Extrapancreatic glucagons

Actions of glucagon-like peptide-1 receptor ligands in the gut

The incretin hormone glucagon-like peptide-1 (GLP-1) is inactivated by the enzyme dipeptidyl peptidase-4 even before it leaves the gut, but it seems to act predominantly via activation of intestinal sensory neurons expressing GLP-1 receptors. Thus, activation of vagal afferents is probably responsible for its effects on appetite and food intake, gastrointestinal secretion and motility, and pancreatic endocrine secretion. However, GLP-1 receptors are widely expressed in the gastrointestinal (GI) tract, including epithelial cells in the stomach, and the Brunner glands, in endocrine cells of the gut epithelium, and on mucosal lymphocytes. In this way, GLP-1 may have important local actions of epithelial protection and endocrine signalling and may interact with the immune system. We review the formation and release of GLP-1 from the endocrine L cells and its fate after release and describe the localization of its receptor throughout the GI tract and discuss its direct or indirect actions in the GI tract.

Investigating Intestinal Glucagon After Roux-en-Y Gastric Bypass Surgery

CONTEXT

After Roux-en-Y gastric bypass (RYGB) surgery, postprandial plasma glucagon concentrations have been reported to increase. This occurs despite concomitant improved glucose tolerance and increased circulating plasma concentrations of insulin and the glucagon-inhibiting hormone glucagon-like peptide 1 (GLP-1).

OBJECTIVE

To investigate whether RYGB-induced hyperglucagonemia may be derived from the gut.

DESIGN AND SETTING

Substudy of a prospective cross-sectional study at a university hospital in Copenhagen, Denmark.

PARTICIPANTS

Morbidly obese individuals undergoing RYGB (n = 8) with or without type 2 diabetes.

INTERVENTIONS

Three months before and after RYGB, participants underwent upper enteroscopy with retrieval of gastrointestinal mucosal biopsy specimens. Mixed-meal tests were performed 1 week and 3 months before and after RYGB.

MAIN OUTCOME MEASURES

The 29-amino acid glucagon concentrations in plasma and in mucosal gastrointestinal biopsy specimens were assessed using mass spectrometry-validated immunoassays, and a new monoclonal antibody reacting with immunoreactive glucagon was used for immunohistochemistry.

RESULTS

Postprandial plasma concentrations of glucagon after RYGB were increased. Expression of the glucagon gene in the small intestine increased after surgery. Glucagon was identified in the small-intestine biopsy specimens obtained after, but not before, RYGB. Immunohistochemically, mucosal biopsy specimens from the small intestine harbored cells costained for GLP-1 and immunoreactive glucagon.

CONCLUSION

Increased concentrations of glucagon were observed in small-intestine biopsy specimens and postprandially in plasma after RYGB. The small intestine harbored cells immunohistochemically costaining for GLP-1 and glucagon-like immunoreactivity after RYGB. Glucagon derived from small-intestine enteroendocrine l cells may contribute to postprandial plasma concentrations of glucagon after RYGB.

From the Incretin Concept and the Discovery of GLP-1 to Today's Diabetes Therapy

Researchers have been looking for insulin-stimulating factors for more than 100 years, and in the 1960ties it was definitively proven that the gastrointestinal tract releases important insulinotropic factors upon oral glucose intake, so-called incretin hormones. The first significant factor identified was the duodenal glucose-dependent insulinotropic polypeptide, GIP, which however, turned out not to stimulate insulin secretion in patients with type 2 diabetes. But resection experiments clearly indicated the presence of an additional incretin, and in 1986, an unexpected processing fragment of the recently identified glucagon precursor, proglucagon, namely truncated glucagon-like peptide 1 (GLP-1 7-36 amide), was isolated from the gut and found to both stimulate insulin secretion and inhibit glucagon secretion. The peptide also inhibited appetite and food intake. Unlike GIP, this peptide had preserved effects in patients with type 2 diabetes and it was soon documented to have powerful antidiabetic effects in clinical studies. Its utility was limited, however, because of an extremely short half-life in humans, but this problem had two solutions, both of which gave rise to important antidiabetic drugs: (1) orally active inhibitors of the enzyme dipeptidylpeptidase 4 (DPP-4 inhibitors), which was responsible for the rapid degradation; the inhibitors protect endogenous GLP-1 from degradation and thereby unfold its antidiabetic activity, and (2) long-acting injectable analogs of GLP-1 protected against DPP-4 degradation. Particularly, the latter, the GLP-1 receptor agonists, either alone or in various combinations, are so powerful that treatment allows more than 2/3 of type 2 diabetes patients to reach glycemic targets. In addition, these agents cause a weight loss which, with the most successful compounds, may exceed 10% of body weight. Most recently they have also been shown to be renoprotective and reduce cardiovascular risk and mortality.

Extrapancreatic glucagon: Present status

Pancreatic alpha cells are generally considered the only source of glucagon secretion in humans. In the 1970s several groups investigating totally pancreatectomised animals reported that glucagon-like immunoreactive material could be detected in the gastrointestinal tract and reopened the question of an extrapancreatic source of glucagon proposed in 1948 when a hyperglycaemic substance was found in the gastrointestinal tract of dogs and rabbits. Nevertheless, over the years, controversy about the existence of extrapancreatic glucagon has flourished as it proved difficult to accurately measure fully processed 29-amino acid glucagon. Recent advances in analytical methods have increased sensitivity and specificity of glucagon assays and, furthermore, technical advances in mass spectrometry-based proteomics have made the detection of low-abundant peptides, such as glucagon, in human plasma more accurate. Here we review new data on extrapancreatic glucagon secretion in the context of historical data and recent analytical breakthroughs. Furthermore, the source, regulation and potential physiological role of extrapancreatic glucagon are discussed and ongoing challenges and knowledge-gaps are outlined.

Glucagon receptor signaling in metabolic diseases

Glucagon is a peptide hormone secreted from the pancreatic alpha cells in response to hypoglycemia but in some patients with type 2 diabetes a paradoxical hypersecretion results from the intake of glucose. In rodent, antagonizing the actions of glucagon have been shown to be effective for lowering blood glucose levels and this has recently have been solidified in patients with type 2 diabetes. Although the reported increases of liver enzymes, hyperglucagonemia, and alpha cell hyperplasia resulting from glucagon receptor antagonism may potentially limit the clinical applicability of glucagon receptor antagonists, they may serve as an instrumental toolbox for delineating the physiology of glucagon. Agonizing glucagon receptor signaling may be relevant, in particular when combined with glucagon-like peptide-1 receptor analogues in the perspective of body weight lowering therapy. Here, we will focus on new conceptual aspects of glucagon biology and how this may led to new diagnostics and treatment of metabolic diseases.

Oxyntomodulin: Actions and role in diabetes

Oxyntomodulin is a product of the glucagon precursor, proglucagon, produced and released from the endocrine L-cells of the gut after enzymatic processing by the precursor prohormone convertase 1/3. It corresponds to the proglucagon sequence 33-69 and thus contains the entire glucagon sequence plus a C-terminal octapeptide, comprising in total 37 amino acids. As might have been expected, it has glucagon-like bioactivity, but also and more surprisingly also activates the receptor for GLP-1. This has given the molecule an interesting status as a glucagon-GLP-1 co-agonist, which is currently attracting considerable interest for its potential in the treatment of diabetes and obesity. Here, we provide an update on oxyntomodulin with a focus on its potential role in metabolic diseases.

Discovery, characterization, and clinical development of the glucagon-like peptides

The discovery, characterization, and clinical development of glucagon-like-peptide-1 (GLP-1) spans more than 30 years and includes contributions from multiple investigators, science recognized by the 2017 Harrington Award Prize for Innovation in Medicine. Herein, we provide perspectives on the historical events and key experimental findings establishing the biology of GLP-1 as an insulin-stimulating glucoregulatory hormone. Important attributes of GLP-1 action and enteroendocrine science are reviewed, with emphasis on mechanistic advances and clinical proof-of-concept studies. The discovery that GLP-2 promotes mucosal growth in the intestine is described, and key findings from both preclinical studies and the GLP-2 clinical development program for short bowel syndrome (SBS) are reviewed. Finally, we summarize recent progress in GLP biology, highlighting emerging concepts and scientific insights with translational relevance.

Insulin and Glucagon: Partners for Life

In August 2016, several leaders in glucagon biology gathered for the European Association for the Study of Diabetes Hagedorn Workshop in Oxford, England. A key point of discussion focused on the need for basal insulin to allow for the therapeutic benefit of glucagon blockade in the treatment of diabetes. Among the most enlightening experimental results presented were findings from studies in which glucagon receptor-deficient mice were administered streptozotocin to destroy pancreatic β cells or had undergone diphtheria toxin-induced β cell ablation. This article summarizes key features of the discussion as a consensus was reached. Agents that antagonize glucagon may be of great benefit for the treatment of diabetes; however, sufficient levels of basal insulin are required for their therapeutic efficacy.

The biology of glucagon and the consequences of hyperglucagonemia

The proglucagon-derived peptide hormone, glucagon, comprises 29 amino acids. Its secretion from the pancreatic α cells is regulated by several factors. Glucagon increases blood glucose levels through gluconeogenesis and glycogenolysis. Elevated plasma concentrations of glucagon, hyperglucagonemia, may contribute to diabetes. However, hyperglucagonemia is also observed in other clinical conditions than diabetes, including nonalcoholic fatty liver disease, glucagon-producing tumors and after gastric bypass surgery. Here, we review the current literature on hyperglucagonemia in disease with a particular focus on diabetes, and finally speculate that the primary physiological importance of glucagon may not reside in glucose homeostasis but in regulation of amino acid metabolism exerted via a hitherto unrecognized hepato-pancreatic feedback loop.

Mechanisms of surgical control of type 2 diabetes: GLP-1 is key factor

GLP-1 secretion in response to meals is dramatically increased after gastric bypass operations. GLP-1 is a powerful insulinotropic and anorectic hormone, and analogs of GLP-1 are widely used for the treatment of diabetes and recently approved also for obesity treatment. It is, therefore, reasonable to assume that the exaggerated GLP-1 secretion contributes to the antidiabetic and anorectic effects of gastric bypass. Indeed, human experiments with the GLP-1 receptor antagonist, Exendin 9-39, have shown that the improved insulin secretion, which is responsible for part of the antidiabetic effect of the operation, is reduced and or abolished after GLP-1 receptor blockade. Also the postoperative improvement of glucose tolerance is eliminated and or reduced by the antagonist, pointing to a key role for the exaggerated GLP-1 secretion. Indeed, there is evidence that the exaggerated GLP-1 secretion is also responsible for postprandial hypoglycemia sometimes observed after bypass. Other operations (biliopancreatic-diversion and or sleeve gastrectomy) appear to involve different and/or additional mechanisms, and so does experimental bariatric surgery in rodents. However, unlike bypass surgery in humans, the rodent operations are generally associated with increased energy metabolism pointing to an entirely different mechanism of action in the animals.

Evidence of Extrapancreatic Glucagon Secretion in Man

Glucagon is believed to be a pancreas-specific hormone, and hyperglucagonemia has been shown to contribute significantly to the hyperglycemic state of patients with diabetes. This hyperglucagonemia has been thought to arise from α-cell insensitivity to suppressive effects of glucose and insulin combined with reduced insulin secretion. We hypothesized that postabsorptive hyperglucagonemia represents a gut-dependent phenomenon and subjected 10 totally pancreatectomized patients and 10 healthy control subjects to a 75-g oral glucose tolerance test and a corresponding isoglycemic intravenous glucose infusion. We applied novel analytical methods of plasma glucagon (sandwich ELISA and mass spectrometry-based proteomics) and show that 29-amino acid glucagon circulates in patients without a pancreas and that glucose stimulation of the gastrointestinal tract elicits significant hyperglucagonemia in these patients. These findings emphasize the existence of extrapancreatic glucagon (perhaps originating from the gut) in man and suggest that it may play a role in diabetes secondary to total pancreatectomy.

Long-term effects of bariatric surgery on meal disposal and β-cell function in diabetic and nondiabetic patients

Gastric bypass surgery leads to marked improvements in glucose tolerance and insulin sensitivity in obese type 2 diabetes (T2D); the impact on glucose fluxes in response to a physiological stimulus, such as a mixed meal test (MTT), has not been determined. We administered an MTT to 12 obese T2D patients and 15 obese nondiabetic (ND) subjects before and 1 year after surgery (10 T2D and 11 ND) using the double-tracer technique and modeling of β-cell function. In both groups postsurgery, tracer-derived appearance of oral glucose was biphasic, a rapid increase followed by a sharp drop, a pattern that was mirrored by postprandial glucose levels and insulin secretion. In diabetic patients, surgery lowered fasting and postprandial glucose levels, peripheral insulin sensitivity increased in proportion to weight loss (~30%), and β-cell glucose sensitivity doubled but did not normalize (compared with 21 nonsurgical obese and lean controls). Endogenous glucose production, however, was less suppressed during the MMT as the combined result of a relative hyperglucagonemia and the rapid fall in plasma glucose and insulin levels. We conclude that in T2D, bypass surgery changes the postprandial response to a dumping-like pattern and improves glucose tolerance, β-cell function, and peripheral insulin sensitivity but worsens endogenous glucose output in response to a physiological stimulus.

Type 2 diabetes mellitus: a possible surgically reversible intestinal dysfunction

Type 2 diabetes mellitus (T2DM) is a global public health problem often associated with obesity. Bariatric surgery is effective for treating serious obesity, and techniques involving intestinal bypass have metabolic benefits, such as complete and early remission of T2DM. We present a literature review of the possible mechanisms of early normalization of glycemic homeostasis after bariatric surgery, including intestinal gluconeogenesis, increased antidiabetogenic signals from L cells located in the distal small intestine, and impaired secretion of diabetogenic signals in the upper part of the small intestine. Adding to these potential mechanisms, unknown factors that regulate insulin sensitivity may be involved and altered by bariatric surgery. This review discusses the various hypotheses about the mechanisms of glycemic control after bariatric surgery involving intestinal bypass. Further research is essential to better understand these mechanisms and to identify potential new mechanisms that might help in developing less invasive and safer alternatives for the treatment of T2DM and reveal novel pharmaceutical targets for glycemic control.

Resolution of type 2 diabetes following gastric bypass surgery: involvement of gut-derived glucagon and glucagonotropic signalling?

Certain types of bariatric surgical procedures have proved not only to be effective with regard to treating obesity, but they also seem to be associated with endocrine changes which independently of weight loss give rise to remission of type 2 diabetes. Currently, it is speculated that surgical re-routing of nutrients triggers changes in the release of gastrointestine-derived hormones, which in turn cause amelioration of the diabetic state. The 'hindgut hypothesis' states that surgical re-routing of nutrients to the distal part of the small intestine results in increased secretion and concomitant glucose-lowering effects of glucagon-like peptide-1, whereas the 'foregut hypothesis' emphasises that surgical bypass of the foregut prevents the release of a hitherto unidentified nutrient-induced diabetogenic signal in susceptible individuals. Recent studies have shown that in patients with type 2 diabetes, glucagon is differentially secreted in response to oral and i.v. glucose, respectively, with lack of suppression (and initial net secretion) during oral glucose administration and a perfectly normal suppression during isoglycaemic i.v. glucose administration. These findings could point towards a role for glucagon or gut-derived glucagonotropic signalling as putative diabetogenic signals of the foregut hypothesis. In the present paper the hypotheses describing the glucose-lowering mechanisms of bariatric surgical procedures sharing the common feature of a bypass of the duodenum and the proximal jejunum are outlined and a possible role for glucagon in these is proposed.

Receptor-mediated activation of gastric vagal afferents by glucagon-like peptide-1 in the rat

The vagus nerve plays a role in mediating effects of the two glucagon-like peptides GLP-1 and GLP-2 on gastrointestinal growth, functions and eating behaviour. To obtain electrophysiological and molecular evidence for the contribution of afferent pathways in chemoreception from the gastrointestinal tract, afferent mass activity in the ventral gastric branch of the vagus nerve and gene expression of GLP-1 receptors and GLP-2 receptors in the nodose ganglion were examined in Sprague-Dawley rats. Intravenous administration of GLP-1 (30-1000 pmol kg(-1)), reaching high physiological plasma concentrations, increased vagal afferent mass activity peaking (13-52% above basal level, P < 0.05) 3-5 min after injection. Repeated administration of GLP-1 (1000 pmol kg(-1); five times, 15 min intervals) elicited similar responses. Pretreatment with GLP-1 receptor antagonist exendin(9-39)amide (500 pmol kg(-1)) abolished the GLP-1 response to doses 30-300 pmol kg(-1) but had no effect on the vagal response to gastric distension. For comparison, GLP-2 (1000 pmol kg(-1)) had no effect on vagal afferent activity. Vagal chemoreception of GLP-1 is supported by expression of the GLP-1 receptor gene in the nodose ganglion. However, the GLP-2 receptor was also expressed. To conclude, our results show that peripherally administered GLP-1, differently from GLP-2, activates vagal afferents, with no evidence of desensitisation. The GLP-1 effect was blocked by exendin(9-39)amide, suggesting that GLP-1 receptors on vagal afferent nerves mediate sensory input from the gastrointestinal tract or pancreas; either directly or indirectly via the release of another mediator. GLP-2 receptors appear not be functionally expressed on vagal afferents.

Glucagon-related peptide 1 (GLP-1): hormone and neurotransmitter

The interest in glucagon-like petide-1 (GLP-1) and other pre-proglucagon derived peptides has risen almost exponentially since seminal papers in the early 1990s proposed to use GLP-1 agonists as therapeutic agents for treatment of type 2 diabetes. A wealth of interesting studies covering both normal and pathophysiological role of GLP-1 have been published over the last two decades and our understanding of GLP-1 action has widened considerably. In the present review, we have tried to cover our current understanding of GLP-1 actions both as a peripheral hormone and as a central neurotransmitter. From an initial focus on glycaemic control, GLP-1 research has been diverted to study its role in energy homeostasis, neurodegeneration, cognitive functions, anxiety and many more functions. With the upcoming introduction of GLP-1 agonists on the pharmaceutical venue, we have witnessed an outstanding example of how initial ideas from basic science laboratories have paved their way to become a novel therapeutic strategy to fight diabetes.

Inhibition of lipid absorption as an approach to the treatment of obesity

A reduction in fat intake may be achieved by making educated choices to reduce total calorie intake, to consume a lower quantity of total fats, or to modify the ratio of saturated-to-polyunsaturated lipids. Leptin agonists or NPY or CCK antagonists may prove to be useful to diminish appetite and thereby reduce the total intake of food. But eating has such cultural, social, and hedonistic attributes that such a single-pronged approach is unlikely to be successful. The use of fat substitutes may prove to be popular to provide a wide range of snack food options, but these are likely to be of minimal use in weight reduction programs because of their distribution of additives in only a limited number of foods. The inhibitors of lipid digestion will be modestly successful in the short term; their long-term success will be influenced by gastrointestinal adverse effects and the need to consume fat-soluble vitamin supplements to prevent the development of fat-soluble vitamin deficiencies. The inhibition of lipid absorption is an attractive targeted approach for the treatment of obesity, since this would reduce the uptake of visible as well as invisible fats, which would potentially offer convenient dosing, and could also be a means to inhibit secondarily the uptake of carbohydrate calories.

Glucagon-like peptide-1, a new hormone of the entero-insular axis

The post-translational processing of proglucagon in the small intestine gives rise to glucagon-like peptide-1 (PG 78-107 amide) which has profound effects on the endocrine pancreas, and in many species also on the stomach. Glucagon-like peptide-1 (PG 78-107 amide) is secreted in man in response to physiological stimuli e.g. a mixed meal. Glucagon-like peptide-1, in concentrations corresponding to those observed in response to meals, strongly stimulates insulin secretion, in all mammals studied, even more potently than the gastric inhibitory peptide. Thus, glucagon-like peptide-1 fulfills the classic criteria for being a hormone and is likely to be a new incretin. The glucagon inhibitory effect of glucagon-like peptide-1 (PG 78-107 amide) probably further potentiates the effect of glucagon-like peptide-1 on glucose metabolism and distinguished this peptide from other intestinal peptides which have been proposed as incretins. Glucagon-like peptide-1 also inhibits gastric acid secretion and gastric emptying in man. The latter delays nutrient entry to the intestine and thereby diminishes meal-induced glucose excursions. Elevated plasma concentrations of immunoreactive glucagon-like peptide-1 have been reported in Type 2 (noninsulin-dependent) diabetic patients, however, the consequences of the elevation are not yet known. However, elevated levels of glucagon-like peptide-1 in patients with increased gastric emptying rate (post-gastrectomy syndromes) may be responsible for the exaggerated insulin secretion seen in these patients.

Morphologic and physiologic studies of canine ileal enteroglucagon-containing cells in short-term culture

Enteroglucagon-containing cells have been maintained in short-term culture, and the morphologic characteristics of these cells and their response to selected agents have been determined. After 48 h in culture the ultrastructural appearance of the enteroglucagon-immunoreactive cells showed evidence of polarization with re-formation of apical microvilli and the secretory granules concentrated at the opposite pole of the cell. The size of the intracellular secretory granules was 370 +/- 15 nm. The release of enteroglucagonlike immunoreactivity was stimulated in a dose-dependent manner by the adrenergic agonists epinephrine and isoproterenol. The response to epinephrine was competitively inhibited by propranolol, producing a rightward shift of the dose-responsive curve. The alpha-adrenergic agonists methoxamine and clonidine did not stimulate enteroglucagon release above basal. The adenyl cyclase activator forskolin also stimulated release of the peptide in a dose-dependent manner. Carbachol and somatostatin produced a dose-dependent inhibition of epinephrine-stimulated release, indicating direct inhibitory modulation of enteroglucagonlike immunoreactive cells. Somatostatin also inhibited forskolin-stimulated release. These data indicate that canine ileal enteroglucagon cells in short-term culture respond to a number of specific stimuli.

Peptide YY release by fatty acids is sufficient to inhibit gastric emptying in dogs

Peptide YY is a candidate enterogastrone localized to endocrine cells of the ileocolonic mucosa. The purpose of the present study was to determine if blood levels of peptide YY observed after perfusion of the intestine with fatty acids are capable of slowing gastric emptying. Gastric emptying of a 300-ml saline meal was monitored during intravenous infusion of normal saline or graded doses of peptide YY. Gastric emptying was significantly inhibited by infusion of peptide YY in doses of 200 and 400 pmol/kg X h. During the saline control study, 229 +/- 12 ml of the 300-ml saline meal emptied by 10 min. This figure was reduced (p less than 0.01) to 110 +/- 28 ml by the infusion of peptide YY at a dose of 200 pmol/kg X h. This dose of peptide YY produced plasma concentrations (delta PYY = 239 +/- 50 pM) that were lower than those seen in response to intestinal perfusion of oleic acid (delta PYY = 395 +/- 55 pM) in the same animals. We conclude that perfusion of the intestine with oleic acid releases peptide YY in amounts sufficient to slow gastric emptying.

Influence of somatostatin and bombesin on plasma enteroglucagon and cell proliferation after intestinal resection in the rat

The possible relationship between enteroglucagon and cellular proliferation in a rat model of intestinal adaptation after suppression and stimulation of enteroglucagon by somatostatin and bombesin has been investigated. Forty eight rats were divided into three groups of 16 animals, each group being further sub-divided into eight animals having intestinal resection and eight having intestinal transection. Group 1 was given somatostatin to suppress enteroglucagon, group 2 was given bombesin to stimulate enteroglucagon and group 3 (control group) had neither peptide. All animals were killed 12 days after operation. Circulating enteroglucagon and crypt cell production rate (CCPR) in the terminal ileum were measured. After administration of somatostatin (group 1) both CCPR and plasma enteroglucagon were lower after resection than controls (group 3) (p less than 0.001). Transected rats receiving somatostatin showed a reduction in both plasma enteroglucagon and CCPR, but only the fall in enteroglucagon was statistically significant (p less than 0.001). Transected rats receiving bombesin (group 2) had raised plasma enteroglucagon and CCPR compared with the control group (group 3) (P less than 0.005) but there was no significant further rise in these already raised parameters in resected animals. This study indicates that cell proliferation in the rat small bowel after surgery can be influenced by regulatory peptides. The changes in enteroglucagon corresponded closely with changes in CCPR, and this peptide remains a favoured candidate for the humorally mediated trophic influence on the small bowel.

Glucagon and small-bowel mucosa

Numerous functional and structural effects of pharmacological dosages of glucagon on the small-intestinal mucosa have been demonstrated. In addition, clinical conditions associated with elevated concentrations of plasma glucagon may go along with alterations of the intestinal mucosa. The physiological and pathophysiological relevance of these findings, however, is questionable in view of the heterogeneity of the findings, of the complexity of the experimental systems used and of the methodological problems involved. With respect to possible trophic effects on the small-bowel mucosa enteroglucagon is of special importance. Numerous diseases in which increased intestinal mucosal growth has been shown are associated with elevated plasma concentrations of enteroglucagon. Our results concerning radiation damage, the time course of plasma enteroglucagon levels during antimitotic treatment, the small intestinal resection and the experimental blind loop syndrome are discussed. An outlook will be given as to the use of monoclonal antibodies in the development of glucagon as well as enteroglucagon deficiency states for the study of the physiological relevance of these two regulatory peptides.

Adenylate cyclase and H+ production of isolated rat parietal cells in response to glucagon and histamine

The effect of glucagon and its interaction with histamine on adenylate cyclase (AC), cellular cAMP and [14C]aminopyrine ( [14C]AP) uptake, a reliable index of parietal cell H+ production, was studied in isolated rat gastric cells. AC activation in response to glucagon and histamine correlated with the number of parietal cells. Glucagon (10(-10)-10(-6) mol/l) increasingly stimulated AC (maximal effect: 92% by 10(-7) mol/l) and cellular cAMP (86% by 10(-9) mol/l) of fractions enriched with 80% parietal cells but did not cause a pronounced change of the histamine-stimulated enzyme. If there was any interaction, the effect of both hormones was additive. Glucagon neither changed basal [14C]AP uptake nor interfered with that in response to histamine. The data suggest that if glucagon activates a parietal cell AC this process is not followed by parietal cell H+ production. Furthermore, unlike other inhibitors such as somatostatin or PGE2, glucagon does not reduce acid secretion via the cAMP system of the parietal cell.

Circulating glucagon after total pancreatectomy in man

In five totally pancreatectomized human subjects the secretion of gut-derived glucagons was stimulated by ingestion of a meal rich in fat and carbohydrates. Glucagon-like immunoreactivity in plasma, measured with an antiserum against the 6-15 sequence, increased fivefold in response to the meal. Glucagon like immunoreactivity measured with a antiserum against the C-terminal sequence was initially normal (12-13 pmol/l), increased slightly (to 20 pmol/l), and then decreased (to approximately 6 pmol/l). The chromatographic profile of glucagon-like immunoreactivity in plasma at maximum stimulation was studied after concentration by affinity chromatography. Both assay systems identified two peaks (at Kd-values of 0.30 and 0.60-0.65, and 0.30 and 0.70, respectively). The position at Kd 0.70 corresponds to that of glucagon 1-29. The same components may be identified in plasma from normal subjects. It is concluded that the human intestine is capable of generating all of the molecular forms of glucagon which normally are present in plasma.

Salivary gland glucagon is a fictitious substance due to tracer-degrading activity resistant to protease inhibitors

A high level of glucagon immunoreactivity was apparently detected in acid-saline extract from rat submandibular glands, but tracer glucagon added to the assay mixture was mostly damaged in spite of the presence of protease inhibitors commonly used in radioimmunoassay. Gel-filtration of the extract on a Bio-Gel P-10 column revealed strong tracer-degrading activity at the void fraction where the apparent immunoreactivity was eluted. Serial changes in apparent immunoreactivity of the extract fit well on the theoretical curve of an exponential tracer degradation. These findings indicate that the salivary gland glucagon is a fictitious substance due to tracer degradation during radioimmunoassay. Further study revealed that the glucagon molecule was hydrolyzed at the arginyl bonds and split into two fragments during incubation with the acid-saline extract from rat submandibular glands.

Evidence that enteroglucagon (II) is identical with the C-terminal sequence (residues 33-69) of glicentin

Enteroglucagon (II) was isolated from extracts of pig ileum mucosa by repeated gel filtrations, and its immunochemical and chromatographic characteristics were compared with those of a synthetic peptide corresponding to the 33-69 sequence of pig glicentin, before and after digestion with trypsin or trypsin followed by carboxypeptidase B, by using five region-specific assays covering most of the glicentin sequence. Enteroglucagon (II) and the synthetic peptide behave identically under three different conditions of chromatography as determined with all five assays (including a highly specific radioreceptor assay), and gave rise to similar fragments after enzyme digestion. It was therefore concluded that enteroglucagon (II) and the 33-69 sequence of glicentin are most probably identical.

Stimulation of feeding in rats by intraperitoneal injection of antibodies to glucagon

Intraperitoneal injections of antibodies to pancreatic glucagon at the onset of the first meal after 12 hours of food deprivation increased meal size 63 percent and meal duration 74 percent in rats. The antibodies also reduced the increase in hepatic vein blood glucose that occurred during meals in control rats, but did not affect the prandial increase in portal vein blood glucose. These results suggest that, under these conditions, pancreatic glucagon is necessary for the normal termination of meals.

The correlation between gastric emptying time and the response of GIP and enteroglucagon to oral glucose in duodenal ulcer patients

Simultaneous 50-g oral glucose tolerance tests and measurements of gastric emptying time were performed in 11 duodenal ulcer patients. Gastric emptying time, measured by the gamma-camera technique, and the response of gastric inhibitory polypeptide (GIP) and enteroglucagon to the oral load showed a significant negative correlation. The GIP response and the insulinogenic index were significantly positively correlated. It is concluded that the increased GIP and insulin response to glucose among duodenal ulcer patients may be explained by increased gastric emptying, known to occur in these patients. The study has not given new information on the possible physiological role of enteroglucagon.

GIP-like immunoreactivity in glucagon cells. Interactions between GIP and glucagon on insulin release

In the present study the cellular and subcellular distribution of immunoreactive glucagon and GIP (gastric inhibitory polypeptide) were studied in the mouse. Furthermore, the effects of pure GIP and glucagon on basal and stimulated insulin secretion were investigated. Immunohistochemistry revealed that immunoreactive GIP occurred in the pancreatic glucagon cells and in endocrine cells, also displaying glucagon immunoreactivity, scattered along the small and large intestines. Electron immunocytochemistry revealed that the GIP-like material and glucagon coexisted in the secretory granules of the pancreatic glucagon cells. Pure porcine GIP and glucagon both stimulated basal insulin release. When equipotent doses of the peptides were given together, the two peptides antagonized each other's effect. Both peptides potentiated glucose- and carbachol-induced insulin release. When equipotent doses of the two peptides were given together prior to the administration of each of these secretagogues their effects on insulin release were additive.

Nervous control of pancreatic endocrine secretion in pigs. III. The effect of acetylcholine on the pancreatic secretion of insulin and glucagon

We studied the effect of intraarterial administration of acetylcholine on insulin and glucagon secretion in anesthetized splanchnicotomized pigs and on insulin, glucagon, and pancreatic polypeptide secretion from the isolated perfused porcine pancreas and the isolated perfused duodeno-pancreatic block of pigs and dogs. In the pigs acetylcholine stimulated insulin and glucagon secretion in a glucose dependent manner similar to vagal stimulation; however, the response was completely resistant to hexamethonium and abolished by atropine. Acetylcholine stimulated insulin and pancreatic polypeptide secretion of the isolated perfused porcine pancreas, and inhibited glucagon secretion, whether the duodenum was present or not, whereas the glucagon secretion of the isolated perfused canine pancreas was stimulated by acetylcholine.

Simultaneous recording of the gastro-entero-pancreatic hormonal peptide response to food in man

The serum or plasma concentrations of gastrin, gastric inhibitory polypeptide (GIP), gut glucagon-like-immunoreactivity (gut GLI), secretin, vasoactive intestinal polypeptide (VIP), insulin, glucagon, and pancreatic polypeptide (PP) were recorded simultaneously following the ingestion of a normal, mixed meal in seven healthy, normal weight men. The concentrations of PP and gastrin increased within 10 min. Subsequently GIP, insulin, glucagon, and gut GLI increased in the order mentioned. The mean concentrations of secretin and VIP were not affected by the meal, athough transient decreases in secretion concentrations could be detected in all subjects. The concentrations of the other hormones remained elevated for 4 hr or more. Perhaps the period of observation following food stimulation of gastro-entero-pancreatic hormones should be extended.

Evidence that glicentin contains the entire sequence of glucagon

Glicentin (a highly purified 100-amino acid peptide with glucagon-like immunoreactivity from porcine gut) was subjected to limited digestion with trypsin and carboxypeptidase B, and the resulting peptides were studied by gel filtration and region-specific glucagon radioimmunoassays. Similar digests of glucagon and purified fragments of glucagon were studied in parallel. Glicentin gave rise to peptides that corresponded closely to the 1-17 and 19-29 fragments of glucagon. Also, 125I-labelled glicentin and 125I-labelled glucagon gave rise to identical fragments after trypsin treatment. On the basis of this and other evidence [Jacobsen, Demandt, Moody & Sundby (1977) Biochim. Biophys. Acta 493, 452-459] it is concluded that glicentin contains the entire glucagon sequence at residues number 64-92 and thus fulfills one of the requirements for being a 'proglucagon'.

Plasma enteroglucagon after jejunoileal bypass with 3:1 or 1:3 jejunoileal ratio

Enteroglucagon concentration in peripheral blood was determined before and after a test meal in 24 morbidly obese patients. Eighteen had jejunoileal bypass, 6 with a 3:1 and 12 with a 1:3 jejunoileal ratio of the functioning segment, and 6 were unoperated. All three groups exhibited an increment of enteroglucagon concentration after the meal. Both the fasting values and the postprandial integrated increments were higher in operated patients than in unoperated patients and higher after 1:3 bypass than after 3:1 bypass. The findings agree with the hypothesis that enteroglucagon secretion is stimulated by exposure of the lower bowel to upper-bowel content, and that the effect of enteroglucagon is, as seen after bypass operation, stimulation of growth and reduction of motility of the intestine.