-hyperinsulinemic clamp (total glucose disposal, 7.21 +/- 0.73 v 11.03 +/- 0.54, P < .001; nonoxidative glucose disposal, 3.17 +/- 0.59 v 6.33 +/- 0.56, P < .002; glucose oxidation, 4.05 +/- 0.46 v 4.71 +/- 0.21, P = NS; hepatic glucose production, 0.29 +/- 0.16 v 0.01 +/- 0.11, P = NS; all mg.min-1.kg-1 fat-free mass, offspring v controls). In conclusion, these data do not support a causal role for amylin in the genesis of insulin resistance in NIDDM.

The role of the agouti gene in the yellow obese syndrome

The yellow obese syndrome in mice encompasses many pleiotropic effects including yellow fur, maturity-onset obesity, hyperinsulinemia, insulin resistance, hyperglycemia, increased skeletal length and lean body mass, and increased susceptibility to neoplasia. The molecular basis of this syndrome is beginning to be unraveled and may have implications for human obesity and diabetes. Normally, the agouti gene is expressed during the hair-growth cycle in the neonatal skin where it functions as a paracrine regulator of pigmentation. The secreted agouti protein antagonizes the binding of the alpha-melanocyte-stimulating hormone to its receptor (melanocortin 1 receptor) on the surface of hair bulb melanocytes, causing alterations in intracellular cAMP levels. Widespread, ectopic expression of the mouse agouti gene is central to the yellow obese phenotype, as demonstrated by the molecular cloning of several dominant agouti mutations and the ubiquitous expression of the wild-type agouti gene in transgenic mice. Recent experiments have revealed that the hypothalamus and adipose tissue are biologically active target sites for agouti in the yellow obese mutant lines.

Activity of amylin at CGRP1-preferring receptors coupled to positive contractile response in rat ventricular cardiomyocytes

Calcitonin gene-related peptide (CGRP) exerts a positive contractile response directly in rat ventricular cardiomyocytes. This response is mediated by receptors of the CGRP1-subtype. Amylin is 46% homologous with CGRP and binds to receptors selective for CGRP in a range of tissues. The ability of amylin to influence ventricular contractility has been assessed using cardiomyocytes isolated from the ventricles of adult rats. Cardiomyocytes were subjected to biphasic electrical stimulation at 0.5 Hz. CGRP produced a concentration-dependent positive contractile response which became maximal 4 min after initial stimulation. CGRP increased the contractile amplitude maximally at 1 nM and to a value which was 23.3% greater than in the absence of peptide (EC50 value = 21 pM). Amylin increased the contractile amplitude maximally at 20 nM and to a value which was 17.3% greater than in the absence of peptide (EC50 value = 216 pM). In the presence of amylin (20 nM), the concentration-dependence of the contractile response to CGRP was shifted to the left, so that the response became maximal when CGRP was present at 50 pM. In the presence of CGRP8-37 (100 nM), a selective antagonist at CGRP1-preferring receptors, the concentration-dependence of the contractile response to CGRP was shifted to the right (dose ratio = 54). Similarly, in the presence of CGRP8-37 (100 nM), the contractile response to amylin was inhibited significantly (P < or = 0.01). Amylin8-37 (100 nM) did not inhibit the concentration-dependence of the contractile responses to CGRP and amylin significantly (dose ratios = 4.2 and 2.4, respectively). In conclusion, these data indicate that amylin exerts a contractile response directly in rat ventricular cardiomyocytes via CGRP1-preferring receptors. This effect could assume greater significance in non-insulin-dependent diabetes mellitus and in hypertensive states, in which the concentration of amylin is elevated in plasma.

Amylin stimulates plasma renin concentration in humans

Although insulin resistance and hypertension are commonly associated, the underlying cause for this association remains unknown. Plasma concentrations of the recently described hormone amylin, which is cosecreted with insulin by the pancreatic beta cell, are reported to be elevated in various states of insulin resistance, including hypertension and obesity. Preliminary studies by our group have suggested that there are amylin binding sites in the kidney. In nine healthy humans an infusion of human amylin that resulted in steady state plasma amylin levels in the subnanomolar range led to significant increases in plasma renin and aldosterone concentrations. These changes occurred in the absence of significant changes in plasma electrolytes, catecholamines, vasopressin, total renin, or osmolality. Diastolic pressure at 30 minutes and plasma glucose at 60 minutes rose modestly. Since amylin has both metabolic and renal actions, this peptide may be an important link between hypertension, insulin resistance, and the renin-angiotensin system.

Dose-dependent elevation of cyclic AMP, activation of glycogen phosphorylase, and release of lactate by amylin in rat skeletal muscle

We report here our investigation of the role of cyclic AMP (cAMP) in amylin signal transduction in isolated strips of soleus muscle. Rat amylin, at 100 nM, increased cAMP levels, from 0.431 +/- 0.047 to a peak of 1.24 +/- 0.01 pmol cAMP/mg wet wt. after 5 min, in the absence of added phosphodiesterase inhibitor. The EC50 of the response was 0.48 nM (+/- 0.12 log units) in the absence of insulin and 0.3 nM (+/- 0.18 log units) in the presence of 7.1 nM insulin. The response seen with a maximally effective concentration of amylin (10 nM) was similar to that seen with a maximally effective concentration of epinephrine (1 microM) under the same conditions. Consistent with the observed rise in cAMP there was an increase in glycogen phosphorylase a (EC50 2.2 nM +/- 0.25 log units), decreased glycogen content (EC50 0.9 nM +/- 0.22 log units) and enhanced production of lactate (EC50 1.5 nM +/- 0.33 log units). These data support the concept that amylin promotes glycogenolysis in skeletal muscle and enhances production of lactate through glycolysis as a result of activation of Gs coupled receptors, stimulation of adenylate cyclase, elevation of cAMP levels and activation of glycogen phosphorylase.

Ectopic expression of the agouti gene in transgenic mice causes obesity, features of type II diabetes, and yellow fur

Mice that carry the lethal yellow (Ay) or viable yellow (Avy) mutation, two dominant mutations of the agouti (a) gene in mouse chromosome 2, exhibit a phenotype that includes yellow fur, marked obesity, a form of type II diabetes associated with insulin resistance, and an increased susceptibility to tumor development. Molecular analyses of these and several other dominant "obese yellow" a-locus mutations suggested that ectopic expression of the normal agouti protein gives rise to this complex pleiotropic phenotype. We have now tested this hypothesis directly by generating transgenic mice that ectopically express an agouti cDNA clone encoding the normal agouti protein in all tissues examined. Transgenic mice of both sexes have yellow fur, become obese, and develop hyperinsulinemia. In addition, male transgenic mice develop hyperglycemia by 12-20 weeks of age. These results demonstrate conclusively that the ectopic agouti expression is responsible for most, if not all, of the phenotypic traits of the dominant, obese yellow mutants.

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.

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.

Amylin regulation of fuel metabolism

The 37-amino acid amylin, co-secreted from the pancreatic beta cells with insulin in response to nutrient stimuli has actions in a number of tissues of metabolic interest. In muscle it opposes glycogen synthesis and activates glycogenolysis, an action likely to underly its stimulation of lactate flux. Amylin therefore appears to have the effect of transposing carbon from peripheral stores to the liver, where it is made available for hepatic synthesis of glucose, glycogen, and lipid. While amylin induces insulin resistance in skeletal muscle, it does not oppose insulin action in fat and may therefore favor fuel deposition in this tissue. Amylin acts on the beta cell to inhibit insulin secretion. Relative impairment of insulin secretion, muscle insulin resistance, relatively preserved insulin sensitivity in fat, increased lactate turnover, and increased hepatic glucose production are features of insulin resistance and early non-insulin-dependent diabetes mellitus. Amylin is elevated in these dysfunctional metabolic states and may be involved in their pathogenesis.

Molecular physiology of amylin

Amylin is a 37-amino acid peptide first isolated, purified, and characterized from the amyloid deposits in the pancrease of type 2 diabetics. It is synthesized and secreted primarily from pancreatic beta cells along with insulin. The ability of amylin to potently reduce insulin-stimulated incorporation of glucose into glycogen in skeletal muscle requires both an intact 2Cys-7Cys disulfide bond and a COOH-terminal amide. Amylin has structural and functional relationships to two other messenger proteins, calcitonin and CGRP. Amylin has relatively potent calcitonin-like activity on bone metabolism and weaker CGRP-like activity on the vasculature. CGRP is a slightly weaker agonist than amylin for metabolic responses. Although rat calcitonins are weak, teleost fish calcitonins are very potent agonists for amylin's metabolic effects. This group of peptides appears to act on a family of related G protein-coupled receptors; several variant calcitonin receptors have recently been cloned and expressed. These receptors appear to be coupled to adenylyl cyclase in many instances; recent evidence supports the view that amylin's effects on skeletal muscle occur, at least in large part, through activation of the cAMP pathway.

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)

Structure and biology of amylin

Amylin is a recently discovered 37 amino acid peptide secreted into the bloodstream, along with insulin, from pancreatic beta-cells. It is about 50% identical to calcitonin gene-related peptides (CGRP alpha and CGRP beta) and structurally related to the calcitonins. Amylin can elicit the vasodilator effects of CGRP and the hypocalcaemic actions of calcitonin, while these peptides can mimic newly discovered actions of amylin on carbohydrate metabolism. The different relative potencies of these peptides suggest that they act with different selectivities at a family of receptors. Amylin is deficient in insulin-dependent diabetes mellitus, while plasma levels are elevated in insulin-resistant conditions such as obesity and impaired glucose tolerance. In this Viewpoint article, Tim Rink and colleagues propose that amylin is an endocrine partner to insulin and glucagon; deficiency or excess of amylin may therefore contribute to important metabolic diseases.

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.

Dose response characteristics for the hyperglycemic, hyperlactemic, hypotensive and hypocalcemic actions of amylin and calcitonin gene-related peptide-I (CGRP alpha) in the fasted, anaesthetized rat

Amylin, a 37 amino-acid peptide secreted from the pancreatic beta-cells, exerts marked effects on carbohydrate metabolism in intact rats. It has approximately 50% amino-acid identity with the calcitonin gene-related peptides (CGRP) as well as certain shared biological actions. In vivo potencies were determined for four responses (increases in plasma glucose, increases in plasma lactate, decreases in plasma calcium, and depression of arterial pressure). These responses were measured in fasted, lightly anaesthetized rats given single intravenous bolus injections of rat amylin or rat CGRP alpha at doses of 0.01, 0.1, 1, 10, 100 and 1000 micrograms (about 7 pmol/kg-700 nmol/kg). Control animals received an equal volume of saline. The order of potency for the different responses was as follows: (i) increase in plasma glucose concentration, amylin approximately 2 times more potent than CGRP (by ED50) with detectable responses occurring at doses 100-fold less; (ii) decrease in plasma total calcium concentration, CGRP of equal or greater potency than amylin; and (iii) decrease in arterial pressure, CGRP 44-fold more potent than amylin. An increase in plasma lactate occurred with amylin doses 1000-fold lower than the CGRP doses producing such effects. Saturation of the dose-dependent increase in lactate was not observed, so median effective doses (ED50) were not obtained. These results are consistent with the existence of separate receptor systems for amylin and CGRP. The effects of amylin on plasma glucose and lactate concentrations were demonstrable at doses of 0.1-1.0 micrograms (70-700 pmol/kg). These doses produced plasma levels that were within the concentration range previously reported for insulin-resistant rats, supporting the proposal that amylin is a physiologic endocrine regulator of carbohydrate metabolism in vivo.

Amylin and insulin in rat soleus muscle: dose responses for cosecreted noncompetitive antagonists

Increasing concentrations of amylin progressively depressed the maximal insulin-stimulated radioglucose incorporation into soleus muscle glycogen, but did not substantively change the EC50 (range 0.78 to 1.52 nM); these findings show noncompetitive, insurmountable antagonism of insulin action by amylin. The results from 36 combinations of different insulin and amylin concentrations were used to construct a response surface that can be used to predict the response for any combination of insulin and amylin concentration. The predicted response to a constant ratio of insulin and amylin concentration is a bell-shaped curve. The experimentally determined response to increasing amounts of an amylin-insulin mixture (molar ratio of 0.14:1, within the range measured for pancreatic secretion and plasma levels) gave a bell-shaped response rather than the sigmoidal response seen with insulin alone. The amylin dose-response relation in the soleus system provides a useful bioassay for amylin agonists. The dose response for highly purified, synthetic human amylin obtained by measuring amylin concentrations by radioimmunoassay in the incubation medium gave an EC50 of 456 pM (+/- 0.18 log units). Human amylin had a potency greater than or equal to that of human insulin in this highly insulin-sensitive preparation.

Induction of acute hyperglycemia, hyperlactemia and hypocalcemia in fed and fasted BALB/c mice by intravenous amylin injection

Amylin has been reported to influence carbohydrate metabolism in rats, dogs and cats. We report here that intravenous injection of 50 micrograms amylin (640 nmol/kg) induced hyperglycemia, hyperlactemia, and hypocalcemia in both fed and 5-hour fasted mice. Peak glucose and lactate increments occurred within 15 minutes of treatment, followed by a slower decline of plasma calcium levels. To determine dose-response characteristics of these effects, fasted animals were given amylin doses ranging from 0.005 micrograms to 500 micrograms (64 pmol/kg to 6.4 mumol/kg). Median effective doses (ED50) for the hyperglycemic, hyperlactemic, and hypocalcemic effects were 155, 16.9 and 190 nmol/kg, respectively, with maximum increases of 6.27 mM for glucose, 1.85 mM for lactate and maximum decrease of 0.37 mM for calcium. The estimated half-life (t1/2) of exogenous amylin in the circulation was 5.0 minutes in fasted mice. These results indicate that amylin is bioactive in mice. The kinetic data predict that biologically effective doses of exogenous amylin result in plasma concentrations comparable to pathophysiological concentrations of endogenous hormone previously reported.

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.