Daily amylin replacement reverses hepatic glycogen depletion in insulin-treated streptozotocin diabetic rats

The endocrine pancreas during exercise in people with and without type 1 diabetes: Beyond the beta-cell

Although important for digestion and metabolism in repose, the healthy endocrine pancreas also plays a key role in facilitating energy transduction around physical exercise. During exercise, decrements in pancreatic β-cell mediated insulin release opposed by increments in α-cell glucagon secretion stand chief among the hierarchy of glucose-counterregulatory responses to decreasing plasma glucose levels. As a control hub for several major glucose regulatory hormones, the endogenous pancreas is therefore essential in ensuring glucose homeostasis. Type 1 diabetes (T1D) is pathophysiological condition characterised by a destruction of pancreatic β-cells resulting in pronounced aberrations in glucose control. Yet beyond the beta-cell perhaps less considered is the impact of T1D on all other pancreatic endocrine cell responses during exercise and whether they differ to those observed in healthy man. For physicians, understanding how the endocrine pancreas responds to exercise in people with and without T1D may serve as a useful model from which to identify whether there are clinically relevant adaptations that need consideration for glycaemic management. From a physiological perspective, delineating differences or indeed similarities in such responses may help inform appropriate exercise test interpretation and subsequent program prescription. With more complex advances in automated insulin delivery (AID) systems and emerging data on exercise algorithms, a timely update is warranted in our understanding of the endogenous endocrine pancreatic responses to physical exercise in people with and without T1D. By placing our focus here, we may be able to offer a nexus of better understanding between the clinical and engineering importance of AIDs requirements during physical exercise.

The Cardiopulmonary Effects of the Calcitonin Gene-related Peptide Family

Cardiopulmonary diseases are very common among the population. They are high-cost diseases and there are still no definitive treatments. The roles of members of the calcitonin-gene related-peptide (CGRP) family in treating cardiopulmonary diseases have been studied for many years and promising results obtained. Especially in recent years, two important members of the family, adrenomedullin and adrenomedullin2/intermedin, have been considered new treatment targets in cardiopulmonary diseases. In this review, the roles of CGRP family members in cardiopulmonary diseases are investigated based on the studies performed to date.

Calcitonin gene-related peptide: neuroendocrine communication between the pancreas, gut, and brain in regulation of blood glucose

Background

Calcitonin gene-related peptide (CGRP), a ubiquitous neuropeptide, plays a diverse and intricate role in chronic low-grade inflammation, including conditions such as obesity, type 2 diabetes, and diabetes of the exocrine pancreas. Diabetes of exocrine pancreas is characterised by chronic hyperglycemia and is associated with persistent low-grade inflammation and altered secretion of certain pancreatic and gut hormones. While CGRP may regulate glucose homeostasis and the secretion of pancreatic and gut hormones, its role in chronic hyperglycemia after acute pancreatitis (CHAP) is not known. The aim of this study was to investigate the association between CGRP and CHAP.

Methods

Fasting blood samples were collected to measure insulin, HbA1c, CGRP, amylin, C-peptide, glucagon, pancreatic polypeptide (PP), somatostatin, gastric inhibitory peptide, glicentin, glucagon-like peptide-1 and 2, and oxyntomodulin. Modified Poisson regression analysis and linear regression analyses were conducted. Five statistical models were used to adjust for demographic, metabolic, and pancreatitis-related risk factors.

Results

A total of 83 patients were recruited. CGRP was significantly associated with CHAP in all five models (P-trend <0.005). Further, it was significantly associated with oxyntomodulin (P<0.005) and glucagon (P<0.030). Oxyntomodulin and glucagon independently contributed 9.7% and 7%, respectively, to circulating CGRP variance. Other pancreatic and gut hormones were not significantly associated with CGRP.

Conclusions

CGRP is involved in regulation of blood glucose in individuals after acute pancreatitis. This may have translational implications in prevention and treatment of diabetes of the exocrine pancreas.

Effects in liver

Amylin and calcitonin gene-related peptide (CGRP) were each shown to stimulate endogenous glucose production in vivo in rats. Neither peptide had any effect on any of several measures of intermediary carbohydrate metabolism in isolated hepatocytes or isolated perfused liver in rats. The possibility exists that augmentation of endogenous glucose production was secondary to release of lactate from muscle into the plasma, thereby stimulating gluconeogenesis by increasing the availability of substrate. Results from hyperlactemic clamp preparations, which allowed for direct measurement of such an effect, suggested that there were additional mechanisms that accounted for amylin stimulation of endogenous glucose production in rats. There is no evidence that amylin increases endogenous glucose production in humans.

Amylin and the integrated control of nutrient influx

The most potent actions of amylin that occur at physiological plasma concentrations include inhibition of food intake, gastric emptying, acid and digestive enzyme secretion, and glucagon secretion. These actions share a common outcome; they each help regulate the rate at which nutrients (including glucose) appear in the blood (Ra). Amylin physiologically orchestrates, via several parallel processes, the rate of entry of nutrient into the circulation, as shown schematically in Fig. 1. In this way, amylin's function may be viewed as complementary to that of insulin (secreted from the same pancreatic beta-cells), which orchestrates the exit of nutrient from blood and its storage in peripheral tissues. The following discussion addresses the emerging picture that, although amylin is co-secreted with an endocrine hormone from endocrine tissue (the pancreatic islets), the target for its most potent and physiologically relevant effects appears to be the central nervous system. Amylin thus may be primarily regarded as a neuroendocrine hormone (Young et al., 2000).

Amylin agonists: a novel approach in the treatment of diabetes

Amylin is a peptide hormone that is cosecreted with insulin from the pancreatic beta-cell and is thus deficient in diabetic people. It inhibits glucagon secretion, delays gastric emptying, and acts as a satiety agent. Amylin replacement could therefore possibly improve glycemic control in some people with diabetes. However, human amylin exhibits physicochemical properties predisposing the peptide hormone to aggregate and form amyloid fibers, which may play a part in beta-cell destruction in type 2 diabetes. This obviously makes it unsuitable for pharmacological use. A stable analog, pramlintide, which has actions and pharmacokinetic and pharmacodynamic properties similar to the native peptide, has been developed. The efficacy and safety of pramlintide administration has been tested in a vast number of clinical trials. Approximately 5,000 insulin-treated patients have received pramlintide and approximately 250 for > or =2 years. The aims of this review are to 1) briefly describe actions of amylin as demonstrated in animal and human models and 2) primarily review results from clinical trials with the amylin analog pramlintide.

A randomized study and open-label extension evaluating the long-term efficacy of pramlintide as an adjunct to insulin therapy in type 1 diabetes

OBJECTIVE

To assess the effect of mealtime amylin replacement with pramlintide on long-term glycemic and weight control in patients with type 1 diabetes.

RESEARCH DESIGN AND METHODS

In a 52-week, double-blind, placebo-controlled, multicenter study, 480 patients with type 1 diabetes were randomized to receive preprandial injections of placebo or 30 microg pramlintide q.i.d., in addition to existing insulin regimens. At week 20, pramlintide-treated patients were re-randomized to 30 or 60 microg pramlintide q.i.d. if decreases from baseline in HbA(1c) were <1% at week 13. Of the 342 patients who completed the 52-week study, 236 individuals ( approximately 70%) elected to participate in a 1-year open-label extension in which all patients received 30 or 60 microg pramlintide q.i.d.

RESULTS

Treatment with pramlintide led to a mean reduction in HbA(1c) of 0.67% from baseline to week 13 that was significantly (P < 0.0001) greater than the placebo reduction (0.16%), and a significant placebo-corrected treatment difference was sustained through week 52 (P = 0.0071). The greater HbA(1c) reduction was associated with an average weight loss, rather than weight gain, and was not accompanied by an increased overall event rate of severe hypoglycemia. In the open-label extension, mean HbA(1c) levels decreased rapidly in patients receiving pramlintide for the first time and remained at reduced levels in patients who continued pramlintide treatment. The most common adverse events reported by the pramlintide group were mild nausea and anorexia, which both occurred during the initial weeks of treatment and dissipated over time.

CONCLUSIONS

Mealtime pramlintide treatment as an adjunct to insulin improved long-term glycemic control without inducing weight gain or increasing the overall risk of severe hypoglycemia in patients with type 1 diabetes.

Amylin receptor agonists: a novel pharmacological approach in the management of insulin-treated diabetes mellitus

Amylin is a peptide hormone which is co-secreted with insulin from the pancreatic beta-cell. Type 1 diabetic individuals and some Type 2 diabetic individuals are characterised by amylin deficiency. Animal experiments have revealed several actions of amylin on intermediary metabolism, of these some have been demonstrated to be of potential physiological relevance in humans. In particular amylin appears to have important actions in controlling prandial glucose homeostasis. The peptide hormone inhibits postprandial glucagon secretion and delays gastric emptying thereby modifying postprandial hyperglycaemia in diabetic individuals which presumably adds to overall glycaemic control without a concomitant increase in the number of severe hypoglycaemic episodes. Moreover, amylin acts as a satiety agent. Amylin replacement may therefore improve glycaemic control in diabetes mellitus. However, human amylin exhibits physicochemical properties predisposing the peptide hormone to aggregate and form amyloid fibres, which makes it unsuitable for pharmacological use. A stable analogue, pramlintide, with actions and pharmacokinetic and pharmacodynamic properties similar to the native peptide has therefore been developed. The efficacy and safety of pramlintide administration to diabetic individuals have been tested in a large number of clinical trials. It is the aim of this review to describe possible (patho)physiological actions of amylin as demonstrated in animal and human models, to discuss the background for potential amylin (analogue) replacement in diabetes mellitus and to review results from clinical trials with the amylin receptor analogue pramlintide.

Effects of the amylin analogue pramlintide on hepatic glucagon responses and intermediary metabolism in Type 1 diabetic subjects

AIMS

Hepatic glycogen stores have been shown to be depleted, and glucagon stimulated hepatic glucose production reduced, in Type 1 diabetic subjects. Co-administration of amylin and insulin has been shown to replete hepatic glycogen stores in diabetic animal models. The aim of the present study was to investigate the effect of amylin replacement on hepatic glucagon responsiveness in humans.

METHODS

Thirteen Type 1 diabetic men were studied in a double-blind, placebo-controlled, cross-over study after 4 weeks of subcutaneous pramlintide (30 microg q.i.d.) or placebo administration. Following an overnight fast, plasma glucose was kept above 5 mmol/l (baseline 210-240 min) with an insulin infusion rate of 0.25 mU x kg(-1) x min(-1). To control portal glucagon levels, somatostatin was infused at a rate of 200 microg/h. Basal growth hormone (2 ng x kg(-1) x min(-1)) and glucagon (0.7 ng x kg(-1) x min(-1)) were replaced. Glucagon infusion was increased to 2.1 ng x kg(-1) x min(-1) at 240-360 min (step 1) and to 4.2 ng x kg(-1) x min(-1) at 360-420 min (step 2).

RESULTS

Baseline plasma glucose (5.59+/-0.16 vs. 5.67+/-0.25 mmol/l) and endogenous glucose production (EGP) (1.32+/-0.22 vs. 1.20+/-0.13 mg x kg(-1). min(-1)) were similar and the response to glucagon was unaffected by pramlintide (glucose: step 1; 6.01+/-0.31 vs. 5.94+/-0.38 mmol/l, step 2; 6.00+/-0.37 vs. 5.96+/-0.50 mmol/l, EGP: step 1; 1.91+/-0.18 vs. 1.83+/-0.15 mg x kg(-1) x min(-1), step 2; 2.08+/-0.17 vs. 1.96+/-0.16 ng x kg(-1) x min(-1), pramlintide vs. placebo). Glucose disposal rates were similar at baseline (2.44+/-0.13 vs. 2.28+/-0.09 mg x kg(-1) x min(-1), pramlintide vs. placebo) as well as during the glucagon challenge (P-values all > 0.2).

CONCLUSIONS

Co-administration of pramlintide and insulin to Type 1 diabetic subjects for 4 weeks does not change the plasma glucose or endogenous glucose production response to a glucagon challenge, following an overnight fast. In addition, pramlintide administration does not appear to alter insulin-mediated glucose disposal.

Lack of effect of calcitonin gene-related peptide and amylin on major markers of glucose metabolism in hepatocytes

Effects of amylin and calcitonin gene-related peptide on several processes involved in carbohydrate metabolism were investigated in rat hepatocytes, non-parenchymal cells (Kupffer, Ito and endothelial cells) and alveolar macrophages. In hepatocytes, cAMP levels were increased 25-fold by glucagon (10 nM), less than 2-fold by calcitonin gene-related peptide (100 nM) and not at all by amylin (100 nM). In non-parenchymal cells and cultured alveolar macrophages, calcitonin gene-related peptide potently, and amylin weakly, stimulated cAMP levels. In hepatocytes neither amylin nor calcitonin gene-related peptide affected glycogen phosphorylase activity, glucose output, lactate uptake, glycogen synthesis, glycogen mass or tyrosine aminotransferase activity. The density of calcitonin gene-related peptide specific binding sites in parenchymal cells was 10-fold less then seen in non-parenchymal cells. We found no significant evidence of specific amylin binding sites. These results are consistent with the notion that amylin does not exert a direct effect in hepatocytes. However, we do not rule out that amylin may affect hepatic glucose output indirectly through Cori cycling of lactate derived from skeletal muscle or from interactions through non-parenchymal cells.

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.

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.

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.

What if Minkowski had been ageusic? An alternative angle on diabetes

Despite decades of intensive investigation, the basic pathophysiological mechanisms responsible for the metabolic derangements associated with diabetes mellitus have remained elusive. Explored here is the possibility that traditional concepts in this area might have carried the wrong emphasis. It is suggested that the phenomena of insulin resistance and hyperglycemia might be more readily understood if viewed in the context of underlying abnormalities of lipid metabolism.

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.

Amylin is more potent and more effective than glucagon in raising plasma glucose concentration in fasted, anesthetized rats

Amylin is a 37 amino-acid peptide secreted from the pancreatic beta-cells. It has actions on carbohydrate metabolism in vivo, including elevation of blood glucose. In this study, the hyperglycemic effect of intravenous bolus injections of amylin was compared with similar injections of glucagon in 20-hour fasted rats lightly anesthetized with halothane. Administered doses ranged from 0.01 micrograms to 1000 micrograms (about 7 pmol/kg--750 nmol/kg for amylin and 8 pmol/kg--800 pmol/kg for glucagon). Control animals received an equal volume of saline. A single intravenous injection of amylin or glucagon led to an increase of plasma glucose levels, which peaked approximately at 1 hour after treatment. The calculated ED50 for amylin was 1.48 nmol whereas that for glucagon was 7.46 nmol; the maximum glucose increment was 4.3 mM for amylin, and 2.9 mM for glucagon. These results show that amylin is a more potent and more effective hyperglycemic agent than glucagon under these experimental conditions.