Exogenous Glucagon-like Peptide-1 Research Articles

Role of central glucagon-like peptide-1 in hypothalamo-pituitary-adrenocortical facilitation following chronic stress

Central glucagon-like peptide-1 (GLP-1) regulates food intake, glucose homeostasis, and behavioral and neuroendocrine responses to acute stress. Given its pronounced role in acute stress regulation, the GLP-1 system is a prime candidate for mediating the prolonged drive of the hypothalamo-pituitary-adrenocortical axis by chronic stress. To test this hypothesis, we evaluated the necessity and sufficiency of GLP-1 for production of chronic stress-induced changes in HPA axis function. Exogenous GLP-1 or the GLP-1 receptor antagonist, dHG-exendin, were delivered into the 3rd ventricle of control animals or animals exposed to chronic variable stress (CVS) for 7 days. Animals in the CVS groups received GLP-1 or dHG-exendin immediately prior to each stress exposure. Prior to and at the end of the 7-day trial, chronically-stressed animals were subjected to a novel stressor to test for HPA axis facilitation. Neither GLP-1 nor dHG-exendin affected CVS-associated increases in adrenal weight or decreases in basal plasma glucose levels. In addition, neither exogenous GLP-1 nor dHG-exendin altered any index of HPA axis activity in unstressed rats. However, GLP-1 enhanced CVS-induced facilitation of corticosterone (but not ACTH) response to an acute stress, whereas dHG-exendin inhibited facilitation. In addition, GLP-1 decreased body weight in chronically-stressed animals. dHG-exendin increased food intake and body weight in unstressed animals, consistent with a tonic role for GLP-1 in body weight regulation. Overall, our data suggest that brain GLP-1 modulates HPA axis activity within the context of chronic stress, perhaps at the level of the adrenal gland.

The enteroinsular axis may mediate the diabetogenic effects of TCF7L2 polymorphisms

In an article in this issue of Diabetologia, Schafer et al. report that the acute insulin response to glucagon-like peptide-1 (GLP-1) is reduced in individuals with normal or impaired oral glucose tolerance undergoing a hyperglycaemic clamp experiment [1]. This effect was specific for individuals with the polymorphisms rs7903146 or rs12255372 in the gene sequence coding for transcription factor 7-like-2 (TCF7L2), polymorphisms associated with an increased risk of type 2 diabetes [2–4], and reduced insulin secretion [3, 5–11]. TCF7L2 plays a role in the WNT signalling pathway [12], which (among other things) influences the synthesis (and possibly secretion) of GLP-1 [13], and the embryological development of the endocrine pancreas [14]. It was therefore of interest to characterise the effect of GLP-1 upon insulin secretion in individuals with these polymorphisms. Schafer et al. started with the hypothesis that GLP-1 secretion, as tested following stimulation with oral glucose, would differ in those who carried diabetogenic TCF7L2 polymorphisms [3, 12]. This was not the case: TCF7L2 polymorphisms did not alter the pattern of GLP-1 secretion in response to oral glucose in any way. Although contrary to the expectations of the authors, this observation was nonetheless consistent with preliminary findings from another study [15]. The authors then went on to examine the influence of these polymorphisms on insulin secretion, using a standardised test that sequentially tested the insulin secretory responses to hyperglycaemia, exogenous GLP-1, and arginine, whilst separating the responses into early and second phases [1]. They found that carriers and controls produced identical responses to glucose and arginine, but the carriers had a reduced ability to secrete insulin in response to exogenous GLP-1. This confirmed that diabetes-associated TCF7L2 polymorphisms are associated with a lower incretin-mediated insulin secretory response. Contrary to the original hypothesis, however, this was mediated by under-responsiveness of endocrine pancreatic beta cells rather than an impairment in nutrient-stimulated GLP-1 secretion. This reduction in response appears to be specific to the insulinotropic action of GLP-1. Glucose and arginine apart, additional stimuli (e.g. β-adrenergic mimetics, sulfonylureas, leucine, gastric inhibitory polypeptide [GIP]) have yet to be studied. Such studies would help to separate specific defects in the response to GLP-1 from a more generalised beta cell secretory failure. Diabetologia (2007) 50:2413–2416 DOI 10.1007/s00125-007-0832-8

Glucagon-like peptide 1 (GLP-1) suppresses ghrelin levels in humans via increased insulin secretion

Introduction Ghrelin is an orexigenic peptide predominantly secreted by the stomach. Ghrelin plasma levels rise before meal ingestion and sharply decline afterwards, but the mechanisms controlling ghrelin secretion are largely unknown. Since meal ingestion also elicits the secretion of the incretin hormone glucagon-like peptide 1 (GLP-1), we examined whether exogenous GLP-1 administration reduces ghrelin secretion in humans. Patients and methods 14 healthy male volunteers were given intravenous infusions of GLP-1(1.2 pmol·kg − 1 min − 1 ) or placebo over 390 min. After 30 min, a solid test meal was served. Venous blood was drawn frequently for the determination of glucose, insulin, C-peptide, GLP-1 and ghrelin. Results During the infusion of exogenous GLP-1 and placebo, GLP-1 plasma concentrations reached steady-state levels of 139 ± 15 pmol/l and 12 ± 2 pmol/l, respectively ( p < 0.0001). During placebo infusion, ghrelin levels were significantly reduced in the immediate postprandial period ( p < 0.001), and rose again afterwards. GLP-1 administration prevented the initial postprandial decline in ghrelin levels, possibly as a result of delayed gastric emptying, and significantly reduced ghrelin levels 150 and 360 min after meal ingestion ( p < 0.05). The patterns of ghrelin concentrations in the experiments with GLP-1 and placebo administration were inversely related to the respective plasma levels of insulin and C-peptide. Conclusions GLP-1 reduces the rise in ghrelin levels in the late postprandial period at supraphysiological plasma levels. Most likely, these effects are indirectly mediated through its insulinotropic action. The GLP-1-induced suppression of ghrelin secretion might be involved in its anorexic effects.

Review: DPP IV inhibitors - current evidence and future directions

Glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are responsible for the higher insulin response after oral versus intravenous glucose administration. This effect is called the incretin effect. An impaired incretin effect in patients with type 2 diabetes focused attention on the possible importance of GIP and GLP-1 in diabetes mellitus. Metabolic control can be markedly improved by administration of exogenous GLP-1, but the native peptide is almost immediately degraded by the enzyme dipeptidyl peptidase IV (DPP IV) and, therefore, has little clinical value. Orally active inhibitors of DPP IV have now been developed and have been shown to enhance endogenous levels of GLP-1, resulting in improved glucose tolerance, lasting improvement of HbA1C and improved beta-cell function. In general the DPP IV inhibitors are weight neutral, and well tolerated. One DPP IV inhibitor, sitagliptin, was approved as a once-daily oral therapy for the treatment of type 2 diabetes mellitus in Mexico and USA in 2006, and Europe in 2007. Other DPP IV inhibitors are in late-stage clinical development.

Dietary Synergies in Appetite Control: Distal Gastrointestinal Tract

AbstractWOODS, STEPHEN C. Dietary synergies in appetite control: distal gastrointestinal tract.

Effects of Intravenous Glucagon-Like Peptide-1 on Gastric Emptying and Intragastric Distribution in Healthy Subjects: Relationships with Postprandial Glycemic and Insulinemic Responses

The inhibitory action of glucagon-like peptide-1 (GLP-1) on gastric emptying (GE) is likely to be important in mediating its effects on postprandial glycemia, appetite, and gastrointestinal symptoms. The objective of the study was to evaluate the effects of "low" and "high" doses of iv GLP-1 on GE, intragastric meal distribution, glycemia, insulinemia, and appetite. Ten healthy males were studied on 3 d. GE of a solid (ground beef)/liquid (glucose) meal, blood glucose, plasma insulin, glucagon and glucose-dependent insulinotropic peptide, appetite perceptions, and gastrointestinal symptoms were evaluated during iv infusion of: 1) GLP-1 at 0.3 pmol x kg(-1) x min(-1) (GLP-1 0.3); 2) GLP-1 at 0.9 pmol x kg(-1) x min(-1) (GLP-1 0.9); and 3) 0.9% saline. GLP-1 0.3 and 0.9 slowed GE of solid (intragastric retention at t = 100 min; saline: 28 +/- 5%; GLP-1 0.3: 53 +/- 6%; GLP-1 0.9: 58 +/- 7%; P < 0.001) and liquid (time for 50% of the liquid to empty, saline: 28 +/- 2 min; GLP-1 0.3: 42 +/- 7 min; GLP-1 0.9: 50 +/- 9 min; P < 0.001). Both doses of GLP-1 induced gastroparesis in about half the cohort and increased meal retention in the distal stomach (P < 0.05). GLP-1 attenuated the rises in glucose, insulin, and glucose-dependent insulinotropic peptide (P < 0.05). There was an inverse relationship between blood glucose at t = 15 min and the time for 50% of the liquid to empty (r = -0.70, P < 0.001). In healthy subjects exogenous GLP-1 increases meal retention in the distal stomach and, even when administered in a "low" dose, frequently induces "gastroparesis," and the effects of GLP-1 on postprandial glycemia are predictable on the basis of its effect on GE, supporting the concept that GE is a major target mechanism for the clinical use of incretin mimetics.

Glucagon-like peptide-1: from extract to agent. The Claude Bernard Lecture, 2005

The incretin hormones are intestinal polypeptides that enhance postprandial insulin secretion. Gastric inhibitory polypeptide (GIP) was initially thought to regulate gastric acid secretion, whereas glucagon-like peptide-1 (GLP-1) was discovered as a result of a systematic search for intestinal insulinotropic products of proglucagon gene expression. The incretin effect is markedly impaired or absent in patients with type 2 diabetes because of decreased secretion of GLP-1 and a loss of the insulinotropic effects of GIP. Metabolic control can be restored or greatly improved by administration of exogenous GLP-1, but this peptide is almost immediately degraded by dipeptidyl peptidase IV (DPP-IV), and therefore has little clinical value. DPP-IV-resistant analogues (incretin mimetics) have been identified or developed, and inhibitors of DPP-IV have also proved effective in protecting endogenous GLP-1 (and GIP) from degradation. Both principles have been tested in clinical studies. The incretin mimetics, administered by sc injection, have demonstrated lasting improvement in HbA(1)c in patients insufficiently treated with conventional oral therapy, and their use has been associated with steady weight loss for up to 2 years. The DPP-IV inhibitors, given once or twice daily by mouth, also appear to provide lasting improvement in HbA(1)c, but are weight-neutral. The first incretin mimetic has reached the market in the US, and applications for approval of the first inhibitors are expected to be filed early in 2006.

Secretion, degradation, and elimination of glucagon-like peptide 1 and gastric inhibitory polypeptide in patients with chronic renal insufficiency and healthy control subjects.

Glucagon-like peptide 1 (GLP-1) and gastric inhibitory polypeptide (GIP) are important factors in the pathogenesis of type 2 diabetes and have a promising therapeutic potential. Alterations of their secretion, in vivo degradation, and elimination in patients with chronic renal insufficiency (CRI) have not yet been characterized. Ten patients with CRI (aged 47 +/- 15 years, BMI 24.5 +/- 2.2 kg/m(2), and serum creatinine 2.18 +/- 0.86 mg/dl) and 10 matched healthy control subjects (aged 44 +/- 12 years, BMI 24.9 +/- 3.4 kg/m(2), and serum creatinine 0.89 +/- 0.10 mg/dl) were included. On separate occasions, an oral glucose tolerance test (75 g), an intravenous infusion of GLP-1 (0.5 pmol. kg(-1). min(-1) over 30 min), and an intravenous infusion of GIP (1.0 pmol. kg(-1). min(-1) over 30 min) were performed. Venous blood samples were drawn for the determination of glucose (glucose oxidase), insulin, C-peptide, GLP-1 (total and intact), and GIP (total and intact; specific immunoassays). Plasma levels of GIP (3-42) and GLP-1 (9-36 amide) were calculated. Statistics were performed using repeated-measures and one-way ANOVA. After the oral glucose load, plasma concentrations of intact GLP-1 and intact GIP reached similar levels in both groups (P = 0.31 and P = 0.87, respectively). The concentrations of GIP (3-42) and GLP-1 (9-36 amide) were significantly higher in the patients than in the control subjects (P = 0.0021 and P = 0.027, respectively). During and after the exogenous infusion, GLP-1 (9-36 amide) and GIP (3-42) reached higher plasma concentrations in the CRI patients than in the control subjects (P < 0.001 and P = 0.0033, respectively), whereas the plasma levels of intact GLP-1 and GIP were not different between the groups (P = 0.29 and P = 0.27, respectively). Plasma half-lives were 3.4 +/- 0.6 and 2.3 +/- 0.4 min for intact GLP-1 (P = 0.13) and 5.3 +/- 0.8 and 3.3 +/- 0.4 min for the GLP-1 metabolite (P = 0.029) for CRI patients vs. healthy control subjects, respectively. Plasma half-lives of intact GIP were 6.9 +/- 1.4 and 5.0 +/- 1.2 min (P = 0.31) and 38.1 +/- 6.0 and 22.4 +/- 3.0 min for the GIP metabolite (P = 0.032) for CRI patients vs. healthy control subjects, respectively. Insulin concentrations tended to be lower in the patients during all experiments, whereas C-peptide levels tended to be elevated. These data underline the importance of the kidneys for the final elimination of GIP and GLP-1. The initial dipeptidyl peptidase IV-mediated degradation of both hormones is almost unaffected by impairments in renal function. Delayed elimination of GLP-1 and GIP in renal insufficiency may influence the pharmacokinetics and pharmacodynamics of dipeptidyl peptidase IV-resistant incretin derivatives to be used for the treatment of patients with type 2 diabetes.

Cardiac function in mice lacking the glucagon-like peptide-1 receptor.

Glucagon-like peptide-1 (GLP-1) acts via its G protein-coupled receptor (GLP-1R) to regulate blood glucose. Although the GLP-1R is widely expressed in peripheral tissues, including the heart, and exogenous GLP-1 administration increases heart rate and blood pressure in rodents, the physiological importance of GLP-1R action in the cardiovascular system remains unclear. We now show that 2-month-old mice with genetic deletion of the GLP-1R (GLP-1R(-/-)) exhibit reduced resting heart rate and elevated left ventricular (LV) end diastolic pressure compared with CD-1 wild-type controls. At the age of 5 months, echocardiography and histology demonstrate increased LV thickness in GLP-1R(-/-) mice. Although baseline hemodynamic parameters of GLP-1R(-/-) did not differ significantly from those of wild type, GLP-1R(-/-) mice displayed impaired LV contractility and diastolic function after insulin administration. The defective cardiovascular response to insulin was not attributable to a generalized defect in the stress response, because GLP-1R(-/-) mice responded appropriately to insulin with increased c-fos expression in the hypothalamus and increased circulating levels of glucagon and epinephrine. Furthermore, LV contractility after exogenous epinephrine infusion was also reduced in GLP-1R(-/-) mice. These findings provide new evidence implicating an essential role for GLP-1R in the control of murine cardiac structure and function in vivo.

Glucagon-like peptide 1 as a regulator of food intake and body weight: therapeutic perspectives

After ingestion of carbohydrate- and fat-rich meals, the incretin hormone glucagon-like peptide 1 (GLP-1) is secreted from the L-cells in the distal put into the circulation. Its major physiological effect lies in a strongly glucose-dependent stimulation of insulin secretion from pancreatic B-cells. Furthermore, GLP-1 suppresses glucagon secretion, stimulates B-cell neogenesis as well as proinsulin biosynthesis and inhibits gastric emptying and acid secretion. Recently, GLP-1 could be shown to reduce caloric intake and to enhance satiety, most likely via specific receptors within the central nervous system, resulting in reduced weight gain in experimental animals. In nondiabetic and Type 2 diabetic human subjects, exogenous GLP-1 reduces hunger, caloric intake and body weight. Therefore, in addition to its well-characterized antidiabetogenic effect, the anorectic effect may offer GLP-1 a potential in the pharmacotherapy of obesity. It is still unknown whether the GLP-1 effect on caloric intake is sustained after long-term treatment. Furthermore, the exact mechanisms by which the peptide exerts its biological effects have not yet been clarified. Due to the rapid degradation of native GLP-1, its therapeutic application is limited by the short half-life. Therefore, suitable modes of administration are needed in order to reach stable plasma concentrations. The present review aims to describe the role of GLP-1 in the central regulation of feeding and to discuss its possible application in the pharmacotherapy of obesity.

Preservation of active incretin hormones by inhibition of dipeptidyl peptidase IV suppresses meal-induced incretin secretion in dogs.

The incretin hormones glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are degraded by dipeptidyl peptidase IV (DPP IV), thereby losing insulinotropic activity. DPP IV inhibition reduces exogenous GLP-1 degradation, but the extent of endogenous incretin protection has not been fully assessed, largely because suitable assays which distinguish between intact and degraded peptides have been unavailable. Using newly developed assays for intact GLP-1 and GIP, the effect of DPP IV inhibition on incretin hormone metabolism was examined. Conscious dogs were given NVP-DPP728, a specific DPP IV inhibitor, at a dose that inhibited over 90% of plasma DPP IV for the first 90 min following treatment. Total and intact incretin concentrations increased (P<0.0001) following a mixed meal, but on control days (vehicle infusion), intact peptide concentrations were lower (P<0.01) than total peptide concentrations (22.6 +/- 1.2% intact GIP; 10.1 +/- 0.4% intact GLP-1). Following inhibitor treatment, the proportion of intact peptide increased (92.5 +/- 4.3% intact GIP, P<0.0001; 99.0 +/- 22.6% intact GLP-1, P<0.02). Active (intact) incretins increased after NVP-DPP728 (from 4797 +/- 364 to 10 649 +/- 106 pM x min for GIP, P<0.03; from 646 +/- 134 to 2822 +/- 528 pM x m in for GLP-1, P<0.05). In contrast, total incretins fell (from 21 632 +/- 654 to 12 084 +/- 1723 pM x min for GIP, P<0.002; from 5145 +/- 677 to 3060 +/- 601 pM x min for GLP-1, P<0.05). Plasma glucose, insulin and glucagon concentrations were unaltered by the inhibitor. We have concluded that DPP IV inhibition with NVP-DPP728 prevents N-terminal degradation of endogenous incretins in vivo, resulting in increased plasma concentrations of intact, biologically active GIP and GLP-1. Total incretin secretion was reduced by DPP IV inhibition, suggesting the possibility of a feedback mechanism.

Lack of germline mutations in the preproglucagon gene region coding for glucagon-like peptide 1 in Type 2 diabetic (NIDDM) patients.

Glucagon-like peptide 1 (GLP-1) has antidiabetic effects and many facets of Type 2 diabetes could theoretically be the consequence of a reduction in or lack of GLP-1 function. Exogenous GLP-1 is exquisitely effective in Type 2 diabetic patients, making receptor defects unlikely. GLP-1 responses after meals as detected by radioimmunoassay are not overtly reduced in Type 2 diabetic patients. Therefore, a sequence analysis of exon 2 of the preproglucagon gene coding for the GLP-1 protein was initiated in order to exclude potential germline mutations. 24 Type 2 diabetic patients and in 14 control subjects with normal oral glucose tolerance (WHO criteria) were studied. In all specimens of peripheral blood leukocyte DNA examined, no germline mutations of the GLP-1 sequence were identified, thus excluding mutations in the GLP-1 sequence as a major contributor to the pathophysiological appearance of the Type 2 diabetic phenotype. Rare mutations, however, cannot be excluded due to the small number of Type 2 diabetic patients examined.

Is glucagon-like peptide 1 an incretin hormone?

Glucagon-like peptide-1 (GLP-1) was predicted, based on the proglucagon gene sequence. It is synthesised by specific post-translational processing in L cells (lower intestine) and secreted mainly as "truncated" GLP-1 [7-36 amide] in response to nutrient ingestion. Glucagon-like peptide-1 stimulates insulin secretion during hyperglycaemia, suppresses glucagon secretion, stimulates (pro)insulin biosynthesis and decelerates gastric emptying and acid secretion. On intracerebroventricular injection, GLP-1 reduces food intake in rodents. A GLP-1 receptor antagonist or GLP-1 antisera have been shown to reduce meal-stimulated insulin secretion in animals, suggesting that GLP-1 has a physiological "incretin" function (augmentation of postprandial insulin secretion due to intestinal hormones) for GLP-1. In healthy human subjects, exogenous GLP-1 slows gastric emptying. Consequently, postprandial insulin secretion is reduced, not augmented. Thus, a participation of this peptide in the incretin effect of non-diabetic humans has not been definitely proven. Nevertheless, it has potent insulinotropic activity, especially during hyperglycaemia. This suggests new therapeutic options for patients with Type II (non-insulin-dependent) diabetes mellitus. On the other hand, most L cells are located in the lower small intestine. Potent inhibitory actions of GLP-1 on upper gastrointestinal motor and digestive functions (e. g. gastric emptying and acid secretion) in response to nutrients placed into the ileal lumen, argue for a role of this peptide as an "ileal brake". Malassimilation and diarrhea leading to the erroneous presence of nutrients in the lower gut may, via GLP-1, delay gastric emptying and reduce upper gut motility and thereby prevent further caloric losses.

Inhibition of the activity of dipeptidyl-peptidase IV as a treatment for type 2 diabetes.

The insulinotropic hormone, glucagon-like peptide 1 (GLP-1), which has been proposed as a new treatment for type 2 diabetes, is metabolized extremely rapidly by the ubiquitous enzyme, dipeptidyl peptidase IV (DPP-IV), resulting in the formation of a metabolite, which may act as an antagonist at the GLP-1 receptor. Because of this, the effects of single injections of GLP-1 are short-lasting, and for full demonstration of its antidiabetogenic effects, continuous intravenous infusion is required. To exploit the therapeutic potential of GLP-1 clinically, we here propose the use of specific inhibitors of DPP-IV. We have demonstrated that the administration of such inhibitors may completely protect exogenous GLP-1 from DPP-IV-mediated degradation, thereby greatly enhancing its insulinotropic effect, and provided evidence that endogenous GLP-1 may be equally protected. Preliminary studies by others in glucose-intolerant experimental animals have shown that DPP-IV inhibition greatly ameliorates the condition. GLP-1 has multifaceted actions, which include stimulation of insulin gene expression, trophic effects on the beta-cells, inhibition of glucagon secretion, promotion of satiety, inhibition of food intake, and slowing of gastric emptying, all of which contribute to normalizing elevated glucose levels. Because of this, we predict that inhibition of DPP-IV, which will elevate the levels of active GLP-1 and reduce the levels of the antagonistic metabolite, may be useful to treat impaired glucose tolerance and perhaps prevent transition to type 2 diabetes. The actions of DPP-IV, other than degradation of GLP-1, particularly in the immune system are discussed, but it is concluded that side effects of inhibition therapy are likely to be mild. Thus, DPP-IV inhibition may be an effective supplement to diet and exercise treatment in attempts to prevent the deterioration of glucose metabolism associated with the Western lifestyle.

Exaggerated secretion of glucagon-like peptide-1 (GLP-1) could cause reactive hypoglycaemia.

The plasma concentrations of the insulinotropic incretin hormone, glucagon-like peptide-1 (GLP-1) are abnormally high after oral glucose in partially gastrectomised subjects with reactive hypoglycaemia, suggesting a causal relationship. Because of the glucose-dependency of its effects, it is impossible to induce hypoglycaemia in normal subjects in the basal state by exogenous GLP-1, regardless of dose. To further assess the role of the incretin hormones in reactive hypoglycaemia, we reproduced the glucose and hormone profiles of the patients with reactive hypoglycaemia in 8 healthy volunteers in 4 separate protocols: 1) i.v. infusion of glucose (25 g) alone, 2) glucose together with i.v. GLP-1 infusion, and 3) and 4) glucose together with i.v. infusion of the other incretin hormone, glucose-dependent insulinotropic polypeptide (GIP), at two different infusion rates. The plasma glucose, GLP-1 and GIP concentrations (low dose) obtained were comparable with those of the patients. With GLP-1, infusion of a total of 33.4 +/- 1.3 g glucose was required to obtain plasma glucose concentrations similar to those obtained by glucose infusion alone; with low GIP, 28.0 +/- 1.2 g and with high GIP 38.4 +/- 3.5 g. Insulin concentrations increased 10-fold with GLP-1 compared with i.v. glucose alone, but less with high and low GIP. In contrast, C-peptide concentrations were similar after GLP-1 and high GIP. After termination of i.v. glucose the lowest glucose concentrations were 4.5 (3.7-4.9) (median, range) for glucose alone; 2.4 (1.9-2.8) mmol/l with GLP-1; 3.7 (2.6-4.0) with low GIP and 3.3 (2.1-4.2) with high GIP. Thus, the exaggerated GLP-1 response to nutrients in patients with accelerated gastric emptying could be responsible for their high incidence of postprandial reactive hypoglycaemia.

Glucagon-Like Peptide-1-(7-36) Amide and Peptide YY Mediate Intraduodenal Fat-Induced Inhibition of Acid Secretion in Dogs

Intraduodenal fat inhibits gastric acid secretion via the release of one or more hormonal enterogastrones thought to arise from ileo-colonic mucosa. This study determined whether glucagon-like peptide-1 (GLP-1)-(7–36) amide and peptide YY (PYY), colocalized in L cells found in the ileum, mediate intraduodenal fat-induced inhibition of stimulated gastric acid, and evaluated the influence of cholecystokinin-A (CCK-A) receptor activation. Gastric acid secretion in response to duodenal perfusions of 8% peptone was measured in conscious dogs with gastric and duodenal cannulas. Intraduodenal administration of a 10% fat emulsion suppressed gastric acid secretion by 72 ± 4% (P < 0.001) and increased plasma levels of GLP-1 and PYY by 44 ± 5 and 46 ± 4 fmol/ml, respectively (both P < 0.01). Pretreatment with the CCK-A receptor antagonist MK-329 completely reversed the inhibition of gastric acid by fat, suppressed rises of plasma GLP-1 (maximum change, 23 ± 4 fmol/ml), and reduced plasma PYY responses to baseline. Intravenous infusions of 50 pmol/kg·h GLP-1 or PYY, which reproduced plasma elevations after intraduodenal fat, inhibited gastric acid secretion by 66 ± 5% and 51 ± 6%, respectively (both P < 0.01); coinfusions of GLP-1 and PYY abolished gastric acid secretion (P < 0.001) without influencing plasma gastrin or somatostatin. Pretreatment with 1500 pmol/kg·h of the GLP-1 antagonist exendin-(9–39) amide did not alter the magnitude of inhibition of gastric acid caused by exogenous GLP-1. These results indicate that GLP-1 and PYY released by intraduodenal fat, in part through CCK-dependent pathways, are major enterogastrones in dogs. This inhibitory action occurs independent of circulating concentrations of somatostatin and gastrin and appears to involve a GLP-1 receptor distinct from that mediating incretin effects.

Glucagon-like peptide-1-(7-36) amide and peptide YY mediate intraduodenal fat-induced inhibition of acid secretion in dogs.

Intraduodenal fat inhibits gastric acid secretion via the release of one or more hormonal enterogastrones thought to arise from ileocolonic mucosa. This study determined whether glucagon-like peptide-1 (GLP-1)-(7-36) amide and peptide YY (PYY), colocalized in L cells found in the ileum, mediate intraduodenal fat-induced inhibition of stimulated gastric acid, and evaluated the influence of cholecystokinin-A (CCK-A) receptor activation. Gastric acid secretion in response to duodenal perfusions of 8% peptone was measured in conscious dogs with gastric and duodenal cannulas. Intraduodenal administration of a 10% fat emulsion suppressed gastric acid secretion by 72 +/- 4% (P < 0.001) and increased plasma levels of GLP-1 and PYY by 44 +/- 5 and 46 +/- 4 fmol/ml, respectively (both P < 0.01). Pretreatment with the CCK-A receptor antagonist MK-329 completely reversed the inhibition of gastric acid by fat, suppressed rises of plasma GLP-1 (maximum change, 23 +/- 4 fmol/ml), and reduced plasma PYY responses to baseline. Intravenous infusions of 50 pmol/kg x h GLP-1 or PYY, which reproduced plasma elevations after intraduodenal fat, inhibited gastric acid secretion by 66 +/- 5% and 51 +/- 6%, respectively (both P < 0.01); coinfusions of GLP-1 and PYY abolished gastric acid secretion (P < 0.001) without influencing plasma gastrin or somatostatin. Pretreatment with 1500 pmol/kg x h of the GLP-1 antagonist exendin-(9-39) amide did not alter the magnitude of inhibition of gastric acid caused by exogenous GLP-1. These results indicate that GLP-1 and PYY released by intraduodenal fat, in part through CCK-dependent pathways, are major enterogastrones in dogs. This inhibitory action occurs independent of circulating concentrations of somatostatin and gastrin and appears to involve a GLP-1 receptor distinct from that mediating incretin effects.

Glucagon-like peptide 1 undergoes differential tissue-specific metabolism in the anesthetized pig.

Glucagon-like peptide 1 (GLP-1) metabolism was studied in halothane-anesthetized pigs (n = 7) using processing-independent (PI) and COOH-terminal (C) radioimmunoassays (RIA) and an enzyme-linked immunosorbent assay (ELISA) specific for biologically active GLP-1. Renal extraction of endogenous GLP-1 was detected by PI-RIA (33.1 +/- 13.3%) and C-RIA (16.0 +/- 6.3%) and by all assays during GLP-1 infusion (ELISA, 69.4 +/- 6.3%; PI-RIA, 32.6 +/- 7.3%; C-RIA, 43.7 +/- 3.4%), indicating substantial fragmentation. Hepatic and pulmonary degradation were undetectable under basal conditions, but exogenous GLP-1 elimination by the liver (43.6 +/- 8.9%) and lungs (10.1 +/- 3.2%) was measured by ELISA, suggesting primarily NH2-terminal degradation. Endogenous GLP-1 extraction by the hindleg was only detected by C-RIA (16.0 +/- 6.3%). During GLP-1 infusion, greater hindleg extraction was measured by ELISA (38.5 +/- 6.8%) and C-RIA (33.0 +/- 6.4%) than by PI-RIA (11.4 +/- 3.2%), indicating limited degradation at each terminus or more substantial COOH-terminal degradation. A shorter (P < 0.01) plasma half-life was revealed by ELISA (1.5 +/- 0.4 min) than by PI-RIA (4.5 +/- 0.6 min) or C-RIA (4.1 +/- 0.5 min). Metabolic clearance rates measured by PI-RIA (20.0 +/- 3.8 ml.min-1.kg-1) and C-RIA (15.5 +/- 1.6 ml.min-1.kg-1) were shorter (P < 0.01) than that measured by ELISA (106.8 +/- 14.7 ml.min-1.kg-1). Tissue-specific differential metabolism of GLP-1 occurs, and NH2-terminal degradation, rendering GLP-1 inactive, is particularly important in its clearance.

Gastric emptying, glucose responses, and insulin secretion after a liquid test meal: effects of exogenous glucagon-like peptide-1 (GLP-1)- (7-36) amide in type 2 (noninsulin-dependent) diabetic patients

Gastric emptying, glucose responses, and insulin secretion after a liquid test meal: effects of exogenous glucagon-like peptide-1 (GLP-1)- (7-36) amide in type 2 (noninsulin-dependent) diabetic patients

Comparison of sandwich enzyme-linked immunoadsorbent assay and radioimmunoassay for determination of exogenous glucagon-like peptide-1(7–36)amide in plasma

A sensitive sandwich enzyme-linked immunoadsorbent assay (ELISA) for determination of exogenous glucagon-like peptide-1(7–36)amide (GLP-1(7–36)amide) in plasma samples from pharmacokinetic studies is described. The assay employs an N-terminally directed antibody and a C-terminally directed antibody. The ELISA has a working range from 10 to 500 pmol 1 −1, and can be applied to plasma samples from humans, dogs, pigs, minipigs, cats, rabbits, and rats. The assay was compared to a validated radioimmunoassay (RIA), employing an antibody directed against the mid-region of GLP-1. After s.c. administration of GLP-1(7–36)amide, the plasma immunoreactivity of GLP-1 (P-GLP-1-IR) measured by ELISA was markedly lower than P-GLP-1-IR measured by RIA. After HPLC fractionation of plasma samples with subsequent RIA and ELISA analyses of the fractions, this difference was shown to be due to cross reaction with biologically inactive fragments of GLP-1(7–36)amide in the RIA but not in the ELISA.