Effects of amylin on human osteoblast-like cells

The Gut-Bone Axis in Diabetes

PURPOSE OF REVIEW

To describe recent advances in the understanding of how gut-derived hormones regulate bone homeostasis in humans with emphasis on pathophysiological and therapeutic perspectives in diabetes.

RECENT FINDINGS

The gut-derived incretin hormone glucose-dependent insulinotropic polypeptide (GIP) is important for postprandial suppression of bone resorption. The other incretin hormone, glucagon-like peptide 1 (GLP-1), as well as the intestinotrophic glucagon-like peptide 2 (GLP-2) has been shown to suppress bone resorption in pharmacological concentrations, but the role of the endogenous hormones in bone homeostasis is uncertain. For ambiguous reasons, both patients with type 1 and type 2 diabetes have increased fracture risk. In diabetes, the suppressive effect of endogenous GIP on bone resorption seems preserved, while the effect of GLP-2 remains unexplored both pharmacologically and physiologically. GLP-1 receptor agonists, used for the treatment of type 2 diabetes and obesity, may reduce bone loss, but results are inconsistent. GIP is an important physiological suppressor of postprandial bone resorption, while GLP-1 and GLP-2 may also exert bone-preserving effects when used pharmacologically. A better understanding of the actions of these gut hormones on bone homeostasis in patients with diabetes may lead to new strategies for the prevention and treatment of skeletal frailty related to diabetes.

Calcitonin and Bone Physiology: In Vitro, In Vivo, and Clinical Investigations

Calcitonin was discovered as a peptide hormone that was known to reduce the calcium levels in the systemic circulation. This hypocalcemic effect is produced due to multiple reasons such as inhibition of bone resorption or suppression of calcium release from the bone. Thus, calcitonin was said as a primary regulator of the bone resorption process. This is the reason why calcitonin has been used widely in clinics for the treatment of bone disorders such as osteoporosis, hypercalcemia, and Paget's disease. However, presently calcitonin usage is declined due to the development of efficacious formulations of new drugs. Calcitonin gene-related peptides and several other peptides such as intermedin, amylin, and adrenomedullin (ADM) are categorized in calcitonin family. These peptides are known for the structural similarity with calcitonin. Aside from having a similar structure, these peptides have few overlapping biological activities and signal transduction action through related receptors. However, several other activities are also present that are peptide specific. In vitro and in vivo studies documented the posttreatment effects of calcitonin peptides, i.e., positive effect on bone osteoblasts and their formation and negative effect on osteoclasts and their resorption. The recent research studies carried out on genetically modified mice showed the inhibition of osteoclast activity by amylin, while astonishingly calcitonin plays its role by suppressing osteoblast and bone turnover. This article describes the review of the bone, the activity of the calcitonin family of peptides, and the link between them.

The Activity of Peptides of the Calcitonin Family in Bone

Calcitonin was discovered over 50 yr ago as a new hormone that rapidly lowers circulating calcium levels. This effect is caused by the inhibition of calcium efflux from bone, as calcitonin is a potent inhibitor of bone resorption. Calcitonin has been in clinical use for conditions of accelerated bone turnover, including Paget's disease and osteoporosis; although in recent years, with the development of drugs that are more potent inhibitors of bone resorption, its use has declined. A number of peptides that are structurally similar to calcitonin form the calcitonin family, which currently includes calcitonin gene-related peptides (αCGRP and βCGRP), amylin, adrenomedullin, and intermedin. Apart from being structurally similar, the peptides signal through related receptors and have some overlapping biological activities, although other activities are peptide specific. In bone, in vitro studies and administration of the peptides to animals generally found inhibitory effects on osteoclasts and bone resorption and positive effects on osteoblasts and bone formation. Surprisingly, studies in genetically modified mice have demonstrated that the physiological role of calcitonin appears to be the inhibition of osteoblast activity and bone turnover, whereas amylin inhibits osteoclast activity. The review article focuses on the activities of peptides of the calcitonin family in bone and the challenges in understanding the relationship between the pharmacological effects and the physiological roles of these peptides.

Diabetes pharmacotherapy and effects on the musculoskeletal system

Persons with type 1 or type 2 diabetes have a significantly higher fracture risk than age-matched persons without diabetes, attributed to disease-specific deficits in the microarchitecture and material properties of bone tissue. Therefore, independent effects of diabetes drugs on skeletal integrity are vitally important. Studies of incretin-based therapies have shown divergent effects of different agents on fracture risk, including detrimental, beneficial, and neutral effects. The sulfonylurea class of drugs, owing to its hypoglycemic potential, is thought to amplify the risk of fall-related fractures, particularly in the elderly. Other agents such as the biguanides may, in fact, be osteo-anabolic. In contrast, despite similarly expected anabolic properties of insulin, data suggests that insulin pharmacotherapy itself, particularly in type 2 diabetes, may be a risk factor for fracture, negatively associated with determinants of bone quality and bone strength. Finally, sodium-dependent glucose co-transporter 2 inhibitors have been associated with an increased risk of atypical fractures in select populations, and possibly with an increase in lower extremity amputation with specific SGLT2I drugs. The role of skeletal muscle, as a potential mediator and determinant of bone quality, is also a relevant area of exploration. Currently, data regarding the impact of glucose lowering medications on diabetes-related muscle atrophy is more limited, although preclinical studies suggest that various hypoglycemic agents may have either aggravating (sulfonylureas, glinides) or repairing (thiazolidinediones, biguanides, incretins) effects on skeletal muscle atrophy, thereby influencing bone quality. Hence, the therapeutic efficacy of each hypoglycemic agent must also be evaluated in light of its impact, alone or in combination, on musculoskeletal health, when determining an individualized treatment approach. Moreover, the effect of newer medications (potentially seeking expanded clinical indication into the pediatric age range) on the growing skeleton is largely unknown. Herein, we review the available literature regarding effects of diabetes pharmacotherapy, by drug class and/or by clinical indication, on the musculoskeletal health of persons with diabetes.

Protein aggregates and proteostasis in aging: Amylin and β-cell function

The ubiquitin-proteasomal-system (UPS) and the autophagy-lysosomal-system (ALS) are both highly susceptible for disturbances leading to the accumulation of cellular damage. A decline of protein degradation during aging results in the formation of oxidatively damaged and aggregated proteins finally resulting in failure of cellular functionality. Besides protein aggregation in response to oxidative damage, amyloids are a different type of protein aggregates able to distract proteostasis and interfere with cellular functionality. Amyloids are clearly linked to the pathogenesis of age-related degenerative diseases such as Alzheimer's disease. Human amylin is one of the peptides forming fibrils in β-sheet conformation finally leading to amyloid formation. In contrast to rodent amylin, human amylin is prone to form amyloidogenic aggregates, proposed to play a role in the pathogenesis of Type 2 Diabetes by impairing β-cell functionality. Since aggregates such as lipofuscin and β-amyloid are known to impair proteostasis, it is likely to assume similar effects for human amylin. In this review, we focus on the effects of IAPP on UPS and ALS and their role in amylin degradation, since both systems play a crucial role in maintaining proteome balance thereby influencing, at least in part, cellular fate and aging.

The Effects of Bariatric Surgery on Bone Metabolism

Most metabolic effects following bariatric surgery are favorable. One area in which the consequences seem to be detrimental is on skeletal health. Mechanisms that have been cited include malabsorption of calcium and vitamin D, decrease in mechanical loading, and changes in gastrointestinal and fat-derived hormone levels. It is important that the impact of these procedures on bone metabolism is closely examined. The significance of the bone loss that occurs, and its possible effect on future fracture risk, should also be evaluated.

Diabetes and disordered bone metabolism (diabetic osteodystrophy): time for recognition

Diabetes and osteoporosis are rapidly growing diseases. The link between the high fracture incidence in diabetes as compared with the non-diabetic state has recently been recognized. While this review cannot cover every aspect of diabetic osteodystrophy, it attempts to incorporate current information from the First International Symposium on Diabetes and Bone presentations in Rome in 2014. Diabetes and osteoporosis are fast-growing diseases in the western world and are becoming a major problem in the emerging economic nations. Aging of populations worldwide will be responsible for an increased risk in the incidence of osteoporosis and diabetes. Furthermore, the economic burden due to complications of these diseases is enormous and will continue to increase unless public awareness of these diseases, the curbing of obesity, and cost-effective measures are instituted. The link between diabetes and fractures being more common in diabetics than non-diabetics has been widely recognized. At the same time, many questions remain regarding the underlying mechanisms for greater bone fragility in diabetic patients and the best approach to risk assessment and treatment to prevent fractures. Although it cannot cover every aspect of diabetic osteodystrophy, this review will attempt to incorporate current information particularly from the First International Symposium on Diabetes and Bone presentations in Rome in November 2014.

Bone and mineral metabolism in patients undergoing Roux-en-Y gastric bypass

Despite effective weight reduction, the impact of bariatric surgery on bone is a major concern. Mechanisms include decreased mechanical loading, calcium and vitamin D malabsorption, deficiency in other nutrients, and alterations in fat- and gut-derived hormones. The evidence to support clinical care pathways to prevent bone loss and fractures is at this point weak.

INTRODUCTION

There is a growing concern regarding the potential deleterious impact of bariatric surgery on bone metabolism. This comprehensive review addresses this controversial topic.

METHODS

We reviewed and analyzed articles evaluating bone metabolism and mechanisms for the ensuing putative bone loss in adult patients exclusively undergoing Roux-en-Y gastric bypass (RYGB) surgery, for the period spanning 1942 till September 2012.

RESULTS

Mechanisms identified to contribute to alterations in bone metabolism after bypass surgery include: decreased mechanical loading, calcium and vitamin D malabsorption with secondary hyperparathyroidism, deficiency in other nutrients, in addition to alterations in adipokines, gonadal steroids, and gut-derived hormones favoring bone loss, with the exception of serotonin and glucagon-like peptide-1. The relative contribution of each of these hormones to changes in bone homeostasis after bypass surgery remains undefined. Bone loss reflected by a decline in bone mineral density (BMD) and an increase in bone turnover markers have been reported in many studies, limited for the most part by the exclusive use of dual energy X-ray absorptiometry. Well-designed long-term prospective trials with fractures as an outcome, and studies investigating the magnitude, reversibility, and impact of the observed metabolic changes on fracture outcomes are lacking.

CONCLUSION

Robust conclusions regarding bone loss and fracture outcome after RYGB surgery cannot be drawn at this time. Although not evidence based, baseline evaluation and sequential monitoring with measurement of BMD and calciotropic hormones seem appropriate, with adequate calcium and vitamin D replacement. Beneficial interventions remain unclear.

Amylin exerts osteogenic actions with different efficacy depending on the diabetic status

Amylin displays osteogenic features, but its role in diabetic osteopenia is unclear. We examined the possible osteogenic action of amylin infusion for 3days into fructose-induced insulin-resistant (IR) and streptozotocin-induced type 2 diabetic (T2D) and normal (N) rats. Amylin failed to affect glycaemia or parathyroid hormone levels in any group, but reduced hyperinsulinemia in IR rats. In N rats, amylin increased bone formation rate and reduced osteoclast surface and erosive surface in the femoral metaphysis, and increased osteoprotegerin (OPG)/receptor activator of NFκB ligand (RANKL) mRNA ratio in the tibia. In T2D rats, amylin normalized trabecular structure parameters and increased osteoblast number and osteocalcin (OC) expression in long bones. In contrast, in IR rats, no apparent osteogenic effect of amylin in the femur was observed, although both OC and OPG/RANKL ratio were increased in the tibia. Our findings demonstrate a different osteogenic efficacy of amylin in two diabetic settings.

Review: The diabetic bone: a cellular and molecular perspective

With the increasing worldwide prevalence of diabetes the resulting complications, their consequences and treatment will lead to a greater social and financial burden on society. One of the many organs to be affected is bone. Loss of bone is observed in type 1 diabetes, in extreme cases mirroring osteoporosis, thus a greater risk of fracture. In the case of type 2 diabetes, both a loss and an increase of bone has been observed, although in both cases the quality of the bone overall was poorer, again leading to a greater risk of fracture. Once a fracture has occurred, healing is delayed in diabetes, including nonunion. The reasons leading to such changes in the state of the bone and fracture healing in diabetes is under investigation, including at the cellular and the molecular levels. In comparison with our knowledge of events in normal bone homeostasis and fracture healing, that for diabetes is much more limited, particularly in patients. However, progress is being made, especially with the use of animal models for both diabetes types. Identifying the molecular and cellular changes in the bone in diabetes and understanding how they arise will allow for targeted intervention to improve diabetic bone, thus helping to counter conditions such as Charcot foot as well as preventing fracture and accelerating healing when a fracture does occur.

Reduced amylin levels are associated with low bone mineral density in women with anorexia nervosa

CONTEXT

Anorexia nervosa, characterized by extreme low body weight due to reduced nutrient intake, is associated with severe bone loss. Peptide hormones, including amylin, GIP, and GLP2, are released immediately after nutrient intake and may be involved in the regulation of bone turnover.

OBJECTIVE

To investigate fasting levels of amylin, GIP, and GLP2 and their relationships with bone mineral density (BMD) in women with anorexia nervosa compared to healthy controls.

DESIGN

Cross-sectional.

SETTING

Clinical Research Center.

STUDY PARTICIPANTS

15 women with anorexia nervosa and 16 healthy controls.

INTERVENTION

None.

MAIN OUTCOME MEASURES

Fasting serum amylin, GIP, and GLP2, and BMD.

RESULTS

Women with anorexia nervosa had significantly lower fasting serum amylin and GIP levels than healthy controls. Fasting serum GLP2 levels were not significantly different between groups. Fasting amylin levels were positively associated with BMD and Z-score at the PA spine, total hip, and femoral neck. Fasting amylin levels were also positively associated with weight and percent fat; after controlling for these variables, amylin was still a significant predictor of BMD and Z-score at the femoral neck and of Z-score at the total hip. In the anorexia nervosa group, there was a trend toward an inverse association between amylin and C-terminal telopeptide (CTX) levels (R=-0.47, p=0.08). GIP and GLP2 levels did not predict BMD at any site.

CONCLUSION

Decreased secretion of amylin may be a mechanism through which reduced nutrient intake adversely affects BMD in anorexia nervosa.

Amylin(1-8) is devoid of anabolic activity in bone

Amylin(1-8), a cyclic peptide consisting of the eight N-terminal amino acids of the 37-amino acid peptide amylin, has been shown to induce proliferation of primary osteoblasts and to induce bone formation in healthy male mice, whereas no data on efficacy in bone disease-related models have been reported. Therefore, we evaluated any effects of amylin(1-8) in ovariectomized rats with established osteopenia, a model for postmenopausal osteoporosis. At doses up to 100 nmol/kg/day, a dose highly effective in healthy mice, amylin(1-8) was unable to increase bone mineral density in ovariectomized rats during an 8-week treatment period. Histomorphometric analysis of the tibia indicated that amylin(1-8) did not change bone histomorphometric parameters. In an attempt to verify any potential biological effects of amylin(1-8), we investigated the efficacy of this peptide in various in vitro assays. Experiments designed to confirm previously published results on the proliferative effects of amylin(1-8) on primary osteoblasts failed to show any response. Amylin(1-8) was able to partially displace (125)I-rat amylin(1-37) from amylin receptors composed of the calcitonin receptor and RAMP1, indicating specific interaction of the peptide with the amylin binding site. However, in vitro efficacy assays with amylin(1-8) in calcitonin receptor-RAMP-positive HEK293T and MCF7 cells failed to reveal any agonist activity of amylin(1-8), whereas amylin(1-37) showed the expected agonist activity. In conclusion, our results indicate that amylin(1-8) does not show agonist activity on amylin receptors, does not affect osteoblast proliferation, and is devoid of anabolic activity in bone.

The role of peptides and receptors of the calcitonin family in the regulation of bone metabolism

The 'calcitonin family' is a group of peptide hormones that share structural similarities with calcitonin, and includes calcitonin gene-related peptide (CGRP), amylin, adrenomedullin and adrenomedullin 2 (intermedin). These hormones are produced by different tissues, with calcitonin being produced in thyroid C cells, alphaCGRP predominantly in neural tissue, amylin in beta-islet cells of the pancreas and adrenomedullin in many tissues and cell types. Bone appears to be a common target for all the peptides of the calcitonin family, although the specific bone effects of the peptides vary. Administration of calcitonin produces rapid lowering of serum calcium levels, mainly through inhibition of bone resorption by osteoclasts. In vitro and in a number of animal experimental models, amylin and CGRP are also effective in inhibiting osteoclast activity and bone resorption. Amylin, adrenomedullin and CGRP can also affect cells of the osteoblast lineage, inducing osteoblast proliferation and promoting bone formation. Receptors for the peptides of the calcitonin family are formed by heterodimerization of the calcitonin receptor (CTR) or calcitonin receptor-like receptor (CLR) with receptor activity modifying proteins (RAMPs). Although the different combinations of these proteins create receptors with distinct ligand specificities, there is a degree of cross-reactivity and the receptors are able to bind other ligands from the family, usually with lower affinity. Analysis of the expression of the receptors for the calcitonin family in 16 samples of human osteoblasts showed high levels of CLR and RAMP1, low levels of RAMP2 and no expression of RAMP3 or CTR. Recent studies of the bone phenotype of knockout animals lacking the calcitonin, alphaCGRP or amylin gene indicated that in this experimental system the main physiological role of amylin in bone is the inhibition of bone resorption, that of CGRP is the activation of bone formation, while calcitonin, unexpectedly appears to be inhibiting bone formation without affecting bone resorption. Further investigations will be required to determine the mechanisms of action of calcitonin peptides in bone and their significance to human bone physiology.

The role of amylin and related peptides in osteoporosis

Osteoporosis is a systemic skeletal disorder that remains a major public health problem due to significant fracture-associated morbidity and mortality. Because it has been shown that individuals having type I diabetes mellitus also suffer from osteopenia or osteoporosis, there is probably a pathophysiological mechanism that links pancreatic beta cell insufficiency with inappropriate bone formation. Many factors have been suggested, including amylin, a product of pancreatic beta cells with structural and functional similarity to calcitonin. Amylin has been shown to stimulate bone development via action on osteoblasts and osteoclasts. Recently, amylin receptors have been identified as complexed calcitonin receptor with receptor activity modifying proteins. Moreover, a synthetic amylin analogue (pramlintide) has been developed for clinical use. These findings including results from in vitro animal and human studies suggest a role for amylin as a potential diagnostic and therapeutical tool in patients with various bone diseases including osteoporosis. However, other structurally and functionally related hormones that affect bone metabolism should also be taken in account including calcitonin, calcitonin gene-related peptide and adrenomedullin.

Human osteoblast-like cell proliferation induced by calcitonin-related peptides involves PKC activity

The calcitonin peptides [calcitonin (CT), calcitonin gene-related peptide (CGRP), amylin] share many biological actions, including activity on bone cells. In the present study, CT (10(-11) to 10(-9) M) stimulated [(3)H]thymidine incorporation in primary cultures of human osteoblasts (hOB), as already demonstrated for CGRP and amylin. RT-PCR analysis showed that the calcitonin receptor and the calcitonin receptor-like receptor are both expressed in hOB. In these cells, CT (10(-10) M) and amylin (10(-9) M), in contrast to CGRP (10(-8) M), did not increase cAMP production. All three peptides stimulated protein kinase C (PKC) activity. To evaluate PKC involvement in hOB proliferation, cells were incubated with phorbol 12,13-dibutyrate, a stimulator of PKC activity; cell proliferation was increased in a dose-dependent manner (EC(50) = 3.4 x 10(-8) M). Staurosporine (10(-9) M), a PKC inhibitor, blocked phorbol 12,13-dibutyrate-induced PKC activity and cell proliferation. Inhibition of PKC by staurosporine also counteracted the stimulatory effect of CT, CGRP, and amylin on hOB proliferation. From these data, it is deduced that the activation of PKC is important for hOB proliferation and that it is involved in the anabolic effect of CT peptides on bone.

Adrenomedullin stimulates the growth of cultured normal human osteoblasts as an autocrine/paracine regulator

Adrenomedullin (AM) is a 52 amino acid peptide that is synthesized in a variety of tissues, including the vessels and bones. This study showed that normal human osteoblast (NHOst) secreted immunoreactive AM and that AM stimulated intracellular cAMP production in these cells. An anti-AM monoclonal antibody, which inhibited endogenous AM, caused the number of NHOst to decrease. The effect of a low concentration AM was inhibited by addition of a cAMP-dependent protein kinase A inhibitor (H89). These data suggest that AM is an autocrine or paracrine regulator that promotes the proliferation of NHOst via the cAMP pathway.

Effects of calcitonin gene-related peptide and amylin on human osteoblast-like cells proliferation

Expression of mRNA for calcitonin gene-related peptide (CGRP) and CGRP receptor has been detected in osteoblasts indicating that CGRP could play a role in bone metabolism. In the present study, we evaluated the effect of CGRP on primary culture of human osteoblast-like cells proliferation. The peptide was able to stimulate [3H]thymidine incorporation in human osteoblast-like cells with a maximal effect at 10(-8) M. The proliferating activity of CGRP was not inhibited by the two antagonists, CGRP-(8-37) or amylin-(8-37), whereas amylin fragment antagonized the proliferating activity of amylin. In human osteoblast-like cells CGRP, but not amylin, was able to stimulate adenylyl cyclase activity and this effect was completely antagonized only by CGRP-(8-37) and not by amylin-(8-37). These data suggest that the CGRP induced stimulation of cAMP is not involved in the peptide proliferating effect in human osteoblast-like cells and that in this cell population there are receptor subtypes for CGRP, distinct from that of amylin.

Systemic administration of a novel octapeptide, amylin-(1---8), increases bone volume in male mice

Amylin increases bone mass when administered systemically to mice. However, because of its size, the full peptide is not an ideal candidate for the therapy of osteoporosis. The fragment, amylin-(1---8), stimulates osteoblast proliferation in vitro, although it is without effect on carbohydrate metabolism. The present study assessed the effects of daily administration of this peptide on sexually mature male mice for 4 wk. Amylin-(1---8) almost doubled histomorphometric indices of osteoblast activity but did not change measures of bone resorption. Trabecular bone volume increased by 36% as a result of increases in both trabecular number and trabecular thickness, and tibial cortical width increased by 8%. On three-point bending tests of bone strength, displacement to fracture was increased by amylin-(1---8), from 0.302 +/- 0.013 to 0.351 +/- 0. 017 mm (P = 0.02). In a separate experiment using dynamic histomorphometry with bone-seeking fluorochrome labels, amylin-(1---8) was administered by local injection over the calvariae of female mice. Amylin-(1---8) (40 nM) increased the double-labeled surface threefold. The effect was dose dependent from 0.4 to 40 nM and was greater than that of an equimolar dose of human parathyroid hormone-(1---34) [hPTH-(1---34)]. Mineral apposition rate was increased by 40 nM amylin-(1---8) but not by hPTH-(1---34). Amylin-(1---8) thus has significant anabolic effects in vivo, suggesting that this peptide or analogs of it should be further evaluated as potential therapies for osteoporosis.

Systemic administration of amylin increases bone mass, linear growth, and adiposity in adult male mice

Amylin is a peptide hormone cosecreted with insulin from the pancreatic beta-cells that can act as an osteoblast mitogen and as an inhibitor of bone resorption. The effects on bone of its systemic administration are uncertain. The present study addresses this question in adult male mice that were given daily subcutaneous injections of amylin (10.5 microgram) or vehicle (n = 20 in each group) for 4 wk. Histomorphometric indices of bone formation increased 30-100% in the amylin-treated group, whereas resorption indices were reduced by approximately 70% (P < 0.005 for all indices). Total bone volume in the proximal tibia was 13.5 +/- 1.4% in control animals and 23.0 +/- 2.0% in those receiving amylin (P = 0.0005). Cortical width, tibial growth plate width, tibial length, body weight, and fat mass were all increased in the amylin-treated group. It is concluded that systemic administration of amylin increases skeletal mass and linear bone growth. This peptide has potential as a therapy for osteoporosis if its bone effects can be dissociated from those on soft tissue mass.

Dissociation of the effects of amylin on osteoblast proliferation and bone resorption

This study assesses the structure-activity relationships of the actions of amylin on bone. In fetal rat osteoblasts, only intact amylin and amylin-(1-8) stimulated cell proliferation (half-maximal concentrations 2.0 x 10(-11) and 2.4 x 10(-10) M, respectively). Amylin-(8-37), COOH terminally deamidated amylin, reduced amylin, and reduced amylin-(1-8) (reduction results in cleavage of the disulfide bond) were without agonist effect but acted as antagonists to the effects of both amylin and amylin-(1-8). Calcitonin gene-related peptide-(8-37) also antagonized the effects of amylin and amylin-(1-8) on osteoblasts but was substantially less potent in this regard than amylin-(8-37). In contrast, inhibition of bone resorption in neonatal mouse calvariae only occurred with the intact amylin molecule and was not antagonized by any of these peptides. The rate of catabolism of the peptides in calvarial cultures was not accelerated in comparison with that of intact amylin. This dissociation of the actions of amylin suggests that it acts through two separate receptors, one on the osteoclast (possibly the calcitonin receptor) and a second on the osteoblast.