GLP-1 inhibits gastric emptying of water but does not influence plasma

BACKGROUND: Glucagon-like peptide-1 (GLP-1) has been shown to inhibit gastric emptying of a caloric load but the effect on a non-caloric load is unknown. METHODS: Seven healthy men were studied after an over-night fast. Thirty min before the intake of 330 ml radioactively labeled water either GLP-1 (0.75 pmol/kg/min) or saline was administered intravenously and continued for 75 min. Scintigraphic gastric emptying was performed for 45 min and plasma samples were obtained for analysis of vasopressin, sodium, osmolality, GLP-1, insulin, and glucose. In addition, electric field stimulation of human gastric muscle strips was done. RESULTS: The median (range) percent water retained in the stomach, 45 min after intake of water, was 96% (68%-98%) and 12% (2%-42%) (P = 0.02) during infusion of GLP-1 and saline, respectively. Additionally, GLP-1 did not affect basal tone or contractile response of gastric muscle strips to electric field stimulation or acetylcholine (ACh). There was no change in plasma concentrations of vasopressin, sodium, or plasma osmolality during GLP-1 compared to saline infusion. CONCLUSION: GLP-1 has a profound inhibitory effect on the gastric emptying of water in man, but no short-term effect on water homeostasis. No effect was seen on contractility of gastric muscle strips suggesting an indirect action on gastric emptying.     

The influence of the colon on postprandial glucagon-like peptide 1 (7-36) amide concentration in man

Glucagon-like peptide (7-36) amide (GLP-1) is an incretin hormone of the enteroinsular axis released rapidly after meals despite the fact that GLP-1 secreting cells (L-cells) occur predominantly in the distal gut. The importance of these colonic L-cells for postprandial GLP-1 was determined in healthy control subjects and in ileostomy patients with minimal small bowel resection (<5 cm). Subjects were fed a high complex carbohydrate test meal (15.3 g starch) followed by two carbohydrate-free, high fat test meals (25 g and 48.7 g fat respectively). Circulating levels of glucose, insulin, glucagon, glucose insulinotrophic peptide (GIP) and GLP-1 were measured over a 9-h postprandial period. For both subject groups the complex carbohydrate test meal failed to elicit a rise in either GIP or GLP-1. However, both hormones were elevated after the fat load although the GLP-1 concentration was significantly reduced in the ileostomist group when compared with controls (P=0.02). Associated with this reduction in circulating GLP-1 was an elevation in glucagon concentration (P=0.012) and a secondary rise in the plasma glucose concentration (P=0.006). These results suggest that the loss of colonic endocrine tissue is an important determinant in the postprandial GLP-1 concentration. Ileostomists should not be assumed to have normal enteroinsular function as the colon appears to have an important role in postprandial metabolism.     

Effects of GLP-1-(7-36)NH2, GLP-1-(7-37), and GLP-1- (9-36)NH2 on intravenous glucose tolerance and glucose-induced insulin secretion in healthy humans

Glucagon-like peptide 1 (GLP-1) is an insulin secretagogue synthesized in the intestine and released in response to meal ingestion. It is secreted primarily in two forms, GLP-1-(7-37) and GLP-1-(7-36)NH(2), both of which bind to a specific GLP-1 receptor (GLP-1r) on the pancreatic beta-cell and augment glucose-stimulated insulin secretion. Once secreted, GLP-1-(7-36)NH(2) is rapidly metabolized to GLP-1-(9-36)NH(2), which is the predominant form of GLP-1 in postprandial plasma because of its relatively slower clearance. Although no clear biological role for GLP-1-(9-36)NH(2) in humans has been identified, recent studies in animals suggest two potential effects: to antagonize the effects of intact GLP-1 and to promote glucose disappearance in peripheral tissues. In the studies reported here we compared the independent effects of GLP-1-(7-36)NH(2), GLP-1-(7-37), and GLP-1-(9-36)NH(2) on parameters of iv glucose tolerance and determined whether GLP-1-(9-36)NH(2) inhibits the insulinotropic actions of GLP-1. Ten healthy subjects underwent 4 separate frequently sampled iv glucose tolerance tests during infusions of GLP-1-(7-37), GLP-1-(7-36)NH(2), GLP-1-(9-36)NH(2), or saline. Results from the iv glucose tolerance test were used to obtain indexes of beta-cell function (acute insulin response to glucose) and iv glucose tolerance (glucose disappearance constant), and the minimal model of glucose kinetics was used to obtain indexes of glucose effectiveness and insulin sensitivity. Compared with control studies, both GLP-1-(7-36)NH(2) and GLP-1-(7-37) significantly increased acute insulin response to glucose, glucose disappearance constant, glucose effectiveness, and glucose effectiveness at zero insulin, but did not change the insulin sensitivity index. In contrast, none of the parameters of glucose tolerance was measurably affected by GLP-1-(9-36) amide. In a second set of experiments, 10 healthy subjects had glucose-stimulated insulin secretion measured during an infusion of GLP-1-(7-36)NH(2) alone or with a simultaneous infusion of GLP-1-(9-36)NH(2) that increased plasma levels approximately 10-fold over those produced by unmetabolized GLP-1. Augmentation of glucose-stimulated insulin secretion by GLP-1-(7-36)NH(2) was not altered by the coadministration of GLP-1-(9-36)NH(2). Based on these results we conclude that GLP-1-(9-36)NH(2) does not regulate insulin release or glucose metabolism in healthy humans.     

GLP-1 and GLP-1(7-36) amide: influences on basal and stimulated insulin and glucagon secretion in the mouse

We studied the cellular distribution of glucagon-like peptide-1 (GLP-1) in the pancreas and gut and the effects of GLP-1 and its truncated form, GLP-1(7-36) amide, on basal and stimulated insulin and glucagon secretion in the mouse. Immunofluorescence staining showed that GLP-1 immunoreactivity occurred within peripheral islet cells and in cells located mainly distally in the small intestine and in the entire large intestine. Double-immunostaining revealed that the GLP-1-immunoreactive cells were identical to the glucagon/glicentin cells. Experiments in vivo revealed that basal insulin secretion was stimulated by GLP-1(7-36) amide at the dose levels of 8 and 32 nmol/kg, and by GLP-1 at 32 nmol/kg. Furthermore, GLP-1(7-36) amide showed additive stimulatory influence with glucose (2.8 mmol/kg), the cholinergic agonist carbachol (0.16 mumol/kg), and the C-terminal octapeptide of cholecystokinin (CCK-8, 5.3 nmol/kg), when injected at 8 or 32 nmol/kg. In contrast, stimulated insulin secretion was unaffected by GLP-1. Moreover, the glucagon secretory responses to carbachol and CCK-8 were inhibited by GLP-1(7-36) amide but were unaffected by the entire GLP-1. We conclude that GLP-1(7-36) has the potential for being a modulator of islet hormone secretion.     

GLP-1(7-36)amide binding in skeletal muscle membranes from streptozotocin diabetic rats

A higher specific binding of GLP-1(7–36)amide is found in skeletal muscle plasma membranes from adult streptozotocin (STZ)-treated rats (insulin-dependent diabetes mellitus model) and from neonatal STZ-treated rats (non insulin-dependent diabetes mellitus model), as compared to that in normal controls; no apparent change in the affinity was observed, that indicating the presence in both diabetic models of an increased number of high affinity binding sites for the peptide. The maximal specific GLP-1(7–16)amide binding in the non insulin-dependent diabetes mellitus model was found to be significantly higher than that in the insulin-dependent diabetes mellitus model. As GLP-1(7–36)amide exerts a glycogenic effect in the rat skeletal muscle, the present data suggest that the action of the peptide in the muscle glucose metabolism may be increased in states of insulin deficiency accompanied or not by insulin resistance.     

GLP-1 does not acutely affect insulin sensitivity in healthy man

Summary Previous studies have suggested that glucagon-like peptide-1 (GLP-1) (7–36 amide) may have the direct effect of increasing insulin sensitivity in healthy man. To evaluate this hypothesis we infused GLP-1 in seven lean healthy men during a hyperinsulinaemic (0.8 mU · kg⁻¹ · min⁻¹), euglycaemic (5 mmol/l) clamp. Somatostatin (450 μg/h) was infused to suppress endogenous insulin secretion, and growth hormone (3 ng · kg⁻¹ · min⁻¹) and glucagon (0.8 ng · kg⁻¹ · min⁻¹) were infused to maintain basal levels. GLP-1 (50 pmol · kg⁻¹ · h⁻¹) or 154 mmol/l NaCl (placebo) was infused after 3 h of equilibration, i.e. from 180-360 min. GLP-1 infusion resulted in GLP-1 levels of approximately 40 pmol/l. Plasma glucose, insulin, growth hormone, and glucagon levels were similar throughout the clamps. The rate of glucose infusion required to maintain euglycaemia was similar with or without GLP-1 infusion (7.69±1.17 vs 7.76±0.95 mg · kg⁻¹ · min⁻¹ at 150–180 min and 8.56±1.13 vs 8.55±0.68 mg · kg⁻¹ · min⁻¹ at 330–360 min) and there was no difference in isotopically determined hepatic glucose production rates (− 0.30±0.23 vs −0.16±0.22 mg · kg⁻¹ · min⁻¹ at 330–360 min). Furthermore, arteriovenous glucose differences across the forearm were similar with or without GLP-1 infusion (1.43±0.23 vs 1.8±0.29 mmol/l), (ANOVA;p>0.60, in all instances). In conclusion, GLP-1 (7–36 amide) administered for 3 h, leading to circulating levels within the physiological range, does not affect insulin sensitivity in healthy man.     

Glucagon-like peptide-1(9-36)amide metabolite inhibits weight gain and attenuates diabetes and hepatic steatosis in diet-induced obese mice

Aims: The metabolic syndrome, a disease arising from the world‐wide epidemic of obesity, is manifested as severe insulin resistance, hyperlipidaemia, hepatic steatosis and diabetes. Previously we reported that GLP‐1(9‐36)amide, derived from the gluco‐incretin hormone, glucagon‐like peptide‐1 (GLP‐1), suppresses gluconeogenesis in isolated hepatocytes. The aims of this study were to determine the effects of GLP‐1(9‐36)amide in diet‐induced obese mice that model the development of the metabolic syndrome. Methods: Mice rendered obese by feeding a very high fat diet were administered GLP‐1(9‐36)amide via subcutaneous osmopumps for 8 weeks. Body weight, energy intake, plasma insulin and glucose levels (insulin‐resistance), and hepatic steatosis were assessed. Results: Eight‐week infusions of GLP‐1(9‐36)amide inhibited weight gain, increased energy intake, prevented the development of fasting hyperinsulinaemia and hyperglycaemia, and curtailed the accumulation of liver triglycerides. The peptide had no effects in mice fed a normal chow diet. Notably, energy intake in the obese mice receiving GLP‐1(9‐36)amide was 20% greater than obese mice receiving vehicle control. Conclusions: GLP‐1(9‐36)amide exerts insulin‐like actions in the presence of insulin resistance and prevents the development of metabolic syndrome. Curtailment of weight gain in the face of increased caloric intake suggests that GLP‐1(9‐36)amide increases energy expenditure. These findings suggest the possibility of the use of GLP‐1(9‐36)amide, or a peptide mimetic derived there from, for the treatment of obesity, insulin resistance and the metabolic syndrome.     

The inhibitory effect of glucagon-like peptide-1 (GLP-1) 7-36 amide on gastric acid secretion in humans depends on an intact vagal innervation.

BACKGROUND: Glucagon-like peptide-1 (GLP-1)(7-36) amide is an intestinal incretin hormone which also inhibits gastric acid secretion in humans. Its mechanism of action is unclear, but it strongly inhibits vagally induced secretion (sham feeding), suggesting that it could influence vagal activity. AIM/METHODS: The effect of intravenous GLP-1 (7-36 amide) (1 pmol/kg/min) was studied on pentagastrin induced acid secretion in otherwise healthy subjects, previously vagotomised for duodenal ulcer (n = 8) and in a group of young (n = 8) and old (n = 6) healthy volunteers. RESULTS: Pentagastrin increased acid secretion significantly in all three groups, but the plateau concentration in the vagotomised subjects was lower than in controls. Infusion of GLP-1 (7-36 amide) significantly inhibited acid secretion in the control groups (to 67 (SEM 6) and 74 (SEM 3)% of plateau concentrations in young and old controls, respectively) but had no effect in the vagotomised subjects. Differences in plasma concentrations of GLP-1 (7-36 amide), recovery of gastric marker, duodenal regurgitation, or Helicobacter pylori status could not explain the lack of effect. Blood glucose was lowered equally by GLP-1 (7-36 amide) in all subjects. CONCLUSION: The inhibitory effect of GLP-1 (7-36 amide) on acid secretion depends on intact vagal innervation of the stomach.     

Lys9 for Glu9 substitution in glucagon-like peptide-1(7-36)amide confers dipeptidylpeptidase IV resistance with cellular and metabolic actions similar to those of established antagonists glucagon-like peptide-1(9-36)amide and exendin (9-39)

The incretin hormone glucagon-like peptide-1(7-36)amide (GLP-1) has been deemed of considerable importance in the regulation of blood glucose. Its effects, mediated through the regulation of insulin, glucagon, and somatostatin, are glucose-dependent and contribute to the tight control of glucose levels. Much enthusiasm has been assigned to a possible role of GLP-1 in the treatment of type 2 diabetes. GLP-1's action unfortunately is limited through enzymatic inactivation caused by dipeptidylpeptidase IV (DPP IV). It is now well established that modifying GLP-1 at the N-terminal amino acids, His(7) and Ala(8), can greatly improve resistance to this enzyme. Little research has assessed what effect Glu(9)-substitution has on GLP-1 activity and its degradation by DPP IV. Here, we report that the replacement of Glu(9) of GLP-1 with Lys dramatically increased resistance to DPP IV. This analogue, (Lys(9))GLP-1, exhibited a preserved GLP-1 receptor affinity, but the usual stimulatory effects of GLP-1 were completely eliminated, a trait duplicated by the other established GLP-1-antagonists, exendin (9-39) and GLP-1(9-36)amide. We investigated the in vivo antagonistic actions of (Lys(9))GLP-1 in comparison with GLP-1(9-36)amide and exendin (9-39) and revealed that this novel analogue may serve as a functional antagonist of the GLP-1 receptor.     

Potent glycogenic effect of GLP-1(7–36)amide in rat skeletal muscle

Summary GLP-1(7–36)amide is an intestinal posttranslational proglucagon product released mainly after carbohydrate ingestion, the glucose dependent insulinotropic and antidiabetogenic actions of which have been documented. In this work, by exploring whether GLP-1(7–36)amide has any effect on the glucose metabolism of the muscle, we have observed that this peptide, at physiological concentrations, exerts in this tissue an increment of the d-[U-¹⁴C] glucose incorporated into glycogen, which is accompanied by an increase in the glycogen synthase a activity; also, it stimulates both glucose oxidation and lactate formation. These data indicate that the skeletal muscle is one of the target tissues for GLP-1(7–36)amide, where its insulin-like effect explains, at least in part, its plasma glucose lowering action; thus, GLP-1(7–36)amide may well be implicated in the physiological control of glucose homeostasis after meals, not only by acting as an incretin, but also by directly promoting glucose disposal.Abbreviations KRB Krebs-Ringer bicarbonate buffer, pH 7.4 - EDTA ethylenedinitrilo tetraacetic acid, disodium salt dihydrate - BSA bovine serum albumin     

alpha-Glucosidase inhibition (acarbose) fails to enhance secretion of glucagon-like peptide 1 (7-36 amide) and to delay gastric emptying in Type 2 diabetic patients.

AIM: Acarbose is able to enhance GLP-1 release and delay gastric emptying in normal subjects. The effect of alpha-glucosidase inhibition on GLP-1 has been less evident in Type 2 diabetic patients. The aim of this study was to investigate the possible influence of acarbose on GLP-1 release and gastric emptying in Type 2 diabetic patients after a mixed test meal. PATIENTS AND METHODS: Ten Type 2 diabetic patients were tested with 100 mg acarbose or placebo served with a mixed meal that was labelled with 100 mg 13C-octanoic acid. Plasma concentrations of glucose, insulin, C-peptide, glucagon, GLP-1 and GIP were determined over 6 h. Gastric emptying was measured by determining breath 13CO2 using infrared absorptiometry. Statistics repeated-measures anova. RESULTS: Gastric emptying rates (t1/2: 162 +/- 45 vs. 163 +/- 62 min, P = 0.65) and plasma concentrations (increasing from approximately 12 to approximately 25 pmol/l, P = 0.37) and integrated responses of GLP-1 (P = 0.37) were not changed significantly by acarbose treatment. Postprandial plasma glucose concentrations (P < 0.0001) and their integrated responses were lowered by acarbose (by 64%; P = 0.016). The plasma concentrations of insulin and C-peptide were reduced (P = 0.007 and 0.057, respectively) by acarbose, while glucagon was not changed (P = 0.96). GIP plasma concentrations (increasing with placebo from approximately 10 to approximately 85 pmol/l and with acarbose to approximately 55 pmol/l (P < 0.0001) and their integrated responses were significantly lowered (by 43%) by acarbose (P = 0.021). After 2 weeks of acarbose treatment (50 mg t.i.d. for the first and 100 mg t.i.d. for the second week, n = 6), similar results were found. CONCLUSIONS: In hyperglycaemic Type 2 diabetic patients, ingestion of acarbose with a mixed test meal failed to enhance GLP-1 release and did not influence gastric emptying.     

Effects of sub-chronic exposure to naturally occurring N-terminally truncated metabolites of glucose-dependent insulinotrophic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1), GIP(3-42) and GLP-1(9-36)amide, on insulin secretion and glucose homeostasis in ob/ob mice

Glucose-dependent insulinotrophic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are important enteroendocrine hormones that are rapidly degraded by an ubiquitous enzyme dipeptidyl peptidase IV to yield truncated metabolites GIP(3-42) and GLP-1(9-36)amide. In this study, we investigated the effects of sub-chronic exposure to these major circulating forms of GIP and GLP-1 on blood glucose control and endocrine pancreatic function in obese diabetic (ob/ob) mice. A once daily injection of either peptide for 14 days had no effect on body weight, food intake or pancreatic insulin content or islet morphology. GLP-1(9-36)amide also had no effect on plasma glucose homeostasis or insulin secretion. Mice receiving GIP(3-42) exhibited small but significant improvements in non-fasting plasma glucose, glucose tolerance and glycaemic response to feeding. Accordingly, plasma insulin responses were unchanged suggesting that the observed enhancement of insulin sensitivity was responsible for the improvement in glycaemic control. These data indicate that sub-chronic exposure to GIP and GLP-1 metabolites does not result in physiological impairment of insulin secretion or blood glucose control. GIP(3-42) might exert an overall beneficial effect by improving insulin sensitivity through extrapancreatic action.     

Glucagon-like peptide-1-(7-36)amide is transformed to glucagon-like peptide-1-(9-36)amide by dipeptidyl peptidase IV in the capillaries supplying the L cells of the porcine intestine.

The insulinotropic hormone glucagon-like peptide-1 (GLP-1) is stored in the intestinal L cell in an active form, GLP-1-(7-36)amide, but more than half of the endogenous peptide circulates in an inactive, N-terminally truncated form, GLP-1-(9-36)amide. This study examined the GLP-1 newly secreted from the porcine ileum, in vitro (isolated perfused preparation) and in vivo (anesthetized pig), to determine where this conversion occurs. Although the GLP-1 extractable from the porcine ileum is predominantly the intact peptide (94.6+/-1.7%), a large proportion of the GLP-1 that is secreted has already been degraded to the truncated form both in vitro (53.8+/-0.9% intact) and in vivo (32.9+/-10.8% intact). In the presence of a specific dipeptidyl peptidase IV (DPP IV) inhibitor (valine-pyrrolidide), the proportion of intact GLP-1 released from the perfused ileum was increased under both basal (99% intact; P < 0.05) and stimulated (86-101% intact; P < 0.05) conditions. Immunohistochemical and histochemical studies revealed specific DPP IV staining in the brush border epithelium as well as in the capillary endothelium. Double staining showed juxtapositioning of DPP IV-positive capillaries and GLP-1-containing L cells. From these results, we suggest that GLP-1 is degraded as it enters the DPP IV containing blood vessels draining the intestinal mucosa.     

Comparison of GLP-1 (7-36amide) and GIP on release of somatostatin-like immunoreactivity and insulin from the isolated rat pancreas

The effect of equimolar doses of GIP and GLP-1 (7-36amide) on insulin and somatostatin secretion in the isolated perfused rat pancreas was compared. At a perfusate glucose concentration of 70 mg/dl GLP-1 (7-36amide) 10(-9) and 10(-8) M and GIP 10(-9) M elicited a significant stimulation of insulin while GIP 10(-8) M and lower doses of both peptides (10(-11) and 10(-10) M) were ineffective. At elevated perfusate glucose levels of 150 mg/dl both peptides stimulated insulin release at 10(-11), 10(-10), 10(-9) and 10(-8) M but not at 10(-12) M. The insulin response at the higher glucose level was significantly greater compared to the effect of the same doses at normoglycemic conditions. Somatostatin release was stimulated significantly by GLP-1 (7-36amide) at 10(-10) and 10(-9) M at perfusate glucose level 70 mg/dl. At a glucose concentration of 150 mg/dl this effect was abolished. GIP did not alter somatostatin release at a perfusate glucose concentration of 70 mg/dl while at 150 mg/dl only the highest dose of GIP (10(-8) M) stimulated somatostatin release significantly. In conclusion, the present data demonstrate that in vitro in the rat pancreas both peptides are equally effective secretagogues of insulin release at normal and moderately elevated perfusate glucose levels. In contrast, somatostatin secretion is stimulated by GLP-1 (7-36amide) at normoglycemic conditions while only a rather high and presumably pharmacological dose of GIP is a stimulus of somatostatin secretion at moderate hyperglycemia.(ABSTRACT TRUNCATED AT 250 WORDS)     

Short-term intestinal lipase inhibition in normal-weight individuals does not affect postprandial peptide YY3-36 and glucagon-like peptide-1 levels,hunger or satiety

Fat malabsorption associated with Roux-en-Y gastric bypass (RYGB) may contribute to elevated postprandial glucagon-like peptide-1 (GLP-1) and peptide YY (PYY) after the procedure, leading to sustained weight loss and appetite reduction. This study investigated whether fat malabsorption via orlistat increases GLP-1 and PYY and if these increases would be proportional to changes in hunger and satiety. Five healthy participants received standardized meals with 120 mg orlistat or placebo in a randomized, double-blinded, crossover design for 3 days. On the final day, glucose, insulin, GLP-1, PYY_3-36 and visual analogue scores for hunger and satiety were measured over a 14-hour period that included three meals. Fasting, 14-hour area under the curve (AUC) and meal-related AUC for glucose and insulin were similar, although postprandial increases in peak insulin and glucose were greater with orlistat. PYY_3-36, GLP-1, hunger and satiety were not different. In conclusion, short-term orlistat administration does not enhance postprandial GLP-1 or PYY_3-36 or affect hunger or satiety in normal-weight individuals. Furthermore, fat malabsorption from RYGB is unlikely to mediate subsequent postprandial increases in GLP-1 and PYY.