Diabetogenic effects of salmon calcitonin are attributable to amylin-like activity

Mediators of Amylin Action in Metabolic Control

Amylin (also called islet amyloid polypeptide (IAPP)) is a pancreatic beta-cell hormone that is co-secreted with insulin in response to nutrient stimuli. The last 35 years of intensive research have shown that amylin exerts important physiological effects on metabolic control. Most importantly, amylin is a physiological control of meal-ending satiation, and it limits the rate of gastric emptying and reduces the secretion of pancreatic glucagon, in particular in postprandial states. The physiological effects of amylin and its analogs are mediated by direct brain activation, with the caudal hindbrain playing the most prominent role. The clarification of the structure of amylin receptors, consisting of the calcitonin core receptor plus receptor-activity modifying proteins, aided in the development of amylin analogs with a broad pharmacological profile. The general interest in amylin physiology and pharmacology was boosted by the finding that amylin is a sensitizer to the catabolic actions of leptin. Today, amylin derived analogs are considered to be among the most promising approaches for the pharmacotherapy against obesity. At least in conjunction with insulin, amylin analogs are also considered important treatment options in diabetic patients, so that new drugs may soon be added to the only currently approved compound pramlintide (Symlin^®). This review provides a brief summary of the physiology of amylin’s mode of actions and its role in the control of the metabolism, in particular energy intake and glucose metabolism.

Mono and dual agonists of the amylin, calcitonin, and CGRP receptors and their potential in metabolic diseases

BACKGROUND

Therapies for metabolic diseases are numerous, yet improving insulin sensitivity beyond that induced by weight loss remains challenging. Therefore, search continues for novel treatment candidates that can stimulate insulin sensitivity and increase weight loss efficacy in combination with current treatment options. Calcitonin gene-related peptide (CGRP) and amylin belong to the same peptide family and have been explored as treatments for metabolic diseases. However, their full potential remains controversial.

SCOPE OF REVIEW

In this article, we introduce this rather complex peptide family and its corresponding receptors. We discuss the physiology of the peptides with a focus on metabolism and insulin sensitivity. We also thoroughly review the pharmacological potential of amylin, calcitonin, CGRP, and peptide derivatives as treatments for metabolic diseases, emphasizing their ability to increase insulin sensitivity based on preclinical and clinical studies.

MAJOR CONCLUSIONS

Amylin receptor agonists and dual amylin and calcitonin receptor agonists are relevant treatment candidates, especially because they increase insulin sensitivity while also assisting weight loss, and their unique mode of action complements incretin-based therapies. However, CGRP and its derivatives seem to have only modest if any metabolic effects and are no longer of interest as therapies for metabolic diseases.

The Dual Amylin and Calcitonin Receptor Agonist, KBP-066, Induces an Equally Potent Weight Loss Across a Broad Dose Range While Higher Doses May Further Improve Insulin Action

Pharmacological treatment with dual amylin and calcitonin receptor agonists (DACRAs) cause significant weight loss and improvement of glucose homeostasis. In this study, the maximally efficacious dose of the novel DACRA, KeyBiosciencePeptide (KBP)-066, was investigated. Two different rat models were used: high-fat diet (HFD)-fed male Sprague-Dawley rats and male Zucker diabetic fatty (ZDF, fa/fa) rats to determine the maximum weight loss and glucose homeostatic effect, respectively. One acute study and one chronic study was performed in HFD rats. Two chronic studies were performed in ZDF rats: a preventive and an interventive. All studies covered a dose range of 5, 50, and 500 µg/kg KBP-066 delivered by subcutaneous injection. Treatment with KBP-066 resulted in a significant weight reduction of 13%-16% and improved glucose tolerance in HFD rats, which was independent of dose concentration. Dosing with 50 and 500 µg/kg led to a transient but significant increase in blood glucose, both in the acute and the chronic study in HFD rats. All doses of KBP-066 significantly improved glucose homeostasis in ZDF rats, both in the preventive and interventive study. Moreover, dosing with 50 and 500 µg/kg preserved insulin secretion to a greater extent than 5 µg/kg when compared with ZDF vehicle rats. Taken together, these results show that maximum weight loss is achieved with 5 µg/kg, which is within the range of previously reported DACRA dosing, whereas increasing dosing concentration to 50 and 500 µg/kg may further improve preservation of insulin secretion compared with 5 µg/kg in diabetic ZDF rats. SIGNIFICANCE STATEMENT: Here we show that KeyBiosciencePeptide (KBP)-066 induces an equally potent body weight loss across a broad dose range in obese rats. However, higher dosing of KBP-066 may improve insulin action in diabetic rats both as preventive and interventive treatment.

The dual amylin- and calcitonin-receptor agonist KBP-042 increases insulin sensitivity and induces weight loss in rats with obesity

OBJECTIVE

In this study, KBP-042, a dual amylin- and calcitonin-receptor agonist, was investigated as a treatment of obesity and insulin resistance in five different doses (0.625 µg/kg-10 µg/kg) compared with saline-treated and pair-fed controls.

METHODS

Rats with obesity received daily s.c. administrations for 56 days, and glucose tolerance was assessed after one acute injection, 3 weeks of treatment, and again after 7 weeks of treatment. To assess the effect on insulin sensitivity, rats received 5 µg/kg KBP-042 for 21 days before hyperinsulinemic-euglycemic clamp.

RESULTS

KBP-042 induced a sustained weight loss of up to 20% without any significant weight reduction in the pair-fed groups. Decreases in adipose tissues and lipid deposition in the liver were observed, while plasma adiponectin was increased and plasma leptin levels were decreased. Acute administration of KBP-042 led to impaired glucose tolerance and increased plasma lactate, while this diabetogenic effect was reversed by chronic treatment. Finally, assessment of insulin sensitivity using the hyperinsulinemic-euglycemic clamp showed that KBP-042 increased the glucose infusion rate.

CONCLUSIONS

The study indicates that KBP-042 combines two highly relevant features, namely weight loss and insulin sensitivity, and is thus an excellent candidate for chronic treatment of obesity and insulin resistance.

Gut-Brain Endocrine Axes in Weight Regulation and Obesity Pharmacotherapy

In recent years, the obesity epidemic has developed into a major health crisis both in the United States as well as throughout the developed world. With current treatments limited to expensive, high-risk surgery and minimally efficacious pharmacotherapy, new therapeutic options are urgently needed to combat this alarming trend. This review focuses on the endogenous gut-brain signaling axes that regulate appetite under physiological conditions, and discusses their clinical relevance by summarizing the clinical and preclinical studies that have investigated manipulation of these pathways to treat obesity.

Regulation of appetite to treat obesity

Obesity has escalated into a pandemic over the past few decades. In turn, research efforts have sought to elucidate the molecular mechanisms underlying the regulation of energy balance. A host of endogenous mediators regulate appetite and metabolism, and thereby control both short- and long-term energy balance. These mediators, which include gut, pancreatic and adipose neuropeptides, have been targeted in the development of anti-obesity pharmacotherapy, with the goal of amplifying anorexigenic and lipolytic signaling or blocking orexigenic and lipogenic signaling. This article presents the efficacy and safety of these anti-obesity drugs.

Oral salmon calcitonin--pharmacology in osteoporosis

IMPORTANCE OF THE FIELD

Osteoporosis is a slow progressive disease with develops over decades, and where intervention is needed for an extended number of years. This highlights the need for safe intervention possibilities, which have sustained beneficial effects post-treatment.

AREAS COVERED IN THIS REVIEW

Articles on salmon calcitonin appearing on Pubmed from 1960 until today, with focus on a newly developed oral formulation showing increased exposure and efficacy compared with nasal formulation is reviewed. The second half focuses on long-term phenomena, such as bone quality and resolution effects. The final part discusses potential additional benefits of salmon calcitonin.

WHAT THE READER WILL GAIN

Insight into the clinical development of an orally formulated peptide, as well as a detailed understanding of why this approach could revive salmon calcitonin as a treatment for osteoporosis.

TAKE HOME MESSAGE

The oral formulation of salmon calcitonin provides additional benefits and increased efficacy on bone based on Phase I and II clinical trials data, as compared with the nasal formulation. Hence, the results on the ongoing Phase III fracture trial are awaited with great interest.

Current trends in targeting the hormonal regulation of appetite and energy balance to treat obesity

With the eruption of the obesity pandemic over the past few decades, much research has been devoted to understanding the molecular mechanisms by which the human body regulates energy balance. These studies have revealed several mediators, including gut/pancreatic/adipose hormones and neuropeptides that control both short- and long-term energy balance by regulating appetite and/or metabolism. These endogenous mediators of energy balance have been the focus of many anti-obesity drug-development programs aimed at either amplifying endogenous anorexigenic/lipolytic signaling or blocking endogenous orexigenic/lipogenic signaling. Here, we discuss the efficacy and safety of targeting these pathways for the pharmacologic treatment of obesity.

Salmon calcitonin reduces food intake through changes in meal sizes in male rhesus monkeys

Amylinergic mechanisms are believed to be involved in the control of appetite. This study examined the effects of the amylin agonist, salmon calcitonin, on food intake and meal patterns in adult male rhesus monkeys. Fifteen minutes before the onset of their 6-h daily feeding period, monkeys received intramuscular injections of various doses of salmon calcitonin (0.032, 0.056, 0.1, 0.32, and 1 microg/kg) or saline. Salmon calcitonin dose dependently reduced total daily and hourly food intake, with significant decreases at the 0.1, 0.32, and 1 microg/kg doses. Daily food intake was reduced by approximately 35%, 62%, and 96%, at these doses, respectively. An analysis of meal patterns revealed that size of the first meal was significantly reduced across the dose range of 0.056 to 1 microg/kg, while average meal size was reduced with the 0.32 and 1 microg/kg doses. Meal number was only affected at the 1 microg/kg dose. Repeated 5-day administration of the 0.1 microg/kg dose resulted in a reduction in daily food intake only on injection day 2, while significant reductions in food intake were observed on all five injection days with a 0.32 microg/kg dose. Daily food intake was also reduced for 1 day after the termination of the 5-day injections of the 0.32 microg/kg salmon calcitonin dose. These sustained reductions in intake were expressed through decreases in meal size. These data demonstrate that salmon calcitonin acutely and consistently decreases food intake mainly through reductions in meal sizes in nonhuman primates.

Bombesin, but not amylin, blocks the orexigenic effect of peripheral ghrelin

The interaction between ghrelin and bombesin or amylin administered intraperitoneally on food intake and brain neuronal activity was assessed by Fos-like immunoreactivity (FLI) in nonfasted rats. Ghrelin (13 microg/kg ip) increased food intake compared with the vehicle group when measured at 30 min (g/kg: 3.66 +/- 0.80 vs. 1.68 +/- 0.42, P < 0.0087). Bombesin (8 microg/kg) injected intraperitoneally with ghrelin (13 microg/kg) blocked the orexigenic effect of ghrelin (1.18 +/- 0.41 g/kg, P < 0.0002). Bombesin alone (4 and 8 microg/kg ip) exerted a dose-related nonsignificant reduction of food intake (g/kg: 1.08 +/- 0.44, P > 0.45 and 0.55 +/- 0.34, P > 0.16, respectively). By contrast, ghrelin-induced stimulation of food intake (g/kg: 3.96 +/- 0.56 g/kg vs. vehicle 0.82 +/- 0.59, P < 0.004) was not altered by amylin (1 and 5 microg/kg ip) (g/kg: 4.37 +/- 1.12, P > 0.69, and 3.01 +/- 0.78, respectively, P > 0.37). Ghrelin increased the number of FLI-positive neurons/section in the arcuate nucleus (ARC) compared with vehicle (median: 42 vs. 19, P < 0.008). Bombesin alone (4 and 8 microg/kg ip) did not induce FLI neurons in the paraventricular nucleus of the hypothalamus (PVN) and coadministered with ghrelin did not alter ghrelin-induced FLI in the ARC. However, bombesin (8 microg/kg) with ghrelin significantly increased neuronal activity in the PVN approximately threefold compared with vehicle and approximately 1.5-fold compared with the ghrelin group. Bombesin (8 microg/kg) with ghrelin injected intraperitoneally induced Fos expression in 22.4 +/- 0.8% of CRF-immunoreactive neurons in the PVN. These results suggest that peripheral bombesin, unlike amylin, inhibits peripheral ghrelin induced food intake and enhances activation of CRF neurons in the PVN.

Effects in skeletal muscle

The first biological action of amylin to be described was the inhibition of insulin-stimulated incorporation of radiolabeled glucose into glycogen in the isolated soleus muscle of the rat. This antagonism of insulin action in muscle was non-competitive, occurring with equal potency and efficacy at all insulin concentrations. Amylin inhibited activation of glycogen synthase, partially accounting for the inhibition of radiolabeled glucose incorporation. However, this did not account for a low rate of labeling at higher amylin concentrations, wherein the radioglycogen accumulation was even less than in incubations where insulin was absent. The principal action of amylin accounting for reduction of insulin-stimulated accumulation of glycogen was activation of glycogen phosphorylase via a cyclic AMP-, protein kinase C-dependent signaling pathway to cause glycogenolysis (glycogen breakdown). At physiological concentrations, amylin activated glycogen phosphorylase at its ED50, but because glycogen phosphorylase is present in such high activity, the resulting flux out of glycogen was estimated to be similar to insulin-mediated flux of glucosyl moieties into glycogen. Thus, in the rat, endogenous amylin secreted in response to meals appeared to mobilize carbon from skeletal muscle. Amylin-induced glycogenolysis resulted in intramuscular accumulation of glucose-6-phosphate and release of lactate from tissue beds that included muscle. When muscle glycogen was pre-labeled with tritium in the three position, amylin could be shown to evoke the release of free glucose. This is made possible by glucosyl moieties cleaved at the branch points in glycogen being released as free glucose, rather than being phosphorylated, as occurs with the bulk of the glycogen glucosyls. Free glucose is free to exit cells via facilitated transport, down a concentration gradient that might exist under such circumstances. When measured by a sensitive technique utilizing efflux of labeled glucose, amylin was reported to not affect muscle glucose transport. In most of the above respects, amylin behaved similarly to catecholamines in skeletal muscle. The pharmacology of amylin's effects on muscle glycogen metabolism was consistent with a classic amylin pharmacology in whole animals and in isolated soleus muscle. In one cell line, the pharmacology was CGRPergic. Amylin, like insulin, stimulated Na+/K+ ATPase activity and enhanced muscle contractility in vitro.

Inhibition of gastric emptying

In studies aimed at defining the role of amylin in glucose control, elevations of postprandial glucose concentration were blunted in subjects infused with the human amylin analog, pramlintide (Kolterman et al., 1995, 1996). An effect similar to blunt glucose excursions was observed by Brown and others during infusions of amylin in dogs trained to drink glucose (Brown et al., 1994). The effect of pramlintide in humans was present when glucose was administered orally, but not when administered intravenously, suggesting that the effect was due to a deceleration of glucose uptake from the meal, rather than an acceleration of its metabolism (Kolterman et al., 1995). Since amylin did not affect the rate of glucose transit across exteriorized gut loops (Young and Gedulin, 2000), it was proposed that blunting of postprandial glucose profiles could reflect effects on gastric emptying. Rates of gastric emptying have been determined using three different approaches: (1) by measurement of remnant dye found in acutely excised stomachs, (2) by the systemic appearance of labels that are not significantly absorbed until they leave the stomach (e.g., labeled glucose, acetaminophen, 13C-labeled volatiles), and (3) by following the passage of radiolabeled meal components scintigraphically, with a gamma-camera. Amylin and/or pramlintide were shown to potently inhibit gastric emptying by the first method in animals (Clementi et al., 1996; Young et al., 1995a, 1996b), by the second method in animals (Gedulin et al., 1995; Young et al., 1995a, 1996a) and in humans, including those with type 1 and type 2 diabetes (Burrell et al., 2003b; Hücking et al., 2000; Kong et al., 1998; Lee et al., 2000; Vella et al., 2002), and by scintigraphy in patients with type 1 diabetes (Kong et al., 1997, 1998) and in nondiabetic subjects (Samsom et al., 2000). Depending upon dose, responses ranged from a slowing of emptying rate (e.g., by approximately 50%) to a complete cessation. In rats, amylin was 15-fold more potent on a molar basis than glucagon-like peptide-1 (GLP-1) and 20-fold more potent than cholecystokinin octapeptide (CCK-8) for inhibition of gastric emptying (Young et al., 1996b). It was the most potent mammalian peptide of 21 tested for this action (Gedulin et al., 1996b). Amylin inhibition of gastric emptying appears to be mediated by a central mechanism (Clementi et al., 1996; Dilts et al., 1997; Young et al.,2000). An intact vagus nerve (Jodka et al., 1996) and an intact area postrema (Edwards et al., 1998) are required for the effect. In rats that underwent total subdiaphragmatic vagotomy or surgical ablation of the area postrema, amylin was no longer effective at inhibiting gastric emptying (Edwards et al.,1998). The effect of amylin and amylin agonists (including pramlintide) to inhibit gastric emptying was reversed by insulin-induced hypoglycemia (Gedulin and Young, 1998; Gedulin et al., 1997b,c,d; Young et al., 1996a). This suggests the existence of a glucose-sensitive "fail-safe" mechanism that safeguards against severe hypoglycemia; nutrients ingested in response to the hunger that accompanies hypoglycemia can pass rapidly through the stomach for immediate digestion and absorption, unimpaired by the physiological restraint of amylin that would normally prevail at normal glucose concentrations. It seems likely that amylinergic control of gastric emptying is mediated via neurons in the area postrema shown in brain slices to be activated by amylin, and inhibited by low glucose (Riediger et al., 1999). Such neurons have been proposed to mediate glucoprivic gut reflexes (Adachi et al., 1995).

Cardiovascular effects

Amylin can lower blood pressure in anesthetized animals (in which reflex bradycardia is absent), or evoke reflex bradycardia. This effect is likely in response to vasodilatation mediated via calcitonin gene-related peptide (CGRP) receptors, and only occurs at concentrations two to three orders of magnitude higher than physiological amylin concentrations. There is suggestive, but not fully established, evidence for an amylin-like pharmacology with cardiotropic effects, consisting of inotropy (stimulation of contractility) and suppression of secretion of atrial natriuretic peptide (ANP).

Inhibition of insulin secretion

Reports of the effects of amylin and amylin agonists on insulin secretion have varied widely. Some confusion can be attributed to the use of human amylin, which has been shown to readily fall out of solution resulting in low estimates of bioactivity. Some confusion can be resolved by assessing the probability that this had happened. The view taken here, supported by authors using reliable and well-characterized ligands (representing the preponderance of recent studies), is that exogenously administered amylin agonists inhibit insulin secretion, at least partly via activation of an amylin-like receptor linked to Gi-mediated inhibition of cAMP in islets. There may additionally be autonomic extrapancreatic effects of amylin on insulin secretion that derive from its action at the area postrema. Studies with amylin receptor antagonists, including human studies, indicate that endogenously secreted amylin may physiologically inhibit beta-cell secretion (insulin and amylin) via feedback inhibition that is characteristic of many other hormones. Part of this inhibition may be local (paracrine), as indicated by the amylin sensitivity of isolated preparations and the fact that the concentration of secreted products in the islet interstitium can be over 100-fold higher than in the circulation (Bendayan, 1993).

Effects on plasma glucose and lactate

Injection of amylin or amylin agonists, including human and rat amylin, pramlintide, salmon calcitonin, and calcitonin gene-related peptide (CGRP), increases the plasma levels of lactate and glucose in non-diabetic fasting rats and mice. This response can be useful in identifying and defining amylin agonists (amylinomimetic agents) (Cooper et al.) and has been investigated in several studies. Increases in plasma glucose and lactate are not present in all species. In humans, for example, increases in lactate are observed at high pramlintide doses but not at doses that would be used to therapeutically regulate plasma glucose. In species where it occurs, the increase in plasma lactate with amylin is comparable to that observed with exercise or adrenergic agents, and it is distinguishable from the very high levels observed during lactic acidosis (as may occur with biguanides). In contrast to lactic acidosis, the plasma lactate with amylin is derived from skeletal muscle rather than liver. Increases in plasma lactate and glucose in some species may initially appear inconsistent with a glucose-lowering effect of amylin agonists. But glycemic effects are due to actions in skeletal muscle and are present only in some species, whereas glucose-lowering actions are attributable to effects in gastrointestinal systems and are present in all species studied to date. And while glycemic effects are most pronounced in the fasted state, glucose-lowering effects are most pronounced in the postprandial state. Since they were discovered first, effects of higher doses of amylin on plasma glucose, especially in the fasted state, are described first and are related to concomitant changes in plasma lactate. These effects are prominent in rodents but are barely discernible in humans. Effects of lower doses of pramlintide to suppress plasma glucose profiles in the postprandial period are also observable in normal and diabetic rats, however, and are covered here as well. The relationship between plasma lactate and glucose concentrations can be confusing. Via some mechanisms, changes in plasma glucose can drive changes in lactate, while via different mechanisms, changes in lactate can drive changes in glucose concentration. The recursive loop created by these separate links, and for which its discoverers received the Nobel prize, is the Cori cycle (Cori, 1931). This cycle of substrate fluxes, simplified as plasma glucose --> muscle glycogen --> plasma lactate --> liver glycogen --> plasma glucose, is important in the redistribution of carbohydrate fuels in some species (Cori and Cori, 1929) and is discussed here in relation to the role of amylin.

Osteoporosis and diabetes

Osteoporosis is the most prevalent metabolic bone disease in the United States. Although the disease has historically been reported mostly in white women, it can affect individuals of both sexes and all ethnic groups. The presence of osteoporosis related to diabetes is not well acknowledged and the impact of osteoporosis in a diabetic patient is often not considered. Routine screening or initiation of preventive medications for osteoporosis in all patients with type 1 or type 2 diabetes is not recommended at the present time. However, in all patients with diabetes, besides optimal glycemic control, general recommendations regarding adequate dietary calcium intake, regular exercise, and avoidance of other potential risk factors such as smoking should be given. In patients who have positive risk factors for osteoporosis, or in those who present with fractures, evaluation of bone density should be done and respective preventive or therapeutic interventions should be applied.

Unresolved challenges with insulin therapy in type 1 and type 2 diabetes: potential benefit of replacing amylin, a second beta-cell hormone

Current insulin therapy still fails to safely restore near-normoglycemia in the majority of patients. Among the barriers to achieving tight long-term glycemic control with insulin in both type 1 and type 2 diabetes are an increased risk of hypoglycemia, undesired weight gain, and a failure to normalize postprandial hyperglycemia and excessive unpredictable diurnal glucose fluctuations. Amylin is a second beta-cell hormone that is cosecreted with insulin in response to meals, and is deficient in patients with type 1 and insulin-requiring type 2 diabetes. Preclinical studies indicate that amylin acts as a neuroendocrine hormone that complements the effects of insulin in postprandial glucose regulation by suppressing postprandial glucagon secretion and slowing the rate of nutrient delivery from the stomach to the small intestine. Human amylin is not optimal for replacement therapy because of its propensity to aggregate; thus, pramlintide, a soluble, nonaggregating synthetic peptide analog of human amylin, was developed that has potency at least equal to that of human amylin. In clinical studies, subcutaneous injections of pramlintide prior to meals, in addition to insulin therapy, significantly reduced postprandial glucose excursions and lowered HbA(1c) levels in patients with type 1 and type 2 diabetes. The improvement in long-term glycemic control was associated with a significant reduction in body weight and occurred without increases in total daily insulin use or in overall severe hypoglycemia event rates. Because of this unique spectrum of clinical effects, amylin replacement with pramlintide as an adjunctive therapy to insulin is a promising approach that may fulfill some of the unmet clinical needs of insulin-using patients with type 1 and type 2 diabetes.

Amylin receptors mediate the anorectic action of salmon calcitonin (sCT)

The teleost salmon calcitonin (sCT), but not mammalian CT, shows similar biologic actions in the skeletal muscle as amylin and calcitonin gene-related peptide (CGRP). The peptides have also been shown to reduce food intake in rams. Because sCT, but not amylin, binds irreversibly to amylin binding sites, the aim of the present study was to compare the anorectic potency of both peptides. To determine whether sCT reduces food intake through interaction with amylin binding sites, we also tested whether appropriate antagonists (CORP 8-37, AC 187) attenuate the anorectic effect of sCT. Finally, we wanted to know whether rat calcitonin (rCT) and sCT reduce food intake to the same extent. Peptides were injected intraperitoneally at dark onset in 24 h food-deprived rats. At doses of 5 or 0.5 microg/kg, the anorectic effect of sCT was more potent and lasted much longer (e.g. 5 microg/kg: sCT > 10 h; amylin approx. 2 h) than that of amylin. Both CORP 8-37 and AC 187 (10 microg/kg) markedly reduced the anorectic action of sCT (0.5 microg/kg). In contrast to sCT, rCT (0.5 microg/kg) had no effect on food intake. It is concluded that sCT s anorectic effect is partly mediated by amylin receptors. Irreversible binding of sCT to amylin receptors may lead to a stronger and prolonged effect in comparison to amylin due to a sustained activation of the binding sites. Similar to other actions of CTs, the anorectic potency of sCT in rats was higher than that of mammalian (rat) CT. This agrees with binding profiles of amylin, sCT, and rCT at amylin binding sites as observed in in vitro studies.

Characterization of binding sites for amylin, calcitonin, and CGRP in primate kidney

Analysis of receptor distributions for 125I-labeled amylin, 125I-labeled calcitonin, and 125I-labeled calcitonin gene-related peptide (CGRP) in Macaca fascicularis kidney by in vitro autoradiography revealed distinct patterns of binding for each peptide. 125I-rat amylin bound primarily to the cortex, being associated with the distal tubule, including apparent binding to the juxtaglomerular apparatus. 125I-salmon calcitonin displayed high-density binding in the cortex with low-density binding to the medulla. Emulsion autoradiography indicated that binding was associated with both distal tubule and thick ascending limb of the loop of Henle. Intense binding was also found often over juxtaglomerular apparatus. 125I-rat CGRP-alpha exhibited low- to moderate-density binding to the inner medulla/papilla with high-density binding over small-, medium-, and large-caliber arteries. Weak binding to the glomerulus was also seen, but no binding was associated with cortical tubules. Competition binding studies, performed with each of the radioligands, revealed peptide specificity profiles for CGRP and calcitonin receptors that were similar to those described in rat. However, the monkey amylin receptors differed from those in rat, exhibiting relatively higher affinity for calcitonin peptides but reduced affinity for CGRP peptides. These studies suggest potential roles for amylin, calcitonin, and CGRP in primate renal function.

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.

Comparison of the in vitro and in vivo pharmacology of adrenomedullin, calcitonin gene-related peptide and amylin in rats

Adrenomedullin has been reported to be structurally similar to a group of peptides that includes amylin, calcitonin and calcitonin gene-related peptide (CGRP). Human and rat adrenomedullin displaced [125I]CGRP from membranes of SK-N-MC cells (CGRP receptors) with affinities intermediate between those of rat amylin and rat CGRP alpha (Ki values 0.12 +/- 0.06, 0.017 +/- 0.007, 3.83 +/- 1.14 and 0.007 +/- 0.001 nM, respectively). In contrast Ki values for displacement of [125I]rat amylin from accumbens membranes (amylin receptors), and [125I]salmon calcitonin from T47D cells (calcitonin receptors) were lower than with rat amylin or rat CGRP alpha in these preparations (51 +/- 5, 34 +/- 2, 0.024 +/- 0.002, 0.31 +/- 0.07 nM, respectively, at amylin receptors; 33 +/- 5, 69 +/- 29, 2.7 +/- 1.5 and 13 +/- 3 nM, respectively, at calcitonin receptors). In anesthetized rats, the hypotensive potency of adrenomedullin was between that of amylin and CGRP alpha. In contrast, for amylin or calcitonin agonist actions (inhibition of [14C]glycogen formation in soleus muscle, hyperlactemia, hypocalcemia and inhibition of gastric emptying), human adrenomedullin was without measurable effect. Thus, in its binding behaviour and in its biological actions, adrenomedullin appeared to behave as a potent CGRP agonist, but as a poor amylin or calcitonin agonist.

Different pharmacological characteristics in L6 and C2C12 muscle cells and intact rat skeletal muscle for amylin, CGRP and calcitonin

1. We compared the ability of rat amylin, rat calcitonin gene-related peptide (CGRP) and rat and salmon calcitonins to elevate cyclic AMP levels and to inhibit [U-14C]-glucose incorporation into glycogen in insulin-stimulated intact rat soleus muscle and in two cell lines derived from rodent skeletal muscle, L6 and C2C12. 2. In intact soleus muscle, both amylin (EC50S of 0.7-6.1 nM) and salmon calcitonin (EC50S of 0.5-1.4 nM) were more potent than CGRP (EC50S of 5.6-15.8 nM) and were much more potent than rat calcitonin (EC50S of 50-137 nM) at stimulating cyclic AMP production, activating glycogen phosphorylase and inhibiting insulin-stimulated [14C]-glycogen formation. 3. In contrast, in both L6 and C2C12 cells, CGRP (EC50S of 0.042-0.12 nM) stimulated cyclic AMP formation and inhibited insulin-stimulated [U-14C]-glucose incorporation into glycogen approximately 1000 times more potently than amylin (EC50S 34-240 nM), while salmon calcitonin was without measurable effect. 4. There was a correlation between elevation of cyclic AMP and inhibition of insulin-stimulated [U-14C]-glucose incorporation into glycogen evoked by these peptides in both intact muscle (r2 = 0.69, P < 0.0004) and muscle cell lines (r2 = 0.96, P < 0.0001). 5. In conclusion, the effects of amylin, CGRP, and calcitonin on soleus muscle glycogen metabolism appear to be mediated by adenylyl cyclase-coupled receptors which show a pharmacological profile similar to high affinity amylin binding sites that have been previously reported in rat brain. In contrast, the effects of amylin and CGRP in L6 and C2C12 rodent muscle cell lines appear to be mediated by adenylyl cyclase-coupled receptors that behave like CGRP receptors.