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Hormonal Control of Nutrient Metabolism and Storage Dr. Mujeeb Ahmed Shaikh Assistant Professor AlMaarefa College Learning Objectives • Describe the effect of insulin on glucose, fat, and protein metabolism. • Explain what is meant by counter-regulatory hormones, and describe the actions of glucagon, amylin, somatostatin, gut-derived hormones, epinephrine, growth hormone, and the adrenal cortical hormones in the regulation of blood glucose levels. Nutrient Metabolism and Storage • The body uses glucose, fatty acids, and other substrates as fuel to satisfy its energy needs. Glucose Metabolism and Storage • The brain and nervous system rely almost exclusively on glucose as a fuel source. • Other tissue can use fatty acids & ketone bodies • Body tissues obtain glucose from the blood. • In normal person the fasting blood glucose levels are tightly regulated between 80 and 90 mg/dL (4.4 and5.0 mmol/L). • After meal 2/3 of the glucose is removed from the blood & stored in liver as glycogen. • Between meals, the liver releases glucose as a means of maintaining the blood glucose within its normal range. • Extra glucose - glycogen / fats. • After muscle & liver get saturated the extra glucose used for FA & triglyceride synthesis. • Between meals, the liver releases glucose as a means of maintaining the blood glucose within its normal range. • The liver synthesizes glucose from amino acids, glycerol, and lactic acid in a process called gluconeogenesis. Glucose Regulating Hormones • The hormonal control of blood glucose resides largely within the endocrine pancreas. • The pancreas is made up of two major tissue types: the acini and the islets of Langerhans. • Acini – digestive juices into duodenum • islets of Langerhans - secrete hormones into the blood. Islet cells • • • • Alpha cells – Glucagon Beta cells – insulin & amylin Delta cells - Somatostatin. PP cells - pancreatic polypeptide Endocrine pancreas: Islets of Langerhans Alpha cells Beta cells Glucagon Insulin and amylin Delta cells PP cells Somatostatin Pancreatic polypeptide Functions of Pancreatic Hormones • Glucagon: causes cells to release stored food into the blood • Insulin: allows cells to take up glucose from the blood • Amylin: slows glucose absorption in small intestine; suppresses glucagon secretion • Somatostatin: decreases GI activity; suppresses glucagon and insulin secretion Anabolism and Catabolism available foodstuffs (in blood) glucose amino acids free fatty acids liver can convert amino acids and free fatty acids into ketones Anabolism Insulin, anabolic steroids Catabolism glucagon, epinephrine, cortisol stored foodstuffs (in cells) glycogen proteins triglycerides Insulin and Glucagon Are the Main Controls available foodstuffs (in blood) glucose Anabolism Insulin , anabolic steroids stored foodstuffs (in cells) glycogen amino acids Catabolism Glucagon, proteins free fatty acids epinephrine, cortisol triglycerides liver can convert amino acids and free fatty acids into ketones Insulin • Insulin is the only hormone known to have a direct effect in lowering blood glucose levels. • The actions of insulin are threefold: 1. it promotes glucose uptake by target cells and provides for glucose storage as glycogen, 2. it prevents fat and glycogen breakdown, and 3. It inhibits gluconeogenesis and increases protein synthesis • Insulin acts to promote – fat storage by increasing the transport of glucose into fat cells – It also facilitates triglyceride synthesis from glucose in cells and inhibits the intracellular breakdown of stored triglycerides. • Insulin also inhibits protein break-down and increases protein synthesis by increasing the active transport of amino acids into body cells, and it inhibits gluconeogenesis, or the building of glucose from new sources, mainly amino acids. Metabolic Effects of Insulin Glycogen Glucose Glucose-P Glucose Pyruvate Free fatty acids Protein Amino acids CO2 Ketoacids Adipose tissue Liver Muscle Plasma Stimulate Inhibit Glucose Free fatty acids Ketoacids Amino acids Approximate Rates of Insulin Secretion at Different Blood Glucose Levels Insulin Response ) ( 100% 50% 0 40 80 120 160 200 Plasma glucose (mg/dl) 240 Insulin Proinsulin α - chain Active insulin β - chain C - Peptide • The C-peptide chains can be measured clinically, and this measurement can be used to study beta cell function (i.e., persons with type 2diabetes with very little or no remaining beta cell function will have very low or nonexistent levels of C-peptide in their blood, and thus will likely need insulin replacement for treatment). Insulin vs Blood Glucose Increase blood glucose Increased insulin secretion • Secretion of insulin occurs in a pulsatile fashion. After exposure to glucose, which is a nutrient secretagogue, a First-phase release of stored preformed insulin occurs, followed by a secondphase release of newly synthesized insulin. • Serum insulin levels begin to rise within minutes after a meal, reach a peak in approximately 3 to 5 minutes, and then return to baseline levels within 2 to 3 hours. • Insulin has a half-life of approximately 15 minutes once it is released into the general circulation. Mechanism of Glucose Stimulated Insulin Secretion Factors Affecting Insulin Secretion Inhibitors Stimulators Glucose Mannose Galactose Amino acids Arginine Lysine Leucine Alanine Fasting Neural influences Vagal activity (acetylcholine) b-adrenergic stimulation Drugs like Sulfonylurea Free fatty acids, Keto acids Glucagon (direct and indirect effects) Gastro intestinal hormones Glucagon like peptide 1 (GLP-1) Glucose dependant Insulinotropic Peptide (GIP) Secretin Exercise Somatostatin Neural influences Sympathetic activity-adrenergic stimulation norepinephrine, epinephrine) Processing of Proinsulin and C-Peptide Connecting peptide HOOC NH2 20 1 1 S S 5 A-chain B-chain 15 10 S SS 5 30 S 25 10 20 15 Proinsulin C-Peptide S S NH2 NH2 20 COOH 1 5 1 5 S S 10 15 S S 15 20 A-chain Insulin COOH 10 B-chain 25 30 The Insulin Receptor & Mechanisms of Insulin Action Insulin Receptors • Cell membranes are impermeable to glucose, they require a special carrier, called a glucose transporter, to move glucose from the blood into the cell. • GLUT 1, GLUT 2, GLUT 3, GLUT 4. • GLUT-4 is the insulin-dependent glucose transporter for skeletal muscle and adipose tissue • GLUT-2 is the major transporter of glucose into beta cells and liver cells. It has a low affinity for glucose and acts as a transporter only when plasma glucose levels are relatively high, such as after a meal. • GLUT-1 is present in all tissues. It does not require the actions of insulin and is important in transport of glucose into cells of the nervous system. Glucagon • A polypeptide molecule produced by the alpha cells of the islets of Langerhans, maintains blood glucose between meals and during periods of fasting. • Glucagon produces an increase in blood glucose. • Initiate breakdown of glycogen between meals • Gluconeogenesis • Makes fatty acids available for use as energy Factors Affecting Glucagon Secretion Inhibition Stimulation Hypoglycemia Glucose Amino acids Arginine Alanine Somatostatin Gastrointestinal hormones Cholecystokinin (CCK) Gastrin Gastrointestinal hormones Secretin Glucagon-like peptide-1 (GLP-1) Fasting Free fatty acids Exercise Ketoacids Neural influences Vagal activity-acetylcholine Sympathetic activityb-adrenergic stimulation (norepinephrine, epinephrine) Neural influences -adrenergic stimulation Insulin (direct effect) Approximate Rates of Insulin & Glucagon Secretion at Different Blood Glucose Levels 100% Glucagon Response ( ) Insulin Response ) ( 100% 50% 50% 0 0 40 80 120 160 200 Plasma glucose (mg/dl) 240 Amylin, Somatostatin, and Gut-Derived Hormones • A polypeptide that is co secreted with insulin from the beta cells. • Plasma levels of amylin increase in response to nutritional stimuli to produce inhibition of gastric emptying and glucagon secretion. • he less soluble and insoluble forms, which may cause degeneration of the beta cells and contribute to the pathogenesis of overt diabetes Somatostatin • Its a polypeptide hormone containing only 14 amino acids that has an extremely short half-life. • Secreted by the delta cells acts locally in the islets of Langerhans to inhibit the release of insulin and glucagon. • It also decreases gastrointestinal activity after ingestion of food. • Almost all factors related to ingestion of food stimulate somatostatin secretion. Incretins • Several gut derived hormones have been identified as having what is termed an incretin effect, meaning that they increase insulin release after an oral nutrient load. • The two hormones that account for about 90% of the incretin effect are glucagon like peptide-1, which is released from L cells in the distal small bowel, and glucose-dependent insulinotropic polypeptide, which is released by K cells in the upper gut (mainly the jejunum). Counter regulatory Hormones • They counteract the storage functions of insulin in regulating blood glucose levels during periods of fasting, exercise, and other situations that either limit glucose intake or deplete glucose stores. 1. The catecholamines, 2. Growth hormone, and 3. Glucocorticoids. Epinephrine • helps to maintain blood glucose levels during periods of stress. • Glycogenolysis in the liver • Inhibits insulin release • Increasing the breakdown of muscle glycogen stores. • A direct lipolytic effect on adipose cells, Growth Hormone • Increases protein synthesis • Mobilizes fatty acids from adipose tissue, and antagonizes the effects of insulin. • Chronic hypersecretion of growth hormone, as occurs in acromegaly, can lead to glucose intolerance and the development of diabetes mellitus. Glucocorticoid Hormones • Gluconeogenesis by the liver • Hypoglycemia is a potent stimulus for cortisol secretion. • In predisposed persons, the prolonged elevation of glucocorticoid hormones can lead to hyperglycemia and the development of diabetes mellitus. Hormonal Interactions in the Maintenance of Blood Glucose Concentration Glucose Production (Liver Hepatic Glucose Output) (-) (+) (+) Insulin Glucagon Blood Glucose (+) Epinephrine Glucocorticoids Growth Hormone (-) Glucose Consumption (Skeletal Muscle and Adipose Tissue) Hormonal Effects on FFA Production in Adipose Tissue Epinephrine Insulin Triglycerides Insulin + Hormone _ Sensitive Lipase -Glycerol-P + _ glucose Fatty Acids Growth hormone Cortisol Epinephrine glycerol FFA