Effect of glucagon-like peptide-1(7-36)-amide on initial splanchnic glucose uptake and insulin action in humans with type 1 diabetes

Effects of GLP-1 Infusion Upon Whole-body Glucose Uptake and Skeletal Muscle Perfusion During Fed-state in Older Men

INTRODUCTION

Ageing skeletal muscles become both insulin resistant and atrophic. The hormone glucagon-like peptide 1 (GLP-1) facilitates postprandial glucose uptake as well as augmenting muscle perfusion, independent of insulin action. We thus hypothesized exogenous GLP-1 infusions would enhance muscle perfusion and positively affect glucose metabolism during fed-state clamps in older people.

METHODS

Eight men (71 ± 1 years) were studied in a randomized crossover trial. Basal blood samples were taken before postprandial (fed-state) insulin and glucose clamps, accompanied by amino acid infusions, for 3 hours. Reflecting this, following insertions of peripheral and femoral vessels cannulae and baseline measurements, peripheral IV infusions of octreotide, insulin (Actrapid), 20% glucose, and mixed amino acids; Vamin 14-EF with or without a femoral arterial GLP-1 infusion were started. GLP-1, insulin, and C-peptide were measured by ELISA. Muscle microvascular blood flow was assessed via contrast enhanced ultrasound. Whole-body glucose handling was assayed by assessing glucose infusion rate parameters.

RESULTS

Skeletal muscle microvascular blood flow significantly increased in response to GLP-1 vs feeding alone (5.0 ± 2.1 vs 1.9 ± 0.7 fold-change from basal, respectively; P = 0.008), while also increasing whole-body glucose uptake (area under the curve 16.9 ± 1.7 vs 11.4 ± 1.8 mg/kg-1/180 minutes-1, P = 0.02 ± GLP, respectively).

CONCLUSIONS

The beneficial effects of GLP-1 on whole-body glycemic control are evident with insulin clamped at fed-state levels. GLP-1 further enhances the effects of insulin on whole-body glucose uptake in older men, underlining its role as a therapeutic target. The effects of GLP-1 in enhancing microvascular flow likely also affects other glucose-regulatory organs, reflected by greater whole-body glucose uptake.

The therapeutic potential of GLP-1 analogues for stress-related eating and role of GLP-1 in stress, emotion and mood: a review

Stress and low mood are powerful triggers for compulsive overeating, a maladaptive form of eating leading to negative physical and mental health consequences. Stress-vulnerable individuals, such as people with obesity, are particularly prone to overconsumption of high energy foods and may use it as a coping mechanism for general life stressors. Recent advances in the treatment of obesity and related co-morbidities have focused on the therapeutic potential of anorexigenic gut hormones, such as glucagon-like peptide 1 (GLP-1), which acts both peripherally and centrally to reduce energy intake. Besides its appetite suppressing effect, GLP-1 acts on areas of the brain involved in stress response and emotion regulation. However, the role of GLP-1 in emotion and stress regulation, and whether it is a viable treatment for stress-induced compulsive overeating, has yet to be established. A thorough review of the pre-clinical literature measuring markers of stress, anxiety and mood after GLP-1 exposure points to potential divergent effects based on temporality. Specifically, acute GLP-1 injection consistently stimulates the physiological stress response in rodents whereas long-term exposure indicates anxiolytic and anti-depressive benefits. However, the limited clinical evidence is not as clear cut. While prolonged GLP-1 analogue treatment in people with type 2 diabetes improved measures of mood and general psychological wellbeing, the mechanisms underlying this may be confounded by associated weight loss and improved blood glucose control. There is a paucity of longitudinal clinical literature on mechanistic pathways by which stress influences eating behavior and how centrally-acting gut hormones such as GLP-1, can modify these. (250).

Endogenously released GIP reduces and GLP-1 increases hepatic insulin extraction

GIP was proposed to play a key role in the development of non- alcoholic fatty liver disease (NAFLD) in response to sugar intake. Isomaltulose, is a 1,6-linked glucose-fructose dimer which improves glucose homeostasis and prevents NAFLD compared to 1,2-linked sucrose by reducing glucose-dependent insulinotropic peptide (GIP) in mice. We compared effects of sucrose vs. isomaltulose on GIP and glucagon-like peptide-1 (GLP-1) secretion, hepatic insulin clearance (HIC) and insulin sensitivity in normal (NGT), impaired glucose tolerant (IGT) and Type 2 diabetes mellitus (T2DM) participants. A randomized crossover study was performed in 15 NGT, 10 IGT and 10 T2DM subjects. In comparison to sucrose, peak glucose concentrations were reduced by 2.3, 2.1 and 2.5 mmol/l (all p < 0.05) and insulin levels were 88% (p < 0.01, NGT), 32% (p < 0.05, IGT) and 55% (T2DM) lower after the isomaltulose load. Postprandial GIP_iAUC concentrations were decreased (56%, p < 0.01 in NGT; 42%, p < 0.05 in IGT and 40%,p < 0.001 in T2DM) whereas GLP-1_iAUC was 77%, 85% and 85% higher compared to sucrose (p < 0.01), respectively. This resulted in ∼35 - 50% improved insulin sensitivity and reduced insulinogenic index after isomaltulose, which correlated closely with improved HIC, respectively (r = 0.62, r=-0.70; p < 0.001). HIC was inversely related to GIP (r=-0.44, p < 0.001) and positively related to GLP-1 levels (r = 0.40, p = 0.001). CONCLUSION: Endogenously released GIP correlated with reduced, and GLP-1 with increased hepatic insulin extraction. Increased peripheral insulin levels may contribute to insulin resistance and obesity. We propose that the unfavorable effects of high glycemic index Western diets are related to increased GIP-release and reduced HIC.

Dissecting the Physiology and Pathophysiology of Glucagon-Like Peptide-1

An aging world population exposed to a sedentary life style is currently plagued by chronic metabolic diseases, such as type-2 diabetes, that are spreading worldwide at an unprecedented rate. One of the most promising pharmacological approaches for the management of type 2 diabetes takes advantage of the peptide hormone glucagon-like peptide-1 (GLP-1) under the form of protease resistant mimetics, and DPP-IV inhibitors. Despite the improved quality of life, long-term treatments with these new classes of drugs are riddled with serious and life-threatening side-effects, with no overall cure of the disease. New evidence is shedding more light over the complex physiology of GLP-1 in health and metabolic diseases. Herein, we discuss the most recent advancements in the biology of gut receptors known to induce the secretion of GLP-1, to bridge the multiple gaps into our understanding of its physiology and pathology.

Centrally located GLP-1 receptors modulate gastric slow waves and cardiovascular function in ferrets consistent with the induction of nausea

Glucagon-like peptide-1 (GLP-1) receptor agonists are indicated for the treatment of Type 2 diabetes and obesity, but can cause nausea and emesis in some patients. GLP-1 receptors are distributed widely in the brain, where they contribute to mechanisms of emesis, reduced appetite and aversion, but it is not known if these centrally located receptors also contribute to a modulation of gastric slow wave activity, which is linked causally to nausea. Our aim was to investigate the potential of the GLP-1 receptor agonist, exendin-4, administered into the 3rd ventricle to modulate emesis, feeding and gastric slow wave activity. Thermoregulation and cardiovascular parameters were also monitored, as they are disturbed during nausea. Ferrets were used as common laboratory rodents do not have an emetic reflex. A guide cannula was implanted into the 3rd ventricle for delivering a previously established dose of exendin-4 (10nmol), which had been shown to induce emesis and behaviours indicative of 'nausea'. Radiotelemetry recorded gastric myoelectric activity (GMA; slow waves), blood pressure and heart rate variability (HRV), and core temperature; food intake and behaviour were also assessed. Exendin-4 (10nmol, i.c.v.) decreased the dominant frequency of GMA, with an associated increase in the percentage of bradygastric power (lasting ~4h). Food intake was inhibited in all animals, with 63% exhibiting emesis. Exendin-4 also increased blood pressure (lasting ~24h) and heart rate (lasting ~7h), decreased HRV (lasting ~24h), and caused transient hyperthermia. None of the above parameters were emesis-dependent. The present study shows for the first time that gastric slow waves may be modulated by GLP-1 receptors in the brain through mechanisms that appear independent from emesis. Taken together with a reduction in HRV, the findings are consistent with changes associated with the occurrence of nausea in humans.

Central Nervous System GLP-1 Receptors Regulate Islet Hormone Secretion and Glucose Homeostasis in Male Rats

The glucagon-like peptide 1 (GLP-1) system plays an important role in blood glucose regulation, in great part through coordinate control of insulin and glucagon secretion. These effects are generally attributed to GLP-1 produced in peripheral sites, principally the intestine. GLP-1 is also produced in hindbrain neurons that signal through GLP-1 receptors (GLP-1rs) expressed in brain regions involved in metabolic regulation. GLP-1 in the central nervous system (CNS) induces satiety, visceral illness, and stress responses. However, recent evidence suggests CNS GLP-1 is also involved in glucose regulation. To test the hypothesis that central GLP-1 regulates islet hormone secretion, conscious rats were given intracerebroventricular (ICV) GLP-1, GLP-1r antagonist exendin-[9-39] (Ex-9), or saline during fasting or hyperglycemia from intravenous glucose. Administration of CNS GLP-1 increased fasting glucose, glucagon, corticosterone, and epinephrine and blunted insulin secretion in response to hyperglycemia. Paradoxically, GLP-1r blockade with ICV Ex-9 also reduced glucose-stimulated insulin secretion, and administration of ICV Ex-9 to freely feeding rats caused mild glucose intolerance. Thus, direct administration of CNS GLP-1 affected islet hormone secretion counter to what is seen with peripherally administered GLP-1, an effect likely due to stimulation of sympathetic nervous system activity. In contrast, blockade of brain GLP-1r supports a role for CNS GLP-1 on glucose-stimulated insulin secretion and glucose control after a meal. These findings suggest a model in which activation of CNS GLP-1r by endogenous peptide promotes glucose tolerance, an effect that can be overridden by stress responses stimulated by exogenous GLP-1.

DREADDing proglucagon neurons: a fresh look at metabolic regulation by the brain

Glucagon-like peptide 1 receptor (GLP-1R) signaling in the CNS has been linked to reduced food intake, lower body weight, improved glucose homeostasis, and activation of CNS stress axes. GLP-1 is produced by cells that express proglucagon (GCG); however, the stimuli that activate GCG+ neurons are not well known, which has made understanding the role of this neuronal population in the CNS a challenge. In this issue of the JCI, Gaykema et al. use designer receptors exclusively activated by designer drugs (DREADD) technology to specifically activate GCG+ neurons in mouse models. While activation of GCG+ neurons did reduce food intake, and variably decreased hepatic glucose production, other GLP-1-associated effects were not observed - e.g., activation of stress axes or stimulation of insulin secretion - in response to GCG+ neuron activation. The authors have provided a valuable model to study this set of neurons in vivo, and their results provide new insights into the function of GCG+ neural activity in the brain and raise questions that will move research on this clinically relevant neural system forward.

Peripheral mechanisms in appetite regulation

Peripheral mechanisms in appetite regulation include the motor functions of the stomach, such as the rate of emptying and accommodation, which convey symptoms of satiation to the brain. The rich repertoire of peripherally released peptides and hormones provides feedback from the arrival of nutrients in different regions of the gut from where they are released to exert effects on satiation, or regulate metabolism through their incretin effects. Ultimately, these peripheral factors provide input to the highly organized hypothalamic circuitry and vagal complex of nuclei to determine cessation of energy intake during meal ingestion, and the return of appetite and hunger after fasting. Understanding these mechanisms is key to the physiological control of feeding and the derangements that occur in obesity and their restoration with treatment (as shown by the effects of bariatric surgery).

Influence of GLP-1 on myocardial glucose metabolism in healthy men during normo- or hypoglycemia

Background and Aims

Glucagon-like peptide-1 (GLP-1) may provide beneficial cardiovascular effects, possibly due to enhanced myocardial energetic efficiency by increasing myocardial glucose uptake (MGU). We assessed the effects of GLP-1 on MGU in healthy subjects during normo- and hypoglycemia.

Materials and Methods

We included eighteen healthy men in two randomized, double-blinded, placebo-controlled cross-over studies. MGU was assessed with GLP-1 or saline infusion during pituitary-pancreatic normo- (plasma glucose (PG): 4.5 mM, n = 10) and hypoglycemic clamps (PG: 3.0 mM, n = 8) by positron emission tomography with ¹⁸fluoro-deoxy-glucose (¹⁸F-FDG) as tracer.

Results

In the normoglycemia study mean (± SD) age was 25±3 years, and BMI was 22.6±0.6 kg/m² and in the hypoglycemia study the mean age was 23±2 years with a mean body mass index of 23±2 kg/m². GLP-1 did not change MGU during normoglycemia (mean (+/− SD) 0.15+/−0.04 and 0.16+/−0.03 µmol/g/min, P = 0.46) or during hypoglycemia (0.16+/−0.03 and 0.13+/−0.04 µmol/g/min, P = 0.14). However, the effect of GLP-1 on MGU was negatively correlated to baseline MGU both during normo- and hypoglycemia, (P = 0.006, r² = 0.64 and P = 0.018, r² = 0.64, respectively) and changes in MGU correlated positively with the level of insulin resistance (HOMA 2IR) during hypoglycemia, P = 0.04, r² = 0.54. GLP-1 mediated an increase in circulating glucagon levels at PG levels below 3.5 mM and increased glucose infusion rates during the hypoglycemia study. No differences in other circulating hormones or metabolites were found.

Conclusions

While GLP-1 does not affect overall MGU, GLP-1 induces changes in MGU dependent on baseline MGU such that GLP-1 increases MGU in subjects with low baseline MGU and decreases MGU in subjects with high baseline MGU. GLP-1 preserves MGU during hypoglycemia in insulin resistant subjects.

ClinicalTrials.gov registration numbers: NCT00418288: (hypoglycemia) and NCT00256256: (normoglycemia).

Direct effect of GLP-1 infusion on endogenous glucose production in humans

AIMS/HYPOTHESIS

Glucagon-like peptide-1 (GLP-1) lowers glucose levels by potentiating glucose-induced insulin secretion and inhibiting glucagon release. The question of whether GLP-1 exerts direct effects on the liver, independently of the hormonal changes, is controversial. We tested whether an exogenous GLP-1 infusion, designed to achieve physiological postprandial levels, directly affects endogenous glucose production (EGP) under conditions mimicking the fasting state in diabetes.

METHODS

In 14 healthy volunteers, we applied the pancreatic clamp technique, whereby plasma insulin and glucagon levels are clamped using somatostatin and hormone replacement. The clamp was applied in paired, 4 h experiments, during which saline (control) or GLP-1(7-37)amide (0.4 pmol min⁻¹ kg⁻¹) was infused.

RESULTS

During the control study, plasma insulin and glucagon were maintained at basal levels and plasma C-peptide was suppressed, such that plasma glucose rose to a plateau of ~10.5 mmol/l and tracer-determined EGP increased by ~60%. During GLP-1 infusion at matched plasma glucose levels, the rise of EGP from baseline was fully prevented. Lipolysis (as indexed by NEFA concentrations and tracer-determined glycerol rate of appearance) and substrate utilisation (by indirect calorimetry) were similar between control and GLP-1 infusion.

CONCLUSIONS/INTERPRETATION

GLP-1 inhibits EGP under conditions where plasma insulin and glucagon are not allowed to change and glucose concentrations are matched, indicating either a direct effect on hepatocytes or neurally mediated inhibition.

Direct effect of GLP-1 infusion on endogenous glucose production in humans

AIMS/HYPOTHESIS

Glucagon-like peptide-1 (GLP-1) lowers glucose levels by potentiating glucose-induced insulin secretion and inhibiting glucagon release. The question of whether GLP-1 exerts direct effects on the liver, independently of the hormonal changes, is controversial. We tested whether an exogenous GLP-1 infusion, designed to achieve physiological postprandial levels, directly affects endogenous glucose production (EGP) under conditions mimicking the fasting state in diabetes.

METHODS

In 14 healthy volunteers, we applied the pancreatic clamp technique, whereby plasma insulin and glucagon levels are clamped using somatostatin and hormone replacement. The clamp was applied in paired, 4 h experiments, during which saline (control) or GLP-1(7-37)amide (0.4 pmol min⁻¹ kg⁻¹) was infused.

RESULTS

During the control study, plasma insulin and glucagon were maintained at basal levels and plasma C-peptide was suppressed, such that plasma glucose rose to a plateau of ~10.5 mmol/l and tracer-determined EGP increased by ~60%. During GLP-1 infusion at matched plasma glucose levels, the rise of EGP from baseline was fully prevented. Lipolysis (as indexed by NEFA concentrations and tracer-determined glycerol rate of appearance) and substrate utilisation (by indirect calorimetry) were similar between control and GLP-1 infusion.

CONCLUSIONS/INTERPRETATION

GLP-1 inhibits EGP under conditions where plasma insulin and glucagon are not allowed to change and glucose concentrations are matched, indicating either a direct effect on hepatocytes or neurally mediated inhibition.

Possible role of GLP-1 and its agonists in the treatment of type 1 diabetes mellitus

Unfortunately, the only approved medical treatment for type 1 diabetes mellitus (DM) is insulin, despite the fact that tight control cannot be reached without some serious side effects such as hypoglycemia and weight gain. More and more importance is now shifted towards developing new drugs that can reach a better glycemic control with lesser side effects. Some of these promising drugs are the glucagon-like peptides 1 (GLP-1) and their agonists, which have been FDA approved for the treatment of type 2 DM. The purpose of this article is to review all of the relevant literature on the potential role of GLP-1 in the treatment of type 1 DM. The major source of data acquisition included Medline search strategies, using the words "type 1 diabetes mellitus" and "GLP-1." Articles published in the last 20 years were screened. GLP-1 increases insulin secretion in humans with existing beta cells; it also decreases glucagon secretion, and blunts appetite. Of note, new animal studies demonstrate a role in beta cell-proliferation and decreased apoptosis. Because of all the effects mentioned above, GLP-1 seems to be a promising drug for type 1 DM treatment, but more studies are still needed before solid conclusions can be drawn.

The effect of colesevelam hydrochloride on insulin sensitivity and secretion in patients with type 2 diabetes: a pilot study

BACKGROUND

This study evaluated the effect of colesevelam hydrochloride on insulin sensitivity, potential binding to glucose, and chronic effect(s) on fasting and postprandial glucose and insulin in patients with type 2 diabetes mellitus.

METHODS

Patients meeting inclusion criteria were withdrawn from all antidiabetes agents for 2 weeks and randomized to colesevelam 3.75 grams/day (n = 17) or placebo (n = 18) for 8 weeks. Hyperinsulinemic-euglycemic clamp studies were performed at baseline (week -1) and weeks 2 and 8. A meal tolerance test was conducted at weeks -1, 0, 2, and 8. The meal tolerance test and study drug were coadministered at week 0 to assess the direct effect of colesevelam on glucose absorption.

RESULTS

Insulin sensitivity as measured by the insulin clamp method did not change, but the Matsuda Index, a measure of whole-body insulin sensitivity calculated from postmeal tolerance test glucose and insulin levels, increased significantly within the colesevelam group from baseline to week 8 with last observation carried forward (P = 0.020). The postprandial area under the curve for glucose decreased with colesevelam versus placebo at weeks 2 and 8 with last observation carried forward (P = 0.012 and P = 0.061, respectively); the area under the curve for insulin did not decrease in concert with the decrease in area under the curve for glucose at week 2 (P = 0.585). Colesevelam had no effect on postmeal tolerance test glucose levels at week 0.

CONCLUSIONS

These results suggest that colesevelam has no effect on peripheral insulin sensitivity or glucose absorption, but may improve glucose control by improving whole-body insulin sensitivity, although not by an acute effect on glucose absorption. CLINICAL TRIAL IDENTIFIER: NCT00361153.

No increased risk of hypoglycaemic episodes during 48 h of subcutaneous glucagon-like-peptide-1 administration in fasting healthy subjects

OBJECTIVE

It is uncertain whether the ability to avoid hypoglycaemia during fasting is preserved, and the risk of reactive hypoglycaemia after an oral glucose stimulus following a prolonged fasting period is increased at augmented glucagon-like peptide-1 (GLP-1) levels.

DESIGN

A randomized, double-blind placebo-controlled cross-over study in eight healthy men to assess the safety, in terms of hypoglycaemia, of a continuously infused pharmacological dose of native GLP-1 during long-term fasting. After an overnight fast the fasting period continued for 48 h and was followed by a 3-h oral glucose tolerance test (OGTT). GLP-1(7-36 amide) or placebo was continuously infused subcutaneously and titrated to a dose of 4.8 pmol/kg per min.

RESULTS

Two subjects in the GLP-1 group and one subject in the placebo group were withdrawn due to protocol specified plasma glucose (PG) < or = 2.8 mm and neuroglycopaenic symptoms. The infusion of GLP-1 resulted in pharmacological levels of intact GLP-1. During the fasting period PG, insulin and C-peptide levels declined and glucagon, GH and free fatty acid (FFA) levels increased with no differences between GLP-1 and placebo. During OGTT circulating levels of insulin and C-peptide were higher with GLP-1 infusion. However, PG was similar during GLP-1 vs. placebo infusions. GLP-1 infusion increased norepinephrine and cortisol levels during OGTT.

CONCLUSION

The counter-regulatory response during 48 h of subcutaneous GLP-1 infusion was preserved despite long-term fasting with no apparent increased risk of hypoglycaemic episodes. No reactive hypoglycaemia was observed when the fast was followed by an OGTT. Thus use of long-acting GLP-1 analogues may not increase the risk of hypoglycaemia.

The backbone of oral glucose-lowering therapy: time for a paradigm shift?

The complex array of metabolic abnormalities associated with type 2 diabetes provides a number of new targets for therapeutic intervention. Although the established oral glucose-lowering therapies, metformin and the sulfonylureas, continue to provide the backbone of therapeutic approaches, the thiazolidinediones (TZDs) also play an important role. Further, a new class of oral agents, the dipeptidyl peptidase-IV (DPP-IV) inhibitors, has recently become available with apparent utility in decreasing postprandial glucose excursions. This review examines how the TZDs and the DPP-IV inhibitors might integrate into current treatment strategies, considering not only glycemic goals, but also longer-term benefits such as durability of glycemic control, effect on metabolic parameters and cardiovascular outcomes. A practical approach is taken, reflecting potential clinical situations in which therapeutic intervention is required.

Inhibition of dipeptidyl peptidase-4 by vildagliptin during glucagon-like Peptide 1 infusion increases liver glucose uptake in the conscious dog

OBJECTIVE

This study investigated the acute effects of treatment with vildagliptin on dipeptidyl peptidase-4 (DPP-4) activity, glucagon-like peptide 1 (GLP-1) concentration, pancreatic hormone levels, and glucose metabolism. The primary aims were to determine the effects of DPP-4 inhibition on GLP-1 clearance and on hepatic glucose uptake.

RESEARCH DESIGN AND METHODS

Fasted conscious dogs were studied in the presence (n = 6) or absence (control, n = 6) of oral vildagliptin (1 mg/kg). In both groups, GLP-1 was infused into the portal vein (1 pmol . kg(-1) . min(-1)) for 240 min. During the same time, glucose was delivered into the portal vein at 4 mg . kg(-1) . min(-1) and into a peripheral vein at a variable rate to maintain the arterial plasma glucose level at 160 mg/dl.

RESULTS

Vildagliptin fully inhibited DPP-4 over the 4-h experimental period. GLP-1 concentrations were increased in the vildagliptin-treated group (50 +/- 3 vs. 85 +/- 7 pmol/l in the portal vein in control and vildagliptin-treated dogs, respectively; P < 0.05) as a result of a 40% decrease in GLP-1 clearance (38 +/- 5 and 22 +/- 2 ml . kg(-1) . min(-1), respectively; P < 0.05). Although hepatic insulin and glucagon levels were not significantly altered, there was a tendency for plasma insulin to be greater (hepatic levels were 73 +/- 10 vs. 88 +/- 15 microU/ml, respectively). During vildagliptin treatment, net hepatic glucose uptake was threefold greater than in the control group. This effect was greater than that predicted by the change in insulin.

CONCLUSIONS

Vildagliptin fully inhibited DPP-4 activity, reduced GLP-1 clearance by 40%, and increased hepatic glucose disposal by means beyond the effects of GLP-1 on insulin and glucagon secretion.

Exenatide can reduce glucose independent of islet hormones or gastric emptying

Exenatide is a long-acting glucagon-like peptide-1 (GLP-1) mimetic used in the treatment of type 2 diabetes. There is increasing evidence that GLP-1 can influence glycemia not only via pancreatic (insulinotropic and glucagon suppression) and gastric-emptying effects, but also via an independent mechanism mediated by portal vein receptors. The aim of our study was to investigate whether exenatide has an islet- and gastric-independent glycemia-reducing effect, similar to GLP-1. First, we administered mixed meals, with or without exenatide (20 microg sc) to dogs. Second, to determine whether exenatide-induced reduction in glycemia is independent of slower gastric emptying, in the same animals we infused glucose intraportally (to simulate meal test glucose appearance) with exenatide, exenatide + the intraportal GLP-1 receptor antagonist exendin-(9-39), or saline. Exenatide markedly decreased postprandial glucose: net 0- to 135-min area under the curve = +526 +/- 315 and -536 +/- 197 mg.dl(-1).min(-1) with saline and exenatide, respectively (P < 0.05). Importantly, the decrease in plasma glucose occurred without a corresponding increase in postprandial insulin but was accompanied by delayed gastric emptying and lower glucagon. Significantly lower glycemia was induced by intraportal glucose infusion with exenatide than with saline (92 +/- 1 vs. 97 +/- 1 mg/dl, P < 0.001) in the absence of hyperinsulinemia or glucagon suppression. The exenatide-induced lower glycemia was partly reversed by intraportal exendin-(9-39): 95 +/- 3 and 92 +/- 3 mg/dl with exenatide + antagonist and exenatide, respectively (P < 0.01). Our results suggest that, similar to GLP-1, exenatide lowers glycemia via a novel mechanism independent of islet hormones and slowing of gastric emptying. We hypothesize that receptors in the portal vein, via a neural mechanism, increase glucose clearance independent of islet hormones.

Beneficial effects of GLP-1 on endothelial function in humans: dampening by glyburide but not by glimepiride

Sulfonylureas (SU) with glucagon-like peptide-1 (GLP-1)-based therapy are an emerging therapeutic combination for type 2 diabetes. Prior human studies have hinted at endothelial effects of GLP-1 and SU. To study the endothelial effects of GLP-1 per se and to evaluate the modulatory effects, if any, of SU agents on GLP-1-induced changes in endothelial function, healthy, nondiabetic, normotensive, nonsmokers, age 18-50 yr with no family history of diabetes, were studied. Subjects were randomized to either placebo (n = 10), 10 mg of glyburide (n = 11), or 4 mg of glimepiride (n = 8) orally. Euglycemic somatostatin pancreatic clamp with replacement basal insulin, glucagon, and growth hormone was performed for 240 min. Forearm blood flow (FBF) was measured by venous occlusion plethysmography with graded brachial artery infusions of acetylcholine (Ach) and nitroprusside (NTP) before and after intravenous infusion of GLP-1. GLP-1 (preinfusion 3.4 +/- 0.2, postinfusion 25.5 +/- 2.8 pM) enhanced (P < 0.03) Ach-mediated vasodilatation (Delta+6.5 +/- 1.1 vs. Delta+9.1 +/- 1.2 ml.100 ml(-1).min(-1), change from baseline FBF) in those on placebo. However, in contrast, glyburide abolished GLP-1-induced Ach-mediated vasodilatation (Delta+11.7 +/- 2.0 vs. Delta+11.7 +/- 2.5 ml.100 ml(-1).min(-1)). On the other hand, glimepiride did not alter the ability of GLP-1 to enhance Ach-mediated vasodilatation (Delta+7.9 +/- 0.5 vs. Delta+10.2 +/- 1.3 ml.100 ml(-1).min(-1), P < 0.04). Neither GLP-1 nor SU altered NTP-induced vasodilatation. These data demonstrate that GLP-1 per se has direct beneficial effects on endothelium-dependent vasodilatation in humans that are differentially modulated by SU.

The glucagon-like peptide-1 metabolite GLP-1-(9-36) amide reduces postprandial glycemia independently of gastric emptying and insulin secretion in humans

Glucagon-like peptide 1 (GLP-1) lowers glycemia by modulating gastric emptying and endocrine pancreatic secretion. Rapidly after its secretion, GLP-1-(7-36) amide is degraded to the metabolite GLP-1-(9-36) amide. The effects of GLP-1-(9-36) amide in humans are less well characterized. Fourteen healthy volunteers were studied with intravenous infusion of GLP-1-(7-36) amide, GLP-1-(9-36) amide, or placebo over 390 min. After 30 min, a solid test meal was served, and gastric emptying was assessed. Blood was drawn for GLP-1 (total and intact), glucose, insulin, C-peptide, and glucagon measurements. Administration of GLP-1-(7-36) amide and GLP-1-(9-36) amide significantly raised total GLP-1 plasma levels. Plasma concentrations of intact GLP-1 increased to 21 +/- 5 pmol/l during the infusion of GLP-1-(7-36) amide but remained unchanged during GLP-1-(9-36) amide infusion [5 +/- 3 pmol/l; P < 0.001 vs. GLP-1-(7-36) amide administration]. GLP-1-(7-36) amide reduced fasting and postprandial glucose concentrations (P < 0.001) and delayed gastric emptying (P < 0.001). The GLP-1 metabolite had no influence on insulin or C-peptide concentrations. Glucagon levels were lowered by GLP-1-(7-36) amide but not by GLP-1-(9-36) amide. However, the postprandial rise in glycemia was reduced significantly (by approximately 6 mg/dl) by GLP-1-(9-36) amide (P < 0.05). In contrast, gastric emptying was completely unaffected by the GLP-1 metabolite. The GLP-1 metabolite lowers postprandial glycemia independently of changes in insulin and glucagon secretion or in the rate of gastric emptying. Most likely, this is because of direct effects on glucose disposal. However, the glucose-lowering potential of GLP-1-(9-36) amide appears to be small compared with that of intact GLP-1-(7-36) amide.

Exenatide: a GLP-1 receptor agonist as novel therapy for Type 2 diabetes mellitus

Exenatide is a glucagon-like peptide 1 receptor agonist, which has recently received FDA approval in the US for the treatment of Type 2 diabetes. Exenatide is an incretin mimetic that improves glycaemic control in patients with diabetes through acute mechanisms, such as glucose-dependent stimulation of insulin secretion, suppression of inappropriate glucagon secretion and slowing of gastric emptying, as well as chronic mechanisms that include enhancement of beta-cell mass in rodent studies and weight loss and inhibition of food intake in humans. This article reviews the mechanisms of exenatide action, as well as its efficacy in the treatment of Type 2 diabetes.

Newer therapeutic options for children with diabetes mellitus: theoretical and practical considerations

Recent studies in adult patients with type 1 diabetes mellitus (T1DM) and T2DM have examined the potential utility, benefits, and side effects of agents that augment insulin secretion after oral ingestion of nutrients in comparison with intravenous nutrient delivery, the so-called incretins. Two families of incretin-like substances are now approved for use in adults. Glucagon-like peptide-1 (GLP-1) or agents that bind to its receptor (exenatide, Byetta) or agents that inhibit its destruction [dipeptidyl peptidase-IV (DPP-IV) inhibitors, Vildagliptin] improve insulin secretion, delay gastric emptying, and suppress glucagon secretion while decreasing food intake without increasing hypoglycemia. Pramlintide, a synthetic amylin analog, also decreases glucagon secretion and delays gastric emptying, improves hemoglobin A1c (HbA1C), and facilitates weight reduction without causing hypoglycemia. We review the historical discovery of these agents, their physiology [corrected] and their current applications. Remarkably, only one or two studies have been reported in children. Pediatricians caring for children with T1DM and T2DM should become familiar with these agents and investigate their applicability, as they seem likely to enhance our therapeutic armamentarium to treat children with diabetes mellitus.

Pharmacologic modifications of hormones to improve their therapeutic potential for diabetes management

Rapid-acting genetically engineered insulin analogues emerging in the last 10 years are now established as more effective prandial insulins than traditional short-acting human insulin. The development of analogues for use as basal insulin, however, has been much slower. Methods of pro-tracting the time-action curve of injected insulin include complexing with proteins, insulin crystal formation, shifting the iso-electric point of the amino acid sequence or attaching a fatty-acid side chain to the molecule. The latter two methods have been more successful in producing physiologic insulin profiles when compared with the former methods. The principle of acylation has also been applied to prolong the action of other hormones, such as glucagon-like peptide 1 (GLP-1), as the native peptide has a very short half-life. Preliminary results with this compound and other GLP-1 analogues show promise in treating patients with type 2 diabetes. In summary, the development of new insulin and other hormone preparations by the manipulation of native peptide structure has recently improved our antidiabetic armamentarium, and further research will continue this fruitful approach.

Effect of somatostatin on duodenal glucose absorption in man

OBJECTIVE

The hyperglycemic hyperinsulinemic clamp technique using intraduodenally infused glucose is an attractive tool for studying postprandial glucose metabolism under strictly controlled conditions. Because it requires the use of somatostatin (SST), we examined, in this study, the effect of SST on intestinal glucose absorption.

CONTEXT

Twenty-six normal volunteers were given a constant 3-h intraduodenal infusion of glucose (6 mg.kg(-1).min(-1)) labeled with [2-(3)H]glucose for glucose absorption measurement. During glucose infusion, 19 subjects received iv SST at doses of 10-100 ng.kg(-1).min(-1) plus insulin and glucagon, and seven subjects were studied under control conditions. In the controls, glucose was absorbed at a rate that, after a 20-min lag period, equaled the infusion rate.

RESULTS

With all the doses of SST tested, absorption was considerably delayed but equaled the rate of infusion after 3 h. At that time, only 5 +/- 2% of the total amount of infused glucose was unabsorbed in the control subjects vs. 36 +/- 2% (P < 0.001) in the SST-infused subjects. In the latter, the intraluminal residue was almost totally absorbed within 40 min of the cessation of SST infusion. At the lowest dose of SST tested (10 ng.kg(-1).min(-1)), suppression of insulin secretion was incomplete.

CONCLUSION

These properties of SST hamper the use of intraduodenal hyperglycemic hyperinsulinemic clamps as a tool for exploring postprandial glucose metabolism.

The regulation of glucose effectiveness: how glucose modulates its own production

PURPOSE OF REVIEW

'Glucose effectiveness' refers to the ability of glucose per se to suppress endogenous glucose production and stimulate glucose uptake. In addition to the inhibitory effects of insulin on endogenous glucose production, rising glucose levels have important direct effects on glucose homeostasis. The loss of glucose effectiveness in type 2 diabetes mellitus contributes importantly to hyperglycemia in those individuals. Given the rapidly increasing incidence and serious complications of type 2 diabetes mellitus, understanding the regulation of glucose effectiveness has great potential therapeutic benefits.

RECENT FINDINGS

The loss of this important regulation appears to be secondary to the chronic 'diabetic milieu' in type 2 diabetes mellitus, which includes elevated plasma glucose and free fatty acid levels. Glucose effectiveness is completely restored by normalizing plasma free fatty acid levels. Increased free fatty acid availability stimulates gluconeogenesis and alters flux through key hepatic enzymes. It is likely that at least part of this regulation is through central pathways. In addition, hormones that may exert important effects on hepatic glucose effectiveness include cortisol, insulin and glucagon-like peptide 1. The effectiveness of glucose to stimulate glucose uptake is impaired by elevated free fatty acid levels and may be enhanced by glucagon-like peptide 1.

SUMMARY

The regulation of glucose effectiveness involves a complex interplay of hormonal and metabolic factors, with free fatty acid and glucoregulatory hormones playing key roles. The loss of this regulation in type 2 diabetes mellitus contributes importantly to hyperglycemia, and may largely be caused by increased free fatty acid levels.

Glucagon-like peptide 1(GLP-1) in biology and pathology

Post-translational proteolytic processing of the preproglucagon gene in the gut results in the formation of glucagon-like peptide 1 (GLP-1). Owing to its glucose-dependent insulinotropic effect, this hormone was postulated to primarily act as an incretin, i.e. to augment insulin secretion after oral glucose or meal ingestion. In addition, GLP-1 decelerates gastric emptying and suppresses glucagon secretion. Under physiological conditions, GLP-1 acts as a part of the 'ileal brake', meaning that is slows the transition of nutrients into the distal gut. Animal studies suggest a role for GLP-1 in the development and growth of the endocrine pancreas. In light of its multiple actions throughout the body, different therapeutic applications of GLP-1 are possible. Promising results have been obtained with GLP-1 in the treatment of type 2 diabetes, but its potential to reduce appetite and food intake may also allow its use for the treatment of obesity. While rapid in vivo degradation of GLP-1 has yet prevented its broad clinical use, different pharmacological approaches aiming to extend the in vivo half-life of GLP-1 or to inhibit its inactivation are currently being evaluated. Therefore, antidiabetic treatment based on GLP-1 may become available within the next years. This review will summarize the biological effects of GLP-1, characterize its role in human biology and pathology, and discuss potential clinical applications as well as current clinical studies.

Effect of intravenous infusion of exenatide (synthetic exendin-4) on glucose-dependent insulin secretion and counterregulation during hypoglycemia

This study assessed whether glucose-dependent insulin secretion and overall counterregulatory response are preserved during hypoglycemia in the presence of exenatide. Twelve healthy fasted volunteers were randomized in a triple-blind crossover study to receive either intravenous exenatide (0.066 pmol. kg(-1). min(-1)) or placebo during a 270-min stepwise hyperinsulinemic-hypoglycemic clamp (insulin infusion 0.8 mU. kg(-1). min(-1)). Plasma glucose was clamped sequentially at 5.0 (0-120 min), 4.0 (120-180 min), 3.2 (180-240 min), and 2.7 mmol/l (240-270 min). At 270 min, insulin infusion was terminated and plasma glucose increased to approximately 3.2 mmol/l. The time to achieve plasma glucose >/=4 mmol/l thereafter was recorded. Insulin secretory rates (ISRs) and counterregulatory hormones were measured throughout. Glucose profiles were superimposable between the exenatide and placebo arms. In the presence of euglycemic hyperinsulinemia, ISRs in the exenatide arm were approximately 3.5-fold higher than in the placebo arm (353 +/- 29 vs. 100 +/- 29 pmol/min [least-square means +/- SE]). However, ISRs declined similarly and rapidly at all hypoglycemic steps (</=4 mmol/l) in both groups. Glucagon was suppressed in the exenatide arm during euglycemia and higher than placebo during hypoglycemia. Plasma glucose recovery time was equivalent for both treatments. The areas under the concentration-time curve from 270 to 360 min for cortisol, epinephrine, norepinephrine, and growth hormone were similar between treatment arms. There were no differences in adverse events. In the presence of exenatide, there was a preserved, glucose-dependent insulin secretory response and counterregulatory response during hypoglycemia.

The gastrointestinal tract and glucose tolerance

PURPOSE OF REVIEW

The development of incretin hormones and incretin analogues for the therapy of diabetes highlights the importance of the gastrointestinal tract in the maintenance of glucose tolerance.

RECENT FINDINGS

The review focuses on recent information on the role of incretins and their breakdown products on insulin secretion, gastric emptying, and satiety. The importance of gastric emptying and its absorptive potential as well as of dietary composition on gastric emptying and glucose tolerance is highlighted. The concept of a portal glucose sensor in humans has been the subject of some controversy but has been recently revisited.

SUMMARY

The gastrointestinal tract plays an important part in glucose tolerance. In this review we have examined how factors altering gastric emptying, insulin secretion in response to meal ingestion, and gastric emptying contribute to the maintenance and deterioration of glucose tolerance.

Glucagon-like peptide 1: evolution of an incretin into a treatment for diabetes

Glucagon-like peptide 1 (GLP-1) is a product of proglucagon that is secreted by specialized intestinal endocrine cells after meals. GLP-1 is insulinotropic and plays a role in the incretin effect, the augmented insulin response observed when glucose is absorbed through the gut. GLP-1 also appears to regulate a number of processes that reduce fluctuations in blood glucose, such as gastric emptying, glucagon secretion, food intake, and possibly glucose production and glucose uptake. These effects, in addition to the stimulation of insulin secretion, suggest a broad role for GLP-1 as a mediator of postprandial glucose homeostasis. Consistent with this role, the most prominent effect of experimental blockade of GLP-1 signaling is an increase in blood glucose. Recent data also suggest that GLP-1 is involved in the regulation of beta-cell mass. Whereas other insulinotropic gastrointestinal hormones are relatively ineffective in stimulating insulin secretion in persons with type 2 diabetes, GLP-1 retains this action and is very effective in lowering blood glucose levels in these patients. There are currently a number of products in development that utilize the GLP-1-signaling system as a mechanism for the treatment of diabetes. These compounds, GLP-1 receptor agonists and agents that retard the metabolism of native GLP-1, have shown promising results in clinical trials. The application of GLP-1 to clinical use fulfills a long-standing interest in adapting endogenous insulinotropic hormones to the treatment of diabetes.

Comparison of intraduodenal and intravenous glucose metabolism under clamp conditions in humans

To determine whether the uptake and metabolic partition of glucose are influenced by its delivery route, 12 normal volunteers underwent two 3-h euglycemic (approximately 93 mg/dl) hyperinsulinemic (approximately 43 mU/l) clamps at a 3- to 5-wk interval, one with intravenous (i.v.) and the other with intraduodenal (i.d.) glucose labeled with [3-3H]- and [U-14C]glucose. Systemic glucose was traced with [6,6-2H2]glucose in eight subjects. During the last hour of the clamps, the average glucose infusion rate (5.85 +/- 0.37 vs. 5.43 +/- 0.43 mg.kg(-1).min(-1); P = 0.02) and exogenous glucose uptake (5.66 +/- 0.37 vs. 5.26 +/- 0.41 mg.kg(-1).min(-1); P = 0.04) were borderline higher in the i.d. than in the i.v. studies. The increased uptake was entirely accounted for by increased glycolysis (3H2O production), which was attributed to the stimulation of gut metabolism by the absorptive process. No difference was observed in glucose storage whether it was calculated as glucose uptake minus glycolysis (i.d. vs. i.v.: 2.44 +/- 0.28 vs. 2.40 +/- 0.31 mg.kg(-1).min(-1)) or as glucose uptake minus net glucose oxidation (2.86 +/- 0.33 vs. 2.81 +/- 0.35 mg.kg(-1).min(-1)). Because peripheral tissues were exposed to identical glucose, insulin, and free fatty acid levels under the two experimental conditions, we assumed that their glucose uptake and storage were similar during the two tests. We therefore suggest that hepatic glycogen storage (estimated as whole body minus peripheral storage) was also unaffected by the route of glucose delivery. On the other hand, in the i.d. tests, the glucose splanchnic extraction ratio calculated by the dual-isotope technique averaged 4.9 +/- 2.3%, which is close to the figures published for i.v. glucose. Despite the limitations related to whole body measurements, these two sets of data do not support the idea that enteral glucose stimulates hepatic uptake more efficiently than i.v. glucose.

Suppression of glucose production by GLP-1 independent of islet hormones: a novel extrapancreatic effect

Glucagon-like peptide-1 (GLP-1) is an intestinal hormone that stimulates insulin secretion and decreases glucagon release. It has been hypothesized that GLP-1 also reduces glycemia independent of its effect on islet hormones. Based on preliminary evidence that GLP-1 has independent actions on endogenous glucose production, we undertook a series of experiments that were optimized to address this question. The effect of GLP-1 on glucose appearance (Ra) and glucose disposal (Rd) was measured in eight men during a pancreatic clamp that was performed by infusing octreotide to suppress secretion of islet hormones, while insulin and glucagon were infused at rates adjusted to maintain blood glucose near fasting levels. After stabilization of plasma glucose and equilibration of [3H]glucose tracer, GLP-1 was given intravenously for 60 min. Concentrations of insulin, C-peptide, and glucagon were similar before and during the GLP-1 infusion (115 +/- 14 vs. 113 +/- 11 pM; 0.153 +/- 0.029 vs. 0.156 +/- 0.026 nM; and 64.7 +/- 11.5 vs. 65.8 +/- 13.8 ng/l, respectively). With the initiation of GLP-1, plasma glucose decreased in all eight subjects from steady-state levels of 4.8 +/- 0.2 to a nadir of 4.1 +/- 0.2 mM. This decrease in plasma glucose was accounted for by a significant 17% decrease in Ra, from 22.6 +/- 2.8 to 19.1 +/- 2.8 micromol. kg-1. min-1 (P < 0.04), with no significant change in Rd. These findings indicate that, under fasting conditions, GLP-1 decreases endogenous glucose production independent of its actions on islet hormone secretion.

Regulation of hepatic and peripheral glucose disposal

Precise regulation of hepatic and peripheral glucose uptake is essential to preserve glucose homeostasis. The liver extracts approximately 1/3 of an oral glucose load, skeletal muscle extracts approximately 1/3, and other tissues, particularly the central nervous system and the formed elements of the blood, take up the balance. The load of glucose reaching the liver, the insulin concentration, and the route of glucose delivery (the hepatic portal or a peripheral vein) are key determinants of the rate of net hepatic glucose uptake. Glucose uptake by muscle requires three steps: delivery of glucose from the blood to the muscle, transport of glucose across the muscle membrane, and phosphorylation of glucose, processes affected by glycaemia and insulinaemia. Exercise stimulates insulin-dependent and -independent muscle glucose uptake, as well as the liver's ability to take up glucose.

Effect of intraportal glucagon-like peptide-1 on glucose metabolism in conscious dogs

Arteriovenous difference and tracer ([3-(3)H]glucose) techniques were used in 42-h-fasted conscious dogs to identify any insulin-like effects of intraportally administered glucagon-like peptide 1-(7-36)amide (GLP-1). Each study consisted of an equilibration, a basal, and three 90-min test periods (P1, P2, and P3) during which somatostatin, intraportal insulin (3-fold basal) and glucagon (basal), and peripheral glucose were infused. Saline was infused intraportally in P1. During P2 and P3, GLP-1 was infused intraportally at 0.9 and 5.1 pmol. kg(-1). min(-1) in eight dogs, at 10 and 20 pmol. kg(-1). min(-1) in seven dogs, and at 0 pmol. kg(-1). min(-1) in eight dogs (control group). Net hepatic glucose uptake was significantly enhanced during GLP-1 infusion at 20 pmol. kg(-1). min(-1) [21.8 vs. 13.4 micromol. kg(-1). min(-1) (control), P < 0.05]. Glucose utilization was significantly increased during infusion at 10 and 20 pmol. kg(-1). min(-1) [87.3 +/- 8.3 and 105.3 +/- 12.8, respectively, vs. 62.2 +/- 5.3 and 74.7 +/- 7.4 micromol. kg(-1). min(-1) (control), P < 0.05]. The glucose infusion rate required to maintain hyperglycemia was increased (P < 0.05) during infusion of GLP-1 at 5.1, 10, and 20 pmol. kg(-1). min(-1) (22, 36, and 32%, respectively, greater than control). Nonhepatic glucose uptake increased significantly during delivery of GLP-1 at 5.1 and 10 pmol. kg(-1). min(-1) (25 and 46% greater than control) and tended (P = 0.1) to increase during GLP-1 infusion at 20 pmol. kg(-1). min(-1) (24% greater than control). Intraportal infusion of GLP-1 at high physiological and pharmacological rates increased glucose disposal primarily in nonhepatic tissues.

Glucagon-like peptide 1 improved glycemic control in type 1 diabetes

BACKGROUND: Glucagon-like peptide-1 (GLP-1) and its agonists are under assessment in treatment of type 2 diabetes, by virtue of their antidiabetic actions, which include stimulation of insulin secretion, inhibition of glucagon release, and delay of gastric emptying. We examined the potential of GLP-1 to improve glycemic control in type 1 diabetes with no endogenous insulin secretion. METHODS: Dose-finding studies were carried out to establish mid range doses for delay of gastric emptying indicated by postponement of pancreatic polypeptide responses after meals. The selected dose of 0.63 micrograms/kg GLP-1 was administered before breakfast and lunch in 8-hour studies in hospital to establish the efficacy and safety of GLP-1. In outside-hospital studies, GLP-1 or vehicle was self-administered double-blind before meals with usual insulin for five consecutive days by five males and three females with well-controlled C-peptide-negative type 1 diabetes. Capillary blood glucose values were self-monitored before meals, at 30 and 60 min after breakfast and supper, and at bedtime. Breakfast tests with GLP-1 were conducted on the day before and on the day after 5-day studies. Paired t-tests and ANOVA were used for statistical analysis. RESULTS: In 8-hour studies time-averaged incremental (delta) areas under the curves(AUC) for plasma glucose through 8 hours were decreased by GLP-1 compared to vehicle (3.2 PlusMinus; 0.9, mean PlusMinus; se, vs 5.4 PlusMinus; 0.8 mmol/l, p <.05), and for pancreatic polypeptide, an indicator of gastric emptying, through 30 min after meals (4.0 PlusMinus; 3.1 vs 37 PlusMinus; 9.6 pmol/l, p <.05) with no adverse effects. Incremental glucagon levels through 60 min after meals were depressed by GLP-1 compared to vehicle (-3.7 PlusMinus; 2.5 vs 3.1 PlusMinus; 1.9 ng/l, p <.04). In 5-day studies, AUC for capillary blood glucose levels were lower with GLP-1 than with vehicle (-0.64 PlusMinus; 0.33 vs 0.34 PlusMinus; 0.26 mmol/l, p <.05). No assisted episode of hypoglycaemia or change in insulin dosage occurred. Breakfast tests on the days immediately before and after 5-day trials showed no change in the effects of GLP-1. CONCLUSION: We have demonstrated that subcutaneous GLP-1 can improve glucose control in type 1 diabetes without adverse effects when self-administered before meals with usual insulin during established intensive insulin treatment programs.

Effect of glucagon-like peptide 1 (7-36 amide) on insulin-mediated glucose uptake in patients with type 1 diabetes

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.

Preclinical developments in type 2 diabetes

Type 2 diabetes is associated with insulin resistance in peripheral tissues, such as muscle and fat, impaired glucose-stimulated insulin secretion from pancreatic beta-cells and elevated hepatic gluconeogenesis. Current pharmacotherapy does not adequately address the metabolic defects underlying this disease. Thus, novel targets are being explored that enhance insulin action at target tissues, stimulate carbohydrate and fat catabolism, decrease endogenous glucose production and increase pancreatic beta-cell neogenesis and glucose-dependent insulin secretion. This article reviews recent developments in research on several of these targets, namely acetyl-CoA carboxylase 2 (ACC2), I kappa kinase (IKK) beta, dipeptidyl peptidase IV (DPP-IV) and glucagon-like peptide-1 receptor (GLP-1R).

Influence of metabolic control on splanchnic glucose uptake, insulin sensitivity, and the time required for glucose absorption in patients with type 1 diabetes

OBJECTIVE

The relationship between splanchnic glucose uptake (SGU) after oral glucose administration and metabolic control in type 1 diabetic patients is controversial. We estimated SGU as well as peripheral glucose uptake and the time required for glucose absorption by a validated method, the oral glucose (OG) clamp, in type 1 diabetic patients with different levels of long-term glycemic control.

RESEARCH DESIGN AND METHODS

An OG clamp (which combines a hyperinsulinemic clamp [120 mU. m(-2). min(-1)] with an OR load [75 g] during steady-state glucose uptake) was performed in eight type 1 diabetic patients with good metabolic control (DG) (HbA(1c) 6.1 +/- 0.2%, BMI 23.1 +/- 0.7 kg/m(2)), eight type 1 diabetic patients with poor metabolic control (DP) (HbA(1c) 8.5 +/- 0.3%, BMI 25.4 +/- 1.4 kg/m(2)), and eight healthy matched control subjects (C) (HbA(1c) 5.1 +/- 0.1%, BMI 25 +/- 1.3 kg/m(2)) to determine SGU, glucose uptake, and glucose absorption.

RESULTS

Glucose uptake calculated from 120 to 180 min during the clamp was 9.13 +/- 0.55 mg. kg(-1). min(-1) in C, 8.18 +/- 0.71 mg. kg(-1). min(-1) in DG, and 7.42 +/- 0.96 mg. kg(-1). min(-1) in DP (NS). Glucose absorption was 140 +/- 6 min in C, 156 +/- 4 min in DG, and 143 +/- 7 min in DP (NS). The respective calculated SGU was 14.5 +/- 5.6% in C, 17.8 +/- 3.1% in DG, and 18.8 +/- 4.2% in DP (NS) and did not correlate with HbA(1c) values.

CONCLUSIONS

Peripheral glucose uptake, SGU after oral glucose administration, and the glucose absorption time were not different in type 1 diabetic patients independent of glycemic control when compared with healthy subjects.

Effect of enteral vs. parenteral glucose delivery on initial splanchnic glucose uptake in nondiabetic humans

To determine if enteral delivery of glucose influences splanchnic glucose metabolism, 10 subjects were studied when glucose was either infused into the duodenum at a rate of 22 micromol x kg(-1) x min(-1) and supplemental glucose given intravenously or when all glucose was infused intravenously while saline was infused intraduodenally. Hormone secretion was inhibited with somatostatin, and glucose (approximately 8.5 mmol/l) and insulin (approximately 450 pmol/l) were maintained at constant but elevated levels. Intravenously infused [6,6-(2)H(2)]glucose was used to trace the systemic appearance of intraduodenally infused [3-(3)H]glucose, whereas UDP-glucose flux (an index of hepatic glycogen synthesis) was measured using the acetaminophen glucuronide method. Despite differences in the route of glucose delivery, glucose production (3.5 +/- 1.0 vs. 3.3 +/- 1.0 micromol x kg(-1) x min(-1)) and glucose disappearance (78.9 +/- 5.7 vs. 85.0 +/- 7.2 micromol x kg(-1) x min(-1)) were comparable on intraduodenal and intravenous study days. Initial splanchnic glucose extraction (17.5 +/- 4.4 vs. 14.5 +/- 2.9%) and hepatic UDP-glucose flux (9.0 +/- 2.0 vs. 10.3 +/- 1.5 micromol x kg(-1) x min(-1)) also did not differ on the intraduodenal and intravenous study days. These data argue against the existence of an "enteric" factor that directly (i.e., independently of circulating hormone concentrations) enhances splanchnic glucose uptake or hepatic glycogen synthesis in nondiabetic humans.

New pharmacologic agents for diabetes

New agents are being developed to address the underlying endocrinopathies and metabolic disturbances of type 2 diabetes. Stimulants of the nuclear peroxisome proliferator-activated receptor gamma (PPAR gamma) are being identified to selectively improve insulin actions, and dual agonists of PPAR gamma and PPAR alpha are being evaluated for enhanced control of hyperglycemia and dyslipidemia. Novel activators of insulin receptor phosphorylation and inhibitors of receptor dephosphorylation are offering encouraging leads for new agents. Analogues of glucagon-like peptide-1 that increase glucose-induced insulin secretion may additionally increase beta-cell neogenesis from progenitor duct cells. The amylin analogue pramlintide, which suppresses glucagon secretion and reduces weight, is advancing in clinical trial. Direct stimulants of glucose utilization and partial inhibitors of gluconeogenesis are providing useful new drug templates. Thus, new pharmacologic approaches are emerging to treat the multiple lesions of type 2 diabetes.