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Biomedical subjects

Andrew Young

Publications and source records attributed to Andrew Young.

At least 19 recordsLinked to original sources
PubMed2006

Investigation of exenatide elimination and its in vivo and in vitro degradation.

Exenatide is a 39 amino acid incretin mimetic for the treatment of type 2 diabetes, with glucoregulatory activity similar to glucagon-like peptide-1 (GLP-1). Exenatide is a poor substrate for the major route of GLP-1 degradation by dipeptidyl peptidase-IV, and displays enhanced pharmacokinetics and in vivo potency in rats relative to GLP-1. The kidney appears to be the major route of exenatide elimination in the rat. We further investigated the putative sites of exenatide degradation and excretion, and identified primary degradants. Plasma exenatide concentrations were elevated and sustained in renal-ligated rats, when compared to sham-operated controls. By contrast, exenatide elimination and degradation was not affected in rat models of hepatic dysfunction. In vitro, four primary cleavage sites after amino acids (AA)-15, -21, -22 and -34 were identified when exenatide was degraded by mouse kidney membranes. The primary cleavage sites of exenatide degradation by rat kidney membranes were after AA-14, -15, -21, and -22. In rabbit, monkey, and human, the primary cleavage sites were after AA-21 and -22. Exenatide was almost completely degraded into peptide fragments <3 AA by the kidney membranes of the species tested. The rates of exenatide degradation by rabbit, monkey and human kidney membranes in vitro were at least 15-fold slower than mouse and rat membranes. Exenatide (1-14), (1-15), (1-22), and (23-39) were not active as either agonists or antagonists to exenatide in vitro. Exenatide (15-39) and (16-39) had moderate-to-weak antagonist activity compared with the known antagonist, exenatide (9-39). In conclusion, the kidney appears to be the primary route of elimination and degradation of exenatide.

Animals↗
PubMed2005

Dynamic activation of cystic fibrosis transmembrane conductance regulator by type 3 and type 4D phosphodiesterase inhibitors.

The diseases of cystic fibrosis, chronic obstructive pulmonary disease (COPD), and chronic bronchitis are characterized by mucus-congested and inflamed airways. Anti-inflammatory agents that can simultaneously restore or enhance mucociliary clearance through cystic fibrosis transmembrane conductance regulator (CFTR) activation may represent new therapeutics in their treatment. Herein, we report the activation of CFTR-mediated chloride secretion by phosphodiesterase (PDE) 4 inhibitors in T84 monolayer using (125)I anion as tracer. In the absence of forskolin, the iodide secretion was insensitive to PDE4 inhibitor L-826,141 [4-[2-(3,4-bis-difluoromethoxyphenyl)-2-[4-(1,1,1,3,3,3-hexafluoro-2-hydroxypropan-2-yl)phenyl]-ethyl]-3-methylpyridine-1-oxide], roflumilast, or to PDE3 inhibitor trequinsin. However, these inhibitors potently augmented iodide secretion after forskolin stimulation, with efficacy coupled to the activation states of adenylyl cyclase. The iodide secretion from PDE3 or PDE4 inhibition was characterized at first by a prolonged efflux duration, followed by progressively elevated peak efflux rates at higher inhibitor concentrations. Paralleled with an increased phosphor-cAMP response element-binding protein formation, the CFTR activation dissociated from a global cAMP elevation and was blocked by H89 [N-[2-((p-bromocinnamyl)amino)ethyl]-5-isoquinolinesulfonamide]. 2-(4-Fluorophenoxy)-N-[(1S)-1-(4-methoxyphenyl)ethyl]nicotinamide, a stereoselective PDE4D inhibitor, augmented iodide efflux more efficiently than its less potent (R)-isomer. The peak efflux from maximal PDE4 and PDE3 inhibition matched that from full adenylyl cyclase activation. These data suggest that PDE3 and PDE4 (mainly PDE4D) form the major cAMP diffusion barrier in T84 cells to ensure a compartmentalized CFTR signaling. Together with their potent anti-inflammatory properties, the potentially enhanced airway mucociliary clearance from CFTR activation may have contributed to the efficacy of PDE4 inhibitors in COPD and asthmatic patients. PDE4 inhibitors may represent new opportunities to combat cystic fibrosis and other respiratory diseases in future.

3',5'-Cyclic-AMP Phosphodiesterases↗
PubMed2005

Effect of ethanol administration on Mg2+ transport across liver plasma membrane.

Acute and chronic ethanol administration results in a decrease in cellular Mg2+ content and an alteration of Mg2+ transport in liver cells. In this study we investigated the extent to which ethanol affects the Mg2+ transport mechanisms in the liver cell membrane. The functionality of these transport mechanisms was assessed in plasma membrane vesicles purified from livers acutely perfused with varying concentrations of alcohol, livers of animals fed with 6% ethanol for 3 weeks, and the respective controls. Acute alcohol administration had little or no effect on the Mg2+ extrusion mechanisms present in the apical and basolateral domains of the hepatocyte but completely impaired the Mg2+ entry mechanism present in the basolateral side of the cell. This effect was already evident at a dose of alcohol as small as 0.01% (approximately 1.5 mM). The chronic administration of ethanol, instead, impaired all the Mg2+ transport mechanisms irrespective of the location and directionality in a time-dependent manner. Taken together, these data indicate a selective sensitivity of the Mg2+ entry mechanism to acute alcohol administration, whereas the Mg2+ extrusion mechanisms are affected only after prolonged exposure to alcohol. These results suggest that the defect in hormone-activated Mg(2+) transport observed in the chronic EtOH model [Young, A., Cefaratti, C., & Romani, A. (2003). Chronic EtOH administration alters liver Mg2+ homeostasis. Am J Physiol 284, G57-G67] depends not only on a reduced cellular Mg2+ content but also on the impaired Mg2+ transport mechanisms present in the hepatocyte plasma membrane, in particular the Mg2+ entry pathway, which prevents the liver cell from restoring cellular Mg2+ homeostasis.

Animals↗
PubMed2005

Apolipoprotein D is down-regulated during malignant transformation of neurofibromas.

Apolipoprotein D (apoD) expression was studied in nonneoplastic peripheral nerve, neurofibromas (NFs), and malignant peripheral nerve sheath tumors (MPNSTs) by quantitative polymerase chain reaction, in situ hybridization, and immunohistochemistry. Multiplex quantitative polymerase chain reaction for messenger RNA was performed on a series of formalin-fixed and paraffin-embedded specimens that included 9 MPNSTs, 12 NFs, and 4 normal peripheral nerves. The average apoD expression was 108-fold decreased (DeltaCt = -7.3) in the MPNSTs compared with the NFs (P < .05). ApoD expression levels were 3.0-fold elevated (DeltaCt = 1.7) in the NFs compared with nonneoplastic peripheral nerve (P < .05). In situ hybridization for apoD RNA was performed on a separate series of 10 cases in which each microscopic section included both MPNST and the NF from which it arose. These studies confirmed elevated apoD expression in NFs compared with MPNSTs and demonstrated that this expression was variable among individual cells within the NFs. Differential expression by immunohistochemistry could only be demonstrated in selected areas, most likely because apoD protein is a small molecule that is secreted out of the cell into the extracellular space and plasma. ApoD expression initially increases a small amount with the formation of NFs from nonneoplastic peripheral nerve and subsequently decreases markedly as NFs transform into MPNSTs. This expression pattern may serve as a marker for cell cycle inhibition during peripheral nerve tumorigenesis.

Apolipoproteins↗
PubMed2005

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.

Amyloid↗
PubMed2005

Receptor pharmacology.

Despite clear evidence for a distinct amylin pharmacology and localization of such pharmacology to sites such as the nucleus accumbens,efforts to clone an amylin receptor were fruitless for over a decade. This enigma led many to doubt the status of amylin as a bona fide hormone. Yet it became apparent during those cloning efforts that, whatever the amylin receptor was, it was somehow similar to a calcitonin receptor. The enigma of the amylin receptor was solved following the identification of receptor activity modifying proteins (RAMPs). These single transmembrane spanning molecules, when associated with a calcitonin receptor, altered its pharmacology from calcitonin-preferring to amylin-preferring. With at least two forms of the calcitonin receptor and three forms of RAMP, there is the potential for six subtypes of amylin receptors. Of these, two appear to predominate. The CTa (shorter form) calcitonin receptor, dimerized with RAMP1 [amylin 1 (a) receptor], appears to represent binding sites at the nucleus accumbens and the subfornical organ. Binding sites at area postrema appear to be composed of CTa + RAMP3 [amylin3 (a) receptors]. Thus far, RAMP proteins have been associated in vivo only with the CT/CLR receptor system. It is presently unknown whether RAMPs are more general modulators of receptor function, dynamically modifying responsivity with time or across other receptor classes. The largest and first identified amylin-binding field was in the nucleus accumbens. The function of these receptors is yet undetermined, but because the nucleus accumbens is within the blood-brain barrier, the cognate ligand is unlikely to be circulating amylin. Dense amylin binding is present at the circumventricular organs, including the subfornical organ, the organum vasculosum lateralis terminalis (OVLT), and the area postrema. There is no diffusional (blood-brain) barrier at these structures, so they most likely respond to circulating (beta-cell-derived) amylin. Despite pharmacological evidence of amylin sensitivity in several peripheral tissues, selective amylin binding outside of the brain is observed only in the renal cortex. The newly designated amylinomimetic drug class was defined on the basis of its unique pharmacology prior to the molecular characterization of amylin receptors. Currently, the class includes any agent that acts as antagonist at characterized amylin receptors. Several peptides, typically analogs of truncated salmon calcitonin, have been developed as potent and selective amylin antagonists and have been useful in identifying amylinergic responses. Of these, AC187 (30Asn32Tyr[8-32]sCT; Amylin Pharmaceuticals Inc.) is particularly selective and potent, and has been most often cited in studies using amylin antagonists. Antagonism of a response with an order of potency of AC187> AC66 > CGRP[8-37] is suggestive that it is mediated via amylin receptors. Activation of a response with salmon calcitonin (sCT) > amylin >calcitonin gene-related peptide (CGRP) > mammalian CT suggests activation via the amylinl (a) receptor, while sCT = amylin >> CGRP >mammalian CT suggests activation via amylin3 (a) receptors. Absence of response to other ligands (e.g., adrenomedullin) is useful for excluding certain pharmacologies.

Animals↗
PubMed2005

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).

Amyloid↗
PubMed2005

Inhibition of food intake.

Over 100 publications, principally from five groups, describe an effect of amylin and amylin analogs in inhibition of food intake in animals and humans. The major groups contributing to this area are those of the following: Chance and Balasubramaniam (Balasubramaniam et al., 1991a,b; Chance et al., 1991a,b, 1992a,b, 1993). Morley, Flood, and Edwards (Edwards and Morley, 1992; Flood and Morley, 1992; Macintosh et al., 2000; Morley and Flood, 1991, 1994; Morley et al., 1992, 1993, 1994, 1995, 1996, 1997). Lutz, Geary, and others (Barth et al., 2003; Del Prete et al., 2002; Lutz et al., 1994, 1995a,b, 1996a,b, 1997a,b, 1998a,b,c, 2000a,b, 2001a,b,c, 2003; Mollet et al., 2001, 2003a,b, 2004; Riediger et al., 2002, 2004; Rushing et al., 2000a,b, 2001, 2002). Workers at Amylin Pharmaceuticals Inc., or their collaborators (Bhavsar et al., 1995, 1996, 1997a, 1998; Birkemo et al., 1995; Chapman et al., 2004a,b; Edwards et al., 1998; Feinle et al., 2002; Mack et al., 2003; Riediger et al., 1999; Roth et al., 2004; Watkins et al., 1996; Weyer et al., 2004; Young, 1997; Young and Bhavsar, 1996). Arnelo, Reidelberger, and others (Arnelo et al., 1996a,b, 1997a,b, 1998, 2000; Fruin et al., 1997; Granqvist et al., 1997; Reidelberger et al., 2001, 2002, 2004). The magnitude of amylin inhibition of food intake, and its potency for this effect when delivered peripherally, suggests a physiological role in satiogenesis. Increases in food intake following disruption of amylin signal-signaling (e.g., with amylin receptor blockade, or with amylin gene knock-out mice) further support a role of endogenous amylin to tonically restrict nutrient intake. In addition, synergies with other endogenous satiety agents may be present, and convey greater physiological importance than is conveyed by single signals. The anorectic effect of amylin is consistent with a classic amylin pharmacology. The anorectic effect of peripheral amylin appears principally due to a direct action at the area postrema/nucleus tractus solitarius, and is not merely a consequence of gastric fullness, for example. Circulating amylin appears to physiologically inhibit secretion of ghrelin, an orexigenic peptide from the stomach. In contrast to the actions of many other anorexigens, amylin appears to stimulate drinking. This disposgenic effect is likely mediated via amylin-sensitive neurones in the subfornical organ, a circumventricular structure, that like the area postrema does not present a blood-brain barrier. Amylin's dipsogenic effect may explain prandial drinking, which has heretofore been regarded as a learned behavior.

Amyloid↗
PubMed2005

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).

Amyloid↗
PubMed2005

Effects on digestive secretions.

Rat amylin subcutaneously injected into rats dose-dependently inhibits pentagastrin-stimulated gastric acid secretion and protects the stomach from ethanol-induced gastritis. The ED50s for these actions (0.050 and 0.036 microg, respectively) are the lowest for any dose-dependent effect of amylin thus far described, and their similar potencies are consistent with a mechanistic (causal) association. At higher amylin doses, inhibition of gastric acid secretion was almost complete (93.4%). Gastric injury (measured by a subjective analog scale) was inhibited by up to 67%. The observation that effective doses of amylin result in plasma concentrations of 7-10 pM (i.e., within the reported range; Pieber et al., 1994) supports the interpretation that inhibition of gastric acid secretion and maintenance of gastric mucosal integrity are physiological actions of endogenous amylin. The pharmacology of these responses fits with one mediated via amylin-like receptors. Rat amylin inhibited CCK-stimulated secretion of pancreatic enzymes,amylase, and lipase by up to approximately 60% without having significant effect in the absence of CCK. ED50s for the effect were in the 0.1-0.2 microg range, calculated to produce plasma amylin excursions within the physiological range. Effects of informative ligands are consistent with the concept of amylin receptor mediation. Amylin was effective in ameliorating the severity of pancreatitis in a rodent model. The amylin analog pramlintide inhibited gallbladder emptying in mice as measured by total weight of acutely excised gallbladders. Amylin inhibition of gastric acid secretion, pancreatic enzyme secretion, and bile secretion likely represents part of an orchestrated control of nutrient appearance. Modulation of digestive function fits with a general role of amylin in regulating nutrient uptake. Rate of ingestion, rate of release from the stomach, and rate of digestion of various food groups appear to be under coordinate control.

Amyloid↗
PubMed2005

Inhibition of glucagon secretion.

This chapter describes a physiological and profound effect of amylin to inhibit meal-related glucagon secretion. Glucagon is processed from a large precursor, proglucagon, in a tissue-specific manner in pancreatic alpha-cells. In addition to amino acid nutrient stimuli, glucagon is also secreted in response to stressful stimuli, such as hypoglycemia and hypovolemia. Glucagon primarily acts on liver to initiate glycogenolysis and gluconeogenesis, resulting in a rapid increase in endogenous production of glucose. With longer stimulation, glucagon action at the liver results in a glucose-sparing activation of free fatty acid oxidation and production of ketones. During hypoglycemia, glucagon secretion is clearly a protective feed-back, defending the organism against damaging effects of low glucose in brain and nerves (neuroglycopenia). Amino acid-stimulated glucagon secretion during meals has a different purpose: amino acids stimulate insulin secretion, which mobilizes amino acid transporters and effects their storage in peripheral tissues. At the same time, insulin obligatorily recruits GLUT4 glucose transporters in muscle and fat. The hypoglycemic potential of such GLUT4 mobilization is averted only by the simultaneous liberation of endogenous glucose in response to feedforward (anticipatory) glucagon secretion. The effect of amylin and its agonists to inhibit amino acid-stimulated glucagon secretion is both potent (EC50 = 18 pM) and profound (approximately 70% inhibition). This glucagonostatic action appears to be extrinsic to the pancreatic islet, occurring in intact animals and in patients, but not in isolated islets or isolated perfused pancreas preparations. On the other hand, the effect of hypoglycemia to stimulate glucagon secretion, which is intrinsic to the islet and occurs in isolated preparations, is not affected by amylin or its agonists. The physiological interpretation of these actions fits with the general concept, illustrated in Fig. 1, that amylin and insulin secreted in response to meals shut down endogenous production as a source of glucose, in favor of that derived from the meal. Amylin and insulin secreted in response to nutrients already absorbed act as a feedback switch for glucose sourcing. The insulinotropic (incretin) gut peptides, GLP-1 and GIP, secreted in response to yet-to-be-absorbed intraluminal nutrients, amplify beta-cell secretion and thereby activate the glucose sourcing switch in a feedforward manner. Hypoglycemia-stimulated glucagon secretion and nutrient (amino acid)-stimulated glucagon secretion are two clearly different processes, differently affected by amylin. The balance of glucose fluxes is disturbed in diabetic states, partly as a result of inappropriate glucagon secretion. Although glucose production due to glucagon secreted in response to hypoglycemia is normal or even reduced in diabetic patients, the secretion of glucagon (and production of endogenous glucose) in response to protein meals is typically exaggerated. Absence of appropriate beta-cell suppression of alpha-cell secretion has been invoked as a mechanism that explains exaggerated glucagon responses, especially prevalent in patients with deficient beta-cell secretion (type 1 diabetes and insulinopenic type 2 diabetes). A proposed benefit of insulin replacement therapy is the reduction of absolute or relative hyperglucagonemia. High glucagon is said to be necessary for ketosis in severe forms of diabetes. A further benefit of reversing hyperglucagonemia is reduction of the excessive endogenous glucose production that contributes to fasting and postprandial hyperglycemia in diabetes. The idea that amylin is a part of the beta-cell drive that normally limits glucagon secretion after meals fits with the observation that glucagon secretion is exaggerated in amylin-deficient states (diabetes characterized by beta-cell failure). This proposal is further supported by the observation that postprandial glucagon suppression is restored following amylin replacement therapy in such states. These observations argue for a therapeutic case for amylin replacement in patients in whom excess glucagon action contributes to fasting and postprandial hyperglycemia and ketosis. The selectivity of amylin's glucagonostatic effect (wherein it is restricted to meal-related glucagon secretion, while preserving glucagon secretion and glucagon action during hypoglycemia) may confer additional benefits; the patient population amenable to amylin replacement therapy is likely to also be receiving insulin replacement therapy, and is thereby susceptible to insulin-induced hypoglycemia. Most explorations of the biology of amylin have used the endogenous hormone in the cognate species (typically rat amylin in rat studies). Clinical studies have typically employed the amylinomimetic agent pramlintide. Studies of amylinomimetic effects on glucagon secretion include effects of rat amylin in anesthetized non-diabetic rats (Jodka et al., 2000; Parkes et al., 1999; Young et al., 1995), effects of rat amylin in isolated perfused rat pancreas (Silvestre et al., 1999), effects of pramlintide in anesthetized non-diabetic rats (Gedulin et al., 1997b,c,d, 1998), effects of pramlintide in patients with type l diabetes (Fineman et al., 1997a,b,c,d, 1998a; Holst, 1997; Nyholm et al., 1996, 1997a,b,c; Orskov et al., 1999; Thompson and Kolterman, 1997), and effects in patients with type 2 diabetes (Fineman et al., 1998b). In addition, effects of amylin antagonists have been observed in isolated preparations (Silvestre et al., 1996), and effects of antagonists or neutralizing antibody have been determined in whole-animal preparations (Gedulin et al., 1997a,e,f).

Amyloid↗
PubMed2005

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).

Amyloid↗
PubMed2005

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.

Amyloid↗
PubMed2005

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.

Amyloid↗
PubMed2005

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.

Amyloid↗
PubMed2005

Effects in fat.

Biological actions in adipocytes can be mediated directly via circulating hormones or indirectly via sympathetic innervation of fat. Direct effects can be usefully assessed in preparations in which adipocytes have been dissociated from each other, and are therefore directly exposed to substrate and hormones. Amylin appears to have no direct effects in isolated adipocytes.

Adipocytes↗
PubMed2005

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).

Amyloid↗