Evidence of Extrapancreatic Glucagon Secretion in Man

The Role of Glucagon in the Pathophysiology and Treatment of Type 2 Diabetes

Type 2 diabetes is a disease involving both inadequate insulin levels and increased glucagon levels. While glucagon and insulin work together to achieve optimal plasma glucose concentrations in healthy individuals, the usual regulatory balance between these 2 critical pancreatic hormones is awry in patients with diabetes. Although clinical discussion often focuses on the role of insulin, glucagon is equally important in understanding type 2 diabetes. Furthermore, an awareness of the role of glucagon is essential to appreciate differences in the mechanisms of action of various classes of glucose-lowering therapies. Newer drug classes such as dipeptidyl peptidase-4 inhibitors and glucagon-like peptide-1 receptor agonists improve glycemic control, in part, by affecting glucagon levels. This review provides an overview of the effect of glucose-lowering therapies on glucagon on the basis of an extensive PubMed literature search to identify clinical studies of glucose-lowering therapies in type 2 diabetes that included assessment of glucagon. Clinical practice currently benefits from available therapies that impact the glucagon regulatory pathway. As clinicians look to the future, improved treatment strategies are likely to emerge that will either use currently available therapies whose mechanisms of action complement each other or take advantage of new therapies based on an improved understanding of glucagon pathophysiology.

Enteroendocrine K and L cells in healthy and type 2 diabetic individuals

AIMS/HYPOTHESIS

Enteroendocrine K and L cells are pivotal in regulating appetite and glucose homeostasis. Knowledge of their distribution in humans is sparse and it is unknown whether alterations occur in type 2 diabetes. We aimed to evaluate the distribution of enteroendocrine K and L cells and relevant prohormone-processing enzymes (using immunohistochemical staining), and to evaluate the mRNA expression of the corresponding genes along the entire intestinal tract in individuals with type 2 diabetes and healthy participants.

METHODS

In this cross-sectional study, 12 individuals with type 2 diabetes and 12 age- and BMI-matched healthy individuals underwent upper and lower double-balloon enteroscopy with mucosal biopsy retrieval from approximately every 30 cm of the small intestine and from seven specific anatomical locations in the large intestine.

RESULTS

Significantly different densities for cells positive for chromogranin A (CgA), glucagon-like peptide-1, glucose-dependent insulinotropic polypeptide, peptide YY, prohormone convertase (PC) 1/3 and PC2 were observed along the intestinal tract. The expression of CHGA did not vary along the intestinal tract, but the mRNA expression of GCG, GIP, PYY, PCSK1 and PCSK2 differed along the intestinal tract. Lower counts of CgA-positive and PC1/3-positive cells, respectively, were observed in the small intestine of individuals with type 2 diabetes compared with healthy participants. In individuals with type 2 diabetes compared with healthy participants, the expression of GCG and PYY was greater in the colon, while the expression of GIP and PCSK1 was greater in the small intestine and colon, and the expression of PCSK2 was greater in the small intestine.

CONCLUSIONS/INTERPRETATION

Our findings provide a detailed description of the distribution of enteroendocrine K and L cells and the expression of their products in the human intestinal tract and demonstrate significant differences between individuals with type 2 diabetes and healthy participants.

TRIAL REGISTRATION

NCT03044860.

Glucagon promotes colon cancer cell growth via regulating AMPK and MAPK pathways

Cancer is one of the major causes of death in diabetic patients, and an association between antidiabetic drugs and cancer risk has been reported. Such evidence implies a strong connection between diabetes and cancer. Recently, glucagon has been recognized as a pivotal factor implicated in the pathophysiology of diabetes. Glucagon acts through binding to its receptor, glucagon receptor (GCGR), and cross-talk between GCGR-mediated signals and signaling pathways that regulate cancer cell fate has been unveiled. In the current study, expression of GCGR in colon cancer cell lines and colon cancer tissue obtained from patients was demonstrated. Glucagon significantly promoted colon cancer cell growth, and GCGR knockdown with small interfering RNA attenuated the proliferation-promoting effect of glucagon on colon cancer cells. Molecular assays showed that glucagon acted as an activator of cancer cell growth through deactivation of AMPK and activation of MAPK in a GCGR-dependent manner. Moreover, a stable GCGR knockdown mouse colon cancer cell line, CMT93, grew significantly slower than control in a syngeneic mouse model of type 2 diabetes with glycemia and hyperglucagonemia. The present observations provide experimental evidence that hyperglucagonemia in type 2 diabetes promotes colon cancer progression via GCGR-mediated regulation of AMPK and MAPK pathways.

Decreased α-cell mass and early structural alterations of the exocrine pancreas in patients with type 1 diabetes: An analysis based on the nPOD repository

Background and aims

Abnormal glucagon secretion and functional alterations of the exocrine pancreas have been described in patients with type 1 diabetes (T1D), but their respective anatomical substrata have seldom been investigated. Our aim was to develop an automated morphometric analysis process to characterize the anatomy of α-cell and exocrine pancreas in patients with T1D, using the publicly available slides of the Network for Pancreatic Organ Donors (nPOD).

Materials and methods

The ratio of β- and α-cell area to total tissue area were quantified in 75 patients with T1D (thereafter patients) and 66 control subjects (thereafter controls), on 2 insulin-stained and 4 glucagon-stained slides from both the head and the tail of the pancreas. The β- and α-cell masses were calculated in the 66 patients and the 50 controls for which the pancreas weight was available. Non-exocrine-non-endocrine tissue area (i.e. non-acinar, non-insular tissue) to total tissue area ratio was evaluated on both insulin- and glucagon-stained slides. Results were expressed as mean ±SD.

Results

An automated quantification method was set up using the R software and was validated by quantification of β-cell mass, a well characterized parameter. β-cell mass was 29.6±112 mg in patients and 628 ±717 mg in controls (p<0.0001). α-cell mass was 181±176 mg in patients and 349 ±241mg in controls (p<0.0001). Non-exocrine-non-endocrine area to total tissue area ratio was 39±9% in patients and 29± 10% in controls (p<0.0001) and increased with age in both groups, with no correlation with diabetes duration in patients.

Conclusion

The absolute α-cell mass was lower in patients compared to controls, in proportion to the decrease in pancreas weight observed in patients. Non-exocrine-non-endocrine area to total tissue area ratio increased with age in both groups but was higher in patients at all ages.

Hyperglucagonemia correlates with plasma levels of non-branched-chain amino acids in patients with liver disease independent of type 2 diabetes

Patients with type 2 diabetes (T2D) and patients with nonalcoholic fatty liver disease (NAFLD) frequently exhibit elevated plasma concentrations of glucagon (hyperglucagonemia). Hyperglucagonemia and α-cell hyperplasia may result from elevated levels of plasma amino acids when glucagon's action on hepatic amino acid metabolism is disrupted. We therefore measured plasma levels of glucagon and individual amino acids in patients with and without biopsy-verified NAFLD and with and without type T2D. Fasting levels of amino acids and glucagon in plasma were measured, using validated ELISAs and high-performance liquid chromatography, in obese, middle-aged individuals with I) normal glucose tolerance (NGT) and NAFLD, II) T2D and NAFLD, III) T2D without liver disease, and IV) NGT and no liver disease. Elevated levels of total amino acids were observed in participants with NAFLD and NGT compared with NGT controls (1,310 ± 235 µM vs. 937 ± 281 µM, P = 0.03) and in T2D and NAFLD compared with T2D without liver disease (1,354 ± 329 µM vs. 511 ± 235 µM, P < 0.0001). Particularly amino acids with known glucagonotropic effects (e.g., glutamine) were increased. Plasma levels of total amino acids correlated to plasma levels of glucagon also when adjusting for body mass index (BMI), glycated hemoglobin (Hb_A1c), and cholesterol levels (β = 0.013 ± 0.007, P = 0.024). Elevated plasma levels of total amino acids associate with hyperglucagonemia in NAFLD patients independently of glycemic control, BMI or cholesterol - supporting the potential importance of a "liver-α-cell axis" in which glucagon regulates hepatic amino acid metabolism. Fasting hyperglucagonemia as seen in T2D may therefore represent impaired hepatic glucagon action with increasing amino acids levels. NEW & NOTEWORTHY Hypersecretion of glucagon (hyperglucagonemia) has been suggested to be linked to type 2 diabetes. Here, we show that levels of amino acids correlate with levels of glucagon. Hyperglucagonemia may depend on hepatic steatosis rather than type 2 diabetes.

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

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

Roles of increased glycaemic variability, GLP-1 and glucagon in hypoglycaemia after Roux-en-Y gastric bypass

OBJECTIVE

Roux-en-Y gastric bypass (RYGB) surgery is currently the most effective treatment for diabetes and obesity. An increasingly recognized and highly disabling complication of RYGB is postprandial hypoglycaemia (PPH). The pathophysiology of PPH remains unclear with multiple mechanisms suggested including nesidioblastosis, altered insulin clearance and increased glucagon-like peptide-1 (GLP-1) secretion. Whilst many PPH patients respond to dietary modification, some have severely disabling symptoms. Multiple treatments are proposed, including dietary modification, GLP-1 antagonism, GLP-1 analogues and even surgical reversal, with none showing a more decided advantage over the others. A greater understanding of the pathophysiology of PPH could guide the development of new therapeutic strategies.

METHODS

We studied a cohort of PPH patients at the Imperial Weight Center. We performed continuous glucose monitoring to characterize their altered glycaemic variability. We also performed a mixed meal test (MMT) and measured gut hormone concentrations.

RESULTS

We found increased glycaemic variability in our cohort of PPH patients, specifically a higher mean amplitude glucose excursion (MAGE) score of 4.9. We observed significantly greater and earlier increases in insulin, GLP-1 and glucagon in patients who had hypoglycaemia in response to an MMT (MMT Hypo) relative to those that did not (MMT Non-Hypo). No significant differences in oxyntomodulin, GIP or peptide YY secretion were seen between these two groups.

CONCLUSION

An early peak in GLP-1 and glucagon may together trigger an exaggerated insulinotropic response to eating and consequent hypoglycaemia in patients with PPH.

Suppression of Extrapancreatic Glucagon by Octreotide May Reduce the Fasting and Postprandial Glucose Levels in a Diabetic Patient after Total Pancreatectomy

A 52-year-old woman was treated with sensor augmented pump therapy after undergoing total pancreatectomy for a nonfunctional pancreatic neuroendocrine tumor (NET). The secretion of both endogenous insulin and pancreatic glucagon were completely depleted. Octreotide long acting repeatable (Oct-LAR) was administered for the treatment of liver metastasis of NET. Both the fasting and postprandial glucagon levels decreased immediately after the administration of Oct-LAR. In a continuous glucose monitoring analysis, episodes of nocturnal hypoglycemia was found to increase and an improvement of postprandial hyperglycemia was observed. This case suggests that octreotide may reduce the glucose level in both the fasting and postprandial states, in part by the suppression of extrapancreatic glucagon.

Genetic determinants of circulating GIP and GLP-1 concentrations

The secretion of insulin and glucagon from the pancreas and the incretin hormones glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic peptide (GIP) from the gastrointestinal tract is essential for glucose homeostasis. Several novel treatment strategies for type 2 diabetes (T2D) mimic GLP-1 actions or inhibit incretin degradation (DPP4 inhibitors), but none is thus far aimed at increasing the secretion of endogenous incretins. In order to identify new potential therapeutic targets for treatment of T2D, we performed a meta-analysis of a GWAS and an exome-wide association study of circulating insulin, glucagon, GIP, and GLP-1 concentrations measured during an oral glucose tolerance test in up to 7,828 individuals. We identified 6 genome-wide significant functional loci associated with plasma incretin concentrations in or near the SLC5A1 (encoding SGLT1), GIPR, ABO, GLP2R, F13A1, and HOXD1 genes and studied the effect of these variants on mRNA expression in pancreatic islet and on metabolic phenotypes. Immunohistochemistry showed expression of GIPR, ABO, and HOXD1 in human enteroendocrine cells and expression of ABO in pancreatic islets, supporting a role in hormone secretion. This study thus provides candidate genes and insight into mechanisms by which secretion and breakdown of GIP and GLP-1 are regulated.

Current Therapies That Modify Glucagon Secretion: What Is the Therapeutic Effect of Such Modifications?

PURPOSE OF REVIEW

Hyperglucagonemia contributes significantly to hyperglycemia in type 2 diabetes and suppressed glucagon levels may increase the risk of hypoglycemia. Here, we give a brief overview of glucagon physiology and the role of glucagon in the pathophysiology of type 2 diabetes and provide insights into how antidiabetic drugs influence glucagon secretion as well as a perspective on the future of glucagon-targeting drugs.

RECENT FINDINGS

Several older as well as recent investigations have evaluated the effect of antidiabetic agents on glucagon secretion to understand how glucagon may be involved in the drugs' efficacy and safety profiles. Based on these findings, modulation of glucagon secretion seems to play a hitherto underestimated role in the efficacy and safety of several glucose-lowering drugs. Numerous drugs currently available to diabetologists are capable of altering glucagon secretion: metformin, sulfonylurea compounds, insulin, glucagon-like peptide-1 receptor agonists, dipeptidyl peptidase-4 inhibitors, sodium-glucose cotransporter 2 inhibitors and amylin mimetics. Their diverse effects on glucagon secretion are of importance for their individual efficacy and safety profiles. Understanding how these drugs interact with glucagon secretion may help to optimize treatment.

Defects in α-Cell Function in Patients With Diabetes Due to Chronic Pancreatitis Compared With Patients With Type 2 Diabetes and Healthy Individuals

OBJECTIVE

Diabetes frequently develops in patients with chronic pancreatitis. We examined the alterations in the glucagon response to hypoglycemia and to oral glucose administration in patients with diabetes due to chronic pancreatitis.

RESEARCH DESIGN AND METHODS

Ten patients with diabetes secondary to chronic pancreatitis were compared with 13 patients with type 2 diabetes and 10 healthy control subjects. A stepwise hypoglycemic clamp and an oral glucose tolerance test (OGTT) were performed.

RESULTS

Glucose levels during the OGTT were higher in patients with diabetes and chronic pancreatitis and lower in control subjects (P < 0.0001). Insulin and C-peptide levels were reduced, and the glucose-induced suppression of glucagon was impaired in both groups with diabetes (all P < 0.0001 vs. control subjects). During hypoglycemia, glucagon concentrations were reduced in patients with chronic pancreatitis and with type 2 diabetes (P < 0.05). The increase in glucagon during the clamp was inversely related to the glucose-induced glucagon suppression and positively related to β-cell function. Growth hormone responses to hypoglycemia were lower in patients with type 2 diabetes (P = 0.0002) but not in patients with chronic pancreatitis.

CONCLUSIONS

α-Cell responses to oral glucose ingestion and to hypoglycemia are disturbed in patients with diabetes and chronic pancreatitis and in patients with type 2 diabetes. The similarities between these defects suggest a common etiology.

The role of somatostatin in GLP-1-induced inhibition of glucagon secretion in mice

AIMS/HYPOTHESIS

Glucagon-like peptide-1 (GLP-1) receptor agonists are currently used for the treatment of type 2 diabetes. Their main mechanism of action is enhancement of glucose-induced insulin secretion (from increased beta cell glucose sensitivity) and inhibition of glucagon secretion. The latter has been demonstrated to account for about half of their blood glucose-lowering activity. Whereas the effect of GLP-1 on insulin secretion is clearly dependent on ambient glucose concentrations and has been described in detail, the mechanism responsible for the inhibitory effect of GLP-1 on glucagon secretion is heavily debated. Glucagon inhibition is also said to be glucose-dependent, although it is unclear what is meant by this. We hypothesise here that GLP-1 does not inhibit glucagon secretion during hypoglycaemia because the inhibition depends on somatostatin secretion, which in turn is dependent on glucose levels.

METHODS

We used the perfused mouse pancreas model to investigate this hypothesis.

RESULTS

We found that, in this model, GLP-1 was able to significantly inhibit glucagon secretion from pancreatic alpha cells at all glucose levels tested: 6.0, 1.5 and 0.5 mmol/l (-27.0%, -37.1%, and -23.6%, respectively), and the decrease in glucagon secretion was invariably accompanied by an increase in somatostatin secretion (+286.8%, +158.7%, and +118.8%, respectively). Specific blockade of somatostatin receptor 2 increased glucagon secretion (+118.8% at 1.5 mmol/l glucose and +162.9% at 6.0 mmol/l glucose) and completely eliminated the inhibitory effect of GLP-1.

CONCLUSIONS/INTERPRETATION

We have shown here that the glucagon-lowering effect of GLP-1 is entirely mediated through the paracrine actions of somatostatin in the perfused mouse pancreas. However, in this model, the inhibitory effect of GLP-1 was preserved at hypoglycaemic levels, leaving unanswered the question of how this is avoided in vivo in individuals treated with GLP-1 receptor agonists.

Short-acting glucagon-like peptide-1 receptor agonists as add-on to insulin therapy in type 1 diabetes: A review

A large proportion of patients with type 1 diabetes do not reach their glycaemic target of glycated hemoglobin (HbA1c) <7.0% (53 mmol/mol) and, furthermore, an increasing number of patients with type 1 diabetes are overweight and obese. Treatment of type 1 diabetes is based on insulin therapy, which is associated with well-described and unfortunate adverse effects such as hypoglycaemia and increased body weight. Glucagon-like peptide-1 (GLP-1) receptor agonists (RAs) are the focus of increasing interest as a possible adjunctive treatment to insulin in type 1 diabetes because of their glucagonostatic and extrapancreatic effects. So far, the focus has mainly been on the long-acting GLP-1RAs, but the risk-benefit ratio emerging from studies evaluating the effect of long-acting GLP-1RAs as adjunctive therapy to insulin therapy in patients with type 1 diabetes has been disappointing. This might be attributable to a lack of glucagonostatic effect of these long-acting GLP-1RAs in type 1 diabetes, alongside development of tachyphylaxis to GLP-1-induced retardation of gastric emptying. In contrast, the short-acting GLP-1RAs seem to have a preserved and sustained effect on glucagon secretion and gastric emptying in patients with type 1 diabetes, which could translate into effective lowering of postprandial glucose excursions; however, these observations regarding short-acting GLP-1RAs are all derived from small open-label trials and should thus be interpreted with caution. In the present paper we review the potential role of GLP-1RAs, in particular short-acting GLP-1RAs, as add-on to insulin in the treatment of type 1 diabetes.

Decreased serum glicentin concentration in patients with severe and morbid obesity

Background

Proglucagon-derived hormones represent a family of peptides mainly produced in the pancreas and the intestine. While several proglucagon-derived peptides play key roles in metabolic diseases, little is known about glicentin. The aim of the present study was to investigate serum glicentin concentrations in individuals with adult obesity and to study its potential link with various metabolic parameters.

Methods

Fifty-two individuals with normal body mass index (BMI < 25 kg/m2) and 39 patients with severe or morbid obesity (BMI > 35 kg/m2) were prospectively included at the University Hospital of Nice between January 2014 and April 2016. Clinical data were recorded, and a fasting blood sample was collected to measure glicentin, glucose, insulin, C-peptide, total cholesterol, triglyceride, LDL and HDL-cholesterol. In addition, a homeostasis model assessment for insulin resistance (HOMA2-IR) was also calculated.

Results

Patients with severe and morbid obesity had significantly higher plasma glucose, together with higher serum concentrations of insulin, C-peptide, HOMA2-IR, triglyceride, LDL-cholesterol and lower serum concentrations of HDL-cholesterol compared with individuals with a normal body mass index. The obese patients displayed significantly lower fasting serum concentrations of glicentin compared with subjects with a normal body mass index (12 pmol/L vs. 24 pmol/L, P < 0.0001). In the total population, fasting glicentin concentrations did not correlate with BMI, glycaemic parameters (glucose, insulin, C-peptide, HOMA-IR) or lipid parameters (total cholesterol, triglyceride, LDL and HDL-cholesterol).

Conclusion

To the best of our knowledge, this is the first study reporting serum glicentin concentrations in healthy lean and obese adult subjects. We found that fasting serum glicentin concentrations are decreased in patients with severe or morbid obesity suggesting the potential interest of this peptide in obesity and metabolic-related disorders.

The Gut: A Key to the Pathogenesis of Type 2 Diabetes?

In this communication we discuss the role of the gut for the development of type 2 diabetes mellitus (T2DM). Gastric emptying rates importantly determine postprandial glucose excursions and regulate postprandial secretion of the incretin hormones, glucose-dependent insulinotropic polypeptide (GIP), and glucagon-like peptide-1 (GLP-1). It thereby also determines their powerful, amplifying effect on glucose-induced insulin secretion and thus the ability of the body to regulate glucose disposal. Although disturbances in gastric emptying are not consistent findings in type 2 diabetes, the incretin system is seriously impaired, probably associated with insulin resistance and obesity. Both of the incretin hormones lose (part of) their insulinotropic activity resulting, together with (genetically) defective beta cell function, in the impaired postprandial insulin secretion of T2DM. In addition, glucagon responses are inappropriately increased and importantly contribute to both fasting and postprandial hyperglycemia. This may involve stimulation by GIP, but evidence also points to a role of circulating amino acids, which are elevated due to steatosis-induced impaired glucagon-mediated hepatic clearance, in line with recent work suggesting that the alpha cells and the liver are linked in a close, amino acid-mediated feedback circuit. Thus, the gut plays an important role in the development of T2DM spurred by overeating and defective beta cells.

A 4-week study assessing the pharmacokinetics, pharmacodynamics, safety, and tolerability of the glucagon receptor antagonist PF-06291874 administered as monotherapy in subjects with type 2 diabetes mellitus

AIMS

The glucagon receptor antagonist PF-06291874 has demonstrated robust glucose reductions in subjects with type 2 diabetes mellitus (T2DM) on background metformin. This study assessed the pharmacokinetics, pharmacodynamics, safety, and tolerability of PF-06291874 administered as monotherapy in subjects with T2DM.

METHODS

After a ≥4-week antidiabetic therapy washout period, 172 subjects were randomized to placebo or PF-06291874 15, 35, 75, or 150mg once daily for 28days. Mean daily glucose (MDG), fasting plasma glucose (FPG), and predefined safety endpoints were assessed at baseline and day 28.

RESULTS

Dose-dependent reductions (placebo-adjusted) from baseline in MDG ranged from 40.3 to 68.8mg/dL and in FPG from 27.1 to 57.2mg/dL after 28days of dosing with PF-06291874. There were no significant changes in low-density lipoprotein cholesterol at doses ≤75mg relative to placebo. Small, dose-dependent increases in alanine aminotransferase and aspartate aminotransferase were observed; however, the incidence of these values >3×upper limit of normal was similar across doses. PF-06291874 exposures were consistent with previous studies and PF-06291874 was well tolerated, with minimal incidence of hypoglycemia.

CONCLUSIONS

PF-06291874 as monotherapy was well tolerated and produced robust reductions in plasma glucose following 4weeks of dosing in subjects with T2DM.

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.

Patients With Long-QT Syndrome Caused by Impaired hERG-Encoded Kv11.1 Potassium Channel Have Exaggerated Endocrine Pancreatic and Incretin Function Associated With Reactive Hypoglycemia

Supplemental Digital Content is available in the text.

Background:

Loss-of-function mutations in hERG (encoding the K_v11.1 voltage-gated potassium channel) cause long-QT syndrome type 2 (LQT2) because of prolonged cardiac repolarization. However, K_v11.1 is also present in pancreatic α and β cells and intestinal L and K cells, secreting glucagon, insulin, and the incretins glucagon-like peptide-1 (GLP-1) and GIP (glucose-dependent insulinotropic polypeptide), respectively. These hormones are crucial for glucose regulation, and long-QT syndrome may cause disturbed glucose regulation. We measured secretion of these hormones and cardiac repolarization in response to glucose ingestion in LQT2 patients with functional mutations in hERG and matched healthy participants, testing the hypothesis that LQT2 patients have increased incretin and β-cell function and decreased α-cell function, and thus lower glucose levels.

Methods:

Eleven patients with LQT2 and 22 sex-, age-, and body mass index–matched control participants underwent a 6-hour 75-g oral glucose tolerance test with ECG recording and blood sampling for measurements of glucose, insulin, C-peptide, glucagon, GLP-1, and GIP.

Results:

In comparison with matched control participants, LQT2 patients had 56% to 78% increased serum insulin, serum C-peptide, plasma GLP-1, and plasma GIP responses (P=0.03–0.001) and decreased plasma glucose levels after glucose ingestion (P=0.02) with more symptoms of hypoglycemia (P=0.04). Sixty-three percent of LQT2 patients developed hypoglycemic plasma glucose levels (<70 mg/dL) versus 36% control participants (P=0.16), and 18% patients developed serious hypoglycemia (<50 mg/dL) versus none of the controls. LQT2 patients had defective glucagon responses to low glucose, P=0.008. β-Cell function (Insulin Secretion Sensitivity Index-2) was 2-fold higher in LQT2 patients than in controls (4398 [95% confidence interval, 2259–8562] versus 2156 [1961–3201], P=0.03). Pharmacological K_v11.1 blockade (dofetilide) in rats had similar effect, and small interfering RNA inhibition of hERG in β and L cells increased insulin and GLP-1 secretion up to 50%. Glucose ingestion caused cardiac repolarization disturbances with increased QTc intervals in both patients and controls, but with a 122% greater increase in QTcF interval in LQT2 patients (P=0.004).

Conclusions:

Besides a prolonged cardiac repolarization phase, LQT2 patients display increased GLP-1, GIP, and insulin secretion and defective glucagon secretion, causing decreased plasma glucose and thus increased risk of hypoglycemia. Furthermore, glucose ingestion increased QT interval and aggravated the cardiac repolarization disturbances in LQT2 patients.

Clinical Trial Registration:

URL: http://clinicaltrials.gov. Unique identifier: NCT02775513.

Chenodeoxycholic acid stimulates glucagon-like peptide-1 secretion in patients after Roux-en-Y gastric bypass

Postprandial secretion of glucagon‐like peptide‐1 (GLP‐1) is enhanced after Roux‐en‐Y gastric bypass (RYGB), but the precise molecular mechanisms explaining this remain poorly understood. Plasma concentrations of bile acids (BAs) increase after RYGB, and BAs may act as molecular enhancers of GLP‐1 secretion through activation of TGR5‐receptors. We aimed to evaluate GLP‐1 secretion after oral administration of the primary bile acid chenodeoxycholic acid (CDCA) and the secondary bile acid ursodeoxycholic acid (UDCA) (which are available for oral use) in RYGB‐operated participants. Eleven participants (BMI 29.1 ± 1.2, age 37.0 ± 3.2 years, time from RYGB 32.3 ± 1.1 months, weight loss after RYGB 37.0 ± 3.1 kg) were studied in a placebo‐controlled, crossover‐study. On three different days, participants ingested (1) placebo (water), (2) UDCA 750 mg, (3) CDCA 1250 mg (highest recommended doses). Oral intake of CDCA increased plasma concentrations of GLP‐1, C‐peptide, glucagon, peptide YY, neurotensin, total bile acids, and fibroblast growth factor 19 significantly compared with placebo (all P < 0.05 for peak and positive incremental area‐under‐the‐curve (piAUC)). All plasma hormone concentrations were unaffected by UDCA. Neither UDCA nor CDCA changed glucose, cholecystokinin or glucose‐dependent insulinotropic polypeptide (GIP) concentrations. In conclusion, our findings demonstrate that the primary bile acid chenodeoxycholic acid is able to enhance secretion of gut hormones when administered orally in RYGB‐operated patients—even in the absence of nutrients.

Glucagon and Amino Acids Are Linked in a Mutual Feedback Cycle: The Liver-α-Cell Axis

Glucagon is usually viewed as an important counterregulatory hormone in glucose metabolism, with actions opposing those of insulin. Evidence exists that shows glucagon is important for minute-to-minute regulation of postprandial hepatic glucose production, although conditions of glucagon excess or deficiency do not cause changes compatible with this view. In patients with glucagon-producing tumors (glucagonomas), the most conspicuous signs are skin lesions (necrolytic migratory erythema), while in subjects with inactivating mutations of the glucagon receptor, pancreatic swelling may be the first sign; neither condition is necessarily associated with disturbed glucose metabolism. In glucagonoma patients, amino acid turnover and ureagenesis are greatly accelerated, and low plasma amino acid levels are probably at least partly responsible for the necrolytic migratory erythema, which resolves after amino acid administration. In patients with receptor mutations (and in knockout mice), pancreatic swelling is due to α-cell hyperplasia with gross hypersecretion of glucagon, which according to recent groundbreaking research may result from elevated amino acid levels. Additionally, solid evidence indicates that ureagenesis, and thereby amino acid levels, is critically controlled by glucagon. Together, this constitutes a complete endocrine system; feedback regulation involving amino acids regulates α-cell function and secretion, while glucagon, in turn, regulates amino acid turnover.

Small Intestinal Glucose Delivery Affects the Lowering of Blood Glucose by Acute Vildagliptin in Type 2 Diabetes

CONTEXT

The rate of gastric emptying is an important determinant of glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) secretion and may influence the magnitude of glucose lowering by dipeptidyl peptidase-4 (DPP-4) inhibitors.

OBJECTIVE

To evaluate the effects of the DPP-4 inhibitor, vildagliptin (VILD), during intraduodenal (ID) glucose infusion at 2 different rates within the physiological range of gastric emptying, in type 2 diabetes.

PARTICIPANTS AND DESIGN

A total of 16 diet-controlled type 2 diabetic patients were studied on 4 separate days in double-blind, randomized, fashion. On each day, either 5-mg VILD or placebo (PLBO) was given 60 minutes before a 120-minute ID glucose infusion at 2 or 4 kcal/min (ID2 or ID4). Plasma glucose and hormones were measured frequently.

RESULTS

Plasma glucose, insulin, C-peptide, glucagon, total GIP, and total and intact GLP-1 concentrations were higher during ID4 than ID2 (P < .01 for each). Compared with PLBO, VILD was associated with higher intact GLP-1, insulin, and C-peptide and lower glucose and total GIP and GLP-1 (P < .01 for each), without affecting glucagon. There were significant interactions between the rate of ID glucose and VILD treatment on plasma glucose, intact and total GLP-1, and GIP (P < .05 for each) but not insulin, C-peptide, or glucagon. The reduction in glucose and the increment in intact GLP-1 after VILD vs PLBO were 3.3- and 3.8-fold greater, respectively, during ID4 compared with ID2.

CONCLUSIONS/INTERPRETATION

These observations warrant further study to clarify whether type 2 diabetic patients with relatively more rapid gastric emptying have greater glucose lowering during treatment with DPP-4 inhibitors.

Vascular Biology of Glucagon Receptor Superfamily Peptides: Mechanistic and Clinical Relevance

Regulatory peptides produced in islet and gut endocrine cells, including glucagon, glucagon-like peptide-1 (GLP-1), GLP-2, and glucose-dependent insulinotropic polypeptide, exert actions with considerable metabolic importance and translational relevance. Although the clinical development of GLP-1 receptor agonists and dipeptidyl peptidase-4 inhibitors has fostered research into how these hormones act on the normal and diseased heart, less is known about the actions of these peptides on blood vessels. Here we review the effects of these peptide hormones on normal blood vessels and highlight their vascular actions in the setting of experimental and clinical vascular injury. The cellular localization and signal transduction properties of the receptors for glucagon, GLP-1, GLP-2, and glucose-dependent insulinotropic polypeptide are discussed, with emphasis on endothelial cells and vascular smooth muscle cells. The actions of these peptides on the control of blood flow, blood pressure, angiogenesis, atherosclerosis, and vascular inflammation are reviewed with a focus on elucidating direct and indirect mechanisms of action. How these peptides traverse the blood-brain barrier is highlighted, with relevance to the use of GLP-1 receptor agonists to treat obesity and neurodegenerative disorders. Wherever possible, we compare actions identified in cell lines and primary cell culture with data from preclinical studies and, when available, results of human investigation, including studies in subjects with diabetes, obesity, and cardiovascular disease. Throughout the review, we discuss pitfalls, limitations, and challenges of the existing literature and highlight areas of controversy and uncertainty. The increasing use of peptide-based therapies for the treatment of diabetes and obesity underscores the importance of understanding the vascular biology of peptide hormone action.

Higher Endogenous Glucose Production During OGTT vs Isoglycemic Intravenous Glucose Infusion

CONTEXT

Oral glucose ingestion elicits a larger insulin response and delayed suppression of glucagon compared to isoglycemic IV glucose infusion (IIGI).

OBJECTIVE

We studied whether these differences translate into effects on endogenous glucose production (EGP) and glucose disposal in patients with type 2 diabetes and nondiabetic control subjects.

DESIGN

This was a single-blinded, randomized, crossover study.

SETTING

The study was conducted at a specialized research unit.

PARTICIPANTS

Ten patients with type 2 diabetes (age, [mean ± SD] 57.1 ± 6.7 years; body mass index, 29.0 ± 4.3 kg/m²; hemoglobin A_1c, 53.8 ± 11.0 mmol/mol; duration of diabetes, 9.2 ± 5.0 years) and 10 matched nondiabetic control subjects (age, 56.0±10.7 years; body mass index, 29.8 ± 2.9 kg/m²; hemoglobin A_1c, 33.8 ± 5.5 mmol/mol) participated.

INTERVENTIONS

Three experimental days: 75 g-oral glucose tolerance test (OGTT), IIGI, and IIGI+glucagon (IIGI with a concomitant IV glucagon infusion [0.8 ng/kg/min from 0 to 25 minutes] designed to mimic portal glucagon concentrations during OGTT in the type 2 diabetic group) were undertaken.

MAIN OUTCOME MEASURES

Glucose kinetics were assessed by tracer methodology.

RESULTS

Glucose rate of disappearance was higher during the OGTT vs IIGI in the control group, but similar on all days in the diabetic group. Surprisingly, in both groups, EGP was more suppressed during IIGI than during OGTT, and exogenous glucagon infusion during IIGI did not restore EGP to the levels observed during OGTT.

CONCLUSION

EGP was less suppressed during OGTT than during IIGI in both patients with type 2 diabetes and in nondiabetic control subjects. Based on the present experimental design, it was not possible to attribute this difference to the delayed glucagon suppression observed in the initial phase of the OGTT.

Insulin Resistance Is Accompanied by Increased Fasting Glucagon and Delayed Glucagon Suppression in Individuals With Normal and Impaired Glucose Regulation

Hyperinsulinemia is an adaptive mechanism that enables the maintenance of normoglycemia in the presence of insulin resistance. We assessed whether glucagon is also involved in the adaptation to insulin resistance. A total of 1,437 individuals underwent an oral glucose tolerance test with measurements of circulating glucose, insulin, and glucagon concentrations at 0, 30 and 120 min. Early glucagon suppression was defined as suppression in the period from 0 to 30 min, and late glucagon suppression as 30 to 120 min after glucose intake. Insulin sensitivity was estimated by the validated insulin sensitivity index. Individuals with screen-detected diabetes had 30% higher fasting glucagon levels and diminished early glucagon suppression, but greater late glucagon suppression when compared with individuals with normal glucose tolerance (P ≤ 0.014). Higher insulin resistance was associated with higher fasting glucagon levels, less early glucagon suppression, and greater late glucagon suppression (P < 0.001). The relationship between insulin sensitivity and fasting glucagon concentrations was nonlinear (P < 0.001). In conclusion, increased fasting glucagon levels and delayed glucagon suppression, together with increased circulating insulin levels, develop in parallel with insulin resistance. Therefore, glucose maintenance during insulin resistance may depend not only on hyperinsulinemia but also on the ability to suppress glucagon early after glucose intake.

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.