The physiology of glucagon-like peptide 1

Glucagon-like peptide 1 (GLP-1) is a 30-amino acid peptide hormone produced in the intestinal epithelial endocrine L-cells by differential processing of proglucagon, the gene which is expressed in these cells. The current knowledge regarding regulation of proglucagon gene expression in the gut and in the brain and mechanisms responsible for the posttranslational processing are reviewed. GLP-1 is released in response to meal intake, and the stimuli and molecular mechanisms involved are discussed. GLP-1 is extremely rapidly metabolized and inactivated by the enzyme dipeptidyl peptidase IV even before the hormone has left the gut, raising the possibility that the actions of GLP-1 are transmitted via sensory neurons in the intestine and the liver expressing the GLP-1 receptor. Because of this, it is important to distinguish between measurements of the intact hormone (responsible for endocrine actions) or the sum of the intact hormone and its metabolites, reflecting the total L-cell secretion and therefore also the possible neural actions. The main actions of GLP-1 are to stimulate insulin secretion (i.e., to act as an incretin hormone) and to inhibit glucagon secretion, thereby contributing to limit postprandial glucose excursions. It also inhibits gastrointestinal motility and secretion and thus acts as an enterogastrone and part of the "ileal brake" mechanism. GLP-1 also appears to be a physiological regulator of appetite and food intake. Because of these actions, GLP-1 or GLP-1 receptor agonists are currently being evaluated for the therapy of type 2 diabetes. Decreased secretion of GLP-1 may contribute to the development of obesity, and exaggerated secretion may be responsible for postprandial reactive hypoglycemia.

Type 2 diabetes mellitus

Type 2 diabetes mellitus (T2DM) is an expanding global health problem, closely linked to the epidemic of obesity. Individuals with T2DM are at high risk for both microvascular complications (including retinopathy, nephropathy and neuropathy) and macrovascular complications (such as cardiovascular comorbidities), owing to hyperglycaemia and individual components of the insulin resistance (metabolic) syndrome. Environmental factors (for example, obesity, an unhealthy diet and physical inactivity) and genetic factors contribute to the multiple pathophysiological disturbances that are responsible for impaired glucose homeostasis in T2DM. Insulin resistance and impaired insulin secretion remain the core defects in T2DM, but at least six other pathophysiological abnormalities contribute to the dysregulation of glucose metabolism. The multiple pathogenetic disturbances present in T2DM dictate that multiple antidiabetic agents, used in combination, will be required to maintain normoglycaemia. The treatment must not only be effective and safe but also improve the quality of life. Several novel medications are in development, but the greatest need is for agents that enhance insulin sensitivity, halt the progressive pancreatic β-cell failure that is characteristic of T2DM and prevent or reverse the microvascular complications. For an illustrated summary of this Primer, visit: http://go.nature.com/V2eGfN.

Preserved incretin activity of glucagon-like peptide 1 [7-36 amide] but not of synthetic human gastric inhibitory polypeptide in patients with type-2 diabetes mellitus

In type-2 diabetes, the overall incretin effect is reduced. The present investigation was designed to compare insulinotropic actions of exogenous incretin hormones (gastric inhibitory peptide [GIP] and glucagon-like peptide 1 [GLP-1] [7-36 amide]) in nine type-2 diabetic patients (fasting plasma glucose 7.8 mmol/liter; hemoglobin A1c 6.3 +/- 0.6%) and in nine age- and weight-matched normal subjects. Synthetic human GIP (0.8 and 2.4 pmol/kg.min over 1 h each), GLP-1 [7-36 amide] (0.4 and 1.2 pmol/kg.min over 1 h each), and placebo were administered under hyperglycemic clamp conditions (8.75 mmol/liter) in separate experiments. Plasma GIP and GLP-1 [7-36 amide] concentrations (radioimmunoassay) were comparable to those after oral glucose with the low, and clearly supraphysiological with the high infusion rates. Both GIP and GLP-1 [7-36 amide] dose-dependently augmented insulin secretion (insulin, C-peptide) in both groups (P < 0.05). With GIP, the maximum effect in type-2 diabetic patients was significantly lower (by 54%; P < 0.05) than in normal subjects. With GLP-1 [7-36 amide] type-2 diabetic patients reached 71% of the increments in C-peptide of normal subjects (difference not significant). Glucagon was lowered during hyperglycemic clamps in normal subjects, but not in type-2 diabetic patients, and further by GLP-1 [7-36 amide] in both groups (P < 0.05), but not by GIP. In conclusion, in mild type-2 diabetes, GLP-1 [7-36 amide], in contrast to GIP, retains much of its insulinotropic activity. It also lowers glucagon concentrations.

Glucagon-like peptide 1 (GLP-1)

BACKGROUND

The glucagon-like peptide-1 (GLP-1) is a multifaceted hormone with broad pharmacological potential. Among the numerous metabolic effects of GLP-1 are the glucose-dependent stimulation of insulin secretion, decrease of gastric emptying, inhibition of food intake, increase of natriuresis and diuresis, and modulation of rodent β-cell proliferation. GLP-1 also has cardio- and neuroprotective effects, decreases inflammation and apoptosis, and has implications for learning and memory, reward behavior, and palatability. Biochemically modified for enhanced potency and sustained action, GLP-1 receptor agonists are successfully in clinical use for the treatment of type-2 diabetes, and several GLP-1-based pharmacotherapies are in clinical evaluation for the treatment of obesity.

SCOPE OF REVIEW

In this review, we provide a detailed overview on the multifaceted nature of GLP-1 and its pharmacology and discuss its therapeutic implications on various diseases.

MAJOR CONCLUSIONS

Since its discovery, GLP-1 has emerged as a pleiotropic hormone with a myriad of metabolic functions that go well beyond its classical identification as an incretin hormone. The numerous beneficial effects of GLP-1 render this hormone an interesting candidate for the development of pharmacotherapies to treat obesity, diabetes, and neurodegenerative disorders.

Glucagon-like peptide 1 promotes satiety and suppresses energy intake in humans

We examined the effect of intravenously infused glucagon-like peptide 1 (GLP-1) on subjective appetite sensations after an energy-fixed breakfast, and on spontaneous energy intake at an ad libitum lunch. 20 young, healthy, normal-weight men participated in a placebo-controlled, randomized, blinded, crossover study. Infusion (GLP-1, 50 pmol/ kg.h or saline) was started simultaneously with initiation of the test meals. Visual analogue scales were used to assess appetite sensations throughout the experiment and the palatability of the test meals. Blood was sampled throughout the day for analysis of plasma hormone and substrate levels. After the energy-fixed breakfast, GLP-1 infusion enhanced satiety and fullness compared with placebo (treatment effect: P < 0.03). Furthermore, spontaneous energy intake at the ad libitum lunch was reduced by 12% by GLP-1 infusion compared with saline (P = 0.002). Plasma GLP-1, insulin, glucagon, and blood glucose profiles were affected significantly by the treatment (P < 0.002). In conclusion, the results show that GLP-1 enhanced satiety and reduced energy intake and thus may play a physiological regulatory role in controlling appetite and energy intake in humans.

Effect of 6-week course of glucagon-like peptide 1 on glycaemic control, insulin sensitivity, and beta-cell function in type 2 diabetes: a parallel-group study

BACKGROUND

Glucagon-like peptide 1 (GLP-1) has been proposed as a treatment for type 2 diabetes. We have investigated the long-term effects of continuous administration of this peptide hormone in a 6-week pilot study.

METHODS

20 patients with type 2 diabetes were alternately assigned continuous subcutaneous infusion of GLP-1 (n=10) or saline (n=10) for 6 weeks. Before (week 0) and at weeks 1 and 6, they underwent beta-cell function tests (hyperglycaemic clamps), 8 h profiles of plasma glucose, insulin, C-peptide, glucagon, and free fatty acids, and appetite and side-effect ratings on 100 mm visual analogue scales; at weeks 0 and 6 they also underwent dexascanning, measurement of insulin sensitivity (hyperinsulinaemic euglycaemic clamps), haemoglobin A(1c), and fructosamine. The primary endpoints were haemoglobin A(1c) concentration, 8-h profile of glucose concentration in plasma, and beta-cell function (defined as the first-phase response to glucose and the maximum insulin secretory capacity of the cell). Analyses were per protocol.

FINDINGS

One patient assigned saline was excluded because no veins were accessible. In the remaining nine patients in that group, no significant changes were observed except an increase in fructosamine concentration (p=0.0004). In the GLP-1 group, fasting and 8 h mean plasma glucose decreased by 4.3 mmol/L and 5.5 mmol/L (p<0.0001). Haemoglobin A(1c) decreased by 1.3% (p=0.003) and fructosamine fell to normal values (p=0.0002). Fasting and 8 h mean concentrations of free fatty acids decreased by 30% and 23% (p=0.0005 and 0.01, respectively). Gastric emptying was inhibited, bodyweight decreased by 1.9 kg, and appetite was reduced. Both insulin sensitivity and beta-cell function improved (p=0.003 and p=0.003, respectively). No important side-effects were seen.

INTERPRETATION

GLP-1 could be a new treatment for type 2 diabetes, though further investigation of the long-term effects of GLP-1 is needed.

Normalization of fasting hyperglycaemia by exogenous glucagon-like peptide 1 (7-36 amide) in type 2 (non-insulin-dependent) diabetic patients

Glucagon-like peptide 1 (GLP-1) (7-36 amide) is a physiological incretin hormone that is released after nutrient intake from the lower gut and stimulates insulin secretion at elevated plasma glucose concentrations. Previous work has shown that even in Type 2 (non-insulin-dependent) diabetic patients GLP-1 (7-36 amide) retains much of its insulinotropic action. However, it is not known whether the magnitude of this response is sufficient to normalize plasma glucose in Type 2 diabetic patients with poor metabolic control. Therefore, in 10 Type 2 diabetic patients with unsatisfactory metabolic control (HbA1c 11.6 +/- 1.7%) on diet and sulphonylurea therapy (in some patients supplemented by metformin or acarbose), 1.2 pmol x kg-1 x min-1 GLP-1 (7-36 amide) or placebo was infused intravenously in the fasting state (plasma glucose 13.1 +/- 0.6 mmol/l). In all patients, insulin (by 17.4 +/- 4.7 nmol x 1-1 x min; p = 0.0157) and C-peptide (by 228.0 +/- 39.1 nmol x 1-1 x min; p = 0.0019) increased significantly over basal levels, glucagon was reduced (by -1418 +/- 308 pmol x 1-1 x min) and plasma glucose reached normal fasting concentrations (4.9 +/- 0.3 mmol/l) within 4 h of GLP-1 (7-36 amide) administration, but not with placebo. When normal fasting plasma glucose concentrations were reached insulin returned towards basal levels and plasma glucose concentrations remained stable despite the ongoing infusion of GLP-1 (7-36 amide). Therefore, exogenous GLP-1 (7-36 amide) is an effective means of normalizing fasting plasma glucose concentrations in poorly-controlled Type 2 diabetic patients.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of gastric inhibitory polypeptide signaling prevents obesity

Secretion of gastric inhibitory polypeptide (GIP), a duodenal hormone, is primarily induced by absorption of ingested fat. Here we describe a novel pathway of obesity promotion via GIP. Wild-type mice fed a high-fat diet exhibited both hypersecretion of GIP and extreme visceral and subcutaneous fat deposition with insulin resistance. In contrast, mice lacking the GIP receptor (Gipr(-/-)) fed a high-fat diet were clearly protected from both the obesity and the insulin resistance. Moreover, double-homozygous mice (Gipr(-/-), Lep(ob)/Lep(ob)) generated by crossbreeding Gipr(-/-) and obese ob/ob (Lep(ob)/Lep(ob)) mice gained less weight and had lower adiposity than Lep(ob)/Lep(ob) mice. The Gipr(-/-) mice had a lower respiratory quotient and used fat as the preferred energy substrate, and were thus resistant to obesity. Therefore, GIP directly links overnutrition to obesity and it is a potential target for anti-obesity drugs.

Reduced postprandial concentrations of intact biologically active glucagon-like peptide 1 in type 2 diabetic patients

Incretin hormones importantly enhance postprandial insulin secretion but are rapidly degraded to inactive metabolites by ubiquitous dipeptidyl peptidase IV. The concentrations of the intact biologically active hormones remain largely unknown. Using newly developed assays for intact glucagon-like peptide (GLP)-1 and glucose-dependent insulinotropic polypeptide (GIP), we measured plasma concentrations after a mixed breakfast meal (566 kcal) in 12 type 2 diabetic patients (age 57 years [range 49-67], BMI 31 kg/m2 [27-38], and HbA1c 9.2% [7.0-12.5]) and 12 matched healthy subjects. The patients had fasting hyperglycemia (10.7 mmol/l [8.0-14.8]) increasing to 14.6 mmol/l (11.5-21.5) 75 min after meal ingestion. Fasting levels of insulin and C-peptide were similar to those of the healthy subjects, but the postprandial responses were reduced and delayed. Fasting levels and meal responses were similar between patients and healthy subjects for total GIP (intact + metabolite) as well as intact GIP, except for a small decrease in the patients at 120 min; integrated areas for intact hormone (area under the curve [AUC]INT) averaged 52 +/- 4% (for patients) versus 56 +/- 3% (for control subjects) of total hormone AUC (AUC(TOT)). AUC(INT) for GLP-1 averaged 48 +/- 2% (for patients) versus 51 +/- 5% (for control subjects) of AUC(TOT). AUC(TOT) for GLP-1 as well as AUC(INT) tended to be reduced in the patients (P = 0.2 and 0.07, respectively); but the profile of the intact GLP-1 response was characterized by a small early rise (30-45 min) and a significantly reduced late phase (75-150 min) (P < 0.02). The measurement of intact incretin hormones revealed that total as well as intact GIP responses were minimally decreased in patients with type 2 diabetes, whereas the late intact GLP-1 response was strongly reduced, supporting the hypothesis that an impaired function of GLP-1 as a transmitter in the enteroinsular axis contributes to the inappropriate insulin secretion in type 2 diabetes.

Antidiabetogenic effect of glucagon-like peptide-1 (7-36)amide in normal subjects and patients with diabetes mellitus

BACKGROUND

Glucagon-like peptide-1 (7-36) amide (glucagon-like insulinotropic peptide, or GLIP) is a gastrointestinal peptide that potentiates the release of insulin in physiologic concentrations. Its effects in patients with diabetes mellitus are not known.

METHODS

We compared the effect of an infusion of GLIP that raised plasma concentrations of GLIP twofold with the effect of an infusion of saline, on the meal-related release of insulin, glucagon, and somatostatin in eight normal subjects, nine obese patients with non-insulin-dependent diabetes mellitus (NIDDM), and eight patients with insulin-dependent diabetes mellitus (IDDM). The blood glucose concentrations in the patients with diabetes were controlled by a closed-loop insulin-infusion system (artificial pancreas) during the infusion of each agent, allowing measurement of the meal-related requirement for exogenous insulin. In the patients with IDDM, normoglycemic-clamp studies were performed during the infusions of GLIP and saline to determine the effect of GLIP on insulin sensitivity.

RESULTS

In the normal subjects, the infusion of GLIP significantly lowered the meal-related increases in the blood glucose concentration (P less than 0.01) and the plasma concentrations of insulin and glucagon (P less than 0.05 for both comparisons). The insulinogenic index (the ratio of insulin to glucose) increased almost 10-fold, indicating that GLIP had an insulinotropic effect. In the patients with NIDDM, the infusion of GLIP reduced the mean (+/- SE) calculated isoglycemic meal-related requirement for insulin from 17.4 +/- 2.8 to 2.0 +/- 0.5 U (P less than 0.001), so that the integrated area under the curve for plasma free insulin was decreased (P less than 0.05) in spite of the stimulation of insulin release. In the patients with IDDM, the GLIP infusion decreased the calculated isoglycemic meal-related insulin requirement from 9.4 +/- 1.5 to 4.7 +/- 1.4 U. The peptide decreased glucagon and somatostatin release in both groups of patients. In the normoglycemic-clamp studies in the patients with IDDM, the GLIP infusion significantly increased glucose utilization (saline vs. GLIP, 7.2 +/- 0.5 vs. 8.6 +/- 0.4 mg per kilogram of body weight per minute; P less than 0.01).

CONCLUSIONS

GLIP has an antidiabetogenic effect, and it may therefore be useful in the treatment of patients with NIDDM:

Tissue and plasma concentrations of amidated and glycine-extended glucagon-like peptide I in humans

Using specific radioimmunoassays, we studied the occurrence of amidated and glycine-extended glucagon-like peptide I (GLP-I) molecules in the human small intestine and pancreas and in the circulation system in response to a breakfast meal. Through gel permeation chromatography of extracts of the human pancreas (n = 5), we found that 71% of the GLP-I immunoreactivity eluted as a large molecule corresponding to the major proglucagon fragment, 24% corresponded to GLP-I 1-36 amide, and 5% to GLP-I 1-37. By gel permeation chromatography of extracts of human small intestine (n = 6), we found that all immunoreactivity eluted in one peak at the common elution position of the two insulin-releasing peptides, GLP-I 7-36 amide and GLP-I 7-37. Of the GLP-I immunoreactivity, 80% corresponded to GLP-I 7-36 amide and 20% to GLP-I 7-37. The mean concentrations of amidated GLP-I and glycine-extended GLP-I in fasting plasma were 7 +/- 1 and 6 +/- 1 pM, respectively (n = 6). In response to a breakfast meal, the concentration of amidated GLP-I rose significantly amounting to 41 +/- 5 pM 90 min after the meal ingestion, whereas the concentration of glycine-extended GLP-I only rose slightly to a maximum of 10 +/- 1 pM. Thus, both amidated and glycine-extended GLP-I molecules are produced in the small intestine and in the pancreas in humans. Both amidated and glycine-extended GLP-I are measurable in fasting plasma.(ABSTRACT TRUNCATED AT 250 WORDS)

Determinants of the impaired secretion of glucagon-like peptide-1 in type 2 diabetic patients

To elucidate the causes of the diminished incretin effect in type 2 diabetes mellitus we investigated the secretion of the incretin hormones, glucagon-like peptide-1 and glucose- dependent insulinotropic polypeptide and measured nonesterified fatty acids, and plasma concentrations of insulin, C peptide, pancreatic polypeptide, and glucose during a 4-h mixed meal test in 54 heterogeneous type 2 diabetic patients, 33 matched control subjects with normal glucose tolerance, and 15 unmatched subjects with impaired glucose tolerance. The glucagon-like peptide-1 response in terms of area under the curve from 0-240 min after the start of the meal was significantly decreased in the patients (2482 +/- 145 compared with 3101 +/- 198 pmol/liter.240 min; P = 0.024). In addition, the area under the curve for glucose-dependent insulinotropic polypeptide was slightly decreased. In a multiple regression analysis, a model with diabetes, body mass index, male sex, insulin area under the curve (negative influence), glucose-dependent insulinotropic polypeptide area under the curve (negative influence), and glucagon area under the curve (positive influence) explained 42% of the variability of the glucagon-like peptide-1 response. The impaired glucose tolerance subjects were hyperinsulinemic and generally showed the same abnormalities as the diabetic patients, but to a lesser degree. We conclude that the meal-related glucagon-like peptide-1 response in type 2 diabetes is decreased, which may contribute to the decreased incretin effect in type 2 diabetes.

Both subcutaneously and intravenously administered glucagon-like peptide I are rapidly degraded from the NH2-terminus in type II diabetic patients and in healthy subjects

To fate of exogenous glucagon-like peptide I (GLP-I)(7-36) amide was studied in nondiabetic and type II diabetic subjects using a combination of high-pressure liquid chromatography (HPLC), specific radioimmunoassays (RIAs), and a sensitive enzyme-linked immunosorbent assay (ELISA), whereby intact biologically active GLP-I and its metabolites could be determined. After GLP-I administration, the intact peptide could be measured using an NH2-terminally directed RIA or ELISA, while the difference in concentration between these assays and a COOH-terminal-specific RIA allowed determination of NH2-terminally truncated metabolites. Subcutaneous GLP-I was rapidly degraded in a time-dependent manner, forming a metabolite, which co-eluted on HPLC with GLP-I(9-36) amide and had the same immunoreactive profile. Thirty minutes after subcutaneous GLP-I administration to diabetic patients (n = 8), the metabolite accounted for 88.5 +/- 1.9% of the increase in plasma immunoreactivity determined by the COOH-terminal RIA, which was higher than the levels measured in healthy subjects (78.4 +/- 3.2%; n = 8; P < 0.05). Intravenously infused GLP-I was also extensively degraded, but no significant differences were seen between the two groups. Intact GLP-I accounted for only 19.9 +/- 3.4% of the increase in immunoreactivity measured with the COOH-terminal RIA in normal subjects (n = 8), and 25.0 +/- 4.8% of the increase in diabetic subjects (n = 8), the remainder being the NH2-terminally truncated metabolite.

Effects of glucagon-like peptide-1 on endothelial function in type 2 diabetes patients with stable coronary artery disease

GLP-1 stimulates insulin secretion, suppresses glucagon secretion, delays gastric emptying, and inhibits small bowel motility, all actions contributing to the anti-diabetogenic peptide effect. Endothelial dysfunction is strongly associated with insulin resistance and type 2 diabetes mellitus and may cause the angiopathy typifying this debilitating disease. Therefore, interventions affecting both endothelial dysfunction and insulin resistance may prove useful in improving survival in type 2 diabetes patients. We investigated GLP-1's effect on endothelial function and insulin sensitivity (S(I)) in two groups: 1) 12 type 2 diabetes patients with stable coronary artery disease and 2) 10 healthy subjects with normal endothelial function and S(I). Subjects underwent infusion of recombinant GLP-1 or saline in a random crossover study. Endothelial function was measured by postischemic FMD of brachial artery, using ultrasonography. S(I) [in (10(-4) dl.kg(-1).min(-1))/(muU/ml)] was measured by hyperinsulinemic isoglycemic clamp technique. In type 2 diabetic subjects, GLP-1 infusion significantly increased relative changes in brachial artery diameter from baseline FMD(%) (3.1 +/- 0.6 vs. 6.6 +/- 1.0%, P < 0.05), with no significant effects on S(I) (4.5 +/- 0.8 vs. 5.2 +/- 0.9, P = NS). In healthy subjects, GLP-1 infusion affected neither FMD(%) (11.9 +/- 0.9 vs. 10.3 +/- 1.0%, P = NS) nor S(I) (14.8 +/- 1.8 vs. 11.6 +/- 2.0, P = NS). We conclude that GLP-1 improves endothelial dysfunction but not insulin resistance in type 2 diabetic patients with coronary heart disease. This beneficial vascular effect of GLP-1 adds yet another salutary property of the peptide useful in diabetes treatment.

Distribution of glucagon-like peptide-1 and other preproglucagon-derived peptides in the rat hypothalamus and brainstem

Central administration of the preproglucagon-derived peptide glucagon-like peptide-1 significantly inhibits ingestion of food and water, and glucagon-like peptide-1 binding sites are present in a multitude of central areas involved in the regulation of ingestional behaviour. To evaluate further the neuroanatomical organization of central glucagon-like peptide-1 containing neuronal circuits with potential implications on ingestional behaviour, we carried out a series of experiments in the rat demonstrating the topographical sites of synthesis and processing of the preproglucagon precursor followed by a chromatographic analysis of the processed fragments. In situ hybridization histochemistry revealed that preproglucagon encoding messenger RNA was expressed in a single population of neurons in the caudal portion of the nucleus of the solitary tract. Gel chromatographic analysis of hypothalamic and brainstem tissue extracts revealed that the preproglucagon precursor is processed in a fashion similar to that seen in the small intestine, preferentially giving rise to glicentin, glucagon-like peptide-1 and glucagon-like peptide-2. This single brain site of glucagon-like peptide-1 synthesis was subsequently confirmed by immunohistochemical demonstration of glucagon-like peptide-1-immunoreactive perikarya in the central and caudal parts of the nucleus of the solitary tract. Numerous sites containing glucagon-like peptide-1 immunoreactive fibres were, however, discovered in the forebrain including hypothalamic, thalamic and cortical areas. The densest innervation by glucagon-like peptide-1 immunoreactive nerve fibres was seen in the hypothalamic dorsomedial and paraventricular nuclei, but numerous glucagon-like peptide-1 immunoreactive fibres were also seen throughout the periventricular strata of the third ventricle. Dual-labelling immunohistochemistry for tyrosine hydroxylase and glucagon-like peptide-1 gave no evidence for co-localization of catecholamines and glucagon-like peptide-1 in neurons of the lower brainstem. To identify neurons of the nucleus of the solitary tract that project to the hypothalamic paraventricular nucleus, the retrograde tracer FluoroGold was injected into this hypothalamic target and dual immunocytochemical identification of glucagon-like peptide-1 and tyrosine hydroxylase-positive neurons was performed on brainstem sections containing retrogradely labelled perikarya. From this experiment it was seen that many of the retrogradely labelled neurons in the central portion of the nucleus of the solitary tract are catecholaminergic, while none is glucagon-like peptide-1 immunoreactive. In contrast, most of the retrogradely labelled neurons of the caudal portion of the nucleus of the solitary tract contain glucagon-like peptide-1. These observations further substantiate that glucagon-like peptide-1 neurons of the solitary tract constitute a distinct non-catecholaminergic cell group which projects to many targets, one of which is the hypothalamic paraventricular nucleus.

Truncated GLP-1 (proglucagon 78-107-amide) inhibits gastric and pancreatic functions in man

We studied the effect of intravenous infusion of synthetic truncated GLP-1 (proglucagon 78-107-amide) on fasting and postprandial gastric acid secretion, gastric emptying, and pancreatic secretion of trypsin and lipase in eight normal volunteers using marker dilution and aspiration technique. The infusion resulted in a plasma concentration of 110 +/- 14 pmol/liter (mean +/- SEM). Truncated GLP-1 significantly inhibited postprandial acid secretion by 43 +/- 11% in spite of unchanged plasma gastrin concentration. Gastric emptying rate decreased significantly; 50% emptying time increased from 16 +/- 2 min to 30 +/- 5 min. Postprandial trypsin and lipase outputs were significantly inhibited by 47 +/- 17% and 40 +/- 9% during truncated GLP-1 infusion. Pancreatic enzyme output was linearly correlated to gastric emptying, and truncated GLP-1 did not affect this relationship, suggesting that the effect on pancreatic secretion was secondary to the effect on gastric emptying. Postprandial insulin and glucagon concentrations were similar with and without truncated GLP-1 infusion in spite of significantly lower blood glucose levels (5.2 +/- 0.2 versus 3.7 +/- 0.3), indicating that GLP-1 stimulated insulin secretion and inhibited glucagon secretion. In conclusion, our results suggest that truncated GLP-1 act as a physiological inhibitor of gastric and pancreatic functions in man.

Lower blood glucose, hyperglucagonemia, and pancreatic alpha cell hyperplasia in glucagon receptor knockout mice

Glucagon, the counter-regulatory hormone to insulin, is secreted from pancreatic alpha cells in response to low blood glucose. To examine the role of glucagon in glucose homeostasis, mice were generated with a null mutation of the glucagon receptor (Gcgr(-/-)). These mice display lower blood glucose levels throughout the day and improved glucose tolerance but similar insulin levels compared with control animals. Gcgr(-/-) mice displayed supraphysiological glucagon levels associated with postnatal enlargement of the pancreas and hyperplasia of islets due predominantly to alpha cell, and to a lesser extent, delta cell proliferation. In addition, increased proglucagon expression and processing resulted in increased pancreatic glucogen-like peptide 1 (GLP-1) (1-37) and GLP-1 amide (1-36 amide) content and a 3- to 10-fold increase in circulating GLP-1 amide. Gcgr(-/-) mice also displayed reduced adiposity and leptin levels but normal body weight, food intake, and energy expenditure. These data indicate that glucagon is essential for maintenance of normal glycemia and postnatal regulation of islet and alpha and delta cell numbers. Furthermore, the lean phenotype of Gcgr(-/-) mice suggests glucagon action may be involved in the regulation of whole body composition.

Exenatide reduces reperfusion injury in patients with ST-segment elevation myocardial infarction

AIMS

Exenatide, a glucagon-like-peptide-1 analogue, increases myocardial salvage in experimental settings with coronary occlusion and subsequent reperfusion. We evaluated the cardioprotective effect of exenatide at the time of reperfusion in patients with ST-segment elevation myocardial infarction (STEMI) treated with primary percutaneous coronary intervention (pPCI).

METHODS AND RESULTS

A total of 172 patients with STEMI and Thrombolysis in Myocardial Infarction flow 0/1 were randomly assigned to exenatide or placebo (saline) intravenously. Study treatment was commenced 15 min before intervention and maintained for 6 h after the procedure. The primary endpoint was salvage index calculated from myocardial area at risk (AAR), measured in the acute phase, and final infarct size measured 90 ± 21 days after pPCI by cardiac magnetic resonance (CMR). In 105 patients evaluated with CMR, a significantly larger salvage index was found in the exenatide group than in the placebo group (0.71 ± 0.13 vs. 0.62 ± 0.16; P= 0.003). Infarct size in relation to AAR was also smaller in the exenatide group (0.30 ± 0.15 vs. 0.39 ± 0.15; P= 0.003). In a regression analysis, there was a significant correlation between the infarct size and the AAR for both treatment groups and an analysis of covariance showed that datapoints in the exenatide group lay significantly lower than for the placebo group (P= 0.011). There was a trend towards smaller absolute infarct size in the exenatide group (13 ± 9 vs. 17 ± 14 g; P= 0.11). No difference was observed in left ventricular function or 30-day clinical events. No adverse effects of exenatide were observed.

CONCLUSION

In patients with STEMI undergoing pPCI, administration of exenatide at the time of reperfusion increases myocardial salvage.

Truncated glucagon-like peptide I, an insulin-releasing hormone from the distal gut

By hydrophobic gel permeation and high pressure liquid chromatography we isolated from pig intestinal mucosa a peptide which corresponds to proglucagon 78-107 as suggested by chromatography and determination of its N-terminal sequence. Natural and synthetic proglucagon 78-107 dose dependently and potently increased insulin secretion from the isolated perfused pig pancreas. Proglucagon 78-107 also secreted by the small intestine may participate in the hormonal control of insulin secretion.

Role of incretin hormones in the regulation of insulin secretion in diabetic and nondiabetic humans

The available evidence suggests that about two-thirds of the insulin response to an oral glucose load is due to the potentiating effect of gut-derived incretin hormones. The strongest candidates for the incretin effect are glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide 1 (GLP-1). In patients with type 2 diabetes, however, the incretin effect is lost or greatly impaired. It is hypothesized that this loss explains an important part of the impaired insulin secretion in patients. Further analysis of the incretin effects in patients has revealed that the secretion of GIP is near normal, whereas the secretion of GLP-1 is decreased. On the other hand, the insulintropic effect of GLP-1 is preserved, whereas the effect of GIP is greatly reduced, mainly because of a complete loss of the normal GIP-induced potentiation of second-phase insulin secretion. These two features, therefore, explain the incretin defect of type 2 diabetes. Strong support for the hypothesis that the defect plays an important role in the insulin deficiency of patients is provided by the finding that administration of excess GLP-1 to patients may completely restore the glucose-induced insulin secretion as well as the beta-cells' sensitivity to glucose. Because of this, analogs of GLP-1 or GLP-1 receptor activations are currently being developed for diabetes treatment, so far with very promising results.

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.

Defective amplification of the late phase insulin response to glucose by GIP in obese Type II diabetic patients

AIMS/HYPOTHESIS

Glucagon-like-peptide-1 (GLP-1) is strongly insulinotropic in patients with Type II (non-insulin-dependent) diabetes mellitus, whereas glucose-dependent insulinotropic polypeptide (GIP) is less effective. Our investigation evaluated "early" (protocol 1) - and "late phase" (protocol 2) insulin and C-peptide responses to GLP-1 and GIP stimulation in patients with Type II diabetes.

METHODS

Protocol 1: eight Type II diabetic patients and eight matched healthy subjects received i.v. bolus injections of GLP-1(2.5 nmol) or GIP(7.5 nmol) concomitant with an increase of plasma glucose to 15 mmol/l. Protocol 2: eight Type II diabetic patients underwent a hyperglycaemic clamp (15 mmol/l) with infusion (per kg body weight/min) of either: 1 pmol GLP-1 (7-36) amide (n=8), 4 pmol GIP (n=8), 16 pmol GIP (n=4) or no incretin hormone (n=5). For comparison, six matched healthy subjects were examined.

RESULTS

Protocol 1: Type II diabetic patients were characterised by a decreased "early phase" response to both stimuli, but their relative response to GIP versus GLP-1 stimulation was exactly the same as in healthy subjects [insulin (C-peptide): patients 59+/-9% (74+/-6%) and healthy subjects 62+/-5% (71+/-9%)]. Protocol 2, "Early phase" (0-20 min) insulin response to glucose was delayed and reduced in the patients, but enhanced slightly and similarly by GIP and GLP-1. GLP-1 augmented the "late phase" (20-120 min) insulin secretion to levels similar to those observed in healthy subjects. In contrast, the "late phase" responses to both doses of GIP were not different from those obtained with glucose alone. Accordingly, glucose infusion rates required to maintain the hyperglycaemic clamp in the "late phase" period (20-120 min) were similar with glucose alone and glucose plus GIP, whereas a doubling of the infusion rate was required during GLP-1 stimulation.

CONCLUSION/INTERPRETATION

Lack of GIP amplification of the late phase insulin response to glucose, which contrasts markedly to the normalising effect of GLP-1, could be a key defect in insulin secretion in Type II diabetic patients.

The incretin system and its role in type 2 diabetes mellitus

The incretin hormones are released during meals from gut endocrine cells. They potentiate glucose-induced insulin secretion and may be responsible for up to 70% of postprandial insulin secretion. The incretin hormones include glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), both of which may also promote proliferation/neogenesis of beta cells and prevent their decay (apoptosis). Both hormones contribute to insulin secretion from the beginning of a meal and their effects are progressively amplified as plasma glucose concentrations rise. The current interest in the incretin hormones is due to the fact that the incretin effect is severely reduced or absent in patients with type 2 diabetes mellitus (T2DM). In addition, there is hyperglucagonaemia, which is not suppressible by glucose. In such patients, the secretion of GIP is near normal, but its effect on insulin secretion, particularly the late phase, is severely impaired. The loss of GIP action is probably a consequence of diabetes, since it is also observed in patients with diabetes secondary to chronic pancreatitis, in whom the incretin effect is also lost. GLP-1 secretion, on the other hand, is also impaired, but its insulinotropic and glucagon-suppressive actions are preserved, although the potency of GLP-1 in this respect is decreased compared to healthy subjects. However, in supraphysiological doses, GLP-1 administration may completely normalize beta as well as alpha cell sensitivity to glucose. The impaired action of GLP-1 and GIP in T2DM may be at least partly restored by improved glycaemic control, as shown in studies involving 4 weeks of intensive insulin therapy. The reduced incretin effect is believed to contribute to impaired regulation of insulin and glucagon secretion in T2DM, and, in support of this, exogenous GLP-1 administration may restore blood glucose regulation to near normal levels. Thus, the pathogenesis of T2DM seems to involve a dysfunction of both incretins. Enhancement of incretin action may therefore represent a therapeutic solution. Clinical strategies therefore include the development of metabolically stable activators of the GLP-1 receptor; and inhibition of DPP-4, the enzyme that destroys native GLP-1 almost immediately. Orally active DPP-4 inhibitors and the metabolically stable activators, exenatide (Byetta), are now on the market, and numerous clinical studies have shown that both principles are associated with durable antidiabetic activity.

Incretin secretion in relation to meal size and body weight in healthy subjects and people with type 1 and type 2 diabetes mellitus

Glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are incretin hormones secreted in response to meal ingestion, thereby enhancing postprandial insulin secretion. Therefore, an attenuated incretin response could contribute to the impaired insulin responses in patients with diabetes mellitus. The aim of the present investigation was to investigate incretin secretion, in obesity and type 1 and type 2 diabetes mellitus, and its dependence on the magnitude of the meal stimulus. Plasma concentrations of incretin hormones (total, reflecting secretion and intact, reflecting potential action) were measured during two meal tests (260 kcal and 520 kcal) in eight type 1 diabetic patients, eight lean healthy subjects, eight obese type 2 diabetic patients, and eight obese healthy subjects. Both in diabetic patients and in healthy subjects, significant increases in GLP-1 and GIP concentrations were seen after ingestion of both meals. The incretin responses were significantly higher in all groups after the large meal, compared with the small meal, with correspondingly higher C-peptide responses. Both type 1 and type 2 diabetic patients had normal GIP responses, compared with healthy subjects, whereas decreased GLP-1 responses were seen in type 2 diabetic patients, compared with matched obese healthy subjects. Incremental GLP-1 responses were normal in type 1 diabetic patients. Increased fasting concentrations of GIP and an early enhanced postprandial GIP response were seen in obese, compared with lean healthy subjects, whereas GLP-1 responses were the same in the two groups. beta-cell sensitivity to glucose, evaluated as the slope of insulin secretion rates vs. plasma glucose concentration, tended to increase in both type 2 diabetic patients (29%, P = 0.19) and obese healthy subjects (22% P = 0.04) during the large meal, compared with the small meal, perhaps reflecting the increased incretin response. We conclude: 1) that a decreased GLP-1 secretion may contribute to impaired insulin secretion in type 2 diabetes mellitus, whereas GIP and GLP-1 secretion is normal in type 1 diabetic patients; and 2) that it is possible to modulate the beta-cell sensitivity to glucose in obese healthy subjects, and possibly also in type 2 diabetic patients, by giving them a large meal, compared with a small meal.

Glucagon-like peptide 2 improves nutrient absorption and nutritional status in short-bowel patients with no colon

BACKGROUND & AIMS

Glucagon-like peptide 2 (GLP-2) is intestinotrophic, antisecretory, and transit-modulating in rodents, and it is mainly secreted from the intestinal mucosa of the terminal ileum and colon after food ingestion. We assessed the effect of GLP-2 on the gastrointestinal function in patients without a terminal ileum and colon who have functional short-bowel syndrome with severe malabsorption of wet weight (>1.5 kg/day) and energy (>2.3 MJ/day) and no postprandial secretion of GLP-2.

METHODS

Balance studies were performed before and after treatment with GLP-2, 400 microg subcutaneously twice a day for 35 days, in 8 patients (4-17 years from last bowel resection; 6 with Crohn's disease). Four patients received home parenteral nutrition (mean residual jejunum, 83 cm), and 4 did not (mean ileum resection, 106 cm). Biopsy specimens were taken from jejunal/ileal stomas, transit was measured by scintigraphy, and body composition was measured by dual-energy x-ray absorptiometry.

RESULTS

Treatment with GLP-2 improved the intestinal absorption of energy 3.5% +/- 4.0% (mean +/- SD) from 49.9% to 53.4% (P = 0.04), wet weight 11% +/- 12% from 25% to 36% (P = 0.04), and nitrogen 4.7% +/- 5.4% from 47.4% to 52.1% (P = 0.04). Body weight increased 1.2 +/- 1.0 kg (P = 0.01), lean body mass increased 2.9 +/- 1.9 kg (P = 0.004), fat mass decreased 1.8 +/- 1.3 kg (P = 0.007), and 24-hour urine creatinine excretion increased (P = 0.02). The time to 50% gastric emptying of solids increased 30 +/- 16 minutes from 89 to 119 minutes (P < 0.05). Small bowel transit time was not changed. Crypt depth and villus height were increased in 5 and 6 patients, respectively.

CONCLUSIONS

Treatment with GLP-2 improves intestinal absorption and nutritional status in short-bowel patients with impaired postprandial GLP-2 secretion in whom the terminal ileum and the colon have been resected.