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Context: Glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide are incretins secreted from enteroendocrine cells postprandially in part to regulate glucose homeostasis. Dysregulation of these hormones is evident in type 2 diabetes mellitus (T2DM). Two new drugs, exenatide (GLP-1 mimetic) and sitagliptin [dipeptidyl peptidase (DPP) 4 inhibitor], have been approved by regulatory agencies for treating T2DM. Liraglutide (GLP-1 mimetic) and vildagliptin (DPP 4 inhibitor) are expected to arrive on the market soon., Evidence Acquisition: The background of incretin-based therapy and selected clinical trials of these four drugs are reviewed. A MEDLINE search was conducted for published articles using the key words incretin, glucose-dependent insulinotropic polypeptide, GLP-1, exendin-4, exenatide, DPP 4, liraglutide, sitagliptin, and vildagliptin., Evidence Synthesis: Exenatide and liraglutide are injection based. Three-year follow-up data on exenatide showed a sustained weight loss and glycosylated hemoglobin (HbA(1c)) reduction of 1%. Nausea and vomiting are common. Results from phase 3 studies are pending on liraglutide. Sitagliptin and vildagliptin are orally active. In 24-wk studies, sitagliptin reduces HbA(1c) by 0.6-0.8% as monotherapy, 1.8% as initial combination therapy with metformin, and 0.7% as add-on therapy to metformin. Vildagliptin monotherapy lowered HbA(1c) by 1.0-1.4% after 24 wk. Their major side effects are urinary tract and nasopharyngeal infections and headaches. Exenatide and liraglutide cause weight loss, whereas sitagliptin and vildagliptin do not., Conclusions: The availability of GLP-1 mimetics and DPP 4 inhibitors has increased our armamentarium for treating T2DM. Unresolved issues such as the effects of GLP-1 mimetics and DPP 4 inhibitors on beta-cell mass, the mechanism by which GLP-1 mimetics lowers glucagon levels, and exactly how DPP 4 inhibitors lead to a decline in plasma glucose levels without an increase in insulin secretion, need further research.

Objective: Glucagon-like peptide-1 (GLP-1) (7-36) amide is a glucoregulatory hormone with insulinotropic and insulinomimetic actions. We determined whether the insulinomimetic effects of GLP-1 are mediated through its principal metabolite, GLP-1 (9-36) amide (GLP-1m)., Methods and Procedures: Glucose turnover during two, 2-h, euglycemic clamps was measured in 12 lean and 12 obese (BMI <25 or >30 kg/m(2)) male and female subject volunteers with normal oral glucose tolerance test. Saline or GLP-1m were infused from 0 to 60 min in each study. Additionally, seven lean and six obese subjects underwent a third clamp in which the GLP-1 receptor (GLP-1R) antagonist, exendin (9-39) amide was infused from -60 to 60 min with GLP-1m from 0 to 60 min., Results: No glucose infusion was required in lean subjects to sustain euglycemia (glucose clamp) during saline or GLP-1m infusions. However, in obese subjects glucose infusion was necessary during GLP-1m infusion alone in order to compensate for a marked (>50%) inhibition of hepatic glucose production (HGP). Plasma insulin levels remained constant in lean subjects but rose significantly in obese subjects after termination of the peptide infusions. During GLP-1R blockade, infusion of glucose was immediately required upon starting GLP-1m infusions in all subjects due to a more dramatic reduction in HGP, as well as a delayed and modest insulinotropic response., Discussion: We conclude that GLP-1m potently inhibits HGP and is a weak insulinotropic agent. These properties are particularly apparent and pronounced in obese but only become apparent in lean subjects during GLP-1 (7-36) receptor blockade. These previously unrecognized antidiabetogenic actions of GLP-1m may have therapeutic usefulness.

Reduced energy intake, or caloric restriction (CR), is known to extend life span and to retard age-related health decline in a number of different species, including worms, flies, fish, mice and rats. CR has been shown to reduce oxidative stress, improve insulin sensitivity, and alter neuroendocrine responses and central nervous system (CNS) function in animals. CR has particularly profound and complex actions upon reproductive health. At the reductionist level the most crucial physiological function of any organism is its capacity to reproduce. For a successful species to thrive, the balance between available energy (food) and the energy expenditure required for reproduction must be tightly linked. An ability to coordinate energy balance and fecundity involves complex interactions of hormones from both the periphery and the CNS and primarily centers upon the master endocrine gland, the anterior pituitary. In this review article we review the effects of CR on pituitary gonadotrope function and on the male and female reproductive axes. A better understanding of how dietary energy intake affects reproductive axis function and endocrine pulsatility could provide novel strategies for the prevention and management of reproductive dysfunction and its associated comorbidities.

In many sensory systems, stimulus sensitivity is dynamically modulated through mechanisms of peripheral adaptation, efferent input, or hormonal action. In this way, responses to sensory stimuli can be optimized in the context of both the environment and the physiological state of the animal. Although the gustatory system critically influences food preference, food intake and metabolic homeostasis, the mechanisms for modulating taste sensitivity are poorly understood. In this study, we report that glucagon-like peptide-1 (GLP-1) signaling in taste buds modulates taste sensitivity in behaving mice. We find that GLP-1 is produced in two distinct subsets of mammalian taste cells, while the GLP-1 receptor is expressed on adjacent intragemmal afferent nerve fibers. GLP-1 receptor knockout mice show dramatically reduced taste responses to sweeteners in behavioral assays, indicating that GLP-1 signaling normally acts to maintain or enhance sweet taste sensitivity. A modest increase in citric acid taste sensitivity in these knockout mice suggests GLP-1 signaling may modulate sour taste, as well. Together, these findings suggest a novel paracrine mechanism for the regulation of taste function.

The level of dietary energy intake influences metabolism, reproductive function, the development of age-related diseases, and even cognitive behavior. Because males and females typically play different roles in the acquisition and allocation of energy resources, we reasoned that dietary energy intake might differentially affect the brains of males and females at the molecular level. To test this hypothesis, we performed a gene array analysis of the hippocampus in male and female rats that had been maintained for 6 months on either ad libitum (control), 20% caloric restriction (CR), 40% CR, intermittent fasting (IF) or high fat/high glucose (HFG) diets. These diets resulted in expected changes in body weight, and circulating levels of glucose, insulin and leptin. However, the CR diets significantly increased the size of the hippocampus of females, but not males. Multiple genes were regulated coherently in response to energy restriction diets in females, but not in males. Functional physiological pathway analyses showed that the 20% CR diet down-regulated genes involved in glycolysis and mitochondrial ATP production in males, whereas these metabolic pathways were up-regulated in females. The 40% CR diet up-regulated genes involved in glycolysis, protein deacetylation, PGC-1alpha and mTor pathways in both sexes. IF down-regulated many genes in males including those involved in protein degradation and apoptosis, but up-regulated many genes in females including those involved in cellular energy metabolism, cell cycle regulation and protein deacetylation. Genes involved in energy metabolism, oxidative stress responses and cell death were affected by the HFG diet in both males and females. The gender-specific molecular genetic responses of hippocampal cells to variations in dietary energy intake identified in this study may mediate differential behavioral responses of males and females to differences in energy availability.

Objective: To verify what information from oral glucose tolerance tests (OGTTs) independently predicts mortality., Research Design and Methods: A total of 1,401 initially nondiabetic participants from the Baltimore Longitudinal Study of Aging aged 17-95 years underwent one or more OGTTs (median 2, range 1-8), with insulin and glucose measurements taken every 20 min over the course of 2 h included in this study. Proportional hazards using the longitudinally collected data and Bayesian model averaging were used to examine the association of OGTT measurements individually and grouped with mortality, adjusting for covariates., Results: Participants were followed for a median 20.3 years (range 0.5-40). The first-hour OGTT glucose and insulin levels increased only modestly with age, whereas levels during the second hour increased 4% per decade. Individually, 100- and 120-min glucose measures and fasting and 100-min insulin levels were all independent predictors of mortality. When all measures were considered together, only higher 120-min glucose was a significant independent risk factor for mortality., Conclusion: The steeper rise with age of the OGTT 2-h glucose values and the prognostic primacy of the 120-min glucose value for mortality is consistent with previous reports and suggests the value of using the OGTT in clinical practice.

Many organ systems are adversely affected by diabetes, including the brain, which undergoes changes that may increase the risk of cognitive decline. Although diabetes influences the hypothalamic-pituitary-adrenal axis, the role of this neuroendocrine system in diabetes-induced cognitive dysfunction remains unexplored. Here we demonstrate that, in both insulin-deficient rats and insulin-resistant mice, diabetes impairs hippocampus-dependent memory, perforant path synaptic plasticity and adult neurogenesis, and the adrenal steroid corticosterone contributes to these adverse effects. Rats treated with streptozocin have reduced insulin and show hyperglycemia, increased corticosterone, and impairments in hippocampal neurogenesis, synaptic plasticity and learning. Similar deficits are observed in db/db mice, which are characterized by insulin resistance, elevated corticosterone and obesity. Changes in hippocampal plasticity and function in both models are reversed when normal physiological levels of corticosterone are maintained, suggesting that cognitive impairment in diabetes may result from glucocorticoid-mediated deficits in neurogenesis and synaptic plasticity.

Caloric restriction (CR) is the most potent intervention known to both protect against carcinogenesis and extend lifespan in laboratory animals. A variety of anticarcinogens and CR mimetics induce and activate the NF-E2-related factor 2 (Nrf2) pathway. Nrf2, in turn, induces a number of antioxidative and carcinogen-detoxifying enzymes. Thus, Nrf2 offers a promising target for anticarcinogenesis and antiaging interventions. We used Nrf2-disrupted (KO) mice to examine its role on the biological effects of CR. Here, we show that Nrf2 is responsible for most of the anticarcinogenic effects of CR, but is dispensable for increased insulin sensitivity and lifespan extension. Nrf2-deficient mice developed tumors more readily in response to carcinogen exposure than did WT mice, and CR was ineffective in suppressing tumors in the KO mice. However, CR extended lifespan and increased insulin sensitivity similarly in KO and WT mice. These findings identify a molecular pathway that dissociates the prolongevity and anticarcinogenic effects of CR.

Huntington's disease (HD) is a tremendously debilitating disorder that strikes relatively young individuals and progresses rapidly over the next ten to fifteen years inducing a loss of cognitive and motor skills and eventually death occurs. The primary locus of the disorder is a polyglutamine expansion of the protein product of the huntingtin (htt) gene. The htt protein appears to be a scaffolding protein that orchestrates the complex assembly of multiple intracellular proteins involved in multiple processes, including vesicular movement and cell metabolism. The htt protein is ubiquitously expressed in human tissues but the predominance of the interest in the pathology lies in its effects on the central nervous system (CNS). Most of the current therapeutics for HD thus have been targeted at preventing neuronal damage in the CNS, however, a considerable body of evidence has been accumulating to suggest that the maintenance of a healthy nervous system is tightly linked with peripheral physiological health. Therefore treatment of both the peripheral and central pathophysiologies of HD could form the basis of a more effective HD therapeutic strategy.

TAS1R- and TAS2R-type taste receptors are expressed in the gustatory system, where they detect sweet- and bitter-tasting stimuli, respectively. These receptors are also expressed in subsets of cells within the mammalian gastrointestinal tract, where they mediate nutrient assimilation and endocrine responses. For example, sweeteners stimulate taste receptors on the surface of gut enteroendocrine L cells to elicit an increase in intracellular Ca(2+) and secretion of the incretin hormone glucagon-like peptide-1 (GLP-1), an important modulator of insulin biosynthesis and secretion. Because of the importance of taste receptors in the regulation of food intake and the alimentary responses to chemostimuli, we hypothesized that differences in taste receptor efficacy may impact glucose homeostasis. To address this issue, we initiated a candidate gene study within the Amish Family Diabetes Study and assessed the association of taste receptor variants with indicators of glucose dysregulation, including a diagnosis of type 2 diabetes mellitus and high levels of blood glucose and insulin during an oral glucose tolerance test. We report that a TAS2R haplotype is associated with altered glucose and insulin homeostasis. We also found that one SNP within this haplotype disrupts normal responses of a single receptor, TAS2R9, to its cognate ligands ofloxacin, procainamide and pirenzapine. Together, these findings suggest that a functionally compromised TAS2R receptor negatively impacts glucose homeostasis, providing an important link between alimentary chemosensation and metabolic disease.

Overall dietary energy intake, particularly the consumption of simple sugars such as fructose, has been increasing steadily in Western societies, but the effects of such diets on the brain are poorly understood. Here, we used functional and structural assays to characterize the effects of excessive caloric intake on the hippocampus, a brain region important for learning and memory. Rats fed with a high-fat, high-glucose diet supplemented with high-fructose corn syrup showed alterations in energy and lipid metabolism similar to clinical diabetes, with elevated fasting glucose and increased cholesterol and triglycerides. Rats maintained on this diet for 8 months exhibited impaired spatial learning ability, reduced hippocampal dendritic spine density, and reduced long-term potentiation at Schaffer collateral--CA1 synapses. These changes occurred concurrently with reductions in levels of brain-derived neurotrophic factor in the hippocampus. We conclude that a high-calorie diet reduces hippocampal synaptic plasticity and impairs cognitive function, possibly through BDNF-mediated effects on dendritic spines., (Published 2008 Wiley-Liss, Inc.)

Isolated postchallenge hyperglycemia (IPH) with normal fasting plasma glucose <100 mg/dL and plasma glucose with diabetic 2-hour plasma glucose >or=200 mg/dL after an oral glucose tolerance test (OGTT) is a common occurrence in the elderly. We sought to understand what unique characteristics this population might have that puts it at risk for this particular metabolic finding. We therefore conducted a longitudinal study of volunteers in the Baltimore Longitudinal Study of Aging (BLSA). All volunteers had an OGTT performed (75 g) on 2 or more occasions. We measured plasma levels of glucose, insulin, C-peptide, glucagon-like peptide-1 (GLP-1), glucose-dependent insulinotropic peptide (GIP), ghrelin, leptin, adiponectin, resistin, C-reactive protein, cytokines, and their soluble receptors, as well as nonesterified free fatty acids (NEFAs). We determined that 22 subjects in BLSA had IPH, accounting for 2.1% of the BLSA population. All 22 were older than 65 years. They were then matched by age, sex, and body mass index to 12 subjects who had isolated impaired glucose tolerance (IGT) and 15 subjects with normal glucose tolerance (NGT). All subjects had normal fasting glucose levels <100 mg/dL in accordance with the American Diabetes Association Expert Committee on the Classification and Diagnosis of Diabetes Mellitus criteria (2003). We found that subjects with IPH had similar plasma insulin levels to the other 2 groups, except at the 2-hour time when their insulin levels were higher than NGT (P < .05). Although there was a clear trend for differences in the insulinogenic index, the areas under the curves for insulin, systolic blood pressure, adiponectin, and C-reactive protein across the glucose tolerance categories revealed no statistical significance. Cytokines and their soluble receptors, gut hormones, and adipokines were similar in all 3 groups. The NEFA levels were significantly elevated in the fasting state (P < .05) in the IPH compared with NGT, with IGT intermediate between the other 2 groups. The rate of clearance of NEFAs after the OGTT decreased progressively from the NGT to the IPH group (in micromoles per liter per minute: NGT, 11.9 vs IGT, 7.6 vs IPH, 3.0). We conclude that the rate of suppression of lipolysis in the elderly determines the sensitivity of glucose uptake to insulin after OGTT.

Glucagon-like peptide-1 (GLP-1), released from gut endocrine L cells in response to glucose, regulates appetite, insulin secretion, and gut motility. How glucose given orally, but not systemically, induces GLP-1 secretion is unknown. We show that human duodenal L cells express sweet taste receptors, the taste G protein gustducin, and several other taste transduction elements. Mouse intestinal L cells also express alpha-gustducin. Ingestion of glucose by alpha-gustducin null mice revealed deficiencies in secretion of GLP-1 and the regulation of plasma insulin and glucose. Isolated small bowel and intestinal villi from alpha-gustducin null mice showed markedly defective GLP-1 secretion in response to glucose. The human L cell line NCI-H716 expresses alpha-gustducin, taste receptors, and several other taste signaling elements. GLP-1 release from NCI-H716 cells was promoted by sugars and the noncaloric sweetener sucralose, and blocked by the sweet receptor antagonist lactisole or siRNA for alpha-gustducin. We conclude that L cells of the gut "taste" glucose through the same mechanisms used by taste cells of the tongue. Modulating GLP-1 secretion in gut "taste cells" may provide an important treatment for obesity, diabetes and abnormal gut motility.

Reduced energy intake, or caloric restriction (CR), is known to extend life-span and to retard age-related health decline in a myriad of species, including nematode worms, flies, fish, mice and rats. The exact mechanism whereby CR exerts its life-extending and health-extending effects is unclear. CR however has been shown to improve insulin sensitivity, reduce oxidative stress and alter neuroendocrine responses and central nervous system (CNS) function in animals. In this review article we provide a comprehensive overview of the effects of CR on animal physiology and we discuss some of the potential molecular mechanisms and pathways whereby reduced energy intake can increase health-span and life-span. A better understanding of how energy intake can influence the aging process could lead to new strategies and therapeutics to reduce age-related decline and increase health-span.

Glucose-dependent insulinotropic polypeptide (GIP) is a gastrointestinal hormone that has a potent stimulatory effect on insulin release under conditions of normal glucose tolerance. However, its insulinotropic effect is reduced or even absent entirely in type 2 diabetic patients. In this study, we addressed the role of glucose concentration in the diabetic range of >or=11 mM, i.e., hyperglycemia per se, as a cause of the lack of response to GIP. Culturing rat and human pancreatic islets in >or=11 mM glucose for up to 24 h resulted in prevention of GIP-mediated intracellular cAMP increase compared with culturing in 5 mM glucose. Western blot analysis revealed a selective 67 +/- 2% (rat) and 60 +/- 8% (human) decrease of GIP-R expression in islets exposed to >or=11 mM glucose compared with 5 mM glucose (P < 0.001). We further immunoprecipitated GIP-R from islets and found that GIP-R was targeted for ubiquitination in a glucose- and time-dependent manner. Downregulation of GIP-R was rescued by treating isolated islets with proteasomal inhibitors lactacystin and MG-132, and the islets were once again capable of increasing intracellular cAMP levels in response to GIP. These results suggest that the GIP-R is ubiquitated, resulting in downregulation of the actions of GIP.

Glucagon-like peptide 1 (GLP-1) is a hormone that is encoded in the proglucagon gene. It is mainly produced in enteroendocrine L cells of the gut and is secreted into the blood stream when food containing fat, protein hydrolysate, and/or glucose enters the duodenum. Its particular effects on insulin and glucagon secretion have generated a flurry of research activity over the past 20 years culminating in a naturally occurring GLP-1 receptor (GLP-1R) agonist, exendin 4 (Ex-4), now being used to treat type 2 diabetes mellitus (T2DM). GLP-1 engages a specific guanine nucleotide-binding protein (G-protein) coupled receptor (GPCR) that is present in tissues other than the pancreas (brain, kidney, lung, heart, and major blood vessels). The most widely studied cell activated by GLP-1 is the insulin-secreting beta cell where its defining action is augmentation of glucose-induced insulin secretion. Upon GLP-1R activation, adenylyl cyclase (AC) is activated and cAMP is generated, leading, in turn, to cAMP-dependent activation of second messenger pathways, such as the protein kinase A (PKA) and Epac pathways. As well as short-term effects of enhancing glucose-induced insulin secretion, continuous GLP-1R activation also increases insulin synthesis, beta cell proliferation, and neogenesis. Although these latter effects cannot be currently monitored in humans, there are substantial improvements in glucose tolerance and increases in both first phase and plateau phase insulin secretory responses in T2DM patients treated with Ex-4. This review will focus on the effects resulting from GLP-1R activation in the pancreas.

Metabolic syndrome (MetS) is a strong risk factor for type 2 diabetes and cardiovascular disease. Conditions associated with hyperandrogenism are often associated with glucose intolerance and other features of MetS in young women. As the prevalence of MetS increases with age and is probably multifactorial, it is reasonable to hypothesize that age-related changes in androgens and other hormones might contribute to the development of MetS in older persons. However, this hypothesis has never been tested in older women. We hypothesized that high levels of testosterone, dehydroepiandrosterone sulfate (DHEA-S), and cortisol and low levels of sex hormone-binding globulin (SHBG) and IGF-I would be associated with MetS in a representative cohort of older Italian women independently of confounders (including inflammatory markers). After exclusion of participants on hormone replacement therapy and those with a history of bilateral oophorectomy, 512 women (>/=65 yr) had complete data on testosterone, cortisol, DHEA-S, SHBG, fasting insulin, total and free IGF-I, IL-6, and C-reactive protein (CRP). MetS was defined according to ATP-III criteria. Insulin resistance was calculated according to HOMA. MetS was found in 145 women (28.3%). Participants with vs. those without MetS had higher age-adjusted levels of bioavailable testosterone (P < 0.001), IL-6 (P < 0.001), CRP (P < 0.001), and HOMA (P < 0.001) and lower levels of SHBG (P < 0.001). After adjustment for potential confounders, participants with decreased SHBG had an increased risk of MetS (P < 0.0001) vs. those with low SHBG. In a further model including all hormones and confounders, log SHBG was the only independent factor associated with MetS (OR: 0.44, 95% CI 0.21-0.91, P = 0.027). In older women, SHBG is negatively associated with MetS independently of confounders, including inflammatory markers and insulin resistance. Further studies are needed to support the notion that raising SHBG is a potential therapeutic target for prevention and treatment of MetS.

Banknote evidence is often submitted after a suspect has attempted to disguise or remove red dye stain that has been released because of an anti-theft device that activates after banknotes have been unlawfully removed from bank premises. Three chlorinated compounds have been synthesized as forensic chemical standards to indicate bank security dye bleaching as a suspect's intentional method for masking a robbery involving dye pack release on banknotes. A novel, facile synthetic method to provide three chlorinated derivatives of 1-(methylamino)anthraquinone (MAAQ) is presented. The synthetic route involved Ultra Clorox bleach as the chlorine source, iron chloride as the catalyst, and MAAQ as the starting material and resulted in a three-component product mixture. Two mono-chlorinated isomers (2-chloro-1-(methylamino)anthraquinone and 4-chloro-1-(methylamino)anthraquinone) and one di-chlorinated compound (2,4-dichloro-1-(methylamino)anthraquinone) of the MAAQ parent molecule were detected by gas chromatography mass spectrometry (GC-MS), and subsequently isolated by liquid chromatography (LC) with postcolumn fraction collection. Although GC-MS is sensitive enough to detect all of the chlorinated products, it is not definitive enough to identify the structural isomers. Liquid-state nuclear magnetic resonance (NMR) spectroscopy was utilized to elucidate structurally the ortho- and para-mono-chlorinated isomers once enough material was properly isolated. A reaction mechanism involving iron is proposed to explain the presence of chlorinated MAAQ species on stolen banknotes after attempted bleaching.

Among the products of enteroendocrine cells are the incretins glucagon-like peptide-1 (GLP-1, secreted by L cells) and glucose-dependent insulinotropic peptide (GIP, secreted by K cells). These are key modulators of insulin secretion, glucose homeostasis, and gastric emptying. Because of the rapid early rise of GLP-1 in plasma after oral glucose, we wished to definitively establish the absence or presence of L cells, as well as the relative distribution of the incretin cell types in human duodenum. We confirmed the presence of proglucagon and pro-GIP genes, their products, and glucosensory molecules by tissue immunohistochemistry and RT-PCR of laser-captured, single duodenal cells. We also assayed plasma glucose, incretin, and insulin levels in subjects with normal glucose tolerance and type 2 diabetes for 120 min after they ingested 75 g of glucose. Subjects with normal glucose tolerance (n=14) had as many L cells (15+/-1), expressed per 1,000 gut epithelial cells, as K cells (13+/-1), with some containing both hormones (L/K cells, 5+/-1). In type 2 diabetes, the number of L and L/K cells was increased (26+/-2; P<0.001 and 9+/-1; P < 0.001, respectively). Both L and K cells contained glucokinase and glucose transporter-1, -2, and -3. Newly diagnosed type 2 diabetic subjects had increased plasma GLP-1 levels between 20 and 80 min, concurrently with rising plasma insulin levels. Significant coexpression of the main incretin peptides occurs in human duodenum. L and K cells are present in equal numbers. New onset type 2 diabetes is associated with a shift to the L phenotype.

Context: Previous studies with small numbers of subjects showed a negative correlation between plasma peptide YY (PYY) levels and obesity. If correct, this would imply that low PYY levels might be involved in the pathogenesis of obesity., Objective: Our objective was to investigate whether plasma PYY levels were correlated with sex and body mass index (BMI)., Design, Setting, and Patients: We conducted a cross-sectional study of 151 normal volunteers (19-90 yr of age) in the Baltimore Longitudinal Study of Aging., Interventions: All subjects had an oral glucose tolerance test (75 g) performed., Main Outcome Measures: Immunostaining of human duodenum, BMI, hemoglobin A1c, plasma glucose, insulin, PYY, glucagon like peptide-1 (GLP-1), ghrelin, and leptin were the main outcome measures., Results: PYY and GLP-1 colocalized in the same cells in human duodenum. Both hormones reached peak plasma levels by 20 min and had similar secretory patterns. The incremental increases in PYY and GLP-1 during that first 20 min were significantly correlated (r2 = 0.388; P < 0.0001). The areas under the curve from 0-120 min for PYY and GLP-1 were similar in both obese and lean participants. Female participants across the range of BMI had significantly higher PYY area under the curve (17,464 +/- 1,240 vs. 14,120 +/- 806 pmol/liter x min, female vs. male; P < 0.05) compared with male participants., Conclusions: Our findings show that PYY and GLP-1 are colocalized and cosecreted from L cells and that total secretion of PYY is higher in females than in males, but fasting PYY levels and PYY secretion in response to oral glucose were not in any way correlated with BMI.

Activation of the glucagon-like peptide-1 (GLP-1) receptor on pancreatic beta cells by GLP-1 and exendin-4 increases insulin secretion. Exendin-4 is 39 amino acids long, unlike GLP-1 which has 30 amino acids. Because of its non-mammalian (lizard) origin and unique C-terminal sequence, exendin-4 may be immunogenic in humans. We showed previously that the C terminally truncated exendin peptide exendin (1-30) has a reduced affinity for the GLP-1 receptor and a diminished ability to increase intracellular cAMP in insulinoma cells. Here we show that daily intraperitoneal injection of exendin (1-30) (1 nmol/kg) for 20 d followed by 31 d twice daily to Lepr(db)/Lepr(db) (db/db) mice significantly reduced the amount of visceral fat relative to saline-treated controls and improved HbA1C (control 9.5 +/- 0.2% vs treated 7.9 +/- 0.2%, p = 0.001) but was not as effective as exendin-4. To examine the ability of exendin (1-30) to stimulate beta-cell growth, we injected one group of 3-mo-old Fisher rats with exendin (1-30) (1 nmol/kg) and another group with saline for 8 d. We observed no change in beta-cell area, but did see a change in the number of islets with nuclei positive for BrdU [10.7 +/- 1.8% exendin (1-30) vs 6.5 +/- 0.5% control].

The purpose of this study is to ascertain the pharmacodynamic properties of exendin-4, a glucose-dependent insulinotropic agent, from plasma glucose and insulin concentration-time profiles following a 60-min intravenous infusion in healthy and type 2 diabetic subjects. Plasma glucose and insulin concentrations were obtained from a previous clinical study, whereby a hyperglycemic clamp was established and maintained in healthy (n = 7) and type 2 diabetic (n = 7) volunteers (plasma glucose raised 5.4 mM above fasting level). Exendin-4 was infused (0.15 pmol/kg/min) during the 2nd hour of a 5-h clamp. A physiological pharmacodynamic model was developed and fitted to individual glucose and insulin responses simultaneously. Because drug concentrations were unavailable, hypothetical pharmacokinetic driving functions were approximated during the modeling process and used to enhance a proportionality constant relating elevated glucose and the rate of second-phase insulin release. Exendin-4 infusions produced substantial insulin release in both subject populations that required higher glucose infusion rates to maintain stable hyperglycemia. Observed plasma glucose-insulin profiles were well characterized by the final pharmacodynamic model. Apparent exendin-4 elimination rate constants for healthy and diabetic subjects were similar (0.0386 +/- 0.0192 and 0.0460 +/- 0.0145 min(-1)). Capacity and sensitivity parameters of drug effect were 2-fold lower in diabetic subjects, but mean differences were not statistically significant. Simulations confirm that diabetic subjects exhibit a reduced capacity to enhance second-phase insulin release in response to exendin-4 compared with healthy subjects. Type 2 diabetic subjects demonstrate a significant response to exendin-4, but to a lesser extent than nondiabetic subjects, despite comparable measures of apparent drug exposure and efficacy.

Objective: The role of gut-derived incretin, glucose-dependent insulinotropic polypeptide (also known as gastric inhibitory peptide [GIP]), in compensatory beta-cell hypersecretion during insulin-resistant states and in transition to beta-cell failure in type 2 diabetes is unknown., Research Design and Methods: We carried out oral glucose tolerance testing followed by blood sampling 10 times for 2 h on 68 age- and BMI-matched participants of the Baltimore Longitudinal Study on Aging (BLSA) with normal glucose tolerance (34 subjects), impaired glucose tolerance (IGT) (18 subjects with both impaired fasting and 2-h plasma glucose levels), and type 2 diabetes (16 subjects with both diabetic fasting and 2-h plasma glucose levels). We assayed plasma glucose, insulin, C-peptide, glucagon, and intact and total GIP levels and quantitated glucose and hormone responses to the oral glucose tolerance test. We also compared GIP and insulin release and sensitivity indexes between groups., Results: After glucose ingestion, subjects with IGT had both hyperinsulinemia and hyperemia, while subjects with type 2 diabetes had both beta- and GIP-cell deficiency. In the former group, there was also a significant positive correlation between the augmented plasma intact and total GIP levels and both fasting and post-oral glucose load plasma insulin levels., Conclusions: Elevated plasma GIP levels are correlated with hyperinsulinemia in the impaired glucose-tolerant state, whereas type 2 diabetes is associated with a failure to secrete adequate amounts of both GIP and insulin, indicating a common pathway of resistance to and eventually failure of glucose responsiveness in beta- and GIP-cells.

Glucagon-like peptide 1 (GLP-1) activates receptors coupled to cAMP production and calcium influx in pancreatic cells, resulting in enhanced glucose sensitivity and insulin secretion. Despite evidence that the GLP-1 receptor is present and active in neurons, little is known of the roles of GLP-1 in neuronal physiology. As GLP-1 modulates calcium homeostasis in pancreatic beta cells, and because calcium plays important roles in neuronal plasticity and neurodegenerative processes, we examined the effects of GLP-1 on calcium regulation in cultured rat hippocampal neurons. When neurons were pre-treated with GLP-1, calcium responses to glutamate and membrane depolarization were attenuated. Whole-cell patch clamp analyses showed that glutamate-induced currents and currents through voltage-dependent calcium channels were significantly decreased in neurons pre-treated with GLP-1. Pre-treatment of neurons with GLP-1 significantly decreased their vulnerability to death induced by glutamate. Acute application of GLP-1 resulted in a transient elevation of intracellular calcium levels, consistent with the established effects of GLP-1 on cAMP production and activation of cAMP response element-binding protein. Collectively, our findings suggest that, by modulating calcium responses to glutamate and membrane depolarization, GLP-1 may play important roles in regulating neuronal plasticity and cell survival.

Objective: Glucagon-like peptide 1 (GLP-1) is an insulinotropic gut hormone that, when given exogenously, may be a useful agent in the treatment of type 2 diabetes. We conducted a 3-month trial to determine the efficacy and safety of GLP-1 in elderly diabetic patients., Research Design and Methods: A total of 16 patients with type 2 diabetes who were being treated with oral hypoglycemic agents were enrolled. Eight patients (aged 75 +/- 2 years, BMI 27 +/- 1 kg/m(2)) remained on usual glucose-lowering therapy and eight patients (aged 73 +/- 1 years, BMI 27 +/- 1 kg/m(2)), after discontinuing hypoglycemic medications, received GLP-1 delivered by continuous subcutaneous infusion for 12 weeks. The maximum dose was 120 pmol x kg(-1). h(-1). Patients recorded their capillary blood glucose (CBG) levels (four times per day, 3 days per week) and whenever they perceived hypoglycemic symptoms. The primary end points were HbA(1c) and CBG determinations. Additionally, changes in beta-cell sensitivity to glucose, peripheral tissue sensitivity to insulin, and changes in plasma ghrelin levels were examined., Results: HbA(1c) levels (7.1%) and body weight were equally maintained in both groups. The usual treatment group had a total of 87 CBG measurements of

Objective: The purpose of this study was to determine the relationships between plasma adiponectin and leptin levels, total and central obesity, and glucose utilization across the adult age span., Research Design and Methods: We studied 148 women aged 18-81 years with a BMI range of 17.2-44.3 kg/m(2). Total percent body fat was determined by dual-energy X-ray absorptiometry and abdominal fat by computed tomography. Glucose tolerance in non-type 2 diabetic volunteers was determined with an oral glucose tolerance test. Glucose utilization (M) was measured during the last 60 min of hyperinsulinemic-euglycemic clamps (240 pmol x m(-2) x min(-1)). Plasma adiponectin levels were measured by radioimmunoassay. The women were separated into three age-groups: young, middle, and old (<40, 40-59, and >or=60 years, respectively), as well as by glucose tolerance status., Results: Adiponectin concentrations did not differ by age-groups. There were significant age effects for BMI, percent body fat, visceral fat, subcutaneous abdominal fat, VO(2max), and M. Adiponectin levels were lower in the prediabetic women (n = 18) than in the normal glucose-tolerant women (n = 108) and the women with type 2 diabetes (n = 22) (both P < 0.05). Univariate correlations revealed significant negative relationships between plasma adiponectin levels and BMI, percent body fat, visceral fat, subcutaneous abdominal fat, fasting leptin, and fasting insulin and positive relationship with M (all P < 0.05). In a multiple stepwise regression model to predict adiponectin, only M remained in the model at P < 0.001. Multivariate analyses revealed a significant relation for M as a function of adiponectin, insulin, and VO(2max)., Conclusions: The data suggest that plasma adiponectin does not change with age but levels are negatively associated with percent body fat, visceral fat, subcutaneous abdominal fat, insulin, and leptin levels in women. Adiponectin is positively associated with M across the age span in women.

Exendin-4, a 39-amino acid (AA) peptide, is a long-acting agonist at the glucagon-like peptide-1 (GLP-1) receptor. Consequently, it may be preferable to GLP-1 as a long-term treatment for type 2 diabetes mellitus. Exendin-4 (Ex-4), unlike GLP-1, is not degraded by dipeptidyl peptidase IV (DPP IV), is less susceptible to degradation by neutral endopeptidase, and possesses a nine-AA C-terminal sequence absent from GLP-1. Here we examine the importance of these nine AAs for biological activity of Ex-4, a sequence of truncated Ex-4 analogs, and native GLP-1 and GLP-1 analogs to which all or parts of the C-terminal sequence have been added. We found that removing these AAs from Ex-4 to produce Ex (1-30) reduced the affinity for the GLP-1 receptor (GLP-1R) relative to Ex-4 (IC50: Ex-4, 3.22+/-0.9 nM; Ex (1-30), 32+/-5.8 nM) but made it comparable to that of GLP-1 (IC50: 44.9+/-3.2 nM). The addition of this nine-AA sequence to GLP-1 improved the affinity of both GLP-1 and the DPP IV resistant analog GLP-1 8-glycine for the GLP-1 receptor (IC50: GLP-1 Gly8 [GG], 220+/-23 nM; GLP-1 Gly8 Ex (31-39), 74+/-11 nM). Observations of the cAMP response in an insulinoma cell line show a similar trend for biological activity.

A gut insulinotropic peptide, glucagon-like peptide-1 (GLP-1), when given continuously subcutaneously, has been shown to be an effective agent to treat type 2 diabetes. Because of inactivation by dipeptidyl peptidase IV (DPP IV), it has a very short half-life (90-120 s), hence the need for continuous administration. Exendin-4 is an agonist of the GLP-1 receptor. It is not a substrate for DPP IV, and we previously demonstrated that intravenous administration has potent insulinotropic properties in type 2 diabetic volunteers. We evaluated the efficacy of bolus subcutaneous exendin-4 in insulin-naive type 2 diabetic volunteers. Ten patients aged 44-72 yr with mean fasting glucose levels of 11.4 +/- 0.9 mmol/l were enrolled, and daily or twice-daily bolus subcutaneous exendin-4 was self-administered for 1 mo. Glycosylated hemoglobin, multiple daily capillary blood glucose, beta-cell sensitivity to glucose, and peripheral tissue sensitivity to insulin were compared before and after treatment. The greatest decline in capillary blood glucose was seen before bed, with a drop from 15.5 to 9.2 mmol/l (P < 0.0001). Glycosylated hemoglobin improved significantly with treatment, from 9.1 to 8.3% (P = 0.009). beta-Cell sensitivity to glucose was improved, as assessed by C-peptide levels during a hyperglycemic clamp. No significant adverse effects were noted or reported. Our data suggest that, even with this short duration of therapy, exendin-4 treatment had a significant effect on glucose homeostasis.

Glucagon-like peptide-1(7-36)-amide (GLP-1) is an endogenous insulinotropic peptide that is secreted from the gastrointestinal tract in response to food. It enhances pancreatic islet beta-cell proliferation and glucose-dependent insulin secretion and lowers blood glucose and food intake in patients with type 2 diabetes mellitus. GLP-1 receptors, which are coupled to the cyclic AMP second messenger pathway, are expressed throughout the brains of rodents and humans. It was recently reported that GLP-1 and exendin-4, a naturally occurring, more stable analogue of GLP-1 that binds at the GLP-1 receptor, possess neurotrophic properties and can protect neurons against glutamate-induced apoptosis. We report here that GLP-1 can reduce the levels of amyloid-beta peptide (Abeta) in the brain in vivo and can reduce levels of amyloid precursor protein (APP) in cultured neuronal cells. Moreover, GLP-1 and exendin-4 protect cultured hippocampal neurons against death induced by Abeta and iron, an oxidative insult. Collectively, these data suggest that GLP-1 can modify APP processing and protect against oxidative injury, two actions that suggest a novel therapeutic target for intervention in Alzheimer's disease., (Copyright 2003 Wiley-Liss, Inc.)

Objective: To examine the insulinomimetic insulin-independent effects of glucagon-like peptide (GLP)-1 on glucose uptake in type 1 diabetic patients., Research Design and Methods: We used the hyperinsulinemic-euglycemic clamp (480 pmol. m(-2) x min(-1)) in paired randomized studies of six women and five men with type 1 diabetes. In the course of one of the paired studies, the subjects also received GLP-1 at a dose of 1.5 pmol. kg(-1) x min(-1). The patients were 41 +/- 3 years old with a BMI of 25 +/- 1 kg/m(2). The mean duration of diabetes was 23 +/- 3 years., Results: Plasma glucose was allowed to fall from a fasting level of approximately 11 mmol/l to 5.3 mmol/l in each study and thereafter was held stable at that level. Plasma insulin levels during both studies were approximately 900 pmol/l. Plasma C-peptide levels did not change during the studies. In the GLP-1 study, plasma total GLP-1 levels were elevated from the fasting level of 31 +/- 3 to 150 +/- 17 pmol/l. Plasma glucagon levels fell from the fasting levels of approximately 14 pmol/l to 9 pmol/l during both paired studies. Hepatic glucose production was suppressed during the glucose clamps in all studies. Glucose uptake was not different between the two studies ( approximately 40 micromol. kg(-1) x min(-1))., Conclusions: GLP-1 does not augment insulin-mediated glucose uptake in lean type 1 diabetic patients.

Blood glucose levels are sensed and controlled by the release of hormones from the islets of Langerhans in the pancreas. The beta-cell, the insulin-secreting cell in the islet, can detect subtle increases in circulating glucose levels and a cascade of molecular events spanning the initial depolarization of the beta-cell membrane culminates in exocytosis and optimal insulin secretion. Here we review these processes in the context of pharmacological agents that have been shown to directly interact with any stage of insulin secretion. Drugs that modulate insulin secretion do so by opening the K(ATP) channels, by interacting with cell-surface receptors, by altering second-messenger responses, by disrupting the beta-cell cytoskeletal framework, by influencing the molecular reactions at the stages of transcription and translation of insulin, and/or by perturbing exocytosis of the insulin secretory vesicles. Drugs acting primarily at the K(ATP) channels are the sulfonylureas, the benzoic acid derivatives, the imidazolines, and the quinolines, which are channel openers, and finally diazoxide, which closes these channels. Methylxanthines also work at the cell membrane level by antagonizing the purinergic receptors and thus increase insulin secretion. Other drugs have effects at multiple levels, such as the calcineurin inhibitors and somatostatin. Some drugs used extensively in research, e.g., colchicine, which is used to study vesicular transport, have no effect at the pharmacological doses used in clinical practice. We also briefly discuss those drugs that have been shown to disrupt beta-cell function in a clinical setting but for which there is scant information on their mechanism of action.

Glucagon-like peptide-1 (GLP-1) is an incretin hormone that, when given exogenously, is capable of normalizing blood glucose in individuals with type 2 diabetes. Until recently most of the research on this compound had been related to its insulinotropic properties. However, GLP-1 also regulates insulin synthesis and proinsulin gene expression, as well as the components of the glucose-sensing machinery. In addition to regulating insulin release, it is involved in regulating the secretion of at least two other islet hormones--glucagon and somatostatin. Extraislet effects of GLP-1 include a role in the central nervous system stress response, hypothalamic-pituitary function, and the suppression of gastric emptying. Recent studies from our own and other laboratories show that GLP-1 can regulate islet growth and is a differentiation factor of the endocrine pancreas. This leads us to propose that GLP-1 and GLP-1 receptor agonists, in the context of long-term treatment of type 2 diabetes, will have broader biological action on the endocrine pancreas than was earlier anticipated. We propose that GLP-1 is a growth factor for pancreatic endocrine cells and can increase islet cell mass. Here we review those reports that have highlighted its role as a factor for islet cell growth and differentiation., (Copyright 2003 John Wiley & Sons, Ltd.)

Exendin-4 (EX-4), a long acting agonist of GLP-1, induces an endocrine phenotype in Capan-1 cells. Under culture conditions which include serum, approximately 10% of the cells contain insulin and glucagon. When exposed to EX-4 (0.1 nM, up to 5 days), the number of cells containing insulin and glucagon increased to approximately 40%. Western blot analysis detected a progressive increase in protein levels of glucokinase and GLUT2 over 3 days of EX-4 treatment. We explored the sequence of activation of certain transcription factors known to be essential for the beta cell phenotype: PDX-1, Beta2/NeuroD, and hepatocyte nuclear factor 3beta (HNF3beta). Double immunostaining showed that PDX-1 coexisted with insulin and glucagon in EX-4-treated cells. Treatment caused an increase in PDX-1 protein levels by 24 h and induced its nuclear translocation. Beta2/NeuroD protein levels also increased progressively over 24 h. HNF3beta protein level increased twofold as early as 6 h after EX-4 treatment. EMSA results indicated that EX-4 caused a 12-fold increase in HNF3beta binding to PDX-1 promoter area II. Beta2/NeuroD protein levels progressively increased after 24 h treatment. Differentiation to insulin-producing cells was also seen when Capan-1 cells were transfected with pdx-1, with 80% of these cells expressing insulin 3 days after transfection. PDX-1 antisense totally inhibited such conversion. During the differentiation of duct cells to endocrine cells, cAMP levels (EX-4 is a ligand for the GLP-1, G-protein coupled receptor) and MAP kinase activity increased. Our results indicate that EX-4 activates adenylyl cyclase and MAP kinase which, in turn, may lead to activation of transcription factors necessary for an endocrine phenotype., (Published 2002 Wiley-Liss, Inc.)

Glucagon-like peptide-1 (7-36)-amide (GLP-1) is an endogenous insulinotropic peptide that is secreted from the L cells of the gastrointestinal tract in response to food. It has potent effects on glucose-dependent insulin secretion, insulin gene expression, and pancreatic islet cell formation. In type 2 diabetes, GLP-1, by continuous infusion, can normalize blood glucose and is presently being tested in clinical trials as a therapy for this disease. More recently, GLP-1 has been found to have central nervous system (CNS) effects and to stimulate neurite outgrowth in cultured cells. We now report that GLP-1, and its longer-acting analog exendin-4, can completely protect cultured rat hippocampal neurons against glutamate-induced apoptosis. Extrapolating these effects to a well defined rodent model of neurodegeneration, GLP-1 and exendin-4 greatly reduced ibotenic acid-induced depletion of choline acetyltransferase immunoreactivity in basal forebrain cholinergic neurons. These findings identify a novel neuroprotective/neurotrophic function of GLP-1 and suggest that such peptides may have potential for halting or reversing neurodegenerative processes in CNS disorders, such as Alzheimer's disease, and in neuropathies associated with type 2 diabetes mellitus.

The insulinotropic hormone, glucagon-like peptide-1 (GLP-1), is being examined as a potential new agent for treatment in type 2 diabetic patients. Whereas the insulinotropic properties of this peptide are well established, another property of the hormone, an insulinomimetic effect per se, is controversial. In the normal glucose-tolerant lean state, it is difficult to demonstrate an insulinomimetic effect. The current study was conducted to examine whether GLP-1 has insulinomimetic effect in the obese state. Ten obese volunteers (body mass index, 34.6 +/- 0.8 kg/m(2)), whose ages were 32.5 +/- 3.0 yr, participated in two euglycemic clamp studies (n = 20 clamps) for 120 min. Five of the volunteers were females. The initial clamp was performed with a primed (0-10)-constant (10-60) infusion of GLP-1 at a final rate of 1.5 pmol x kg(-1) x min(-1). At 60 min, the GLP-1 infusion was terminated, and euglycemic was maintained from 60-120 min. After the GLP-1 study, each individual's plasma insulin level was measured. A second study was performed that was identical to the first, with the infusion of regular insulin in place of GLP-1. Insulin infusion rates were regulated in each individual to simulate plasma insulin levels produced during the GLP-1 infusion. The rate of disappearance of glucose was calculated for each subject. Fasting plasma insulin levels were similar between studies. In response to the GLP-1 infusion, with maintenance of plasma glucose level clamped at fasting level, significant increases in plasma insulin occurred in all subjects (P < 0.001). The insulin levels during the insulin infusion study were similar to that induced by GLP-1. The rate of disappearance of glucose (insulin-mediated glucose uptake) progressively increased in response to both the GLP-1 and insulin infusion. However, the rate of disappearance of glucose during the GLP-1 study was significantly higher (P = 0.033) than during the insulin study. We conclude that in insulin-resistant states, GLP-1 has insulinomimetic properties per se.

Exendin-4 is a potent and long-acting agonist of the glucagon-like peptide-1 (GLP-1) receptor. GLP-1 is an insulinotropic gut peptide and is being evaluated for the regulation of plasma glucose in type 2 diabetes. The purpose of the present study was to ascertain whether exendin-4 is insulinotropic and whether it has long-lived biological effects in nondiabetic and type 2 diabetic subjects. Because incretins are glucose dependent with respect to their insulin-releasing capacity, we used the hyperglycemic glucose clamp technique to begin to address these issues in two separate protocols. In one protocol, we infused exendin-4 (0.15 pmol x kg(-1) x min(-1)) in seven nondiabetic and seven type 2 diabetic subjects during the second hour of a 5-h hyperglycemic clamp in which fasting plasma glucose was raised by 5.4 mmol/liter. The second protocol was identical to the first except that plasma glucose was allowed to fall to the fasting levels during the fourth hour and again raised by 5.4 mmol/liter during the fifth hour in four nondiabetic and four diabetic subjects. With the initiation of exendin-4 infusion at 60 min, plasma insulin response was potentiated 4- to 5-fold in both groups. Despite termination of exendin-4 at the end of the second hour, the insulin levels remained elevated for several hours and hyperglycemia was maintained. All volunteers ate a meal 5.5 h after inducing hyperglycemia. Postprandial plasma glucose, insulin, and GLP-1 did not rise in any subject, possibly because of delayed gastric emptying by exendin-4 even though its infusion had been terminated 4 h previously. We concluded that exendin-4 is a potent and long-lasting insulinotropic agent in nondiabetic and diabetic subjects.

The insulinotropic hormone glucagon-like peptide-1 (7-36)-amide (GLP-1) has potent effects on glucose-dependent insulin secretion, insulin gene expression, and pancreatic islet cell formation and is presently in clinical trials as a therapy for type 2 diabetes mellitus. We report on the effects of GLP-1 and two of its long-acting analogs, exendin-4 and exendin-4 WOT, on neuronal proliferation and differentiation, and on the metabolism of two neuronal proteins in the rat pheochromocytoma (PC12) cell line, which has been shown to express the GLP-1 receptor. We observed that GLP-1 and exendin-4 induced neurite outgrowth in a manner similar to nerve growth factor (NGF), which was reversed by coincubation with the selective GLP-1 receptor antagonist exendin (9-39). Furthermore, exendin-4 could promote NGF-initiated differentiation and may rescue degenerating cells after NGF-mediated withdrawal. These effects were induced in the absence of cellular dysfunction and toxicity as quantitatively measured by 3-(4,5-cimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide and lactate dehydrogenase assays, respectively. Our findings suggest that such peptides may be used in reversing or halting the neurodegenerative process observed in neurodegenerative diseases, such as the peripheral neuropathy associated with type 2 diabetes mellitus and Alzheimer's and Parkinson's diseases. Due to its novel twin action, GLP-1 and exendin-4 have therapeutic potential for the treatment of diabetic peripheral neuropathy and these central nervous system disorders.

Ageing is associated with an increased incidence of hypertension, macrovascular disease and type 2 diabetes (non-insulin-dependent diabetes). It has been suggested that a common mechanism may be responsible for all of these pathological states since all of these conditions often cluster in the same individual. Epidemiological and clinical data have consistently demonstrated an association between insulin resistance and/or hyperinsulinaemia and glucose intolerance, dyslipidaemia and elevated systolic blood pressures. Therefore, insulin resistance and hyperinsulinaemia have been proposed as the causal link among the elements of the clusters. The elderly are more glucose intolerant and insulin resistant, but it remains controversial whether this decrease in function is due to an inevitable consequence of 'biological ageing' or due to environmental or lifestyle variables, noticeably increased adiposity/altered fat distribution and physical inactivity. An increase of these modifiable factors has been shown to result in increases in insulin resistance and hyperinsulinaemia and vice versa. However, insulin secretion appears to decrease with age even after adjustments for differences in adiposity, fat distribution and physical activity. The glucose intolerance of ageing may be due, in part, to decreased insulin sensitivity of pancreatic / cells to insulinotropic gut hormones (GLP1/GIP) and in part to alterations of hepatic glucose production.

Background: Glucagon-like peptide-1 (GLP-1) is an intestinal insulinotropic hormone that augments glucose-induced insulin secretion in patients with type 2 diabetes. It has also been proposed that a substantial component of the glucose-lowering effects of GLP-1 occurs because this hormone enhances insulin-mediated glucose disposal. However, interpretations of the studies have been controversial. This study determines the effect of GLP-1 on insulin-mediated glucose disposal in elderly patients with type 2 diabetes., Methods: Studies were conducted on 8 elderly patients with type 2 diabetes (age range, 76 +/- 1 years; body mass index, 28 +/- 1 kg/m(2)). Each subject underwent two 180-minute euglycemic (insulin infusion rate, 40 mU/m(2)/min) insulin clamps in random order. Glucose production (Ra) and disposal (Rd) rates were measured using tritiated glucose methodology. In one study, glucose and insulin alone were infused. In the other study, a primed-continuous infusion of GLP-1 was administered at a final rate of 1.5 pmol x kg(-1) x min(-1) from 30 to 180 minutes., Results: Glucose values were similar between the control and GLP-1 infusion studies. 120- to 180-minute insulin values appeared to be higher during the GLP-1 infusion study (control, 795 +/- 63 pmol/l; GLP-1, 1140 +/- 275 pmol/l; p = not significant [NS]). The higher insulin values were largely due to 2 subjects who had substantial insulin responses to GLP-1 despite euglycemia and hyperinsulinemia. The 120- to 180-minute insulin values were similar in the other 6 subjects (control, 746 +/- 35 pmol/l; GLP-1, 781 +/- 41 pmol/l; p = NS). Basal (control, 2.08 +/- 0.05 mg/kg/min; GLP-1, 2.13 +/- 0.04 mg/kg/min; p = NS) and 120- to 180-minute (control, 0.50 +/- 0.18 mg/kg/min; GLP-1, 0.45 +/- 0.14 mg/kg/min; p = NS) Ra was similar between studies. The 120- to 180-minute Rd values were higher during the GLP-1 infusion studies (control, 4.73 +/- 0.39 mg/kg/min; GLP-1, 5.52 +/- 0.43 mg/kg/min; p <.01). When the 2 subjects who had significant insulin responses to GLP-1 during the euglycemic clamp were excluded, the 120- to 180-minute Rd values were still higher in the GLP-1 infusion study (control, 5.22 +/- 0.32 mg/kg/min; GLP-1, 6.05 +/- 0.37 mg/kg/min; p <.05)., Conclusions: We conclude that GLP-1 may enhance insulin sensitivity in elderly patients with diabetes.

An important cause of elevated glucose levels in elderly patients with diabetes is an alteration in non-insulin-mediated glucose uptake (NIMGU). Glucagon-like peptide 1 (GLP-1) is an intestinal insulinotropic hormone. It has been proposed that this hormone also lowers glucose levels by enhancing NIMGU. This study was conducted to determine whether GLP-1 augments NIMGU in elderly patients with diabetes, a group in which NIMGU is known to be impaired. Studies were conducted on 10 elderly patients with type 2 diabetes (aged 75 +/- 2 years, BMI 27 +/- 1 kg/m(2)) who underwent paired 240-min glucose clamp studies. In each study, octreotide was infused to suppress endogenous insulin release, and tritiated glucose methodology was used to measure glucose production and disposal rates. For the first 180 min, no glucose was infused. From 180 to 240 min, glucose was increased to 11 mmol/l using the glucose clamp protocol. In the GLP-1 study, GLP-1 was infused from 30 to 240 min. In a subsequent control study, insulin was infused using the glucose clamp protocol from 30 to 240 min to match the insulin levels that occurred during the GLP-1 infusion study. During hyperglycemia, GLP-1 enhanced glucose disposal (control study: 2.52 +/- 0.19 mg x kg(-1) x min(-1); GLP-1 study: 2.90 +/- 0.17 mg x kg(-1) x min(-1); P < 0.0001). Hepatic glucose output was not different between studies. We conclude that GLP-1 may partially reverse the defect in NIMGU that occurs in elderly patients with diabetes.

The use of glucagon-like peptide-1 (GLP-1) as a routine treatment for type 2 diabetes mellitus is undermined by its short biological half-life. A cause of degradation is its cleavage at the N-terminal HAE sequence by the enzyme dipeptidyl peptidase IV (DPP IV). To protect from DPP IV, we have studied the biological activity of a GLP-1 analog in which 6-aminohexanoic acid (Aha) is inserted between histidine and alanine at positions 7 and 8. We have compared the biological activity of this new compound, GLP-1 Aha(8), with the previously described GLP-1 8-glycine (GLP-1 Gly(8)) analog. GLP-1 Aha(8) (10 nM) was equipotent with GLP-1 (10 nM) in stimulating insulin secretion in RIN 1046-38 cells. As with GLP-1 Gly(8), the binding affinity of GLP-1 Aha(8) for the GLP-1 receptor in intact Chinese hamster ovary (CHO) cells expressing the human GLP-1 receptor (CHO/GLP-1R cells) was reduced (IC(50): GLP-1, 3.7 +/- 0.2 nM; GLP-1 Gly(8), 41 +/- 9 nM; GLP-1 Aha(8), 22 +/- 7 nM). GLP-1 Aha(8) was also shown to stimulate intracellular cAMP production 4-fold above basal at concentrations as low as 0.5 nM. However, it exhibited a higher ED(50) when compared to GLP-1 and GLP-1 Gly(8) (ED(50): GLP-1, 0.036 +/- 0.002 nM, GLP-1 Gly(8), 0.13 +/- 0.02 nM, GLP-1 Aha(8), 0.58 +/- 0.03 nM). A series of D-amino acid-substituted GLP-1 compounds were also examined to assess the importance of putative peptidase-sensitive cleavage sites present in the GLP-1 molecule. They had poor binding affinity for the GLP-1 receptor, and none of these compounds stimulated the production of intracellular cAMP in CHO/GLP-1R cells or insulin secretion in RIN 1046-38 cells. GLP-1 Aha(8) (24 nmol/kg) administered sc to fasted Zucker (fa/fa) rats (mean blood glucose, 195 +/- 32 mg/dl) lowered blood glucose levels to a nadir of 109 +/- 3 mg/dl, and it remained significantly lower for 8 h. Matrix-assisted linear desorption ionization-time of flight mass spectrometry of GLP-1 Aha(8) incubated with DPP IV (37 C, 2 h) did not exhibit an N-terminal degradation product. Taken together, these results show that insertion of Aha after the 7 position in GLP-1 produces an effective, long-acting GLP-1 analog, which may be useful in the treatment of type 2 diabetes mellitus.

Background: The current studies were designed to examine the effect of aging and diabetes on the enteroinsular axis., Methods: Healthy young control subjects (n = 10 young; age 23 +/- 1 years; body mass index [BMI] 24 +/- 1 kg/m(2)), healthy elderly subjects (n = 10; age 80 +/- 2 years; BMI 26 +/- 1 kg/m(2)), and elderly patients with type 2 diabetes (n = 10; age 76 +/- 2 years; BMI 26 +/- 2 kg/m(2)) underwent a 3-hour oral glucose tolerance test (glucose dose 40 gm/m(2))., Results: Insulin responses were not different between young controls and elderly patients with diabetes but were significantly lower in elderly patients with diabetes and young controls than in elderly controls (young control: 178 +/- 27 pM; elderly control: 355 +/- 57 pM; elderly diabetes: 177 +/- 30 pM; p <.05 elderly control vs young control and elderly diabetes). Total glucagon-like peptide 1 (GLP-1) responses were not significantly different between young and elderly controls and patients with diabetes (young control: 15 +/- 2 pM; old control: 8 +/- 2 pM; elderly diabetes: 12 +/- 3 pM; p = ns). Active GLP-1 responses were also not different between young and elderly controls and patients with diabetes (young control: 5 +/- 1 pM; old control: 6 +/- 1 pM; elderly diabetes: 7 +/- 1 pM; p = ns). However, the difference between total and active GLP levels was significantly greater in the young controls (young control: 10 +/- 2 pM; old control: 2 +/- 2 pM; elderly diabetes: 4 +/- 2 pM; p <.05, young vs elderly). Glucose-dependent insulinotropic polypeptide responses were not different between young and elderly controls and between elderly controls and patients with diabetes but were significantly higher in elderly patients with diabetes than in young controls (young control: 97 +/- 12 pM; elderly control: 121 +/- 16 pM; elderly diabetes: 173 +/- 27 pM; p <.05, young vs elderly diabetes). Glucagon responses were reduced in elderly controls but were similar in young controls and elderly patients with diabetes (young control: 15 +/- 1 pM; elderly control: 9 +/- 1 pM; elderly diabetes: 16 +/- 1 pM; p <.01 elderly control vs young control and elderly diabetes). Dipeptidyl peptidase IV levels were lower in both elderly controls and patients with diabetes when compared with young controls (young control: 0.17 +/- 0.01; elderly control: 0.15 +/- 0.01; elderly diabetes: 0.15 +/- 0.01 DeltaOD/20 minutes; p <.05, elderly vs young)., Conclusions: We conclude that normal aging and diabetes are associated with multiple changes in the enteroinsular axis.

The gut hormone, glucagon-like peptide-1 (GLP-1), which is secreted in nanomolar amounts in response to nutrients in the intestinal lumen, exerts cAMP/protein kinase A-mediated insulinotropic actions in target endocrine tissues, but its actions in heart cells are unknown. GLP-1 (10 nmol/L) increased intracellular cAMP (from 5.7+/-0.5 to 13.1+/-0.12 pmol/mg protein) in rat cardiac myocytes. The effects of cAMP-doubling concentrations of both GLP-1 and isoproterenol (ISO, 10 nmol/L) on contraction amplitude, intracellular Ca(2+) transient (CaT), and pH(i) in indo-1 and seminaphthorhodafluor (SNARF)-1 loaded myocytes were compared. Whereas ISO caused a characteristic increase (above baseline) in contraction amplitude (160+/-34%) and CaT (70+/-5%), GLP-1 induced a significant decrease in contraction amplitude (-27+/-5%) with no change in the CaT after 20 minutes. Neither pertussis toxin treatment nor exposure to the cGMP-stimulated phosphodiesterase (PDE2) inhibitor erythro-9-(2-hydroxy-3-nonyl)adenine or the nonselective PDE inhibitor 3-isobutyl-1-methylxanthine nor the phosphatase inhibitors okadaic acid or calyculin A unmasked an ISO-mimicking response of GLP-1. In SNARF-1-loaded myocytes, however, both ISO and GLP-1 caused an intracellular acidosis (DeltapH(i) -0.09+/-0.02 and -0.08+/-0.03, respectively). The specific GLP-1 antagonist exendin 9-39 and the cAMP inhibitory analog Rp-8CPT-cAMPS inhibited both the GLP-1-induced intracellular acidosis and the negative contractile effect. We conclude that in contrast to beta-adrenergic signaling, GLP-1 increases cAMP but fails to augment contraction, suggesting the existence of functionally distinct adenylyl cyclase/cAMP/protein kinase A compartments, possibly determined by unique receptor signaling microdomains that are not controlled by pertussis toxin-sensitive G proteins or by enhanced local PDE or phosphatase activation. Furthermore, GLP-1 elicits a cAMP-dependent modest negative inotropic effect produced by a decrease in myofilament-Ca(2+) responsiveness probably resulting from intracellular acidification.

Glucagon-like peptide-1 (GLP-1) enhances insulin secretion and synthesis. It also regulates the insulin, glucokinase, and GLUT2 genes. It mediates increases in glucose-stimulated insulin secretion by activating adenylyl cyclase and elevating free cytosolic calcium levels in the beta-cell. In addition, GLP-1 has been shown, both in vitro and in vivo, to be involved in regulation of the transcription factor, pancreatic duodenal homeobox-1 protein (PDX-1), by increasing its total protein levels, its translocation to the nucleus and its binding and resultant increase in activity of the insulin gene promoter in beta-cells of the pancreas. Here we have investigated the role of protein kinase A (PKA) in these processes in RIN 1046-38 cells. Three separate inhibitors of PKA, and a cAMP antagonist, inhibited the effects of GLP-1 on PDX-1. Furthermore, forskolin, (which stimulates adenylyl cyclase and insulin secretion), and 8-Bromo-cAMP, (an analog of cAMP which also stimulates insulin secretion), mimicked the effects of GLP-1 on PDX-1. These effects were also prevented by PKA inhibitors. Glucose-mediated increases in nuclear translocation of PDX-1 were not prevented by PKA inhibitors. Our results suggest that regulation of PDX-1 by GLP-1 occurs through activation of adenylyl cyclase and the resultant increase in intracellular cAMP, in turn, activates PKA, which ultimately leads to increases in PDX-1 protein levels and translocation of the protein to the nuclei of beta-cells.

There is a progressive impairment in beta-cell function with age. As a result, 19 percent of the U.S. population over the age of 65 is diagnosed with type 2 diabetes mellitus (DM). Glucagon-like peptide-1 (GLP-1) is a potent insulin secretagogue that has multiple synergetic effects on the glucose-dependent insulin secretion pathways of the beta-cell. This peptide and its longer-acting analog exendin-4 are currently under review as treatments for type 2 DM. In our work on the rodent model of glucose intolerance in aging, we found that GLP-1 is capable of rescuing the age-related decline in beta-cell function. We have shown that this is due to the ability of GLP-1 to 1) recruit beta-cells into a secretory mode; 2) upregulate the genes of the beta-cell glucose-sensing machinery; and 3) cause beta-cell differentiation and neogenesis. Our investigations into the mechanisms of action of GLP-1 began by using the reverse hemolytic plaque assay to quantify insulin secretion from individual cells of the RIN 1046-38 insulinoma cell line in response to acute treatment with the peptide. GLP-1 increases both the number of cells secreting insulin and the amount secreted per cell. This response to GLP-1 is retained even in the beta cell of the old (i.e., 22-month), glucose-intolerant Wistar rat, which exhibits a normal, first-phase insulin response to glucose following an acute bolus of GLP-1. Preincubation with GLP-1 (24 hours) potentiates glucose- and GLP-1-dependent insulin secretion and increases insulin content in the insulinoma cells. Treatment of old Wistar rats for 48 hours with GLP-1 leads to normalization of the insulin response and an increase in islet insulin content and mRNA levels of GLUT 2 and glucokinase. PDX-1, a transcriptional factor activator of these three genes, also is upregulated in the insulinoma cell line in aged rats and diabetic mice following treatment with GLP-1. Administration of GLP-1 to old rats leads to pancreatic cell proliferation, insulin-positive clusters, and an increase in beta-cell mass. This evidence led us to believe that GLP-1 is an endocrinotrophic factor. We used an acinar cell line to show that GLP-1 can directly cause the conversion of a putative pro-endocrine cell into an endocrine one. Thus, the actions of GLP-1 on the beta-cell are complex, with possible benefits to the diabetic patient that extend beyond a simple glucose-dependent increase in insulin secretion. The major limitation to GLP-1 as a clinical treatment is its short biological half-life. We have shown that the peptide exendin-4, originating in the saliva of the Gila monster, exhibits the same insulinotropic and endocrinotrophic properties as GLP-1 but is more potent and longer acting in rodents and humans.

Objective: A spring network can be used to represent the load transfer from a prosthetic stem into its surrounding bone. The study seeks to test the hypothesis that clinical patterns of bone remodelling can be simulated using a feedback that modifies the properties of the network depending on the load transfer., Design: A mathematical model is used to simulate the initial properties of the linear system and its subsequent remodelling behaviour., Background: A stable and pain-free transfer of physiological forces is essential for a clinically successful arthroplasty. Following surgery, bone remodelling and osteolysis can modify this load transfer., Methods: The combined effect of all factors that influence prosthesis-bone load transfer are summarised in the properties of 'inter-link' springs that connect springs representing the prosthesis and bone in the linear network. It is on these inter-links that a remodelling feedback operates, and their properties can be varied with time in response to deformation or force values., Results: Reducing inter-link stiffness leads to a broad distribution of load transfer, whilst an iso-elastic stem concentrates this transfer through the proximal and distal portions of a prosthesis. Physiological patterns of bone resorption and osteolysis become apparent in a time-series analysis of the feedback in the linear system. Specifically, osseo-integration requires a fixation of sufficient stiffness otherwise loosening will occur. Simulated osteolysis following osseo-integration loosens the implant from a distal to a proximal direction., Conclusions: Complex physiological bone remodelling patterns can emerge from a simple feedback within a linear system. Relevance. Implant loosening is presented here as an adverse response of a stable dynamic system caused by mechanical or biological stimuli.

Glucose homeostasis in mammals is maintained by insulin secretion from the beta-cells of the islets of Langerhans. Type 2 diabetes results either from primary beta-cell failure alone and/or a failure to secrete enough insulin to overcome insulin resistance. Here, we show that continuous infusion of glucagon-like peptide-1 (7-36) (GLP-1; an insulinotropic agent), to young and old animals, had effects on the beta-cell of the pancreas other than simply on the insulin secretory apparatus. Our previous studies on a rodent model of glucose intolerance, the aging Wistar rat, show that a plateau in islet size, insulin content, and beta-cell mass is reached at 13 months, despite a continuing increase in body weight. Continuous sc infusion of GLP-1 (1.5 pM/kg x min), over 5 days, resulted in normal glucose tolerance. Our current results in both young and old rats demonstrate that treatment caused an up-regulation of pancreatic-duodenum homeobox-1 (PDX-1) expression in islets and total pancreas, induced pancreatic cell proliferation, and beta-cell neogenesis. The effects on levels of PDX-1 messenger RNA were abrogated by simultaneous infusion of Exendin (9-39), a specific antagonist of GLP-1. PDX-1 protein levels increased 4-fold in whole pancreata and 6-fold in islets in response to treatment. Beta-cell mass increased to 7.2 +/- 0.58 from 4.88 +/- 0.38 mg, treated vs. control, respectively, P < 0.02. Total pancreatic insulin content also increased from 0.55 +/- 0.02 to 1.32 +/- 0.11 microg/mg total pancreatic protein. Therefore, GLP-1 would seem to be a unique therapy that can stimulate pancreatic cell proliferation and beta-cell differentiation in the pancreas of rodents.