Oxyntomodulin inhibits pancreatic secretion through the nervous system in rats

Response to lowering plasma glucose is characterised by decreased oxyntomodulin: Results from a randomised controlled trial

BACKGROUND

With the prevalence of diabetes reaching an epidemic level, there is a growing interest in the investigation of its remission. Proglucagon-derived peptides (PGDP) have been shown to have a glucose-regulating effect. However, whether they play a role in diabetes remission remains poorly understood.

AIM

To investigate changes in plasma levels of PGDP in glycaemic responders versus non-responders.

METHODS

The study was a randomised placebo-controlled trial comprising 18 adults with prediabetes (registered at www.

CLINICALTRIALS

gov as NCT03889210). Following an overnight fast, participants consumed ketone β-hydroxybutyrate (KEβHB)-supplemented beverage and placebo beverage in crossover manner. Serial blood samples were collected from baseline to 150 min at 30-min intervals. The endpoints were changes in glucagon-like peptide-1 (GLP-1), glicentin, oxyntomodulin, glucagon, and major proglucagon fragment (MPGF). Participants were stratified into the 'responders' and 'non-responders' subgroups based on their glycaemic changes following the ingestion of KEβHB. The area under the curve (AUC) was calculated to estimate the accumulated changes in the studied PGDP and compared using paired-t test between the KEβHB and placebo beverages.

RESULTS

Responders had a significantly greater reduction in plasma glucose compared with non-responders following acute ketosis (p < 0.001). The AUC0-150 for oxyntomodulin was significantly lower following the KEβHB beverage compared with the placebo (p = 0.045) in responders, but not in non-responders (p = 0.512). No significant differences in AUCs0-150 were found for GLP-1, glicentin, glucagon, and MPGF in either responders or non-responders.

CONCLUSION

Oxyntomodulin is involved in lowering plasma glucose and may play an important role in diabetes remission.

Distinguishing diabetes secondary to pancreatic diseases from type 2 diabetes mellitus

PURPOSE OF REVIEW

Diabetes secondary to pancreatic diseases (i.e., acute pancreatitis, chronic pancreatitis, and pancreatic cancer) is increasingly studied, but remains challenging to distinguish from type 2 diabetes (T2DM). We review the clinical significance and potential biomarkers that may help differentiate these types of diabetes.

RECENT FINDINGS

Recent studies have identified several complications (including nonvascular) that occur more frequently in patients with diabetes secondary to acute and chronic pancreatitis than T2DM, and biomarkers to differentiate these types of diabetes. There have been advances that may enable the enrichment of a population of adults with new onset diabetes to potentially screen for occult pancreatic cancer, but efforts are needed to identify and validate promising diagnostic biomarkers.

SUMMARY

High-quality studies are needed to more precisely understand the risk factors and natural course of diabetes secondary to pancreatic diseases. Mechanistic and interventional studies are awaited to provide insights that will distinguish diabetes secondary to pancreatic diseases and refine the management of hyperglycemia in this patient population.

Post-pancreatitis diabetes mellitus: investigational drugs in preclinical and clinical development and therapeutic implications

Introduction: Post-pancreatitis diabetes mellitus is one of the most common types of secondary diabetes. The pharmaceutical armamentarium in the field of diabetology can be broadened if the design of novel drugs is informed by pathogenetic insights from studies on post-pancreatitis diabetes mellitus.Areas covered: The article provides an overview of preclinical and clinical studies of compounds selectively antagonizing the gastric inhibitory peptide receptor, simultaneously stimulating both the glucagon-like peptide-1 and glucagon receptors, and activating ketogenesis.Expert opinion: The current pharmacotherapy for post-pancreatitis diabetes mellitus is relatively ineffective. This type of diabetes represents a unique platform for rigorous, efficient, and practical search for glucose-lowering therapeutic candidates. Various methods of gastric inhibitory peptide receptor (expressed in the pancreas) antagonism have undergone extensive preclinical testing in diabetes, with promising compounds being trialed in man. Molecular mimicry with oxyntomodulin ─ an extra-pancreatic hormone homologous with pancreatic hormone glucagon and involved in the regulation of exocrine pancreatic function ─ could be harnessed. The emerging findings of a salutary effect of ketosis mimetics in people with prediabetes set the stage for a novel approach to preventing diabetes.

Neuroendocrine Peptides of the Gut and Their Role in the Regulation of Food Intake

The regulation of food intake encompasses complex interplays between the gut and the brain. Among them, the gastrointestinal tract releases different peptides that communicate the metabolic state to specific nuclei in the hindbrain and the hypothalamus. The present overview gives emphasis on seven peptides that are produced by and secreted from specialized enteroendocrine cells along the gastrointestinal tract in relation with the nutritional status. These established modulators of feeding are ghrelin and nesfatin-1 secreted from gastric X/A-like cells, cholecystokinin (CCK) secreted from duodenal I-cells, glucagon-like peptide 1 (GLP-1), oxyntomodulin, and peptide YY (PYY) secreted from intestinal L-cells and uroguanylin (UGN) released from enterochromaffin (EC) cells. © 2021 American Physiological Society. Compr Physiol 11:1679-1730, 2021.

Panorama of mediators in postpancreatitis diabetes mellitus

PURPOSE OF REVIEW

To provide an overview of mediators involved in the pathogenesis of postacute pancreatitis diabetes mellitus.

RECENT FINDINGS

The 'holistic prevention of pancreatitis' framework has brought to the fore the sequelae of not only end-stage chronic pancreatitis and extensive pancreatic necrosis but also mild acute pancreatitis. Insights from the DORADO project have provided a wealth of information on the signalling molecules that do and do not affect glucose metabolism in individuals after mild acute pancreatitis and have challenged conventional views of the pathogenesis of postpancreatitis diabetes mellitus.

SUMMARY

Growing evidence compels a reconsideration of the dogma that mechanical β-cell destruction (and the resulting insulin deficiency) is the only underlying mechanism of postpancreatitis diabetes mellitus. Chronic low-grade inflammation, β-cell compensation, lipolysis, altered secretion of gut hormones, and changes in iron metabolism characterize postacute pancreatitis diabetes mellitus. Some of these are druggable targets that offer novel opportunities to reduce the burden of pancreatitis through tertiary prevention.

Dipeptidyl peptidase inhibitor therapy in type 2 diabetes: Control of the incretin axis and regulation of postprandial glucose and lipid metabolism

Dipeptidyl peptidase 4 (DPP4) is a widely expressed, serine protease which regulates the bioactivity of many peptides through cleavage and inactivation including the incretin hormones, glucagon like peptide -1 (GLP-1) and glucose dependent insulinotropic polypeptide (GIP). Inhibitors of DPP4 are used therapeutically to treat patients with Type 2 Diabetes Mellitus (T2DM) as they potentiate incretin action to regulate islet hormone secretion and improve glycemia and post-prandial lipid excursions. The widespread clinical use of DPP4 inhibitors has increased interest in the molecular mechanisms by which these drugs mediate their beneficial effects. Traditionally, focus has remained on inhibiting the catalytic activity of DPP4 within the plasma compartment, however evidence is emerging on the importance of inactivation of membrane-bound DPP4 in selective tissue beds to potentiate local hormone gradients. Here we review the recent advances in identifying the cellular sources of both circulating and membrane-bound DPP4 important for cleavage of the incretin hormones and regulation of glucose and lipoprotein metabolism.

Peptide hormones and lipopeptides: from self-assembly to therapeutic applications

This review describes the properties and activities of lipopeptides and peptide hormones and how the lipidation of peptide hormones could potentially produce therapeutic agents combating some of the most prevalent diseases and conditions. The self-assembly of these types of molecules is outlined, and how this can impact on bioactivity. Peptide hormones specific to the uptake of food and produced in the gastrointestinal tract are discussed in detail. The advantages of lipidated peptide hormones over natural peptide hormones are summarised, in terms of stability and renal clearance, with potential application as therapeutic agents. © 2017 The Authors Journal of Peptide Science published by European Peptide Society and John Wiley & Sons Ltd.

Proglucagon-Derived Peptides Do Not Significantly Affect Acute Exocrine Pancreas in Rats

OBJECTIVES

Reports have suggested a link between treatment with glucagon-like peptide 1 (GLP-1) analogs and an increased risk of pancreatitis. Oxyntomodulin, a dual agonist of both GLP-1 and glucagon receptors, is currently being investigated as a potential antiobesity therapy, but little is known about its pancreatic safety. The aim of the study was to investigate the acute effect of oxyntomodulin and other proglucagon-derived peptides on the rat exocrine pancreas.

METHODS

Glucagon-like peptide 1, oxyntomodulin, glucagon, and exendin-4 were infused into anesthetized rats to measure plasma amylase concentration changes. In addition, the effect of each peptide on both amylase release and proliferation in rat pancreatic acinar (AR42J) and primary isolated ductal cells was determined.

RESULTS

Plasma amylase did not increase postpeptide infusion, compared with vehicle and cholecystokinin; however, oxyntomodulin inhibited plasma amylase when coadministered with cholecystokinin. None of the peptides caused a significant increase in proliferation rate or amylase secretion from acinar and ductal cells.

CONCLUSIONS

The investigated peptides do not have an acute effect on the exocrine pancreas with regard to proliferation and plasma amylase, when administered individually. Oxyntomodulin seems to be a potent inhibitor of amylase release, potentially making it a safer antiobesity agent regarding pancreatitis, compared with GLP-1 agonists.

Action and therapeutic potential of oxyntomodulin

Oxyntomodulin (OXM) is a peptide hormone released from the gut in post-prandial state that activates both the glucagon-like peptide-1 receptor (GLP1R) and the glucagon receptor (GCGR) resulting in superior body weight lowering to selective GLP1R agonists. OXM reduces food intake and increases energy expenditure in humans. While activation of the GCGR increases glucose production posing a hyperglycemic risk, the simultaneous activation of the GLP1R counteracts this effect. Acute OXM infusion improves glucose tolerance in T2DM patients making dual agonists of the GCGR and GLP1R new promising treatments for diabetes and obesity with the potential for weight loss and glucose lowering superior to that of GLP1R agonists.

Ghrelin, the proglucagon-derived peptides and peptide YY in nutrient homeostasis

Dysregulation of nutrient homeostasis is implicated in the current epidemics of obesity and type 2 diabetes mellitus. The maintenance of homeostasis in the setting of repeated cycles of feeding and fasting occurs through complex interactions between metabolic, hormonal and neural factors. Although pancreatic islets, the liver, muscle, adipocytes and the central nervous system are all key players in this network, the gastrointestinal tract is the first tissue exposed to ingested nutrients and thus has an important role. This Review focuses on several of the endocrine hormones released by the gastrointestinal tract prior to or during nutrient ingestion that have key roles in maintaining energy balance. These hormones include the gastric orexigenic hormone, ghrelin, and the distal L cell anorexigenic and metabolic hormones, glucagon-like peptide (GLP)-1, GLP-2, oxyntomodulin and peptide YY. Each of these hormones exerts a distinct set of biological actions to maintain nutrient homeostasis, the properties of which are currently, or might soon be, exploited in the clinic for the treatment of obesity and type 2 diabetes mellitus.

Unraveling oxyntomodulin, GLP1's enigmatic brother

Oxyntomodulin (OXM) is a peptide secreted from the L cells of the gut following nutrient ingestion. OXM is a dual agonist of the glucagon-like peptide-1 receptor (GLP1R) and the glucagon receptor (GCGR) combining the effects of GLP1 and glucagon to act as a potentially more effective treatment for obesity than GLP1R agonists. Injections of OXM in humans cause a significant reduction in weight and appetite, as well as an increase in energy expenditure. Activation of GCGR is classically associated with an elevation in glucose levels, which would be deleterious in patients with T2DM, but the antidiabetic properties of GLP1R agonism would be expected to counteract this effect. Indeed, OXM administration improved glucose tolerance in diet-induced obese mice. Thus, dual agonists of the GCGR and GLP1R represent a new therapeutic approach for diabetes and obesity with the potential for enhanced weight loss and improvement in glycemic control beyond those of GLP1R agonists.

Ileal brake: a sensible food target for appetite control. A review

With the rising prevalence of obesity and related health problems increases, there is increased interest in the gastrointestinal system as a possible target for pharmacological or food-based approaches to weight management. Recent studies have shown that under normal physiological situations undigested nutrients can reach the ileum, and induce activation of the so-called "ileal brake", a combination of effects influencing digestive process and ingestive behaviour. The relevance of the ileal brake as a potential target for weight management is based on several findings: First, activation of the ileal brake has been shown to reduce food intake and increase satiety levels. Second, surgical procedures that increase exposure of the ileum to nutrients produce weight loss and improved glycaemic control. Third, the appetite-reducing effect of chronic ileal brake activation appears to be maintained over time. Together, this evidence suggests that activation of the ileal brake is an excellent long-term target to achieve sustainable reductions in food intake. This review addresses the role of the ileal brake in gut function, and considers the possible involvement of several peptide hormone mediators. Attention is given to the ability of macronutrients to activate the ileal brake, and particularly variation attributable to the physicochemical properties of fats. The emphasis is on implications of ileal brake stimulation on food intake and satiety, accompanied by evidence of effects on glycaemic control and weight loss.

Pro-protein convertases in intermediary metabolism: islet hormones, brain/gut hormones and integrated physiology

Many peptide hormones implicated in the regulation of intermediary metabolism arise from larger precursors called prohormones. These precursors are cut into pieces by proprotein convertases, more precisely those called prohormone convertases (PCs) that cleave at the C terminus of basic doublets. The remaining basic amino acids are eliminated by a specialized carboxypeptidase, leading to the active hormone. This processing may provide, from a single precursor, several peptides with different biological activities depending on the site(s) of cleavage on the precursor. When the processing is tissue-specific, this mechanism allows to produce, from a single protein, different sets of hormones depending on the tissue considered, leading to novel regulatory processes. The archetype of such a pluripotent prohormone in the field of intermediary metabolism is pro-glucagon that, when cut by PC1 in intestinal L cells, produces four different peptides with different specificities [glicentin, oxyntomodulin (OXM), glucagon-like peptide-1, and glucagon-like peptide-2], whereas, when cut by PC2 in the alpha cells of the endocrine pancreas, glucagon is produced and, through the supplementary action of NRD convertase, a fragment of glucagon (miniglucagon) with original properties.

Oxyntomodulin increases intrinsic heart rate in mice independent of the glucagon-like peptide-1 receptor

Oxyntomodulin (OXM), a postprandially released intestinal hormone, inhibits food intake via the glucagon-like peptide-1 receptor (GLP-1R). Although OXM may have clinical value in treating obesity, the cardiovascular effects of OXM are not well understood. Using telemetry to measure heart rate (HR), body temperature (Tb), and activity in conscious and freely moving mice, we tested 1) whether OXM affects HR and 2) whether this effect is mediated by the GLP-1R. We found that peripherally administered OXM significantly increased HR in wild-type mice, raising HR by >200 beats/min to a maximum of 728 ± 11 beats/min. To determine the extent to which the sympathetic nervous system mediates the tachycardia of OXM, we delivered this hormone to mice deficient in dopamine-β-hydroxylase [ Dbh(−/−) mice], littermate controls [ Dbh(+/−) mice], and autonomically blocked C57Bl mice. OXM increased HR equally in all groups (192 ± 13, 197 ± 21, and 216 ± 11 beats/min, respectively), indicating that OXM elevated intrinsic HR. Intrinsic HR was also vigorously elevated by OXM in Glp-1R(−/−) mice (200 ± 28 beats/min). In addition, peripherally administered OXM inhibited food intake and activity levels in wild-type mice and lowered Tb in autonomically blocked mice. None of these effects were observed in Glp-1R(−/−) mice. These data suggest multiple modes of action of OXM: 1) it directly elevates murine intrinsic HR through a GLP-1R-independent mechanism, perhaps via the glucagon receptor or an unidentified OXM receptor, and 2) it lowers food intake, activity, and Tb in a GLP-1R-dependent fashion.

Gut peptides and the regulation of appetite

There is a growing worldwide epidemic of obesity. Obese people have a higher incidence of type 2 diabetes and cardiovascular disease, and hence present increasing social, financial and health burdens. Weight loss is always difficult to achieve through lifestyle changes alone, and currently licensed anti-obesity drug treatments, such as orlistat and sibutramine, if tolerated, only achieve modest weight loss. Therefore, there is a need to identify more potent pharmacological targets. In the last 10 years, discoveries of new hormones such as leptin and ghrelin, together with greater understanding of previously described hormones such as cholecystokinin (CCK), pancreatic polypeptide (PP), peptide YY (PYY) and glucagon-like peptide 1 (GLP-1), have led to a rapid increase in our knowledge of the regulation of energy balance. Among the most important factors, controlling appetite and satiety are peptide hormones released from the gut. In this paper, we provide a full up-to-date overview of the current state of knowledge of this field, together with the potential of these peptides as drugs, or as other therapeutic targets, in the treatment of obesity. Finally, we propose an integrated model to describe the complex interplay of these hormones in the broader physiology of energy balance.

Oxyntomodulin

The prevalence of obesity is increasing rapidly and the associated morbidity and mortality has led to an urgent need for potential therapeutic targets to reduce appetite and food intake. Gut hormones released after eating that coordinate digestive activity and promote satiety are novel potential treatments for obesity. Oxyntomodulin is a gut hormone that is produced by the L cells in the small intestine and reduces food intake. It is timely to review some of the original literature on oxyntomodulin, to evaluate what is already known about the peptide, and also to set the recent findings on its effects on food intake and bodyweight into context.Recent studies have shown that long-term peripheral administration of oxyntomodulin to rats leads to reduced food intake and reduced weight gain. Studies in humans have demonstrated that acute administration reduces food intake by 19%. When given preprandially by subcutaneous injection three times daily, oxyntomodulin resulted in a reduction in food intake and mean weight loss of 2.8kg over 4 weeks. Oxyntomodulin thus represents a potential therapy for obesity.The mechanism of action of oxyntomodulin is not known. Current evidence suggests that it acts via the glucagon-like peptide 1 (GLP-1) receptor. There may be an additional receptor in the gastric mucosa mediating its effects on gastric acid secretion. Although oxyntomodulin probably acts via the GLP-1 receptor, the two peptides differentially regulate food intake and energy expenditure in the mouse.Oxyntomodulin represents a potential therapy for obesity. Further work will help to clarify its mechanisms of action.

Proglucagon-derived peptides: mechanisms of action and therapeutic potential

Glucagon is used for the treatment of hypoglycemia, and glucagon receptor antagonists are under development for the treatment of type 2 diabetes. Moreover, glucagon-like peptide (GLP)-1 and GLP-2 receptor agonists appear to be promising therapies for the treatment of type 2 diabetes and intestinal disorders, respectively. This review discusses the physiological, pharmacological, and therapeutic actions of the proglucagon-derived peptides, with an emphasis on clinical relevance of the peptides for the treatment of human disease.

Contraction induced by glicentin on smooth muscle cells from the human colon is abolished by exendin (9-39)

Glicentin and glucagon-like peptide-1 (7-36) amide (GLP-1) are gut hormones released during digestion. Glicentin and GLP-1 slow down gastric emptying and glicentin can switch off the duodenojejunal fed motor pattern. The effect of glicentin on the motor activity of colon has never been reported in humans. Our aim was to determine if circular smooth muscle cells (SMC) from the human colon are target cells for glicentin or GLP-1, and if their motility is dependent upon these digestive hormones.

METHODS

Twenty-two resections were performed on patients operated for colon adenocarcinoma. The SMC were isolated from colonic circular muscle layer and cell contraction was assessed.

RESULTS

Glicentin caused a dose-related contraction of SMC, when GLP-1 determined a contraction of weak amplitude. Exendin-(9-39), described as a GLP-1 receptor antagonist, inhibited contraction due to glicentin or GLP-1. In contrast, on antral SMC from rabbit, GLP-1 exerts neither relaxation nor contraction; however, exendin-(9-39) dose dependently reduced the contractile activity of glicentin [glicentin EC(50) = 5 pM, exendin-(9-39) pA(2) = -9.36].

CONCLUSIONS

The circular muscle from the human colon is a target tissue for glicentin and GLP-1. Whereas glicentin is a long-life digestive hormone which would contribute to segmental contraction, the biological activity of GLP-1 remains unknown on this tissue. On the digestive smooth muscle, exendin-(9-39) behaved as an antagonist for two members of the glucagon-receptor family, GLP-1 and glicentin.

Relationships between the autonomic nervous system and the pancreas including regulation of regeneration and apoptosis: recent developments

Substantial new information has accumulated on the mechanisms of secretion, the development, and regulation of the gene expression, and the role of growth factors in the differentiation, growth, and regeneration of the pancreas. Many genes that are required for pancreas formation are active after birth and participate in endocrine and exocrine cell functions. Although the factors that normally regulate the proliferation of the pancreas largely remain elusive, several factors to influence the growth have been identified. It was also reported that the pancreas was sensitive to a number of apoptotic stimuli. The autonomic nervous system influences many of the functions of the body, including the pancreas. In fact, the parasympathetic nervous system and the sympathetic nervous system have opposing effects on insulin secretion from islet beta cells; feeding-induced parasympathetic neural activity to the pancreas stimulates insulin secretion, whereas stress-induced sympathetic neural activity to the pancreas inhibits insulin secretion. Moreover, it has been reported that the autonomic nervous system is one of the important factors that regulate pancreatic regeneration and stimulate the carcinogenesis. The present review focuses on the relationships between the autonomic nervous system and the pancreas, and furthermore, presents evidence of the autonomic nervous system-related pancreatic regeneration and carcinogenesis.

Oxyntomodulin and glicentin are potent inhibitors of the fed motility pattern in small intestine

Glicentin (GLIC) and oxyntomodulin (OXM or GLIC 33-69) are gut hormones which regulate digestion. They are known to reduce digestive secretions and to delay gastric emptying. Their biological activities on intestinal motility are still unknown. The effect of a systemic GLIC or OXM increase was investigated in rats on the food intake, the postprandial myoelectrical activity of small intestine and the orocaecal transit. An OXM or GLIC i.v. infusion was applied during the 5 min preceding food onset and during the first 15 min of food intake. This determined a three- to fourfold increase of the preprandial OXM-GLIC level. The OXM or GLIC plasma increase did not modify food intake. OXM infusion slowed down gastric emptying when the stomach contained 3/4 of the ingested food (before T 3 h). The quantity of food delivered in jejunum was subsequently smaller (P < 0.05). In the small intestine, the duration of postprandial myoelectrical activity (50-60 min g(-1) of ingested food) was reduced by 70% (P < 0.001) on duodenum or jejunum and by 54% (P < 0.01) on ileum in OXM-treated rats. An interdigestive motility profile was settled and an acceleration of both gastric emptying and transit rate was thereafter evidenced (after T 3 h). GLIC also reduced the duration of the postprandial myoelectrical activity on duodenum and jejunum (65 and 63% respectively, P < 0.05), but was not as efficient as OXM on ileum. In pathological states such as acute adult gastroenteritis, OXM and GLIC exhibit a two- to fivefold increase in their plasma concentrations. The present findings suggest that OXM and GLIC could, in that disease, contribute to exclude pathogens, due to their joined action on gut motility.

Oxyntomodulin and glucagon-like peptide-1 differentially regulate murine food intake and energy expenditure

BACKGROUND & AIMS

Gut-derived peptides including ghrelin, cholecystokinin (CCK), peptide YY (PYY), glucagon-like peptide (GLP-1), and GLP-2 exert overlapping actions on energy homeostasis through defined G-protein-coupled receptors (GPCRs). The proglucagon-derived peptide (PGDP) oxyntomodulin (OXM) is cosecreted with GLP-1 and inhibits feeding in rodents and humans; however, a distinct receptor for OXM has not been identified.

METHODS

We examined the mechanisms mediating oxyntomodulin action using stable cell lines expressing specific PGDP receptors in vitro and both wild-type and knockout mice in vivo.

RESULTS

OXM activates signaling pathways in cells through glucagon or GLP-1 receptors (GLP-1R) but transiently inhibits food intake in vivo exclusively through the GLP-1R. Both OXM and the GLP-1R agonist exendin-4 (Ex-4) activated neuronal c-fos expression in the paraventricular nucleus of the hypothalamus, the area postrema, and the nucleus of the solitary tract following intraperitoneal (i.p.) injection. However, OXM transiently inhibited food intake in wild-type mice following intracerebroventricular (i.c.v.) but not i.p. administration, whereas Ex-4 produced a more potent and sustained inhibition of food intake following both i.c.v. and i.p. administration. The anorectic effects of OXM were preserved in Gcgr(-/-) mice but abolished in GLP-1R(-/-) mice. Although central Ex-4 and OXM inhibited feeding via a GLP-1R-dependent mechanism, Ex-4 but not OXM reduced VO2 and respiratory quotient in wild-type mice.

CONCLUSIONS

These findings demonstrate that structurally distinct PGDPs differentially regulate food intake and energy expenditure by interacting with a GLP-1R-dependent pathway. Hence ligand-specific activation of a common GLP-1R increases the complexity of gut-central nervous system pathways regulating energy homeostasis and metabolic expenditure.

Neurohormonal control of pancreatic exocrine secretion

The neurohormonal control of pancreatic exocrine secretion is a complex interaction of multiple pathways involving a large number of gut hormones, neurotransmitters, and neuropeptides. Recent studies have elucidated a role for cholecystokinin in the regulation of bicarbonate and fluid secretion from pancreatic duct cells and suggested that cholecystokinin stimulation of human pancreatic acinar cells is likely regulated by an indirect mechanism of stimulation of afferent neurons. Evidence supports the regulation of potassium channels in rat pancreatic acinar cells by the cyclic AMP-mediated agonist secretin. Mechanisms for the regulation of cholecystokinin and secretin release by releasing factors have also been elucidated. The area postrema has been implicated in the mediation of inhibition of pancreatic secretion by the gut hormones peptide YY and pancreatic polypeptide. The neurotransmitter serotonin has been demonstrated to play a role in acid-induced secretin release and in pancreatic secretion stimulated by luminal factors. The regulation of pancreatic exocrine secretion by purines, nitric oxide, and gamma-aminobutyric acid as well as by the neuropeptides pituitary adenylate cyclase-activating peptide, gastrin-releasing peptide, and substance P is reviewed. The role of the central nervous system in modulating pancreatic secretion is also described. This review highlights the recent advances in knowledge of the neurohormonal regulation of pancreatic exocrine secretion.

Neural hormonal regulation of exocrine pancreatic secretion

Exocrine pancreatic secretion is regulated by hormone-hormonal and neural-hormonal interactions involving several regulatory peptides and neurotransmitter from the gut, the pancreas and the vagus nerve. The roles of the gastrointestinal peptides including secretin, CCK, neurotensin, motilin, PYY and pancreatic islet hormones including insulin, pancreatic polypeptide and somatostatin have been established. Interactions among secretin, CCK and neurotensin produce synergistic stimulatory effect. Motilin modulates the cyclic pattern of pancreatic secretion while local insulin provides a permissive role for the action of secretin and CCK at physiological concentration. Somatostatin, PYY and pancreatic polypeptide are inhibitory regulators, acting either on the release of secretin and CCK or on the action of the two stimulatory hormones. The vagal afferent-efferent pathway mediates the actions of many of these regulatory peptides, particularly of secretin and CCK. Acetylcholine and nitric oxide are the neurotransmitters known to mediate the actions of secretin and CCK. Serotonin (5-HT) released from enterochromaffin cells in the intestinal mucosa and nerve terminals of the enteric nervous system and intrapancreatic nerves may be involved in both stimulatory and inhibitory mechanism through its various receptor subtypes. 5-HT also mediates the action of secretin and CCK. The regulatory roles of neuropeptides, PACP and GRP, are now established, whereas those of others are being uncovered. Pancreatic juice provides both positive and negative feedback regulation of pancreatic secretion through mediation of both secretin- and CCK-releasing peptides. Three CCK-releasing peptides have been purified: monitor peptide from pancreatic juice, diazepam-binding inhibitor from porcine intestine, and luminal CCK-releasing factor from rat intestinal secretion. All have been shown to stimulate CCK release and pancreatic enzyme secretion. Pancreatic phospholipase A2 from pancreatic juice and intestinal secretion appears to function as a secretin-releasing peptide. However, the detailed map of neurohormonal regulatory pathways of exocrine pancreatic secretion is yet to be constructed.

Neurohormonal control of exocrine pancreas

The exocrine pancreas is regulated by various hormonal factors derived from the gut through hormone-hormonal and neurohormonal interactions. Physiologic stimuli entering the upper small intestine elicit the release of intestinal hormones and activate sensory reflex mechanisms from the intestinal mucosa to stimulate or inhibit exocrine pancreatic secretion. In addition, the endocrine pancreas, intrapancreatic nerves, and some extrapancreatic neural pathways, with or without mediation by the vagus nerve, are known to participate in regulation of exocrine pancreatic secretion. It has been established that two key intestinal hormones, secretin and cholecystokinin (CCK), in physiologic doses, act through the vagal afferent pathway and interact with each other as well as with other gut hormones. The releases of these two hormones are mediated through the corresponding releasing peptides. In the past few years, the roles of secretin- and CCK-releasing peptides have become more clearly defined. The participation of several neurotransmitters and regulatory peptides in the regulation of exocrine pancreatic secretion has also been established. In addition, neurotransmitters and neuropeptides released from the central nervous system may participate in the regulation of pancreatic secretion. It is conceivable that a few neurotransmitters and neuropeptides are involved in each neural regulatory pathway. However, their roles and sites of action in each pathway remain to be determined.