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Nutrient-Induced Cellular Mechanisms of Gut Hormone Secretion.
Lu, VB, Gribble, FM, Reimann, F
Nutrients. 2021;(3)
Abstract
The gastrointestinal tract can assess the nutrient composition of ingested food. The nutrient-sensing mechanisms in specialised epithelial cells lining the gastrointestinal tract, the enteroendocrine cells, trigger the release of gut hormones that provide important local and central feedback signals to regulate nutrient utilisation and feeding behaviour. The evidence for nutrient-stimulated secretion of two of the most studied gut hormones, glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), along with the known cellular mechanisms in enteroendocrine cells recruited by nutrients, will be the focus of this review. The mechanisms involved range from electrogenic transporters, ion channel modulation and nutrient-activated G-protein coupled receptors that converge on the release machinery controlling hormone secretion. Elucidation of these mechanisms will provide much needed insight into postprandial physiology and identify tractable dietary approaches to potentially manage nutrition and satiety by altering the secreted gut hormone profile.
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Potential for Gut Peptide-Based Therapy in Postprandial Hypotension.
Borg, MJ, Xie, C, Rayner, CK, Horowitz, M, Jones, KL, Wu, T
Nutrients. 2021;(8)
Abstract
Postprandial hypotension (PPH) is an important and under-recognised disorder resulting from inadequate compensatory cardiovascular responses to meal-induced splanchnic blood pooling. Current approaches to management are suboptimal. Recent studies have established that the cardiovascular response to a meal is modulated profoundly by gastrointestinal factors, including the type and caloric content of ingested meals, rate of gastric emptying, and small intestinal transit and absorption of nutrients. The small intestine represents the major site of nutrient-gut interactions and associated neurohormonal responses, including secretion of glucagon-like peptide-1, glucose-dependent insulinotropic peptide and somatostatin, which exert pleotropic actions relevant to the postprandial haemodynamic profile. This review summarises knowledge relating to the role of these gut peptides in the cardiovascular response to a meal and their potential application to the management of PPH.
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Evidence for the existence and potential roles of intra-islet glucagon-like peptide-1.
Campbell, SA, Johnson, J, Light, PE
Islets. 2021;(1-2):32-50
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Abstract
Glucagon-Like Peptide-1 (GLP-1) is an important peptide hormone secreted by L-cells in the gastrointestinal tract in response to nutrients. It is produced by the differential cleavage of the proglucagon peptide. GLP-1 elicits a wide variety of physiological responses in many tissues that contribute to metabolic homeostasis. For these reasons, therapies designed to either increase endogenous GLP-1 levels or introduce exogenous peptide mimetics are now widely used in the management of diabetes. In addition to GLP-1 production from L-cells, recent reports suggest that pancreatic islet alpha cells may also synthesize and secrete GLP-1. Intra-islet GLP-1 may therefore play an unappreciated role in islet health and glucose regulation, suggesting a potential functional paracrine role for islet-derived GLP-1. In this review, we assess the current literature from an islet-centric point-of-view to better understand the production, degradation, and actions of GLP-1 within the endocrine pancreas in rodents and humans. The relevance of intra-islet GLP-1 in human physiology is discussed regarding the potential role of intra-islet GLP-1 in islet health and dysfunction.
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Gut-Based Strategies to Reduce Postprandial Glycaemia in Type 2 Diabetes.
Kamruzzaman, M, Horowitz, M, Jones, KL, Marathe, CS
Frontiers in endocrinology. 2021;:661877
Abstract
Postprandial glycemic control is an important target for optimal type 2 diabetes management, but is often difficult to achieve. The gastrointestinal tract plays a major role in modulating postprandial glycaemia in both health and diabetes. The various strategies that have been proposed to modulate gastrointestinal function, particularly by slowing gastric emptying and/or stimulating incretin hormone GLP-1, are summarized in this review.
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5.
GLP-1 and insulin regulation of skeletal and cardiac muscle microvascular perfusion in type 2 diabetes.
Love, KM, Liu, J, Regensteiner, JG, Reusch, JEB, Liu, Z
Journal of diabetes. 2020;(7):488-498
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Abstract
Muscle microvasculature critically regulates skeletal and cardiac muscle health and function. It provides endothelial surface area for substrate exchange between the plasma compartment and the muscle interstitium. Insulin fine-tunes muscle microvascular perfusion to regulate its own action in muscle and oxygen and nutrient supplies to muscle. Specifically, insulin increases muscle microvascular perfusion, which results in increased delivery of insulin to the capillaries that bathe the muscle cells and then facilitate its own transendothelial transport to reach the muscle interstitium. In type 2 diabetes, muscle microvascular responses to insulin are blunted and there is capillary rarefaction. Both loss of capillary density and decreased insulin-mediated capillary recruitment contribute to a decreased endothelial surface area available for substrate exchange. Vasculature expresses abundant glucagon-like peptide 1 (GLP-1) receptors. GLP-1, in addition to its well-characterized glycemic actions, improves endothelial function, increases muscle microvascular perfusion, and stimulates angiogenesis. Importantly, these actions are preserved in the insulin resistant states. Thus, treatment of insulin resistant patients with GLP-1 receptor agonists may improve skeletal and cardiac muscle microvascular perfusion and increase muscle capillarization, leading to improved delivery of oxygen, nutrients, and hormones such as insulin to the myocytes. These actions of GLP-1 impact skeletal and cardiac muscle function and systems biology such as functional exercise capacity. Preclinical studies and clinical trials involving the use of GLP-1 receptor agonists have shown salutary cardiovascular effects and improved cardiovascular outcomes in type 2 diabetes mellitus. Future studies should further examine the different roles of GLP-1 in cardiac as well as skeletal muscle function.
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The incretin system in healthy humans: The role of GIP and GLP-1.
Holst, JJ
Metabolism: clinical and experimental. 2019;:46-55
Abstract
The incretin effect, the amplification of insulin secretion occurring when glucose is taken in orally as compared to infused intravenously, is one of the factors that help the body to tolerate carbohydrate/glucose ingestion. These include 1) amount and type of carbohydrates; 2) gastric emptying rate; 3) digestion and absorption of the carbohydrates; 4) secretion and effect of the incretin hormones; 5) disposition of absorbed nutrients/glucose. The incretin effect can also be viewed as the fraction of the ingested glucose load handled via gastrointestinal mechanisms (including the incretin effect); it is calculated by comparison of the amount of glucose required to copy, by intravenous infusion, the oral load. Typically, for 75 g of oral glucose, about 25 g are required. This means that the GastroIntestinal Glucose Disposal (GIGD) is 66%. Both the GIGD and the incretin effect depend on the amount of glucose ingested: for higher doses the GIGD may amount to 80%, which shows that this effect is a major contributor to glucose tolerance. The main mechanism behind it is stimulation of insulin secretion by a proportional secretion of the insulinotropic hormones GIP and GLP-1. Recently it has become possible to estimate their contributions in healthy humans using specific and potent receptor antagonists. Both hormones act to improve glucose tolerance (i.e. the antagonists impair tolerance) and their effects are additive. GIP seems to be quantitatively the most important, particularly regarding insulin secretion, whereas the action of GLP-1 is mainly displayed via inhibition of glucagon secretion.
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The effect of sodium-glucose cotransporter 2 inhibitors and glucagon-like peptide 1 agonists on cardiovascular disease in patients with type 2 diabetes.
Dey, AK, Groenendyk, J, Mehta, NN, Gourgari, E
Clinical cardiology. 2019;(3):406-412
Abstract
Patients with type 2 diabetes have a significantly increased risk of cardiovascular disease (CVD) compared to the general population-with CVD accounting for two out of every three deaths in patients with diabetes. In 2008, the FDA suggested that CVD risk should be evaluated for any new antidiabetic therapy, leading to a multitude of large CVD outcome trials to assess CVD risk from these medications. Interestingly, several of these outcome trials with new novel antidiabetic therapies have demonstrated a clear and definite CVD advantage at mid-term follow up in high-risk patients with T2DM. In this review, we discuss two relatively new classes of diabetic drugs, sodium-glucose cotransporter 2 inhibitors and glucagon-like peptide 1 agonists, and their efficacy in improving cardiovascular outcomes.
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Ghrelin, CCK, GLP-1, and PYY(3-36): Secretory Controls and Physiological Roles in Eating and Glycemia in Health, Obesity, and After RYGB.
Steinert, RE, Feinle-Bisset, C, Asarian, L, Horowitz, M, Beglinger, C, Geary, N
Physiological reviews. 2017;(1):411-463
Abstract
The efficacy of Roux-en-Y gastric-bypass (RYGB) and other bariatric surgeries in the management of obesity and type 2 diabetes mellitus and novel developments in gastrointestinal (GI) endocrinology have renewed interest in the roles of GI hormones in the control of eating, meal-related glycemia, and obesity. Here we review the nutrient-sensing mechanisms that control the secretion of four of these hormones, ghrelin, cholecystokinin (CCK), glucagon-like peptide-1 (GLP-1), and peptide tyrosine tyrosine [PYY(3-36)], and their contributions to the controls of GI motor function, food intake, and meal-related increases in glycemia in healthy-weight and obese persons, as well as in RYGB patients. Their physiological roles as classical endocrine and as locally acting signals are discussed. Gastric emptying, the detection of specific digestive products by small intestinal enteroendocrine cells, and synergistic interactions among different GI loci all contribute to the secretion of ghrelin, CCK, GLP-1, and PYY(3-36). While CCK has been fully established as an endogenous endocrine control of eating in healthy-weight persons, the roles of all four hormones in eating in obese persons and following RYGB are uncertain. Similarly, only GLP-1 clearly contributes to the endocrine control of meal-related glycemia. It is likely that local signaling is involved in these hormones' actions, but methods to determine the physiological status of local signaling effects are lacking. Further research and fresh approaches are required to better understand ghrelin, CCK, GLP-1, and PYY(3-36) physiology; their roles in obesity and bariatric surgery; and their therapeutic potentials.
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Dipeptidyl-peptidase (DPP)-4 inhibitors and glucagon-like peptide (GLP)-1 analogues for prevention or delay of type 2 diabetes mellitus and its associated complications in people at increased risk for the development of type 2 diabetes mellitus.
Hemmingsen, B, Sonne, DP, Metzendorf, MI, Richter, B
The Cochrane database of systematic reviews. 2017;(5):CD012204
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Abstract
BACKGROUND The projected rise in the incidence of type 2 diabetes mellitus (T2DM) could develop into a substantial health problem worldwide. Whether dipeptidyl-peptidase (DPP)-4 inhibitors or glucagon-like peptide (GLP)-1 analogues are able to prevent or delay T2DM and its associated complications in people at risk for the development of T2DM is unknown. OBJECTIVES To assess the effects of DPP-4 inhibitors and GLP-1 analogues on the prevention or delay of T2DM and its associated complications in people with impaired glucose tolerance, impaired fasting blood glucose, moderately elevated glycosylated haemoglobin A1c (HbA1c) or any combination of these. SEARCH METHODS We searched the Cochrane Central Register of Controlled Trials; MEDLINE; PubMed; Embase; ClinicalTrials.gov; the World Health Organization (WHO) International Clinical Trials Registry Platform; and the reference lists of systematic reviews, articles and health technology assessment reports. We asked investigators of the included trials for information about additional trials. The date of the last search of all databases was January 2017. SELECTION CRITERIA We included randomised controlled trials (RCTs) with a duration of 12 weeks or more comparing DPP-4 inhibitors and GLP-1 analogues with any pharmacological glucose-lowering intervention, behaviour-changing intervention, placebo or no intervention in people with impaired fasting glucose, impaired glucose tolerance, moderately elevated HbA1c or combinations of these. DATA COLLECTION AND ANALYSIS Two review authors read all abstracts and full-text articles and records, assessed quality and extracted outcome data independently. One review author extracted data which were checked by a second review author. We resolved discrepancies by consensus or the involvement of a third review author. For meta-analyses, we planned to use a random-effects model with investigation of risk ratios (RRs) for dichotomous outcomes and mean differences (MDs) for continuous outcomes, using 95% confidence intervals (CIs) for effect estimates. We assessed the overall quality of the evidence using the GRADE instrument. MAIN RESULTS We included seven completed RCTs; about 98 participants were randomised to a DPP-4 inhibitor as monotherapy and 1620 participants were randomised to a GLP-1 analogue as monotherapy. Two trials investigated a DPP-4 inhibitor and five trials investigated a GLP-1 analogue. A total of 924 participants with data on allocation to control groups were randomised to a comparator group; 889 participants were randomised to placebo and 33 participants to metformin monotherapy. One RCT of liraglutide contributed 85% of all participants. The duration of the intervention varied from 12 weeks to 160 weeks. We judged none of the included trials at low risk of bias for all 'Risk of bias' domains and did not perform meta-analyses because there were not enough trials.One trial comparing the DPP-4 inhibitor vildagliptin with placebo reported no deaths (very low-quality evidence). The incidence of T2DM by means of WHO diagnostic criteria in this trial was 3/90 participants randomised to vildagliptin versus 1/89 participants randomised to placebo (very low-quality evidence). Also, 1/90 participants on vildagliptin versus 2/89 participants on placebo experienced a serious adverse event (very low-quality evidence). One out of 90 participants experienced congestive heart failure in the vildagliptin group versus none in the placebo group (very low-quality evidence). There were no data on non-fatal myocardial infarction, stroke, health-related quality of life or socioeconomic effects reported.All-cause and cardiovascular mortality following treatment with GLP-1 analogues were rarely reported; one trial of exenatide reported that no participant died. Another trial of liraglutide 3.0 mg showed that 2/1501 in the liraglutide group versus 2/747 in the placebo group died after 160 weeks of treatment (very low-quality evidence).The incidence of T2DM following treatment with liraglutide 3.0 mg compared to placebo after 160 weeks was 26/1472 (1.8%) participants randomised to liraglutide versus 46/738 (6.2%) participants randomised to placebo (very low-quality evidence). The trial established the risk for (diagnosis of) T2DM as HbA1c 5.7% to 6.4% (6.5% or greater), fasting plasma glucose 5.6 mmol/L or greater to 6.9 mmol/L or less (7.0 mmol/L or greater) or two-hour post-load plasma glucose 7.8 mmol/L or greater to 11.0 mmol/L (11.1 mmol/L). Altogether, 70/1472 (66%) participants regressed from intermediate hyperglycaemia to normoglycaemia compared with 268/738 (36%) participants in the placebo group. The incidence of T2DM after the 12-week off-treatment extension period (i.e. after 172 weeks) showed that five additional participants were diagnosed T2DM in the liraglutide group, compared with one participant in the placebo group. After 12-week treatment cessation, 740/1472 (50%) participants in the liraglutide group compared with 263/738 (36%) participants in the placebo group had normoglycaemia.One trial used exenatide and 2/17 participants randomised to exenatide versus 1/16 participants randomised to placebo developed T2DM (very low-quality evidence). This trial did not provide a definition of T2DM. One trial reported serious adverse events in 230/1524 (15.1%) participants in the liraglutide 3.0 mg arm versus 96/755 (12.7%) participants in the placebo arm (very low quality evidence). There were no serious adverse events in the trial using exenatide. Non-fatal myocardial infarction was reported in 1/1524 participants in the liraglutide arm and in 0/55 participants in the placebo arm at 172 weeks (very low-quality evidence). One trial reported congestive heart failure in 1/1524 participants in the liraglutide arm and in 1/755 participants in the placebo arm (very low-quality evidence). Participants receiving liraglutide compared with placebo had a small mean improvement in the physical component of the 36-item Short Form scale showing a difference of 0.87 points (95% CI 0.17 to 1.58; P = 0.02; 1 trial; 1791 participants; very low-quality evidence). No trial evaluating GLP-1-analogues reported data on stroke, microvascular complications or socioeconomic effects. AUTHORS' CONCLUSIONS There is no firm evidence that DPP-4 inhibitors or GLP-1 analogues compared mainly with placebo substantially influence the risk of T2DM and especially its associated complications in people at increased risk for the development of T2DM. Most trials did not investigate patient-important outcomes.
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[Theoretical Investigations into the Quantitative Mechanisms Underlying the Regulation of [cAMP]i, Membrane Excitability and [Ca(2+)]i during GLP-1 Stimulation in Pancreatic β Cells].
Takeda, Y
Yakugaku zasshi : Journal of the Pharmaceutical Society of Japan. 2016;(3):467-71
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Abstract
Upon elevation of plasma glucose concentration, pancreatic β-cells generate bursts of action potentials to induce cyclic changes in [Ca(2+)]i regulating insulin release. Glucose-dependent insulin secretion is synergistically enhanced by glucagon-like peptide-1 (GLP-1), which increases [cAMP]i and activates protein kinase A (PKA) and exchange protein activated by cAMP (Epac). The insulinotropic effect of GLP-1 is mediated, at least in part, by modulating multiple ion channels/transporters at the plasma membrane and ER through PKA- and EPAC-dependent mechanisms, which increase membrane excitability and intracellular Ca(2+) release. However, because of complex interactions between multiple cellular factors involved in the GLP-1 effects, quantitative aspects of the molecular/ionic mechanisms have not yet been determined. We thus performed simulation studies and mathematical analysis to investigate how GLP-1 signals control [cAMP]i and subsequently modify the bursting activities and Ca(2+) dynamics. First, a GLP-1 receptor signal transduction model was developed and introduced to our β-cells model. Secondly, modulatory effects of PKA/Epac on ion channels/transporters were incorporated based on experimental studies. Increases in the frequency and duration of the bursting activity observed during GLP-1 stimulation were well reconstructed by our model, and lead potential analysis quantitatively determined the functional role of each ion channel/transporter in modifying the burst pattern. Finally, an IP3R model was developed to reproduce GLP-1-induced Ca(2+) transients/oscillations. Instantaneous equilibrium analysis and bifurcation analysis also elucidated the quantitative mechanisms involved in generating IP3R-mediated Ca(2+) mobilization. The results of this theoretical analysis of the effects of GLP-1 on membrane excitability/Ca(2+) dynamics are discussed in this review.