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

PACAP contributes to insulin secretion after gastric glucose gavage in mice

Pituitary adenylate cyclase-activating polypeptide (PACAP) is localized to pancreatic ganglia governing the parasympathetic nerves, which contribute to prandial insulin secretion. We hypothesized that this contribution involves PACAP and show here that the PACAP receptor antagonist PACAP-(6---27) (1.5 nmol/kg iv) reduces the 15-min insulin response to gastric glucose (150 mg/mouse) by 18% in anesthetized mice (P = 0.041). The reduced insulinemia was not due to inhibited release of the incretin factor glucagon-like peptide 1 (GLP-1) because PACAP-(6---27) enhanced the GLP-1 response to gastric glucose. Furthermore, the GLP-1 antagonist exendin-3-(9---39) (30 nmol/kg) exerted additive inhibitory effect on the insulin response when combined with PACAP-(6---27). The PACAP antagonism was specific because intravenous PACAP-(6---27) inhibited the insulin response to intravenous PACAP-27 plus glucose without affecting the insulin response to intravenous glucose alone (1 g/kg) or glucose together with other insulin secretagogues of potential incretin relevance of intestinal (GLP-1, gastric inhibitory polypeptide, cholecystokinin) and neural (vasoactive intestinal peptide, gastrin-releasing peptide, cholinergic agonism) origin. We conclude that PACAP contributes to the insulin response to gastric glucose in mice and suggest that PACAP is involved in the regulation of prandial insulin secretion.

Glucagon-like peptide-1 improves insulin and proinsulin binding on RINm5F cells and human monocytes

Glucagon-like peptide-1-(7---36) amide (GLP-1) is a potent incretin hormone secreted from distal gut. It stimulates basal and glucose-induced insulin secretion and proinsulin gene expression. The present study tested the hypothesis that GLP-1 may modulate insulin receptor binding. RINm5F rat insulinoma cells were incubated with GLP-1 (0.01-100 nM) for different periods (1 min-24 h). Insulin receptor binding was assessed by competitive ligand binding studies. In addition, we investigated the effect of GLP-1 on insulin receptor binding on monocytes isolated from type 1 and type 2 diabetes patients and healthy volunteers. In RINm5F cells, GLP-1 increased the capacity and affinity of insulin binding in a time- and concentration-dependent manner. The GLP-1 receptor agonist exendin-4 showed similar effects, whereas the receptor antagonist exendin-(9---39) amide inhibited the GLP-1-induced increase in insulin receptor binding. The GLP-1 effect was potentiated by the adenylyl cyclase activator forskolin and the stable cAMP analog Sp-5, 6-dichloro-1-beta-D-ribofuranosyl-benzimidazole-3', 5'-monophosphorothioate but was antagonized by the intracellular Ca(2+) chelator 1,2-bis(0-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-AM. Glucagon, gastric inhibitory peptide (GIP), and GIP-(1---30) did not affect insulin binding. In isolated monocytes, 24 h incubation with 100 nM GLP-1 significantly (P<0.05) increased the diminished number of high-capacity/low-affinity insulin binding sites per cell in type 1 diabetics (9,000+/-3,200 vs. 18,500+/-3,600) and in type 2 diabetics (15,700+/-2,100 vs. 28,900+/-1,800) compared with nondiabetic control subjects (25,100+/-2,700 vs. 26,200+/-4,200). Based on our previous experiments in IEC-6 cells and IM-9 lymphoblasts indicating that the low-affinity/high-capacity insulin binding sites may be more specific for proinsulin (Jehle, PM, Fussgaenger RD, Angelus NK, Jungwirth RJ, Saile B, and Lutz MP. Am J Physiol Endocrinol Metab 276: E262-E268, 1999 and Jehle, PM, Lutz MP, and Fussgaenger RD. Diabetologia 39: 421-432, 1996), we further investigated the effect of GLP-1 on proinsulin binding in RINm5F cells and monocytes. In both cell types, GLP-1 induced a significant increase in proinsulin binding. We conclude that, in RINm5F cells and in isolated human monocytes, GLP-1 specifically increases the number of high-capacity insulin binding sites that may be functional proinsulin receptors.

Somatostatin restrains the secretion of glucagon-like peptide-1 and -2 from isolated perfused porcine ileum

Suspecting that paracrine inhibition might influence neuronal regulation of the endocrine L cells, we studied the role of somatostatin (SS) in the regulation of the secretion of the proglucagon-derived hormones glucagon-like peptide-1 and -2 (GLP-1 and GLP-2). This was examined using the isolated perfused porcine ileum stimulated with acetylcholine (ACh, 10(-6) M), neuromedin C (NC, 10(-8) M), and electrical nerve stimulation (NS) with or without alpha-adrenergic blockade (phentolamine 10(-5) M), and perfusion with a high-affinity monoclonal antibody against SS. ACh and NC significantly increased GLP secretion, whereas NS had little effect. SS immunoneutralization increased GLP secretion eight- to ninefold but had little influence on the GLP responses to ACh, NC, and NS. Basal SS secretion (mainly SS28) was unaffected by NS alone. Phentolamine + NS and NC abstract strongly stimulated release mainly of SS14, whereas ACh had little effect. Infused intravascularly, SS14 weakly and SS28 strongly inhibited GLP secretion. We conclude that GLP secretion is tonically inhibited by a local release of SS28 from epithelial paracrine cells, whereas SS14, supposedly derived from enteric neurons, only weakly influences GLP secretion.

Glucagon-like peptide (GLP-1) is involved in the central modulation of fecal output in rats

In addition to its insulinotropic action, exogenously administered glucagon-like peptide (GLP-1) inhibits gastropancreatic motility and secretion via central pathways. The aims of the present study were to evaluate the effects of exogenous GLP-1-(7-36) amide on fecal output and to investigate the role of endogenous GLP-1 on stress-induced colonic activity. With the use of a stereotaxic instrument, adult male Sprague-Dawley rats weighing 200-250 g were fitted with stainless steel cerebroventricular guide cannulas under ketamine anesthesia. A group of rats were placed in Bollman-type cages to induce restraint stress. Fecal output monitored for 2 h was increased significantly by intracerebroventricular GLP-1 to 500, 1, 000, and 3,000 pmol/rat (P < 0.05-0.01), whereas intraperitoneal GLP-1 had no effect. Intracerebroventricular administration of the GLP-1 receptor antagonist exendin-(9-39) (10 nmol/rat) reversed the increases induced by GLP-1 (500 pmol/rat; P<0.01). Similar results were also observed with the injection of corticotropin-releasing factor receptor antagonist astressin (10 microg/rat icv). The significant increase in fecal pellet output induced by restraint stress was also decreased by both intracerebroventricular exendin (10 nmol/rat) and astressin (10 microg/rat; P<0.01-0.001). These results suggest that GLP-1 participates in the central, but not peripheral, regulation of colonic motility via its own receptor and that GLP-1 is likely to be a candidate brain-gut peptide that acts as a physiological modulator of stress-induced colonic motility.

1,5-Anhydro-D-fructose increases glucose tolerance by increasing glucagon-like peptide-1 and insulin in mice

Besides being degraded to glucose-6-phosphate and to free glucose, glycogen is degraded by alpha-1,4-glucan lyase to 1, 5-anhydro-D-fructose. We examined the influence of 1, 5-anhydro-D-fructose on glucose-stimulated insulin secretion in vivo and in vitro in mice. When administered together with i.v. glucose (1 g/kg), 1,5-anhydro-D-fructose did not affect (at 0.2 g/kg) or inhibited (at 1 g/kg) insulin secretion without affecting glucose elimination. When incubated with isolated islets, 1, 5-anhydro-D-fructose at <16.7 mmol/l, did not affect glucose (11.1 mM)-stimulated insulin secretion but inhibited insulin secretion at 16.7 mmol/l. When given through a gastric gavage (150 mg/mouse) together with glucose (150 mg/mouse), 1,5-anhydro-D-fructose increased glucose tolerance and insulin secretion. Furthermore, 1, 5-anhydro-D-fructose potentiated the increase in plasma levels of the gut hormone, glucagon-like peptide-1 (GLP-1). We therefore conclude that when given enterally, but not parenterally, 1, 5-anhydro-D-fructose increases glucose tolerance in mice by increasing insulin secretion due to increased plasma levels of GLP-1. The sugar may therefore be explored for increasing endogenous GLP-1 secretion in the treatment of type 2 diabetes.

Deficiency of the intestinal growth factor, glucagon-like peptide 2, in the colon of SCID mice with inflammatory bowel disease induced by transplantation of CD4+ T cells

BACKGROUND

Glucagon-like peptide 2 (GLP-2) is produced in endocrine L-cells of the intestinal mucosa. Recently, GLP-2 was found to stimulate intestinal mucosal growth. Our objective was to study the content of GLP-2 in the large intestine in a murine model of T-cell-induced inflammatory bowel disease.

METHODS

Inflammation was induced by adoptive transfer of CD4+ blast T cells from BALB/c mice to SCID mice. The amount of GLP-2 (1-33) was measured with a specific, NH2-terminally directed radioimmunoassay in tissue extracts from the large intestine of transplanted mice developing colitis and from BALB/c and SCID control mice.

RESULTS

In the middle and descending colon segments showing the most severe signs of inflammatory lesions in the CD4+ T-cell-transplanted mice, the amount of GLP-2 was significantly lower than in similar colon segments in both untransplanted SCID mice and normal BALB/c mice (P = 0.0013 and 0.0033). In the descending colon the amount of GLP-2 was 6.7 +/- 1.0 pmol/g protein in the CD4+ transplanted mice compared with 68.4 +/- 20.3 and 42.7 +/- 4.3 in the two groups of control mice. Similar findings were made with regard to the contents of the two other proglucagon-derived intestinal peptides, glicentin and GLP-1.

CONCLUSION

The amount of GLP-2 is markedly reduced in the colon of mice with a T-cell-induced inflammatory bowel disease histopathologically resembling both Crohn disease and ulcerative colitis. This observation may provide a pathophysiologic rationale for administration of GLP-2 as a trophic factor in inflammatory bowel disease.

The effect of glucagon-like peptide-1 on energy expenditure and substrate metabolism in humans

OBJECTIVE

To investigate the effects of a near-physiological peripheral glucagon-like peptide-1 (GLP-1) infusion, during and after a breakfast of fixed energy content, on resting energy expenditure, substrate oxidation and metabolism and the desire to eat specific types of food in humans.

DESIGN

A placebo-controlled, randomized, blinded, cross-over study. Infusion (GLP-1, 50 pmol/kg x h or saline) was started simultaneously with initiation of the test meals.

SUBJECTS

20 healthy, normal weight (body mass index 20.3-25.7 kg/m2) men of 20-31 y of age.

MEASUREMENTS

Energy expenditure and substrate oxidations were measured before and for 4 h after standard breakfast (20% of calculated daily energy requirements, 50% of energy from carbohydrates, 37% of energy from fat and 13% of energy from protein) using a ventilated hood system. Visual analogue scales were used throughout the experiment to assess the desire to eat specific types of food and the palatability of the test meals. Blood was sampled throughout the day for analysis of plasma hormone and substrate concentrations.

RESULTS

Diet-induced thermogenesis (DIT) was lower (47%) on the GLP-1 infusion than on the saline infusion (P < 0.0001). This was due to a lower carbohydrate oxidation (P < 0.01). No differences in fat oxidation or total 4 h protein oxidation were observed. All hormone and substrate profiles except non-esterified fatty acids (NEFA) and cholecystokinin (CCK) were significantly suppressed (GLP-2 completely suppressed) during the GLP-1 infusion, whereas profiles of NEFA and CCK differed in time course during the two treatments (treatment x time effect), P < 0.0001). GLP-1 infusion also suppressed the desire to eat all food types following the breakfast (treatment effect: P < 0.05).

CONCLUSION

Peripheral GLP-1 decreased DIT and carbohydrate oxidation, probably secondary to a delayed absorption of nutrients, since substrate and hormone concentrations in plasma were suppressed during GLP-1 infusion. Endogenous secretion of GLP-1 and GLP-2 was completely suppressed by GLP-1 infusion. Finally, the desire to eat any type of food was decreased by exogenous administrated GLP-1.

Antidiabetogenic action of cholecystokinin-8 in type 2 diabetes

Cholecystokinin (CCK) is a gut hormone and a neuropeptide that has the capacity to stimulate insulin secretion. As insulin secretion is impaired in type 2 diabetes, we explored whether exogenous administration of this peptide exerts antidiabetogenic action. The C-terminal octapeptide of CCK (CCK-8) was therefore infused i.v. (24 pmol/kg x h) for 90 min in six healthy postmenopausal women and in six postmenopausal women with type 2 diabetes. At 15 min after start of infusion, a meal was served and ingested during 10 min. On a separate day, saline was infused instead of CCK-8. In both healthy subjects and subjects with type 2 diabetes, CCK-8 reduced the increase in circulating glucose after meal ingestion and potentiated the increase in circulating insulin. The ratio between the area under the curves for serum insulin and plasma glucose during the 15- to 75-min period after meal ingestion was increased by CCK-8 by 198 +/- 18% in healthy subjects (P = 0.002) and by 474 +/- 151% (P = 0.038) in subjects with type 2 diabetes. In contrast, the increase in the circulating levels of gastric inhibitory polypeptide (GIP), glucagon-like peptide-1 (GLP-1), or glucagon after meal ingestion was not significantly affected by CCK-8. The study therefore shows that CCK-8 exerts an antidiabetogenic action in both healthy subjects and type 2 diabetes through an insulinotropic action that most likely is exerted trough a direct islet effect. As at the same time, CCK-8 was infused without any adverse effects, the study suggests that CCK is a potential treatment for type 2 diabetes.

Effect of glucagon-like peptide 1(7-36)amide in insulin-treated patients with diabetes mellitus secondary to chronic pancreatitis

Diabetes mellitus secondary to chronic pancreatitis is characterized by a progressive destruction of the pancreas, including loss of the islet cells, leading to a form of diabetes that can mimic both type 1 and type 2 diabetes. Glucagon-like peptide 1(7-36)amide (GLP-1), an intestinally derived insulinotropic hormone, represents a potential therapeutic agent for type 2 diabetes, because exogenous GLP-1 has been shown to increase the insulin and reduce the glucagon concentrations in these patients, and thus induce lower blood glucose, but without causing hypoglycemia. Ten patients with diabetes mellitus secondary to chronic pancreatitis and five normal subjects were studied. Nine patients were treated with insulin and one patient with sulfonylurea. In the fasting state, saline or GLP-1 in doses of 0.4 or 1.2 pmol/min/kg body weight were infused intravenously for 4 hours. Blood glucose was reduced in all patients with both doses of GLP-1; plasma C-peptide increased (p<0.02), and plasma glucagon decreased (p<0.02) compared with basal levels, also in three patients with normoglycemia and high levels of presumably exogenous insulin. Similar results were obtained in the normal subjects. In conclusion, GLP-1 treatment may be considered in patients with diabetes mellitus secondary to chronic pancreatitis, provided that a certain amount of alpha- and beta-cell secretory capacity is still present.

GLP-1 slows solid gastric emptying and inhibits insulin, glucagon, and PYY release in humans

The aim of the present study was to assess the effect of glucagon-like peptide-1 (GLP-1) on solid gastric emptying and the subsequent release of pancreatic and intestinal hormones. In eight men [age 33.6 +/- 2.5 yr, body mass index 24.1 +/- 0.9 (means +/- SE)], scintigraphic solid gastric emptying during infusion of GLP-1 (0.75 pmol. kg(-1). min(-1)) or saline was studied for 180 min. Concomitantly, plasma concentrations of C- and N-terminal GLP-1, glucose, insulin, C-peptide, glucagon, and peptide YY (PYY) were assessed. Infusion of GLP-1 resulted in a profound inhibition of both the lag phase (GLP-1: 91.5, range 73.3-103.6 min vs. saline: 19. 5, range 10.2-43.4 min) and emptying rate (GLP-1: 0.34, range 0.06-0. 56 %/min vs. saline: 0.84, range 0.54-1.33 %/min; P < 0.01 for both) of solid gastric emptying. Concentrations of both intact and total GLP-1 were elevated to supraphysiological levels. Plasma glucose and glucagon concentrations were below baseline during infusion of GLP-1 in contrast to saline infusion, where concentrations were elevated above baseline (both P < 0.001). The insulin and C-peptide responses were lower during infusion with GLP-1 than with saline (P < 0.004 and P < 0.001, respectively). Plasma PYY concentrations decreased below baseline during GLP-1 infusion in contrast to saline, where concentrations were elevated above baseline (P = 0.04). Infusion of GLP-1 inhibits solid gastric emptying with secondary effects on the release of insulin, C-peptide, and glucagon, resulting in lower plasma glucose concentrations. In addition, the release of PYY into the circulation is inhibited by GLP-1 infusion, suggesting a negative feedback of GLP-1 on the function of the L-cell.

Inhibition of sham feeding-stimulated human gastric acid secretion by glucagon-like peptide-2

Glucagon-like peptide (GLP)-2 is formed from proglucagon in the intestinal L cells and is secreted postprandially in parallel with the insulinotropic hormone GLP-1, the latter of which, in addition, acts to inhibit gastric secretion and motility by inhibiting central parasympathetic outflow. We now studied the effect of GLP-2 on gastric secretion stimulated by sham feeding to test the hypothesis that also GLP-2 acts as an enterogastrone. Eight healthy volunteers were studied twice on separate days. They were sham fed with and without GLP-2 infused iv at a rate of 0.8 pmol/kg x min. Gastric contents were aspirated continuously by a nasogastric tube for determination of acid secretion, volume, and osmolarity. Sham feeding increased gastric acid secretion nearly 5-fold. Infusion of GLP-2 reduced incremental acid secretion by 65+/-6%, compared with saline infusion (delta8.75+/-0.37 vs. delta3.04+/-0.47 mmol x 60 min; P<0.01). Plasma concentrations of GLP-2 rose from a basal mean of 3.3+/-0.9 to a mean of 115+/-8 pmol/L (range, 57-149 pmol/L) during infusion of GLP-2 and remained at basal level during saline infusion. Plasma concentrations of GLP-1, gastrin, cholecystokinin, and secretin remained low and unchanged on both study days. We conclude that GLP-2 is a powerful inhibitor of gastric acid secretion in man. Further investigations will show to what extent GLP-2 contributes to the inhibitory effects on gastric secretion exerted by hormones from the distal small intestine, under physiological circumstances.

Effect of GIP and GLP-1 antagonists on insulin release in the rat

Glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide 1 (GLP-1) are potent insulinotropic peptides released from the small intestine. To examine their relative contribution to postprandial insulin release, a specific GIP antagonist (ANTGIP) and a GLP-1 antagonist, exendin-(9-39)-NH2, were infused into rats after an intragastric glucose meal. In control rats, plasma glucose and insulin levels rose gradually during the first 20 min and then decreased. Exendin-(9-39)-NH2 administration inhibited postprandial insulin secretion by 32% at 20 min and concomitantly increased plasma glucose concentrations. In contrast, ANTGIP treatment not only induced a 54% decrease in insulin secretion but also a 15% reduction in plasma glucose levels 20 min after the glucose meal. In vivo studies in rats demonstrated that glucose uptake in the upper small intestine was significantly inhibited by the ANTGIP, an effect that might account for the decrease in plasma glucose levels observed in ANTGIP-treated rats. When the two antagonists were administered to rats concomitantly, no potentiating effect on either insulin release or plasma glucose concentration was detected. Glucose meal-stimulated GLP-1 release was not affected by ANTGIP administration, whereas postprandial glucagon levels were diminished in rats receiving exendin-(9-39)-NH2. The results of these studies suggest that GIP and GLP-1 may share a common mechanism in stimulating pancreatic insulin release. Furthermore, the GIP receptor appears to play a role in facilitating glucose uptake in the small intestine.

Effects of age on concentrations of plasma cholecystokinin, glucagon-like peptide 1, and peptide YY and their relation to appetite and pyloric motility

BACKGROUND

Aging is associated with a decrease in appetite and a slowing of gastric emptying. The gastrointestinal hormones cholecystokinin (CCK), glucagon-like peptide 1 (GLP-1), and peptide YY (PYY) may mediate these changes.

OBJECTIVE

We investigated whether aging influenced the secretion of CCK, GLP-1, and PYY and their effects on appetite and pyloric motility.

DESIGN

Eight healthy older (65-80 y) and 7 younger (20-34 y) men received isoenergetic (12.1 kJ/min) intraduodenal infusions of lipid and glucose for 120 min on separate days. Plasma CCK, GLP-1, and PYY concentrations were measured.

RESULTS

Plasma CCK concentrations were higher in older than in younger subjects (P = 0.004) as a result of higher baseline values (4.7+/-0.2 compared with 3.2+/-0.2 pmol/L; P < 0.0001) and a greater rise during lipid infusion (increase from baseline: 7.1+/-0.5 compared with 5.3+/-0.6 pmol/L; P = 0.048). Plasma GLP-1 and PYY concentrations were not significantly different between groups. The decrease in hunger during intraduodenal lipid infusion was inversely related to the increase in CCK, GLP-1, and PYY in younger but not older subjects. During intraduodenal lipid infusion, the increase in isolated pyloric pressure wave (IPPW) frequency was positively related to GLP-1 and PYY and the increase in IPPW amplitude was positively related to CCK in older but not younger subjects, whereas the increase in IPPW amplitude and pyloric tone was negatively related to GLP-1 and PYY in younger subjects.

CONCLUSIONS

Human aging is associated with increased CCK concentrations, which may contribute to the slowing of gastric emptying, mediated by increased pyloric motility. The role of increased plasma CCK concentrations in mediating the age-related decrease in appetite remains to be established.

Structure-function analysis of a series of glucagon-like peptide-1 analogs

We have used NMR in conjunction with measurements of functional bioactivity to define the receptor-binding structure of glucagon-like peptide-1 (GLP-1.) Identification of the important residues for binding was accomplished by the substitution of amino acids at sites that seemed likely, from an examination of the amino acid sequence and from previously published observations, to be important in the three-dimensional (3D) structure of the molecule. Identification of the receptor-bound conformation of GLP-1, because it is a flexible peptide, required constraint of the peptide backbone into a predetermined 3D structure. Constraint was achieved by the introduction of disulfide bonds and specific side chain-side chain cross-links. The biological relevance of the synthetic structure of each rigidified peptide was assessed by measurement of its ability to bind to the receptor present on RINm5F cells and to elicit a functional response, cyclic AMP production. NMR solution structures were obtained for the most biologically relevant of these analogs. The results of this study indicated that the residues necessary for the biological activity of GLP-1 occupy approximately three equally-spaced regions of the peptide 3D structure, at the corners of an equilateral triangle whose sides are, at a minimum estimate, 12-15A.

Glucagon-like peptide-1 inhibits gastropancreatic function by inhibiting central parasympathetic outflow

Glucagon-like peptide (GLP)-1 inhibits acid secretion and gastric emptying in humans, but the effect on acid secretion is lost after vagotomy. To elucidate the mechanism involved, we studied its effect on vagally stimulated gastropancreatic secretion and motility in urethan-anesthetized pigs with cut splanchnic nerves, in which insulin-induced hypoglycemia elicited a marked stimulation of gastropancreatic secretion and antral motility. In addition, we studied vagally stimulated motility and pancreatic secretion in isolated perfused preparations of the porcine antrum and pancreas. GLP-1 infusion (2 pmol. kg-1. min-1) strongly and significantly inhibited hypoglycemia-induced antral motility, gastric acid secretion, pancreatic bicarbonate and protein secretion, and pancreatic polypeptide (PP) secretion. GLP-1 (at 10(-10)-10(-8) mol/l) did not inhibit vagally induced antral motility, pancreatic exocrine secretion, or gastrin and PP secretion in isolated perfused antrum and pancreas. We conclude that the inhibitory effect of peripheral GLP-1 on upper gastrointestinal secretion and motility is exerted via interaction with centers in the brain or afferent neural pathways relaying to the vagal motor nuclei.

Amidated and non-amidated glucagon-like peptide-1 (GLP-1): non-pancreatic effects (cephalic phase acid secretion) and stability in plasma in humans

The incretin and enterogastrone hormone, GLP-1, occurs in an amidated (GLP-1 (7-36) amide; 75%) and a glycine-extended (GLP-1 (7-37); 25%) form. Their effects on the endocrine pancreas are similar and their overall (mainly renal) elimination rates appear to equal. Assuming that they might differentially affect non-pancreatic targets we investigated the effect of GLP-1 (7-37) infused at 0.7 pmol/kg/min on sham-feeding induced acid secretion in six healthy volunteers. The infusion increased the plasma concentrations from 16+/-2 pmol/l to 45+/-2 pmol/l. This was associated with a 61+/-14% decrease in acid output compared to saline and was not significantly different from that previously observed with GLP-1 (7-36) amide infused at the same rate. We then compared the degradation of the two forms in human plasma at 37 degrees C in vitro. T1/2 values were 32+/-3 (7-37) and 42+/-2 min (7-36) amide (P=0.007). The difference in metabolism persisted after addition of diprotin A, an inhibitor of dipeptidyl peptidase IV, the enzyme responsible for the initial degradation of GLP-1 in plasma, and broader enzyme inhibitors. Thus, the only effect of the amidation of GLP-1 seems to be to enhance its survival in plasma.

Glucagon-like peptide-1 (7-36) amide as a novel neuropeptide

Although earlier studies indicated that GLP-1 (7-36) amide was an intestinal peptide with a potent effect on glucose-dependent insulin secretion, later on it was found that several biological effects of this peptide occur in the brain, rather than in peripheral tissues. Thus, proglucagon is expressed in pancreas, intestine, and brain, but post translational processing of the precursor yields different products in these organs, glucagon-like peptide-1 (7-36) amide being one of the forms produced in the brain. Also, GLP-1 receptor cDNA from human and rat brains has been cloned and sequenced, and the deduced amino acid sequences are the same as those found in pancreatic islets. Through these receptors, GLP-1 (7-36) amide from gut or brain sources induces its effects on the release of neurotransmitters from selective brain nuclei, the inhibition of gastric secretion and motility, the regulation of food and drink intake, thermoregulation, and arterial blood pressure. Central administration (icv) of GLP-1 (7-36) amide produces a marked reduction in food and water intake, and the colocalization of the GLP-1 receptor, GLUT-2, and glucokinase mRNAs in hypothalamic neurons involved in glucose sensing suggests that these cells may be involved in the transduction of signals needed to produce a state of fullness. In addition, GLP-1 (7-36) amide inhibits gastric acid secretion and gastric emptying, but these effects are not found in vagotomized subjects, suggesting a centrally mediated effect. Similar results have been found with the action of this peptide on arterial blood pressure and heart rate in rats. Synthesis of GLP-1 (7-36) amide and its own receptors in the brain together with its abovementioned central physiological effects imply that this peptide may be considered a neuropeptide. Also, the presence of GLP-1 (7-36) amide in the synaptosome fraction and its calcium-dependent release by potassium stimulation, suggest that the peptide may act as a neurotransmitter although further electrophysiological and ultrastructural studies are needed to confirm this possibility.

Glucagon-like peptide-1 retards gastric emptying and small bowel transit in the rat: effect mediated through central or enteric nervous mechanisms

This study investigated effects of glucagon-like peptide-1(7-36)amide (GLP-1) on gastric emptying, small intestinal transit, and contractility of smooth muscle strips in rats. GLP-1 at doses of 10 and 20 pmol/kg/min administered intravenously dose-dependently retarded transit of the small intestine (P < 0.001), while only the higher dose of 20 pmol/kg/min retarded gastric emptying (P < 0.01). GLP-1 at concentrations up to 10(-4) M did not affect the basal tone or contractility of the gastrointestinal muscle strips that were stimulated with electric field stimulation or acetylcholine. Our results demonstrate that small intestinal transit seems more sensitive than gastric emptying to inhibition by GLP-1 at physiologic levels in plasma. Furthermore, this inhibition appears to be mediated through central mechanisms rather than through peripheral actions. Thus, GLP-1 is suggested to inhibit gastric emptying and small intestinal transit through an indirect effect via central or enteric nervous mechanisms.

Interaction of insulin, glucagon-like peptide 1, gastric inhibitory polypeptide, and appetite in response to intraduodenal carbohydrate

The relation between gastrointestinal incretin hormones in the control of insulin release and short-term satiety by intestinal carbohydrate was investigated in 8 fasted, healthy male volunteers. Insulin, gastric inhibitory polypeptide (GIP), glucagon-like peptide 1 (GLP-1), and appetite ratings were measured during, and food intake was measured after, intraduodenal infusions of glucose or saline. Studies were conducted under hyperinsulinemic and euglycemic conditions. Raising plasma insulin with intravenous insulin infusion to concentrations slightly above usual postprandial concentrations (356.4 +/- 4.8 pmol/L) had no effect on GIP, GLP-1, or appetite ratings before the intraduodenal infusions began. Intraduodenal glucose infusion resulted in a further increase in plasma insulin to a peak of 779.4 +/- 114.0 pmol/L, caused an early increase in plasma GIP and a later increase in GLP-1 concentrations (P < 0.01), suppressed appetite (P < 0.05), and reduced energy intake (P < 0.01) compared with intraduodenal infusion of saline. There was a close association between the increase in GLP-1 and decrease in appetite. Infusion of octreotide to suppress the release of gastrointestinal hormones prevented the rise in insulin, GIP, and GLP-1 induced by intraduodenal glucose infusion and reversed the suppression of appetite and reduction in energy intake. These results suggest that 1) when infused to result in plasma concentrations slightly above usual postprandial concentrations, insulin does not inhibit its own release and 2) the effects of intraduodenal glucose on appetite may be mediated through the release of GLP-1 and not insulin.

Inhibitory effect of glucagon-like peptide-1 on small bowel motility. Fasting but not fed motility inhibited via nitric oxide independently of insulin and somatostatin

Effects of glucagon-like peptide-1 (GLP-1)(7-36)amide on fasted and fed motility in the rat small intestine were investigated in relation to its dependence on nitric oxide (NO), insulin, and somatostatin. Small bowel electromyography was performed using bipolar electrodes implanted 15, 25, and 35 cm distal to pylorus, and transit was studied with a radioactive marker. In the fasted state, GLP-1 (5-20 pmol kg-1min-1), reaching physiological plasma levels, prolonged the migrating myoelectric complex (MMC) cycle length along with slowed transit. This effect was antagonized by exendin(9-39)amide. The NO synthase inhibitor Nomega-nitro- L-arginine (L-NNA) also blocked the response to GLP-1, whereas L-arginine restored the response. Insulin (80-200 pmol kg-1min-1) induced irregular spiking, whereas somatostatin (100-500 pmol kg-1min-1) increased the MMC cycle length, independently of NO. In the fed state, GLP-1 (20-40 pmol kg-1min-1) reduced motility, an inhibition unaffected by L-NNA, whereas motility was stimulated by exendin(9-39)amide. Infusion of GLP-1 (20-100 pmol kg-1min-1) did not affect plasma insulin, but somatostatin was increased. In conclusion, GLP-1 seems to inhibit small bowel motility directly via the GLP-1 receptor. Inhibition of fasting motility is dependent of NO and not mediated via insulin or somatostatin, whereas inhibition of fed motility is independent of NO.

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.

The inhibitory effect of glucagon-like peptide-1 (7-36)amide on antral motility is antagonized by its N-terminally truncated primary metabolite GLP-1 (9-36)amide

In plasma, glucagon-like peptide-1 7-36 amide (GLP-1) is rapidly degraded from the N terminus, generating the endogenous metabolite GLP-1 9-36 amide. This cleavage of GLP-1 eliminates its incretin effect, and the metabolite even may act as an antagonist. We have shown previously that GLP-1 strongly inhibited cephalic-induced antral motility in pigs. We decided, therefore, to examine the effect of GLP-1 9-36 amide, with and without GLP-1, on cephalic-induced motility in pigs. In one series of experiments, we studied the effect of three different doses of GLP-1 9-36 amide (2, 4, and 10 pmol/kg/min) on insulin-induced (hypoglycemia) antral motility in anaesthetized pigs (n = 9). In another series, we studied the effect of infusion of GLP-1 9-36 amide in two different doses (1 and 5 pmol/kg/min) in six pigs in which the antral motility was inhibited by GLP-1 7-36 amide in a dose of 2 pmol/kg/min. Plasma levels of intact GLP-1 7-36 amide and GLP-1 9-36 amide were determined using specific radioimmunoassays. Insulin-induced hypoglycemia increased the antral motility index from 0.4 +/- 0.1 to 8.3 +/- 3.5 (cm/min). The motility was constant throughout the experimental period and was absolutely unaffected by the infusion of GLP-1 9-36 amide at 10 pmol/kg/min, which resulted in a plasma concentration of 351 +/- 60 pmol/l. The inhibitory effect of GLP-1 7-36 amide on antral motility was reduced from 93 +/- 3% to 33 +/- 9% (p < 0.05) by concomitant infusion of GLP-1 9-36 amide in a dose of 5 pmol/kg/min. The metabolite GLP-1 9-36 amide has no effect on antral motility in pigs but is able to antagonize the inhibitory effect of GLP-1. Thus, an intact N terminus is essential for the gastrointestinal actions of GLP-1. Its primary metabolite may act as an endogenous antagonist.

Glucagon-like peptide 1 inhibition of gastric emptying outweighs its insulinotropic effects in healthy humans

Glucagon-like peptide 1 (GLP-1) has been shown to inhibit gastric emptying of liquid meals in type 2 diabetic patients. It was the aim of the present study to compare the action of physiological and pharmacological doses of intravenous GLP-1-(7-36) amide and GLP-1-(7-37) on gastric emptying in normal volunteers. Nine healthy subjects participated (26 +/- 3 yr; body mass index 22.9 +/- 1.6 kg/m2; hemoglobin A1C 5.0 +/- 0.2%) in five experiments on separate occasions after an overnight fast. A nasogastric tube was positioned for the determination of gastric volume by use of a dye-dilution technique (phenol red). GLP-1-(7-36) amide (0.4, 0.8, or 1.2 pmol.kg-1.min-1), GLP-1-(7-37) (1.2 pmol.kg-1.min-1), or placebo was infused intravenously from -30 to 240 min. A liquid meal (50 g sucrose, 8% amino acids, 440 ml, 327 kcal) was administered at 0 min. Glucose, insulin, and C-peptide were measured over 240 min. Gastric emptying was dose dependently slowed by GLP-1-(7-36) amide (P < 0.0001). Effects of GLP-1-(7-37) at 1.2 pmol.kg-1.min-1 were virtually identical. GLP.1 dose dependently stimulated fasting insulin secretion (-30 to 0 min) and slightly reduced glucose concentrations. After the meal (0-240 min), integrated incremental glucose (P < 0.0001) and insulin responses (P = 0.01) were reduced (dose dependently) rather than enhanced. In conclusion, 1) GLP-1-(7-36) amide or -(7-37) inhibits gastric emptying also in normal subjects, 2) physiological doses (0.4 pmol.kg-1.min-1) still have a significant effect, 3) despite the known insulinotropic actions of GLP-1-(7-36) amide and -(7-37), the net effect of administering GLP-1 with a meal is no change or a reduction in meal-related insulin responses. These findings suggest a primarily inhibitory function for GLP-1 (ileal brake mechanisms).

Circulating and tissue forms of the intestinal growth factor, glucagon-like peptide-2

Glucagon-like peptide-2 (GLP-2) has recently been identified as a stimulator of intestinal epithelial growth, prompting the development of RIA and HPLC methodologies to study this peptide in more detail. A GLP-2-specific antiserum (UTTH-7) was developed that recognizes amino acids 25-30 of human and rat GLP-2-(1-33). UTTH-7 cross-reacts with N- and C-terminally modified forms of GLP-2, proglucagon, and the major proglucagon fragment. Analysis of rat ileal extracts demonstrated the presence of GLP-2-(1-33) as well as significant amounts of GLP-2-(3-33) (16 +/- 7% of total GLP-2). The level of total immunoreactive GLP-2 in plasma from fasted rats was 700 +/- 71 pg/ml, and this increased 3.6-fold (P < 0.001) in 24-h fed rats. HPLC analysis demonstrated the presence of both GLP-2-(1-33) and GLP-2-(3-33) in plasma from fasted rats, with increments in both peptides in plasma from fed rats. Immunoreactive GLP-2 increased in plasma from human subjects 2 h after a meal, rising from 851 +/- 230 to 1106 +/- 211 pg/ml (P < 0.05); 15 +/- 4% of this immunoreactivity was accounted for by the presence of intact GLP-2. HPLC showed the presence of both GLP-2-(1-33) and GLP-2-(3-33) in plasma from fed humans. Incubation of human GLP-2-(1-33) with the enzyme dipeptidylpeptidase IV resulted in liberation of GLP-2-(3-33), whereas replacement of Ala2 with Gly2 prevented this cleavage. Thus, while GLP-2-(1-33) is a major circulating and tissue form of GLP-2, GLP-2-(3-33) is a significant component ofimmunoreactive GLP-2 in both intestine and plasma.

A radioimmunoassay for LY315902, an analog of glucagon-like insulinotropic peptide, and its application in the study of canine pharmacokinetics

Glucagon-like insulinotropic peptide (GLP-1) and its analogs are of interest because of their therapeutic potential in type II diabetes. LY315902 is a GLP-1-(7-37)-OH analog with a modified N-terminus (IP7), an octanoic acid (C8) acylated on the lysine residue at position 34, and a substitution with arginine at position 26. We developed a sensitive and specific radioimmunoassay (RIA) for the determination of immunoreactive LY315902 in the plasma of animals. A homobifunctional cross-linker was used to couple the nonacylated form of LY315902 [IP7-R26-GLP-1-(7-37)-OH] to carrier proteins to enhance its immunogenicity. Following immunization, animal antisera were screened by RIA for the presence of LY315902 antibodies. One rabbit produced a high-affinity antiserum that display insignificant cross-reactivity against two forms of native GLP-1 and possible major metabolites of LY315902. In this RIA method, plasma samples were combined with radioiodinated LY315902 and rabbit anti-IP7-R26-GLP-1-(7-37)-OH serum, and then incubated overnight at room temperature. The bound forms of LY315902 were separated by polyethylene glycol assisted second antibody precipitation. The sensitivity of the assay was estimated to be 19 pM. Inter-assay precision (%CV) and accuracy (recovery) for quality control samples in dog plasma ranged from 8.0% to 14.7% and 92.8% to 107.3%, respectively. By applying this assay to measure plasma concentrations of immunoreactive LY315902 in dogs following twice daily subcutaneous injections of LY315902, we determined that the plasma half-life of LY315902 is significantly longer than that of native GLP-1-(7-37)-OH. We concluded that the structural modifications which were made to produce LY315902 prolonged its plasma half-life. The extended plasma half-life of LY315902 correlated well with its prolonged pharmacology in dogs.

Glucagon-like peptide-1 7-36 amide and peptide YY have additive inhibitory effect on gastric acid secretion in man

BACKGROUND

Glucagon-like peptide-1 7-36 amide (GLP-1) and peptide YY (PYY) are colocalized in the L-cell of the ileal mucosa, and both peptides may function as enterogastrone hormones. However, it is not known whether they interact with regard to the effect on acid secretion.

METHODS

The effect of intravenous infusion of GLP-1 and PYY, either alone or in combination, on pentagastrin-induced acid secretion in eight healthy volunteers was examined. The peptides were infused at two different rates: 0.25 pmol/kg/min (low rate) and 0.5 pmol/kg/min (high rate).

RESULTS

Given alone, GLP-1 and PYY inhibited acid secretion by 26 +/- 5% and 18 +/- 5% (low rate) and 45 +/- 8% and 38 +/- 7% (high rate), respectively. Combined infusion resulted in an inhibition of 32 +/- 5% (low rate) and 62 +/- 7% (high rate). Both infusion rates resulted in GLP-1 and PYY plasma concentrations below or similar to postprandial levels.

CONCLUSION

The present study suggests that the interaction between GLP-1 and PYY in man is of the additive type. The results indicate that GLP-1 and PYY have an important role in the physiologic control of gastric acid secretion.

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.

Rapid oscillations in plasma glucagon-like peptide-1 (GLP-1) in humans: cholinergic control of GLP-1 secretion via muscarinic receptors

The mechanisms involved in the rapid glucagon-like peptide-1 (GLP-1) release following glucose ingestion are poorly defined. Besides a direct intestinal stimulation of L cells, humoral and neuronal mechanisms have been discussed. We investigated the temporal pattern of GLP-1 release in five healthy men (aged 27.8 +/- 3.6 yr, body mass index, 23.4 +/- 1.2 kg/m2) after an overnight fast for 60 min under basal conditions and for 60 min after an oral glucose load (OGL; 100 g) in both the presence and absence of atropine (80 ng/kg min, iv). Blood was sampled every 2 min, and data were evaluated for the temporal pattern of GLP-1 secretion by several computer-assisted programs (deconvolution, Pulsar analysis, and Fourier transformation). With all methods a pulsatile pattern of plasma GLP-1 levels with a frequency of five to seven per h was detected; this remained unchanged in the different metabolic states and during atropine treatment. Glucose and GLP-1 plasma levels showed a parallel increase after OGL (OGL without atropine = control: 8.4 +/- 2.9 and 7.9 +/- 3.0 min, respectively). Atropine infusion delayed this increase significantly (16.8 +/- 8.07 and 17.4 +/- 6.61 min, respectively; P < 0.02). In contrast to plasma glucose concentrations (82.7 +/- 0.3% of control; P < 0.05), atropine infusion reduced the integrated GLP-1 pulse amplitude to 56.0 +/- 11.3% of the control levels (P < 0.05). In conclusion, GLP-1 is secreted in a pulsatile manner with a frequency comparable to that of pancreatic hormones. Mean GLP-1 plasma concentrations increase after OGL due to augmented GLP-1 pulse amplitudes but not frequency. The differential effect of atropine on glucose and GLP-1 plasma levels suggest a direct cholinergic muscarinic control of L cells.

The hepatic vagal nerve is receptive to incretin hormone glucagon-like peptide-1, but not to glucose-dependent insulinotropic polypeptide, in the portal vein

To examine whether incretin hormones, truncated glucagon-like peptide-1 (tGLP-1) and glucose-dependent insulinotropic polypeptide (GIP), are recognized by the hepatic vagal nerve, changes of the impulse discharge rate in the afferent vagus upon their intraportal administrations were measured in situ in rats anesthetized with urethan and chloralose. One-min injection of tGLP-1 at a periphysiological dose of 0.2 pmol or a pharmacological dose of 4.0 pmol, but not of the vehicle, significantly facilitated the hepatic vagal afferents. However, the injection of GIP at either a physiological dose of 0.2 pmol, a periphysiological dose of 4.0 pmol, or an even much larger dose of 40.0 pmol did not change the afferents at all. The present results indicate that the hepatic vagus specifically recognizes an intraportal appearance of tGLP-1 in the hepatoportal area, suggesting that the vagal monitoring system for intraportal levels of the incretin hormone operates on ingestion of a mixed meal.

Vagal hepatopancreatic reflex effect evoked by intraportal appearance of tGLP-1

Among proglucagon-derived peptides, the truncated form of glucagon-like peptide-1, GLP-1(7-36)amide (tGLP-1), is known as the most likely physiological humoral incretin. To examine whether there exists any relationship between tGLP-1 levels in the portal vein and activities of the hepatic and pancreatic vagal system, changes of the impulse discharge rate in the hepatic afferent vagus and the pancreatic efferent vagus upon intraportal tGLP-1 injection were measured in situ in rats anesthetized with urethan and chloralose. First, a 1-min bolus tGLP-1 injection at a periphysiological dose of 0.2 pmol or a pharmacological dose of 4.0 pmol, but not the vehicle injection, significantly facilitated the hepatic vagal afferents for > 90 min, showing weaker facilitation at the 0.05 pmol dose. Notably, the injection of noninsulinotropic full-length GLP-1 failed to facilitate the afferents at the 4.0 or 40.0 pmol dose. Second, the intraportal tGLP-1 injections at the 0.05 and 0.2 pmol dose facilitated marginally and significantly the pancreatic vagal efferents in normal rats, respectively, but had no effect on the hepatic vagotomized rats, even at the 40.0 pmol dose. The present results indicate that an intraportal appearance of tGLP-1 is specifically recognized by the hepatic vagal nerve, and this recognition further augments the pancreatic vagal efferent activity in a reflex way, suggesting another nature of tGLP-1 as neuroincretin in the enteroinsular axis.

The enteroinsular axis in dipeptidyl peptidase IV-negative rats

Evidence has accumulated that the incretins glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1(7-36) amide) are degraded and rendered biologically inactive in plasma by the enzyme dipeptidyl peptidase IV (DPIV). A strain of Fischer rats lacking the DPIV enzyme were used in the current investigation as a model for examining the enteroinsular axis under conditions in which normal inactivation of GIP and GLP-1(7-36) does not occur. This was assessed by comparing GIP and GLP-1(7-36) responses following oral glucose in normal versus DPIV-deficient Fischer rats, and by comparing the insulinotropic potency of both peptides in the perfused pancreas of both groups. The insulin response to an oral glucose challenge was decreased slightly in DPIV-negative rats compared with control animals. Of the two incretins, the GIP response to oral glucose was reduced by 50% compared with controls, whereas GLP-1(7-36) release in response to glucose was unchanged. A decrease of 30% in the sensitivity of the perfused pancreas of DPIV-negative rats to GIP was observed, whereas the insulin response to GLP(7-36) was identical in both groups. Incubation of both peptides in plasma from DPIV-positive and -negative rats was performed to determine the effect of the presence or absence of DPIV on the insulinotropic activity of GLP-1(7-36) and GIP in the isolated perfused rat pancreas. Incubation in plasma from DPIV-positive rats resulted in a 65% decrease in insulinotropic activity of both incretins compared with incubation in plasma from DPIV-deficient rats. It was hypothesized that the reduced GIP response and decreased sensitivity of the pancreas to GIP are compensatory mechanisms that maintain insulin and glucose levels within a normal range despite abnormal degradation of GIP. An explanation of the lack of effect of the absence of DPIV on the GLP-1(7-36) response to oral glucose and insulinotropic action of this peptide must await further study.

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

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 hyper insulinaemic (0.8 mU.kg-1.min-1), euglycaemic (5 mmol/l) clamp. Somatostatin (450 micrograms/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 undergoes differential tissue-specific metabolism in the anesthetized pig

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

Secretion of the incretin hormones glucagon-like peptide-1 and gastric inhibitory polypeptide correlates with insulin secretion in normal man throughout the day

BACKGROUND

The insulinotropic hormones gastric inhibitory polypeptide (GIP) and glucagon-like peptide-1 (GLP-1), secreted from the K-cells of the upper small intestine and from the L-cells of the lower small intestine, respectively, are thought to be responsible for intestinal stimulation of insulin secretion. If true, their plasma concentrations should parallel the meal-related diurnal changes in plasma insulin concentrations.

METHODS

Using COOH-terminal assays, thought to reflect accurately their rates of secretion, we measured circulating levels of GIP and GLP-1 in six normal subjects for 15 h of a day, during which they ate three mixed meals.

RESULTS

Both GIP and GLP-1 concentrations increased significantly and in parallel with insulin in response to all three meals. The plasma insulin concentrations correlated significantly with both GIP and GLP-1 values throughout the study period (correlation coefficients, 0.49 +/- 0.07 and 0.56 +/- 0.05; p < 0.001).

CONCLUSIONS

These results support the notion that GLP-1 and GIP are important incretin hormones.

Glucagon-like peptide-1(7-36)amide enhances insulin-stimulated glucose uptake and decreases intracellular cAMP content in isolated rat adipocytes

We investigated the effect of GLPs on glucose uptake in isolated rat adipocytes. GLP-1(7-36)amide significantly enhanced glucose uptake in the presence of 1 nM insulin. GLP-1(7-36)amide at 15 nM increased glucose uptake maximally by 56.4% as compared with 1 nM insulin alone (P < 0.01). In contrast, with less than 1 nM insulin or without insulin GLP-1(7-36)amide showed no effect on glucose uptake. Full-sequence GLP-1(1-37) at 15 nM in the presence of 1 nM insulin increased glucose uptake by 24.6% as compared with 1 nM insulin alone (P < 0.05). GLP-2 showed no effect on glucose uptake. Further, we examined the effect of GLP-1(7-36)amide on cAMP content in isolated rat adipocytes. Insulin at 1 nM caused a significant decrease of cAMP content. The combination of 15 nM GLP-1(7-36)amide and 1 nM insulin caused a further reduction of cAMP content. These data indicate that GLP-1(7-36)amide possesses augmentative effects on insulin action in isolated rat adipocytes. Furthermore, it is suggested that the stimulatory effect of GLP-1(7-36)amide occurs through the reduction of intracellular cAMP content.

Effects of glucagon-like peptide-I on glucose turnover in rats

The influences of glucagon-like peptide-I-(7-36) amide (GLP-I; 15 mumol. kg-1.min-1) on glucose turnover were studied in freely moving Wistar rats. In fed rats, GLP-1 reduced plasma glucose (from 7.3 +/- 0.2 to 5.6 +/- 0.3 mmol/l; P = 0.017), increased plasma insulin (from 20 +/- 3 to 89 +/- 11 mU/l; P = 0.002), and reduced plasma glucagon (from 44 +/- 1 to 35 +/- 2 pg/ml; P = 0.009) and glucose appearance rate (Ra; from 3.9 +/- 0.2 to 1.7 +/- 0.7 micromol.min-1. 100 g-1 after 30 min; P = 0.049) without affecting glucose disappearance rate (Rd). The glucose clearance rate (MCR) was increased (P = 0.048). In 48-h-fasted rats, GLP-I reduced plasma glucose (from 5.0 +/- 0.2 to 4.4 +/- 0.3 mmol/l; P = 0.035) and increased plasma insulin (from 4 +/- 1 to 25 +/- 10 mU/l; P = 0.042) and plasma glucagon (from 43 +/- 3 to 61 +/- 7 pg/ml; P = 0.046). Ra and Rd were not significantly affected, although Ra was lower than Rd after 15-30 min (P = 0.005) and MCR was increased (P = 0.049). Thus GLP-I reduces Ra in fed rats and increases MCR in fed and fasted rats. The reduced Ra seems mediated by an increased insulin-to-glucagon ratio; the increased glucose clearance seems dependent on insulin and a peripheral effect of GLP-I.

Elimination of the action of glucagon-like peptide 1 causes an impairment of glucose tolerance after nutrient ingestion by healthy baboons

Glucagon-like peptide 1 (GLP-1) is an insulinotropic hormone released after nutrient ingestion which is known to augment insulin secretion, inhibit glucagon release, and promote insulin-independent glucose disposition. To determine the overall effect of GLP-1 on glucose disposition after a meal we studied a group of healthy, conscious baboons before and after intragastric glucose administration during infusions of saline, and two treatments to eliminate the action of GLP-1: (a) exendin-[9-39] (Ex-9), a peptide receptor antagonist of GLP-1; or (b) an anti-GLP-1 mAb. Fasting concentrations of glucose were higher during infusion of Ex-9 than during saline (4.44 +/- 0.05 vs. 4.16 +/- 0.05 mM, P < 0.01), coincident with an elevation in the levels of circulating glucagon (96 +/- 10 vs. 59 +/- 3 ng/liter, P < 0.02). The postprandial glycemic excursions during administration of Ex-9 and mAb were greater than during the control studies (Ex-9 13.7 +/- 2.0 vs. saline 10.0 +/- 0.8 mM, P = 0.07; and mAb 13.6 +/- 1.2 vs. saline 10.6 +/- 0.9 mM, P = 0.044). The increments in insulin levels throughout the absorption of the glucose meal were not different for the experimental and control conditions, but the insulin response in the first 30 min after the glucose meal was diminished significantly during treatment with Ex-9 (Ex-9 761 +/- 139 vs. saline 1,089 +/- 166 pM, P = 0.044) and was delayed in three of the four animals given the neutralizing antibody (mAb 946 +/- 262 vs. saline 1,146 +/- 340 pM). Thus, elimination of the action of GLP-1 impaired the disposition of an intragastric glucose meal and this was at least partly attributable to diminished early insulin release. In addition to these postprandial effects, the concurrent elevation in fasting glucose and glucagon during GLP-1 antagonism suggests that GLP-1 may have a tonic inhibitory effect on glucagon output. These findings demonstrate the important role of GLP-1 in the assimilation of glucose absorbed from the gut.

Human colon produces fully processed glucagon-like peptide-1 (7-36) amide

The human colon contains many open-type endocrine cells which express the preproglucagon gene and possess glucagon-like peptide-1 (GLP-1) immunoreactivity, but the molecular form of the peptide is unknown. Acid ethanol extracts of human colon (n = 4) were subjected to gel filtration and successive purification by high-pressure liquid chromatography, monitored by specific RIAs. A single GLP-1-immunoreactive peak was isolated and identified as GLP-1 (7-36)amide by amino acid sequence analysis and mass spectrometry. We conclude that proglucagon is processed in the large intestine in the same manner as in the small intestine, and results in the formation of fully processed biologically active GLP-1.

Pharmacokinetic, insulinotropic, and glucagonostatic properties of GLP-1 [7-36 amide] after subcutaneous injection in healthy volunteers. Dose-response-relationships

Intravenous infusions of glucagon-like peptide 1 (GLP-1) [7-36 amide] are glucose-dependently insulinotropic and glucagonostatic and normalize plasma glucose concentrations in non-insulin-dependent diabetic patients. It was the aim of this study to investigate whether subcutaneous GLP-1 [7-36 amide] also has an influence on insulin and glucagon secretion, and which doses are required for significant effects. Therefore, eight healthy volunteers (24 +/- 2 years, body mass index [BMI] 21.9 +/- 2.3 kg/m2) were studied in the fasting state on five occasions in randomized order. Placebo (0.9% NaCl with 1% human serum albumin) or GLP-1 [7-36 amide] in doses of 0.15, 0.5, 1.5 or 4.5 nmol/kg body weight (volume 1 ml or, at the highest dose, 2 ml) was administered subcutaneously. An intravenous glucose bolus (0.33 g/kg body weight) was injected 30 min later. Blood was drawn for the measurement of glucose, insulin, C-peptide, GLP-1 [7-36 amide], and glucagon using specific radioimmunoassays. There were dose-related increments in GLP-1 [7-36 amide] concentrations (p < 0.0001). However, basal values were reached again after 90-120 min. Before glucose administration, insulin (p < 0.0001) and C-peptide (p < 0.0004) increased, whereas glucagon (p = 0.0018) and glucose (p < 0.0001) decreased in a dose-dependent manner. After glucose stimulation, integrated increments in insulin (p = 0.0007) and C-peptide (p = 0.02) were augmented and kG-values increased (p < 0.0001) in a dose-related fashion. The extent of reactive hypoglycaemia was related to the GLP-1 [7-36 amide] dose.(ABSTRACT TRUNCATED AT 250 WORDS)

Physiological augmentation of amino acid-induced insulin secretion by GIP and GLP-I but not by CCK-8

It was the aim of this study to test insulinotropic actions of cholecystokinin octapeptide (CCK-8), gastric inhibitory polypeptide (GIP), and glucagon-like peptide I (GLP-I)-(7--36) amide at basal glucose but physiologically elevated amino acid concentrations. Therefore, in nine fasting healthy volunteers, an amino acid mixture was infused intravenously (12.6 g/h over 120 min). On separate occasions, from 30 to 120 min, placebo (0.9% NaCl-1% human serum albumin), synthetic sulfated CCK-8 (0.5 pmol.kg-1.min-1), human GIP (1 pmol.kg-1.min-1), or GLP-I-(7--36) amide (0.3 pmol.kg-1.min-1) was infused intravenously to mimic physiological increments after a meal. The amino acid infusion lead to a small increment in plasma glucose from 4.8 +/- 0.2 to 5.0 +/- 0.2 mmol/l and significantly elevated insulin and C-peptide concentrations. GIP and GLP-I-(7--36) amide further stimulated insulin (1.8-fold, P = 0.0001 and 0.004, respectively) and C-peptide (1.3-fold, P = 0.0003 and 0.013, respectively), with a subsequent slight reduction in plasma glucose (P < 0.0001). Insulin and C-peptide then decreased again in parallel. CCK-8 was without effect on insulin and C-peptide levels. In conclusion, GIP and GLP-I-(7--36) amide are not only able to interact with elevated plasma glucose but are insulinotropic also with physiologically raised amino acid concentrations. Such an interaction could play a role after the ingestion of mixed meals. Cholecystokinin, on the other hand, is not a physiological incretin also under these conditions.

Effects of glucose-dependent insulinotropic polypeptide and glucagon-like peptide-I-(7-36) on insulin secretion

Glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide (GLP)-I-(7-36) are probably the most important "incretins," but there is controversy as to their relative insulinotropic activities. The effects of natural (np) and synthetic porcine (sp) GIP, synthetic human (sh) GIP, and GLP-I-(7-36) on insulin secretion from the perfused rat pancreas were compared using gradient perfusion. Insulin secretion was increased by both spGIP and GLP-I-(7-36) at concentrations of approximately 16 pM. Maximal responses to GLP-I-(7-36) in the presence of 16.7 mM glucose were slightly greater than with npGIP or spGIP, but with 10 mM glucose spGIP and GLP-I-(7-36) exerted equivalent effects. Responses to shGIP were greatly reduced compared with spGIP. In the presence of 50 pM spGIP or GLP-I-(7-36) the glucose threshold was 4.5 +/- 0.11 mM. The data indicate that GLP-I-(7-36) and porcine GIP are equally insulinotropic and share the same glucose threshold for activity, whereas shGIP is less active. At the concentrations found postprandially, however, GIP is likely to be the more important incretin.

Glucagonlike peptide 1: a newly discovered gastrointestinal hormone

Glucagonlike peptide (GLP) 1, a peptide of 30 amino acids with 50% sequence homology to glucagon, results from expression of the glucagon gene in the L cells of the distal intestinal mucosa. It is secreted early in response to mixed meals by mechanisms involving the presence of unabsorbed nutrients in the gut lumen or the absorptive process itself, but other mechanisms may also be involved. GLP-1 has two important actions. First, it stimulates insulin secretion and inhibits glucagon secretion and thereby inhibits hepatic glucose production and lowers blood glucose levels. It may have effects on glucose clearance independent of its pancreatic effects. It acts on recently cloned G protein-coupled specific receptors and seems to increase insulin secretion via cyclic adenosine monophosphate-dependent increases in intracellular calcium. It has been suggested that activation of the beta cells by GLP-1 is a prerequisite for glucose-induced insulin secretion. Second, it also potently inhibits gastrointestinal secretion and motility and is likely to act as an "ileal brake," possibly after activation of cerebral receptors. Therefore, GLP-1 physiologically seems to signal nutritional abundancy and enhance deposition of nutrients. Because of these effects, however, the peptide can completely normalize blood glucose levels in type 2 diabetics and is therefore of considerable pharmaceutical interest.