7.1 Insulin and glucagon: structure, secretion, and action

Insulin is a peptide hormone produced by the pancreas that plays a crucial role in regulating blood glucose levels and metabolism. It facilitates the uptake of glucose by cells, promotes glycogen synthesis, and aids in lipid and protein metabolism, making it essential for maintaining energy balance in the body.

Glucagon: A hormone produced by the pancreas that works in opposition to insulin by raising blood glucose levels, primarily through promoting glycogen breakdown in the liver.

Diabetes Mellitus: A metabolic disorder characterized by chronic high blood sugar levels due to inadequate insulin production or action, leading to various health complications.

Glycogen: A stored form of glucose in the liver and muscles that can be quickly mobilized to meet energy needs, regulated by insulin and glucagon.

Glucagon is a peptide hormone produced by the alpha cells of the pancreas that plays a critical role in glucose metabolism by increasing blood glucose levels. It is primarily released during fasting states when blood glucose levels are low, signaling the liver to convert glycogen into glucose and release it into the bloodstream, thus ensuring a continuous supply of energy for the body.

Insulin: A hormone produced by the beta cells of the pancreas that lowers blood glucose levels by promoting the uptake of glucose by cells and stimulating glycogen synthesis in the liver.

Glycogenolysis: The biochemical process of breaking down glycogen into glucose-1-phosphate, which is then converted to glucose, primarily occurring in the liver.

Liver Metabolism: The series of biochemical processes that occur in the liver, including gluconeogenesis, glycogen synthesis and breakdown, and lipid metabolism, which are crucial for maintaining energy homeostasis.

Preproinsulin is the initial, inactive form of insulin synthesized in the beta cells of the pancreas. It is a precursor molecule that undergoes several processing steps to become active insulin, essential for glucose metabolism and regulation. Understanding preproinsulin is critical as it illustrates the complex pathway from synthesis to secretion of insulin, highlighting its structural features and functional importance in maintaining glucose homeostasis.

Insulin: A peptide hormone produced by the beta cells of the pancreas that regulates blood glucose levels by facilitating cellular glucose uptake.

Proinsulin: The processed form of preproinsulin that consists of insulin and C-peptide, which is cleaved during the conversion to active insulin.

C-peptide: A peptide that connects the A and B chains of insulin in proinsulin and is released into the bloodstream when proinsulin is cleaved to form insulin.

Beta cells are specialized pancreatic cells that play a crucial role in glucose metabolism by producing and secreting insulin. They are located in the islets of Langerhans within the pancreas and are responsible for regulating blood sugar levels by facilitating the uptake of glucose into cells. Insulin secretion from beta cells occurs in response to rising blood glucose levels, highlighting their essential function in maintaining energy balance and homeostasis in the body.

Insulin: A hormone produced by beta cells that helps regulate blood glucose levels by promoting glucose uptake and storage in tissues.

Glucagon: A hormone produced by alpha cells in the pancreas that raises blood glucose levels by promoting the release of glucose from the liver.

Islets of Langerhans: Clusters of endocrine cells in the pancreas, including beta cells, alpha cells, and delta cells, which secrete hormones involved in glucose regulation.

Alpha cells are specialized cells located in the pancreatic islets, also known as the islets of Langerhans, responsible for producing and secreting glucagon. These cells play a crucial role in regulating blood glucose levels by promoting the release of glucose from stored glycogen in the liver when blood sugar levels are low. The action of glucagon counteracts insulin, helping to maintain homeostasis in energy metabolism.

Glucagon: A hormone produced by alpha cells that raises blood glucose levels by stimulating glycogen breakdown and glucose release from the liver.

Beta cells: Another type of cell found in the pancreatic islets that produces insulin, a hormone that lowers blood glucose levels by facilitating cellular uptake of glucose.

Pancreatic islets: Clusters of endocrine cells in the pancreas that include alpha cells, beta cells, and other cell types involved in hormone production.

Proinsulin is a precursor molecule to insulin, formed in the pancreatic beta cells and consisting of a single-chain polypeptide that undergoes proteolytic cleavage to produce active insulin. This molecule plays a crucial role in the synthesis and secretion of insulin, which is essential for glucose metabolism and maintaining blood sugar levels.

Insulin: A hormone produced by the pancreas that regulates glucose levels in the blood by facilitating cellular uptake of glucose.

C-peptide: A short peptide that is released into the bloodstream during the cleavage of proinsulin into insulin, serving as a marker for insulin production.

Beta cells: Specialized cells in the pancreas that produce and secrete insulin and proinsulin, playing a vital role in glucose homeostasis.

C-peptide is a short peptide that is released into the bloodstream when proinsulin is cleaved to form insulin and C-peptide. This process occurs in the pancreatic beta cells and serves as a crucial marker for insulin production, helping to assess the function of insulin secretion in the body.

Proinsulin: Proinsulin is the precursor molecule to insulin, consisting of insulin and C-peptide linked together. It is synthesized in the pancreas and converted into active insulin and C-peptide before release into circulation.

Insulin: Insulin is a hormone produced by the pancreatic beta cells that regulates glucose levels in the blood by facilitating the uptake of glucose into tissues, thereby lowering blood sugar levels.

Diabetes Mellitus: Diabetes Mellitus is a chronic condition characterized by high blood sugar levels due to either insufficient insulin production or ineffective use of insulin by the body.

GLP-1, or Glucagon-Like Peptide-1, is an incretin hormone produced in the intestines that plays a crucial role in glucose metabolism and insulin secretion. It enhances insulin release from the pancreas in response to meals, suppresses glucagon release, and slows gastric emptying, which together help regulate blood sugar levels. GLP-1 is integral to understanding how insulin and glucagon function in maintaining glucose homeostasis.

Incretins: Hormones released by the gut in response to food intake that enhance insulin secretion from the pancreas.

Insulin: A peptide hormone produced by the pancreas that lowers blood glucose levels by promoting glucose uptake in tissues.

Glucagon: A hormone produced by the alpha cells of the pancreas that raises blood glucose levels by promoting glycogen breakdown and gluconeogenesis.

GLP-2, or Glucagon-Like Peptide-2, is an intestinal hormone produced by the L cells of the intestinal mucosa in response to nutrient intake. It plays a significant role in enhancing gut growth, promoting intestinal absorption, and regulating nutrient metabolism, especially in the context of insulin and glucagon signaling.

GLP-1: GLP-1, or Glucagon-Like Peptide-1, is a hormone that stimulates insulin secretion, inhibits glucagon release, and increases satiety, playing a key role in glucose homeostasis.

Insulin: Insulin is a peptide hormone produced by the pancreas that regulates blood glucose levels by facilitating cellular glucose uptake and promoting storage of glucose as glycogen.

Glucagon: Glucagon is a peptide hormone also produced by the pancreas that raises blood glucose levels by promoting glycogen breakdown and gluconeogenesis in the liver.

Glucose levels refer to the concentration of glucose in the bloodstream, which is a vital source of energy for the body’s cells. Maintaining appropriate glucose levels is crucial for metabolic processes and overall health, as they are tightly regulated by hormones such as insulin and glucagon. When glucose levels rise after eating, insulin facilitates its uptake by cells, while glucagon plays a key role in increasing glucose levels during fasting or low-energy states.

Insulin: A hormone produced by the pancreas that lowers blood glucose levels by promoting glucose uptake in cells.

Glucagon: A hormone produced by the pancreas that raises blood glucose levels by stimulating glycogen breakdown and glucose release from the liver.

Homeostasis: The process through which biological systems maintain stability while adjusting to conditions that are optimal for survival.

ATP-sensitive potassium channels (K_ATP channels) are a type of potassium channel that opens in response to low intracellular ATP levels, allowing potassium ions to flow out of the cell. These channels play a crucial role in linking cellular metabolism to electrical activity, influencing insulin secretion from pancreatic beta cells and regulating various physiological processes.

Pancreatic beta cells: Cells in the pancreas responsible for producing and secreting insulin, which regulates blood glucose levels.

Insulin: A hormone produced by pancreatic beta cells that lowers blood glucose levels by promoting cellular uptake of glucose.

Glucose metabolism: The process by which glucose is broken down to produce energy, with its regulation being critical for maintaining normal cellular function.

Hypoglycemia refers to a condition characterized by abnormally low levels of glucose in the blood, typically defined as a blood glucose level below 70 mg/dL. This state can lead to various physiological responses as the body attempts to restore normal glucose levels, often involving hormonal regulation. Hormones like insulin and glucagon play critical roles in maintaining glucose homeostasis, highlighting the importance of these hormones in metabolic control.

Insulin: A hormone produced by the pancreas that lowers blood glucose levels by promoting the uptake of glucose into cells and stimulating glycogenesis.

Glucagon: A hormone secreted by the alpha cells of the pancreas that raises blood glucose levels by promoting gluconeogenesis and glycogenolysis in the liver.

Glycogenesis: The metabolic process of converting glucose into glycogen for storage, primarily occurring in the liver and muscles.

A glucose transporter is a type of protein that facilitates the transport of glucose across cell membranes, allowing cells to take up glucose for energy and metabolism. These transporters are essential for maintaining glucose homeostasis in the body, responding to hormonal signals and the energy needs of different tissues and organs.

GLUT: GLUT refers to a family of glucose transporter proteins, with different members (like GLUT1, GLUT2, GLUT4) having specific roles and tissue distributions.

Insulin: Insulin is a hormone produced by the pancreas that regulates blood glucose levels by promoting the uptake of glucose into cells, particularly through the action of glucose transporters.

Homeostasis: Homeostasis refers to the physiological processes that maintain stable internal conditions in the body, including the regulation of blood glucose levels.

Glycogenolysis is the biochemical process of breaking down glycogen into glucose molecules, which can then be used as a source of energy by the body. This process is particularly important during fasting or periods of intense physical activity, where the body requires quick access to glucose. The regulation of glycogenolysis is closely linked to hormonal signals, energy demands, and metabolic states, making it a crucial component of overall carbohydrate metabolism.

Glycogen: A polysaccharide that serves as a form of energy storage in animals, primarily found in the liver and muscles.

Glucagon: A hormone produced by the pancreas that raises blood glucose levels by stimulating glycogenolysis and gluconeogenesis.

AMP-activated protein kinase (AMPK): An enzyme that plays a key role in cellular energy homeostasis and can activate glycogenolysis when energy levels are low.

Gluconeogenesis is the metabolic process that generates glucose from non-carbohydrate precursors, such as lactate, glycerol, and certain amino acids. This process is crucial during fasting or intense exercise when blood glucose levels are low, ensuring a continuous supply of glucose for vital functions, particularly in the brain and red blood cells.

Cori Cycle: A metabolic pathway that interconverts lactate and glucose between the liver and muscle, allowing for the recycling of lactate produced during anaerobic metabolism.

Glycogenesis: The process of converting glucose into glycogen for storage in the liver and muscle tissues, acting as a readily available energy source.

Ketogenesis: The metabolic pathway that produces ketone bodies from fatty acids, providing an alternative energy source when glucose is scarce.

Glycogen synthesis is the biochemical process by which glucose molecules are polymerized to form glycogen, a polysaccharide that serves as a major energy reserve in animals. This process is primarily regulated by insulin, which promotes the uptake of glucose and its conversion into glycogen in liver and muscle cells, highlighting the intricate hormonal control of metabolism.

Glycogen: A branched polysaccharide that acts as a form of energy storage in animals, primarily found in the liver and muscles.

Insulin: A peptide hormone produced by the pancreas that plays a key role in regulating glucose levels in the blood and promoting glycogen synthesis.

Glycogenolysis: The biochemical process of breaking down glycogen into glucose molecules to be used as an energy source during periods of fasting or intense exercise.

Lipogenesis is the metabolic process through which fatty acids and triglycerides are synthesized from acetyl-CoA and glycerol, primarily occurring in the liver and adipose tissue. This process plays a crucial role in energy storage and helps maintain lipid homeostasis during periods of excess caloric intake.

Fatty Acid Synthase: An enzyme complex that catalyzes the synthesis of fatty acids from acetyl-CoA and malonyl-CoA in a multi-step reaction.

Insulin: A hormone secreted by the pancreas that promotes glucose uptake and lipogenesis, facilitating the conversion of excess nutrients into stored fat.

Triglycerides: The main form of stored fat in the body, composed of three fatty acids attached to a glycerol backbone, serving as a key energy reserve.

Lipolysis is the metabolic process of breaking down lipids, specifically triglycerides, into glycerol and free fatty acids. This process is essential for energy production and plays a crucial role in the regulation of fat metabolism in response to hormonal signals, linking the breakdown of fats to the body's energy needs.

Triglycerides: The main form of stored fat in the body, composed of three fatty acids esterified to a glycerol backbone.

Hormonal Regulation: The control of physiological processes by hormones, which can stimulate or inhibit metabolic pathways like lipolysis.

Beta-oxidation: The metabolic process that breaks down fatty acids into acetyl-CoA units for energy production after lipolysis has occurred.

Fatty acid oxidation is the metabolic process by which fatty acids are broken down to produce energy, primarily in the form of ATP. This process occurs mainly in the mitochondria and involves the sequential removal of two-carbon units from the fatty acid chain, converting them into acetyl-CoA, which then enters the citric acid cycle for further energy extraction. Understanding this process is crucial for grasping how the body adapts to different energy states, regulates metabolism through hormones, and manages energy production and storage.

Beta-Oxidation: The series of enzymatic reactions that degrade fatty acids into acetyl-CoA units, forming NADH and FADH2 in the process.

Ketogenesis: The metabolic pathway through which excess acetyl-CoA is converted into ketone bodies, providing an alternative energy source during fasting or low-carbohydrate intake.

Lipid Metabolism: The complex biochemical processes involving the synthesis and degradation of lipids, including fatty acids, triglycerides, and phospholipids.

Amino acid uptake refers to the process by which cells absorb amino acids from their surrounding environment, primarily through specialized transporters in the cell membrane. This process is crucial for protein synthesis, cellular metabolism, and overall cellular function. Insulin and glucagon significantly influence amino acid uptake, as they regulate various metabolic pathways that dictate how cells utilize nutrients.

Transporters: Proteins embedded in the cell membrane that facilitate the movement of molecules, including amino acids, into and out of cells.

Insulin: A hormone produced by the pancreas that promotes glucose uptake and stimulates amino acid transport into cells, playing a vital role in anabolic metabolism.

Glucagon: A hormone secreted by the pancreas that has the opposite effect of insulin, stimulating gluconeogenesis and affecting amino acid release from tissues to maintain blood glucose levels.

mRNA translation is the process through which messenger RNA (mRNA) is decoded by ribosomes to synthesize proteins. This essential step in gene expression involves the conversion of the genetic information carried by mRNA into a specific sequence of amino acids, forming a polypeptide chain that ultimately folds into a functional protein. The translation process is critical for the proper functioning of hormones such as insulin and glucagon, as it directly influences their production and activity in the body.

Ribosome: A complex molecular machine found within all living cells that performs the task of translating mRNA into proteins by facilitating the assembly of amino acids.

tRNA (Transfer RNA): Molecules that transport specific amino acids to the ribosome during protein synthesis, ensuring the correct sequence is assembled according to the mRNA template.

Codon: A sequence of three nucleotides in mRNA that specifies a particular amino acid or signals the termination of protein synthesis.

Potassium uptake refers to the process by which cells absorb potassium ions (K+) from their surrounding environment, a crucial function for maintaining cellular function and overall homeostasis. This process is especially important in the context of insulin and glucagon, as these hormones influence potassium levels in the body by regulating the movement of potassium into cells, impacting various metabolic pathways and electrical activities within tissues such as muscle and nerve cells.

Insulin: A hormone produced by the pancreas that promotes the uptake of glucose and potassium into cells, playing a vital role in regulating blood sugar levels.

Glucagon: A hormone secreted by the pancreas that raises blood glucose levels by stimulating glycogen breakdown and influencing potassium distribution in the body.

Homeostasis: The maintenance of stable internal conditions within an organism, including electrolyte balance and fluid regulation.

Insulin resistance is a condition where cells in the body become less responsive to the hormone insulin, leading to impaired glucose uptake and increased blood sugar levels. This phenomenon plays a crucial role in the development of metabolic disorders such as type 2 diabetes and is closely linked to obesity, hormonal regulation, and the overall energy metabolism in the body.

Type 2 Diabetes: A chronic condition characterized by insulin resistance, leading to high blood sugar levels due to the body's inability to effectively use insulin.

Leptin: A hormone produced by adipose (fat) tissue that helps regulate energy balance by inhibiting hunger, which can be disrupted in obesity and contribute to insulin resistance.

Metabolic Syndrome: A cluster of conditions, including hypertension, high blood sugar, excess body fat around the waist, and abnormal cholesterol levels, that occur together and increase the risk of heart disease and diabetes.

Diabetes mellitus is a chronic metabolic disorder characterized by high blood sugar levels due to either insufficient insulin production or the body's cells not responding effectively to insulin. This condition impacts the body's ability to regulate glucose, leading to various metabolic adaptations during fed and fasting states, as well as influencing hormonal control and nutrient sensing pathways.

Insulin: A hormone produced by the pancreas that regulates blood glucose levels by facilitating the uptake of glucose into cells.

Glucose: A simple sugar that is an important energy source in living organisms and is a primary product of carbohydrate metabolism.

Type 1 Diabetes: A form of diabetes where the immune system attacks insulin-producing beta cells in the pancreas, leading to little or no insulin production.

A counterregulatory response is a physiological mechanism that occurs in the body to counteract the effects of insulin, particularly during hypoglycemia or low blood sugar levels. This response involves the secretion of hormones like glucagon and epinephrine, which work to increase blood glucose levels by stimulating glycogenolysis and gluconeogenesis in the liver. Understanding this response is crucial for grasping how the body maintains glucose homeostasis and how insulin and glucagon interact to regulate energy metabolism.

Glucagon: A hormone produced by the alpha cells of the pancreas that raises blood glucose levels by promoting glycogen breakdown and gluconeogenesis.

Insulin: A hormone produced by the beta cells of the pancreas that lowers blood glucose levels by facilitating the uptake of glucose into cells.

Hypoglycemia: A condition characterized by abnormally low blood glucose levels, which can trigger a counterregulatory response to restore normal glucose levels.