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What Stimulates The Pancreas To Secrete Glucagon?

Glucagon

Glucagon

Glucagon has a major role in maintaining normal concentrations of glucose in blood, and is often described as having the opposite effect of insulin. That is, glucagon has the effect of increasing blood glucose levels. Glucagon is a linear peptide of 29 amino acids. Its primary sequence is almost perfectly conserved among vertebrates, and it is structurally related to the secretin family of peptide hormones. Glucagon is synthesized as proglucagon and proteolytically processed to yield glucagon within alpha cells of the pancreatic islets. Proglucagon is also expressed within the intestinal tract, where it is processed not into glucagon, but to a family of glucagon-like peptides (enteroglucagon). Physiologic Effects of Glucagon The major effect of glucagon is to stimulate an increase in blood concentration of glucose. As discussed previously, the brain in particular has an absolute dependence on glucose as a fuel, because neurons cannot utilize alternative energy sources like fatty acids to any significant extent. When blood levels of glucose begin to fall below the normal range, it is imperative to find and pump additional glucose into blood. Glucagon exerts control over two pivotal metabolic pathways within the liver, leading that organ to dispense glucose to the rest of the body: Glucagon stimulates breakdown of glycogen stored in the liver. When blood glucose levels are high, large amounts of glucose are taken up by the liver. Under the influence of insulin, much of this glucose is stored in the form of glycogen. Later, when blood glucose levels begin to fall, glucagon is secreted and acts on hepatocytes to activate the enzymes that depolymerize glycogen and release glucose. Glucagon activates hepatic gluconeogenesis. Gluconeogenesis is the pathway by which non-hexose Continue reading >>

Endocrine Pancreas

Endocrine Pancreas

This page outlines information on the pancreas. Several hormones participate in the regulation of carbohydrate metabolism. Four of them are secreted by the cells of the islets of Langerhans in the pancreas: two, insulin and glucagon, with major actions on glucose metabolism and two, somatostatin and pancreatic polypeptide, with modulating actions on insulin and glucagon secretion. Other hormones affecting carbohydrate metabolism include: epinephrine, thyroid hormones, glucocorticoids, and growth hormone. Structure and Function of the Pancreas The pancreas lies inferior to the stomach, in a bend of the duodenum. It is both an endocrine and an exocrine gland. The exocrine functions are concerned with digestion. The endocrine function consists primarily of the secretion of the two major hormones, insulin and glucagon. Four cell types have been identified in the islets, each producing a different hormone with specific actions: * A cells produce glucagon; * B cells produce insulin; * D cells produce somatostatin; and * F or D1 cells produce pancreatic polypeptide. These hormones are all polypeptides. Insulin is secreted only by the B cells whereas the other hormones are also secreted by the gastrointestinal mucosa and somatostatin is also found in the brain. Both insulin and glucagon are important in the regulation of carbohydrate, protein and lipid metabolism: Insulin is an anabolic hormone, that is, it increases the storage of glucose, fatty acids and amino acids in cells and tissues. Glucagon is a catabolic hormone, that is, it mobilizes glucose, fatty acids and amino acids from stores into the blood. Somatostatin may regulate, locally, the secretion of the other pancreatic hormones; in brain (hypothalamus) and spinal cord it may act as a neurohormone and neurotransmitter Continue reading >>

114 17.9 The Endocrine Pancreas

114 17.9 The Endocrine Pancreas

Learning Objectives By the end of this section, you will be able to: Describe the location and structure of the pancreas, and the morphology and function of the pancreatic islets Compare and contrast the functions of insulin and glucagon The pancreas is a long, slender organ, most of which is located posterior to the bottom half of the stomach (Figure 1). Although it is primarily an exocrine gland, secreting a variety of digestive enzymes, the pancreas has an endocrine function. Its pancreatic islets—clusters of cells formerly known as the islets of Langerhans—secrete the hormones glucagon, insulin, somatostatin, and pancreatic polypeptide (PP). Figure 1. Pancreas. The pancreatic exocrine function involves the acinar cells secreting digestive enzymes that are transported into the small intestine by the pancreatic duct. Its endocrine function involves the secretion of insulin (produced by beta cells) and glucagon (produced by alpha cells) within the pancreatic islets. These two hormones regulate the rate of glucose metabolism in the body. The micrograph reveals pancreatic islets. LM × 760. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) View the University of Michigan WebScope at to explore the tissue sample in greater detail. View the University of Michigan WebScope at to explore the tissue sample in greater detail. Cells and Secretions of the Pancreatic Islets The pancreatic islets each contain four varieties of cells: The alpha cell produces the hormone glucagon and makes up approximately 20 percent of each islet. Glucagon plays an important role in blood glucose regulation; low blood glucose levels stimulate its release. The beta cell produces the hormone insulin and makes up approximately 75 percent of each islet. Elevated Continue reading >>

Glucagon

Glucagon

This article is about the natural hormone. For the medication, see Glucagon (medication). Glucagon is a peptide hormone, produced by alpha cells of the pancreas. It works to raise the concentration of glucose and fat in the bloodstream, and is considered to be the main catabolic hormone of the body [3]. It is also used as a medication to treat a number of health conditions. Its effect is opposite to that of insulin, which lowers the extracellular glucose.[4] The pancreas releases glucagon when the concentration of glucose in the bloodstream falls too low. Glucagon causes the liver to convert stored glycogen into glucose, which is released into the bloodstream.[5] High blood-glucose levels, on the other hand, stimulate the release of insulin. Insulin allows glucose to be taken up and used by insulin-dependent tissues. Thus, glucagon and insulin are part of a feedback system that keeps blood glucose levels stable. Glucagon increases energy expenditure and is elevated under conditions of stress.[6] Glucagon belongs to the secritin family of hormones. Function[edit] Glucagon generally elevates the concentration of glucose in the blood by promoting gluconeogenesis and glycogenolysis [7]. Glucagon also decreases fatty acid synthesis in adipose tissue and the liver, as well as promoting lipolysis in these tissues, which causes them to release fatty acids into circulation where they can be catabolised to generate energy in tissues such as skeletal muscle when required [8]. Glucose is stored in the liver in the form of the polysaccharide glycogen, which is a glucan (a polymer made up of glucose molecules). Liver cells (hepatocytes) have glucagon receptors. When glucagon binds to the glucagon receptors, the liver cells convert the glycogen into individual glucose molecules and re Continue reading >>

The Endocrine Pancreas

The Endocrine Pancreas

Learning Objectives By the end of this section, you will be able to: Describe the location and structure of the pancreas, and the morphology and function of the pancreatic islets Compare and contrast the functions of insulin and glucagon The pancreas is a long, slender organ, most of which is located posterior to the bottom half of the stomach (Figure 1). Although it is primarily an exocrine gland, secreting a variety of digestive enzymes, the pancreas has an endocrine function. Its pancreatic islets—clusters of cells formerly known as the islets of Langerhans—secrete the hormones glucagon, insulin, somatostatin, and pancreatic polypeptide (PP). Cells and Secretions of the Pancreatic Islets The pancreatic islets each contain four varieties of cells: The alpha cell produces the hormone glucagon and makes up approximately 20 percent of each islet. Glucagon plays an important role in blood glucose regulation; low blood glucose levels stimulate its release. The beta cell produces the hormone insulin and makes up approximately 75 percent of each islet. Elevated blood glucose levels stimulate the release of insulin. The delta cell accounts for four percent of the islet cells and secretes the peptide hormone somatostatin. Recall that somatostatin is also released by the hypothalamus (as GHIH), and the stomach and intestines also secrete it. An inhibiting hormone, pancreatic somatostatin inhibits the release of both glucagon and insulin. The PP cell accounts for about one percent of islet cells and secretes the pancreatic polypeptide hormone. It is thought to play a role in appetite, as well as in the regulation of pancreatic exocrine and endocrine secretions. Pancreatic polypeptide released following a meal may reduce further food consumption; however, it is also released Continue reading >>

Physiology Of The Pancreatic Α-cell And Glucagon Secretion: Role In Glucose Homeostasis And Diabetes

Physiology Of The Pancreatic Α-cell And Glucagon Secretion: Role In Glucose Homeostasis And Diabetes

Abstract The secretion of glucagon by pancreatic α-cells plays a critical role in the regulation of glycaemia. This hormone counteracts hypoglycaemia and opposes insulin actions by stimulating hepatic glucose synthesis and mobilization, thereby increasing blood glucose concentrations. During the last decade, knowledge of α-cell physiology has greatly improved, especially concerning molecular and cellular mechanisms. In this review, we have addressed recent findings on α-cell physiology and the regulation of ion channels, electrical activity, calcium signals and glucagon release. Our focus in this review has been the multiple control levels that modulate glucagon secretion from glucose and nutrients to paracrine and neural inputs. Additionally, we have described the glucagon actions on glycaemia and energy metabolism, and discussed their involvement in the pathophysiology of diabetes. Finally, some of the present approaches for diabetes therapy related to α-cell function are also discussed in this review. A better understanding of the α-cell physiology is necessary for an integral comprehension of the regulation of glucose homeostasis and the development of diabetes. Introduction The principal level of control on glycaemia by the islet of Langerhans depends largely on the coordinated secretion of glucagon and insulin by α- and β-cells respectively. Both cell types respond oppositely to changes in blood glucose concentration: while hypoglycaemic conditions induce α-cell secretion, β-cells release insulin when glucose levels increase (Nadal et al. 1999, Quesada et al. 2006a). Insulin and glucagon have opposite effects on glycaemia as well as on the metabolism of nutrients. Insulin acts mainly on muscle, liver and adipose tissue with an anabolic effect, inducing th Continue reading >>

Pancreatic Regulation Of Glucose Homeostasis

Pancreatic Regulation Of Glucose Homeostasis

Go to: The pancreas is an exocrine and endocrine organ The pancreas has key roles in the regulation of macronutrient digestion and hence metabolism/energy homeostasis by releasing various digestive enzymes and pancreatic hormones. It is located behind the stomach within the left upper abdominal cavity and is partitioned into head, body and tail. The majority of this secretory organ consists of acinar—or exocrine—cells that secrete the pancreatic juice containing digestive enzymes, such as amylase, pancreatic lipase and trypsinogen, into the ducts, that is, the main pancreatic and the accessory pancreatic duct. In contrast, pancreatic hormones are released in an endocrine manner, that is, direct secretion into the blood stream. The endocrine cells are clustered together, thereby forming the so-called islets of Langerhans, which are small, island-like structures within the exocrine pancreatic tissue that account for only 1–2% of the entire organ (Figure 1).1 There are five different cell types releasing various hormones from the endocrine system: glucagon-producing α-cells,2 which represent 15–20% of the total islet cells; amylin-, C-peptide- and insulin-producing β-cells,2 which account for 65–80% of the total cells; pancreatic polypeptide (PP)-producing γ-cells,3 which comprise 3–5% of the total islet cells; somatostatin-producing δ-cells,2 which constitute 3–10% of the total cells; and ghrelin-producing ɛ-cells,4 which comprise <1% of the total islet cells. Each of the hormones has distinct functions. Glucagon increases blood glucose levels, whereas insulin decreases them.5 Somatostatin inhibits both, glucagon and insulin release,6 whereas PP regulates the exocrine and endocrine secretion activity of the pancreas.3, 7 Altogether, these hormones regul Continue reading >>

Glucagon

Glucagon

Glucagon is a 29 amino acid peptide hormone liberated in the α cells of the islets of Langerhans. For an overview of glucagon action, see the section on the Glucagon receptor Glucagon-producing α cells represent one of the earliest populations of detectable islet cells in the developing endocrine pancreas. Glucagon is generally viewed as a hormone that opposes the action of insulin in peripheral tissues, predominantly the liver, where the insulin:glucagonratio determines the intricate control of gluconeogenesis and glycogenolysis. The action of glucagon in the liver is complex and involves coordinate regulation of transcription factors and signal transduction networks which converge on regulation of amino acid, lipid and carbohydrate metabolism. For an overview of selected recent insights into the downstream signals activated following engagement of the hepatic glucagon receptor, see CREB regulates hepatic gluconeogenesis through the coactivator PGC-1. Nature. 2001 Sep 13;413(6852):179-83 and CREB controls hepatic lipid metabolism through nuclear hormone receptor PPAR-mgama. Nature. 2003 Nov 13;426(6963):190-3. and Glucagon represses signaling though the mammalian target of rapamycin in rat liver by activating AMP-activated protein kinase. J Biol Chem. 2004 Oct 19 [Epub ahead of print]. Although meals generally suppress glucagon secretion from the normal α cell, subjects with diabetes frequently exhibit disordered control of glucagon secretion leading to excess hepatic glucose production. Given the structural relationships between glucagon, oxyntomodulin, and glicentin, many commercial assays that claim to measure 29 amino acid pancreatic glucagon may exhibit lack of specificity, and occasionally, problems with sensitivty. Hyperglucagonaemia analysed by glucagon sand Continue reading >>

An Overview Of The Pancreas

An Overview Of The Pancreas

Pancreas Essentials The pancreas maintains the body’s blood glucose (sugar) balance. Primary hormones of the pancreas include insulin and glucagon, and both regulate blood glucose. Diabetes is the most common disorder associated with the pancreas. The pancreas is unique in that it’s both an endocrine and exocrine gland. In other words, the pancreas has the dual function of secreting hormones into blood (endocrine) and secreting enzymes through ducts (exocrine). The pancreas belongs to the endocrine and digestive systems—with most of its cells (more than 90%) working on the digestive side. However, the pancreas performs the vital duty of producing hormones—most notably insulin—to maintain the balance of blood glucose (sugar) and salt in the body. Without this balance, your body is susceptible to serious complications, such as diabetes. Anatomy of the Pancreas The pancreas is a 6 inch-long flattened gland that lies deep within the abdomen, between the stomach and the spine. It is connected to the duodenum, which is part of the small intestine. Only about 5% of the pancreas is comprised of endocrine cells. These cells are clustered in groups within the pancreas and look like little islands of cells when examined under a microscope. These groups of pancreatic endocrine cells are known as pancreatic islets or more specifically, islets of Langerhans (named after the scientist who discovered them). Hormones of the Pancreas The production of pancreatic hormones, including insulin, somatostatin, gastrin, and glucagon, play an important role in maintaining sugar and salt balance in our bodies. Gastrin: This hormone aids digestion by stimulating certain cells in the stomach to produce acid. Glucagon: Glucagon helps insulin maintain normal blood glucose by working in the Continue reading >>

Pancreas

Pancreas

The pancreas is a glandular organ in the upper abdomen, but really it serves as two glands in one: a digestive exocrine gland and a hormone-producing endocrine gland. Functioning as an exocrine gland, the pancreas excretes enzymes to break down the proteins, lipids, carbohydrates, and nucleic acids in food. Functioning as an endocrine gland, the pancreas secretes the hormones insulin and glucagon to control blood sugar levels throughout the day. Both of these diverse functions are vital to the body’s survival. Continue Scrolling To Read More Below... Click To View Large Image Related Anatomy: Body of Pancreas Common Bile Duct Head of Pancreas Kidneys Neck of Pancreas Pancreatic Notch Small Intestine Tail of Pancreas Continued From Above... Anatomy of the Pancreas The pancreas is a narrow, 6-inch long gland that lies posterior and inferior to the stomach on the left side of the abdominal cavity. The pancreas extends laterally and superiorly across the abdomen from the curve of the duodenum to the spleen. The head of the pancreas, which connects to the duodenum, is the widest and most medial region of the organ. Extending laterally toward the left, the pancreas narrows slightly to form the body of the pancreas. The tail of the pancreas extends from the body as a narrow, tapered region on the left side of the abdominal cavity near the spleen. Glandular tissue that makes up the pancreas gives it a loose, lumpy structure. The glandular tissue surrounds many small ducts that drain into the central pancreatic duct. The pancreatic duct carries the digestive enzymes produced by endocrine cells to the duodenum. The pancreas is classified as a heterocrine gland because it contains both endocrine and exocrine glandular tissue. The exocrine tissue makes up about 99% of the pancrea Continue reading >>

What Is Glucagon?

What Is Glucagon?

Blood sugar levels are an important part of overall health. When blood sugar levels drop, an individual may feel lethargic. If they drop too low, the individual may become disoriented, dizzy or even pass out. Blood sugar control involves a complex system of hormones, and one of those hormones is glucagon. Glucagon is a hormone that works with other hormones and bodily functions to control glucose levels in the blood. It comes from alpha cells found in the pancreas and is closely related to insulin-secreting beta cells, making it a crucial component that keeps the body’s blood glucose levels stable. What does glucagon do? Although secreted by the pancreas, glucagon directly impacts the liver as it works to control blood sugar levels. Specifically, glucagon prevents blood glucose levels from dropping to a dangerous point by stimulating the conversion of stored glycogen to glucose in the liver. This glucose can be released into the bloodstream, a process known as glycogenolysis. Secondly, glucagon stops the liver from consuming some glucose. This helps more glucose to enter the bloodstream, rather than being consumed by the liver, to keep levels stable. Finally, glucagon works in a process known as gluconeogenesis, which is the production of glucose in the amino acid molecules. In each of these processes, glucagon and insulin work together. Insulin will prevent glucose levels from increasing to a point that is too high, while glucagon prevents it from dropping too low. Glucagon production is stimulated when an individual eats a protein-rich meal, experiences a surge in adrenaline, or has a low blood sugar event. Potential problems with glucagon function Glucagon function is crucial to proper blood glucose levels, so problems with glucagon production will lead to problems Continue reading >>

Insulin And Glucagon

Insulin And Glucagon

Acrobat PDF file can be downloaded here. The islets of Langerhans The pancreatic Islets of Langerhans are the sites of production of insulin, glucagon and somatostatin. The figure below shows an immunofluorescence image in which antibodies specific for these hormones have been coupled to differing fluorescence markers. We can therefore identify those cells that produce each of these three peptide hormones. You can see that most of the tissue, around 80 %, is comprised of the insulin-secreting red-colored beta cells (ß-cells). The green cells are the α-cells (alpha cells) which produce glucagon. We see also some blue cells; these are the somatostatin secreting γ-cells (gamma cells). Note that all of these differing cells are in close proximity with one another. While they primarily produce hormones to be circulated in blood (endocrine effects), they also have marked paracrine effects. That is, the secretion products of each cell type exert actions on adjacent cells within the Islet. An Introduction to secretion of insulin and glucagon The nutrient-regulated control of the release of these hormones manages tissue metabolism and the blood levels of glucose, fatty acids, triglycerides and amino acids. They are responsible for homeostasis; the minute-to-minute regulation of the body's integrated metabolism and, thereby, stabilize our inner milieu. The mechanisms involved are extremely complex. Modern medical treatment of diabetes (rapidly becoming "public enemy number one") is based on insight into these mechanisms, some of which are not completely understood. I will attempt to give an introduction to this complicated biological picture in the following section. Somewhat deeper insight will come later. The Basics: secretion Let us begin with two extremely simplified figur Continue reading >>

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Glucagon, a pancreatic hormone produced by cells in the islets of Langerhans. Glucagon is a 29-amino-acid peptide that is produced specifically by the alpha cells of the islets. It has a high degree of similarity with several glucagon-like peptides that are secreted by cells scattered throughout the gastrointestinal tract. Glucagon secretion is stimulated by the ingestion of protein, by low blood glucose concentrations (hypoglycemia), and by exercise. It is inhibited by the ingestion of carbohydrates, an effect that may be mediated by the resultant increase in blood glucose concentrations and insulin secretion. Glucagon strongly opposes the action of insulin; it raises the concentration of glucose in the blood by promoting glycogenolysis, which is the breakdown of glycogen (the form in which glucose is stored in the liver), and by stimulating gluconeogenesis, which is the production of glucose from amino acids and glycerol in the liver. By increasing the concentration of glucose in the bloodstream, glucagon plays a critical role in maintaining blood glucose concentrations during fasting and exercise. Gastrointestinal glucagon, another form, is secreted into the blood when glucose is ingested; its only action appears to be to stimulate the secretion of insulin. Continue reading >>

You And Your Hormones

You And Your Hormones

What is glucagon? Glucagon is a hormone that is involved in controlling blood sugar (glucose) levels. It is secreted into the bloodstream by the alpha cells, found in the islets of langerhans, in the pancreas. The glucagon-secreting alpha cells surround a core of insulin-secreting beta cells, which reflects the close relationship between the two hormones. Glucagon’s role in the body is to prevent blood glucose levels dropping too low. To do this, it acts on the liver in several ways: It stimulates the conversion of stored glycogen (stored in the liver) to glucose, which can be released into the bloodstream. This process is called glycogenolysis. It promotes the production of glucose from amino acid molecules. This process is called gluconeogenesis. It reduces glucose consumption by the liver so that as much glucose as possible can be secreted into the bloodstream to maintain blood glucose levels. Glucagon also acts on adipose tissue to stimulate the breakdown of fat stores into the bloodstream. How is glucagon controlled? Glucagon works along with the hormone insulin to control blood sugar levels and keep them within set levels. Glucagon is released to stop blood sugar levels dropping too low, while insulin is released to stop blood sugar levels rising too high. Release of glucagon is stimulated by low blood glucose (hypoglycaemia), protein-rich meals and adrenaline (another important hormone for combating low glucose). Release of glucagon is prevented by raised blood glucose and carbohydrate in meals, detected by cells in the pancreas. In the longer-term, glucagon is crucial to the body’s response to lack of food. For example, it encourages the use of stored fat for energy in order to preserve the limited supply of glucose. What happens if I have too much glucagon? Continue reading >>

What Is Glucagon?

What Is Glucagon?

Tweet The effects of glucagon are the opposite of the effects induced by insulin. The two hormones need to work in partnership with each other to keep blood glucose levels balanced. Glucagon is a hormone that is produced by alpha cells in a part of the pancreas known as the islets of Langerhans. The role of glucagon in the body Glucagon plays an active role in allowing the body to regulate the utilisation of glucose and fats. Glucagon is released in response to low blood glucose levels and to events whereby the body needs additional glucose, such as in response to vigorous exercise. When glucagon is released it can perform the following tasks: Stimulating the liver to break down glycogen to be released into the blood as glucose Activating gluconeogenesis, the conversion of amino acids into glucose Breaking down stored fat (triglycerides) into fatty acids for use as fuel by cells Glucagon and blood glucose levels Glucagon serves to keep blood glucose levels high enough for the body to function well. When blood glucose levels are low, glucagon is released and signals the liver to release glucose into the blood. Glucagon secretion in response to meals varies depending on what we eat: In response to a carbohydrate based meal, glucagon levels in the blood fall to prevent blood glucose rising too high. In response to a high protein meal, glucagon levels in the blood rise. Glucagon in diabetes In people with diabetes, glucagon’s presence can raise blood glucose levels too high. The reason for this is either because not enough insulin is present or, as is the case in type 2 diabetes, the body is less able to respond to insulin. In type 1 diabetes, high levels of circulating insulin can inhibit the release of glucagon in response to hypoglycemia. Medications which affect gluca Continue reading >>

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