Endocrine Pancreas

Introduction

The endocrine pancreas is central to metabolic balance through insulin regulation of glucose and fat metabolism. Its dysfunction causes hyperglycemia, dyslipidemia, and ketosis. Hemoglobin A1c reflects long-term glycemic control, while C-peptide helps assess endogenous beta-cell function.

• The pancreas is a vital organ that regulates digestion, nutrient storage, and energy utilization. It has two major components: exocrine pancreas and endocrine pancreas.
• The exocrine part forms most of the gland, about 80%, and secretes digestive enzymes and bicarbonate into the intestine.
• The endocrine part forms about 1–2% of pancreatic tissue and consists of islets that secrete hormones controlling metabolism.
• The endocrine pancreas secretes insulin, glucagon, somatostatin, and pancreatic polypeptide.
• Insulin is the main hormone that lowers blood glucose and prevents excessive ketone formation.
• Glucagon usually exerts effects opposite to insulin.
• The endocrine pancreas secretes insulin, glucagon, somatostatin, and pancreatic polypeptide.
• Insulin is the main hormone that lowers blood glucose and prevents excessive ketone formation.
• Glucagon usually exerts effects opposite to insulin.
• Insulin is anabolic and promotes storage of glucose, fatty acids, and amino acids. It stimulates glycogen synthesis, lipogenesis, and protein synthesis.
• Glucagon is catabolic and increases release of nutrients into blood. It stimulates glycogen breakdown and gluconeogenesis, mainly in the liver.
• Together, these hormones maintain continuous energy supply in resting and active states.

• Insulin deficiency causes diabetes mellitus and may lead to ketosis.
• Excess insulin may produce hypoglycemia.
• Glucagon deficiency can also contribute to hypoglycemia.
• Excess glucagon may worsen diabetes by increasing hepatic glucose output.
• In many diabetic states, insulin deficiency is accompanied by relative glucagon excess.

Physiological Anatomy

Islets of Langerhans

• The endocrine pancreas forms less than 2% of total pancreatic tissue. It is composed of the islets of Langerhans, which are clusters of hormone-secreting cells. These islets are more numerous in the pancreatic tail than in the head or body.
• The human pancreas contains nearly 1 million islets.
• Each islet is oval and measures about 50–300 micrometers in diameter.
• An average islet contains around 2,500 cells.
• Beta cells secrete insulin, which lowers blood glucose levels.
• Alpha cells release glucagon.
• Delta cells produce somatostatin.
• F cells secrete pancreatic polypeptide.

β Cells

• Beta cells form about 70–80% of islet cells and secrete insulin. These cells are mainly located in the central region of the islet.
• Each beta cell measures approximately 10–20 micrometers in diameter.
• Insulin is stored within cytoplasmic secretory granules about 0.25 micrometers wide.
• The granules are enclosed in vesicles with a clear halo between the vesicle wall and insulin core.
• Cytoplasmic microtubules and microfilaments assist movement and exocytosis of secretory vesicles.

α Cells

• Alpha cells constitute nearly 15–20% of islet cells. They are mainly situated at the islet periphery.
• Alpha cells release glucagon, which increases blood glucose concentration.

d Cells

• Delta cells account for about 10% of islet cells. They are usually found between beta and alpha cells toward the peripheral zone.
• Delta cells secrete somatostatin, which inhibits release of several pancreatic and gastrointestinal hormones.

F Cells

• F cells form about 1–2% of the cells in pancreatic islets. They secrete pancreatic polypeptide, a hormone involved in regulating pancreatic exocrine secretion, gastrointestinal motility, and appetite.
• These cells are scattered irregularly among delta cells within the islets.

Fate of Islet Hormones

• Hormones released from islet cells first enter the pancreatic venous blood. They are then carried to the portal vein and reach the liver before entering systemic circulation. This vascular pathway allows the liver immediate exposure to newly secreted hormones.
• The liver is the principal organ for nutrient metabolism and responds rapidly to insulin and glucagon.
• Early hepatic exposure helps maintain normal blood glucose and metabolic balance.

Blood Supply of Islets

• Pancreatic islets are highly vascular endocrine structures.
• Although they constitute only about 1% of pancreatic tissue, they receive nearly 10% of total pancreatic blood flow.
• Dense capillary networks support rapid hormone exchange with blood.
• A local portal arrangement allows hormones from one group of islet cells to influence neighboring cells. This mechanism is important for paracrine regulation within the islet.

Innervation of Islets

• Islet cells receive both parasympathetic and sympathetic autonomic nerve supply.
• Neural input modifies hormone secretion according to nutritional and metabolic needs.
• Beta-adrenergic receptor stimulation usually increases insulin release.
• Alpha-adrenergic receptor stimulation generally reduces insulin secretion.
• Increased vagal cholinergic activity enhances insulin secretion, especially during meals.

Insulin

Structure of Insulin

• Insulin is a peptide hormone produced by pancreatic beta cells. It consists of two polypeptide chains, A chain and B chain, linked by disulfide bonds.
• The approximate molecular weight of insulin is 6,000 daltons.
• The A chain contains 21 amino acids.
• The B chain contains 30 amino acids.
• The specific arrangement of amino acids and disulfide linkages determines the final three-dimensional structure of insulin.
• Amino acids near the carboxyl terminal region of the B chain are important for biological activity and receptor binding.
• Insulin is initially synthesized as single molecules called monomers.
• In storage granules, insulin molecules associate with zinc ions to form stable crystalline hexamers.

Synthesis, Secretion and Metabolism

Synthesis of Insulin

• Insulin is synthesized as a peptide precursor called preproinsulin, which contains about 110 amino acids.
• The insulin gene is located on the short arm of chromosome 11. This gene is related to the family that also includes insulin-like growth factors.
• Preproinsulin is formed on ribosomes attached to the rough endoplasmic reticulum of pancreatic beta cells.
• Preproinsulin contains four sequential parts:
  • An amino-terminal signal peptide
  • The B chain of insulin
  • The connecting C peptide
  • The A chain of insulin

• The signal peptide is rapidly removed after synthesis, converting preproinsulin into proinsulin, which contains 86 amino acids.
• Proinsulin then enters the Golgi apparatus for further processing.

Processing in the Golgi Apparatus

• In the Golgi apparatus, disulfide bonds are formed between the A and B chains. These bonds help the molecule fold into its active three-dimensional structure.
• The C peptide links the A and B chains during folding and assists correct structural alignment.
• During packaging into secretory granules, proinsulin is enzymatically cleaved to produce mature insulin and C peptide.

Storage in Secretory Granules

• Mature insulin associates with zinc ions inside granules.
• Zinc stabilizes insulin as crystalline hexamers, forming the dense central core of the granule.
• C peptide remains in the surrounding clear space of the granule.
• A small quantity of unprocessed proinsulin is also stored and later secreted.

Substances Released from Beta Cells

• Beta-cell granules release three principal substances: insulin, proinsulin, and C peptide.

Insulin

• Insulin constitutes nearly 95% of beta-cell secretory products.
• It is the major hormone regulating carbohydrate, fat, and protein metabolism.

Proinsulin

• Proinsulin has approximately 10% of the biological activity of insulin.
• It is secreted in small amounts, roughly 3% of secreted insulin.
• Its plasma half-life is longer (15–30 minutes) than insulin, so fasting plasma levels may be 10–15% of insulin concentration.

C Peptide

• C peptide is a 31 amino acid peptide released in equimolar amounts with endogenous insulin. It has no significant glucose-lowering action.
• Because hepatic metabolism of C peptide is minimal, its plasma level reflects pancreatic beta-cell secretion more accurately than insulin levels.
• C peptide measurement is useful in distinguishing endogenous insulin production from injected insulin therapy.
• It is clinically valuable in evaluating residual beta-cell function in diabetes mellitus.

Substances Having Insulin Like Activity

• Human plasma contains a few substances with insulin-like activity, but their glucose-lowering effect is weak compared with insulin. Therefore, insulin deficiency or insulin resistance can still result in diabetes mellitus.
• Major insulin-like substances include:
  • Insulin
  • Proinsulin
  • Non-suppressible insulin-like activities, mainly insulin-like growth factor I and insulin-like growth factor II, along with their binding proteins.

• These factors are not neutralized by anti-insulin antibodies.
• Plasma glucose concentration is the main regulator of insulin synthesis and secretion.
• Feeding increases insulin release, while fasting decreases it.
• Glucose enhances insulin gene transcription and translation of insulin messenger ribonucleic acid.

Secretion of Insulin

• Insulin secretion is strongly regulated by plasma glucose concentration.
• Increased blood glucose is the major stimulus for insulin release from pancreatic beta cells.
• Multiple hormonal and neural factors also modify secretion.

Table 60.1: Important factors that regulate insulin secretion.

Effect on Insulin Secretion	Major Regulators
Increase	Hyperglycemia (strongest stimulus), glucose/galactose, amino acids (arginine, leucine, alanine), ketoacids, long-chain free fatty acids, gastrointestinal hormones (GIP, GLP-1, gastrin, secretin, cholecystokinin), vagal stimulation, acetylcholine, beta-adrenergic agonists, potassium, calcium, theophylline
Decrease	Hypoglycemia, fasting, exercise, somatostatin, leptin, interleukin-1, alpha-adrenergic agonists, beta blockers, hypokalemia, beta-cell toxins (streptozotocin, alloxan), microtubule inhibitors such as colchicine

Regulation of Insulin Secretion

• Insulin secretion is primarily regulated by feedback from plasma nutrient levels.
• When glucose and other nutrients rise, insulin release increases to promote uptake and metabolism.
• When nutrient levels fall, insulin secretion decreases accordingly.

Plasma Glucose

• Glucose is the most important physiological stimulus for insulin secretion from pancreatic beta cells.
• As plasma glucose rises, plasma insulin concentration increases almost linearly within a glucose range of approximately 50–300 milligrams per deciliter.
• Insulin secretion is minimal when plasma glucose falls below 50 milligrams per deciliter.
• Above 300 milligrams per deciliter, further glucose elevation produces little additional insulin release because secretion approaches maximal capacity.
• After a sudden increase in blood glucose, insulin secretion occurs in two phases. This biphasic pattern is especially evident after intravenous glucose administration.

First Phase Response

• The first phase begins immediately after glucose rises.
• Insulin secretion increases rapidly and reaches a peak within 1–2 minutes. It then declines toward basal levels within the next 2–3 minutes. This rapid response results from exocytosis of preformed insulin granules already stored in beta cells.
• The first phase helps limit the initial postprandial rise in blood glucose.

Second Phase Response

• The second phase begins after the early rapid response.
• Plasma insulin concentration rises gradually and reaches a peak in about 60 minutes.
• Elevated secretion may persist for 3–5 hours while glucose remains increased. This phase depends on continued release of stored insulin plus new insulin synthesis. It is important for sustained glucose disposal after meals.

Mechanism of Glucose-induced Insulin Secretion

• Glucose enters pancreatic beta cells through glucose transporters, mainly GLUT2 in many species. In humans, GLUT1 and GLUT3 also contribute.
• Inside beta cells, glucose undergoes glycolysis and mitochondrial oxidation, producing adenosine triphosphate.
• Rising intracellular adenosine triphosphate closes ATP-sensitive potassium channels.
• Reduced potassium efflux causes membrane depolarization of the beta cell.
• Depolarization opens voltage-gated calcium channels.
• Calcium enters the cell and increases cytoplasmic calcium concentration.
• Elevated intracellular calcium triggers exocytosis of insulin-containing secretory granules. Thus, calcium is the immediate signal for insulin release.
• Plasma potassium influences insulin secretion.
• Hypokalemia reduces insulin release and may impair glucose tolerance.
• Severe potassium depletion should be corrected during insulin therapy.
• Insulin secretion is greater after oral glucose than after intravenous glucose producing the same plasma glucose level. This difference is called the incretin effect.
• Oral glucose stimulates release of gastrointestinal hormones such as glucagon-like peptide-1 and glucose-dependent insulinotropic peptide. These hormones enhance glucose-stimulated insulin secretion.
• Therefore, oral nutrient intake produces a stronger physiological insulin response than intravenous glucose infusion.

Other Stimuli

• Amino acids formed during protein digestion stimulate insulin release.
• Arginine, leucine, lysine, and alanine are strong insulin secretagogues.
• Glucose and amino acids enhance each other’s insulin-stimulating effects.
• Triglycerides, free fatty acids, and ketoacids can also increase insulin secretion.
• Cholinergic vagal stimulation markedly promotes insulin release and contributes to the cephalic phase of digestion.
• Catecholamines acting through beta receptors increase insulin secretion, whereas alpha receptors inhibit it.
• During exercise, sympathetic alpha-adrenergic activity suppresses insulin release and helps prevent hypoglycemia.
• Obesity often increases insulin secretion initially.
• Chronic excess weight leads to receptor down-regulation and insulin resistance. This mechanism contributes to type 2 diabetes mellitus.
• Regular exercise and weight control reduce risk.

Metabolism of Insulin

• Insulin circulates freely in plasma and is not significantly bound to carrier proteins.
• Because of rapid clearance, its plasma half-life is about 5–8 minutes.
• The metabolic clearance rate is approximately 800 milliliters per minute.
• Basal insulin secretion into circulation is about 0.5–1 unit per hour (20–40 micrograms per hour).
• After food intake, insulin release may increase nearly tenfold.
• Total daily insulin secretion into peripheral circulation is about 30 units in healthy adults.
• Insulin is metabolized mainly by the liver and kidneys.

C peptide Activity

• C peptide is released in equimolar amounts with endogenous insulin.
• Basal plasma C peptide concentration is about five times higher than insulin because it is cleared more slowly.

• Plasma C peptide is therefore a better indicator of beta-cell secretory function than plasma insulin.

Mechanism of Action

• Insulin acts by binding to specific insulin receptors on target cells.
• Major target tissues are the liver, skeletal muscle, and adipose tissue. These tissues regulate glucose uptake, storage, and energy metabolism.

Insulin Receptor

• The insulin receptor is a membrane glycoprotein tetramer made of two alpha and two beta subunits.
• Alpha subunits are located extracellularly and bind insulin.
• Beta subunits span the cell membrane and contain intracellular signaling domains.
• The subunits are linked by disulfide bonds and are glycosylated proteins.
• The insulin receptor gene is located on chromosome 19.
• Binding of insulin to alpha subunits causes a conformational change in the receptor. This activates intracellular tyrosine kinase activity of the beta subunits.
• Activated receptors initiate signaling pathways that increase glucose transport, glycogen synthesis, lipid synthesis, and protein synthesis.
• Binding of insulin to alpha subunits causes a conformational change in the receptor. This activates intracellular tyrosine kinase activity of the beta subunits.
• Activated receptors initiate signaling pathways that increase glucose transport, glycogen synthesis, lipid synthesis, and protein synthesis.

Hormone Mechanisms

• Binding of insulin to the alpha subunits of its receptor causes a structural change in the beta subunits. This activates the intrinsic tyrosine kinase activity present in the intracellular part of the beta subunits.
• The receptor then undergoes autophosphorylation on tyrosine residues, increasing signaling activity.
• Activated receptors phosphorylate insulin receptor substrates, mainly IRS-1 and IRS-2. These substrates function as docking proteins for multiple intracellular signaling molecules.
• IRS proteins help transmit signals to kinases, phosphatases, phospholipases, and ion channel regulatory pathways.
• IRS signaling activates pathways such as phosphatidylinositol 3-kinase and protein kinase B. These pathways promote translocation of GLUT4 transporters to the plasma membrane in skeletal muscle and adipose tissue.
• Increased GLUT4 on the membrane enhances glucose entry into cells.
• Insulin also increases cellular uptake of amino acids, potassium, magnesium, and phosphate. It stimulates glycogen synthesis by activating glycogen synthase and inhibiting glycogen breakdown.
• Lipid synthesis and protein synthesis are also promoted.
• Some enzymes are activated, whereas others are inhibited through phosphorylation or dephosphorylation.

• Insulin signaling can induce or suppress specific nuclear genes. This leads to synthesis of proteins required for metabolism and cellular maintenance.
• Additional pathways activate mitogenic signals involved in cell growth and differentiation.
• Adapter proteins such as growth receptor-bound protein 2 can activate Ras-related signaling cascades.
• These pathways mainly regulate growth responses rather than the direct metabolic actions of insulin.

Action through GLUT 4

• Insulin binding to its receptor rapidly increases glucose transport into skeletal muscle, cardiac muscle, and adipose tissue.
• Glucose entry into these cells may rise nearly 20-fold after insulin stimulation. This effect occurs mainly through GLUT4, the insulin-responsive glucose transporter.
• In the resting state, most GLUT4 molecules are stored inside cytoplasmic vesicles and endosomes.
• Only a small number of transporters remain on the cell surface during low insulin levels.
• Insulin activates intracellular signaling pathways, including phosphatidylinositol 3-kinase. This signaling moves GLUT4-containing vesicles toward the plasma membrane.
• Vesicles fuse with the membrane and insert GLUT4 transporters into the cell surface.
• Increased membrane GLUT4 enhances glucose uptake from blood into cells.
• After insulin levels fall, GLUT4 is removed from the membrane and recycled into intracellular vesicles.
• Defective GLUT4 translocation contributes to insulin resistance.
• In the liver, insulin promotes glucose utilization mainly by activating glucokinase, not by GLUT4.

Table 60.2: Types of glucose transporters and their functions.

Transporter Type	Main Function / Site
SGLT1	Sodium-dependent cotransporter that absorbs glucose and galactose in the small intestine; also reabsorbs glucose in renal tubules.
SGLT2	Major sodium-dependent transporter responsible for glucose reabsorption in the proximal renal tubule.
GLUT1	Basal glucose uptake in erythrocytes, blood-brain barrier, placenta, and many tissues.
GLUT2	Low-affinity transporter in liver, intestine, kidney, and pancreatic beta cells; acts as a glucose sensor.
GLUT3	High-affinity glucose uptake, especially in neurons and brain tissue.
GLUT4	Insulin-responsive transporter in skeletal muscle, cardiac muscle, and adipose tissue.
GLUT5	Primarily transports fructose in the jejunum and sperm cells.

Glucose Transporters

• Glucose enters cells by facilitated diffusion or by sodium-linked secondary active transport.
• In the intestine and kidney, glucose uptake occurs mainly through sodium-glucose cotransporters.
• In skeletal muscle, cardiac muscle, and adipose tissue, insulin increases glucose entry by raising the number of membrane glucose transporters.
• GLUTs are membrane proteins that mediate facilitated diffusion of glucose.
• Most GLUT proteins span the cell membrane 12 times.
• Multiple transporter isoforms exist, with at least 14 members identified.
• Different tissues express different GLUT types according to metabolic needs.

Important Types

• GLUT1 provides basal glucose uptake in many tissues, including erythrocytes and the blood-brain barrier.
• GLUT3 has high affinity for glucose and supports neuronal glucose uptake.
• GLUT4 is the principal insulin-responsive transporter in skeletal muscle, cardiac muscle, and adipose tissue.
• Insulin stimulates translocation of GLUT4 from intracellular vesicles to the plasma membrane.
• GLUT2 is present in liver, intestine, kidney, and pancreatic beta cells, where it functions as a glucose sensor.
• GLUT5 mainly transports fructose in the jejunum and sperm cells.

Clinical Importance

• Defects in glucose transport contribute to diabetes mellitus, insulin resistance, and inherited transport disorders.

Physiological Actions of Insulin

• Insulin is mainly secreted in the fed state when nutrients are abundant. It is often called the hormone of abundance because it promotes nutrient storage.
• Insulin stimulates storage of glucose, fat, and protein when energy supply exceeds immediate needs. It suppresses breakdown of endogenous fuel stores.
• Major target tissues are the liver, adipose tissue, and skeletal muscle.

On Carbohydrate Metabolism

• Insulin increases glucose uptake, promotes glucose utilization, and enhances storage of excess glucose. It simultaneously suppresses endogenous glucose production.
• Its primary physiological role is to lower plasma glucose concentration.
• Insulin is the principal hormone that reduces blood glucose in both fasting and fed states.
• Major metabolic actions occur in the liver, skeletal muscle, and adipose tissue.
• Brain tissue, erythrocytes, intestinal epithelium, and renal tubules largely use insulin-independent glucose transporters.

In Liver

• In hepatocytes, insulin promotes glucose trapping by increasing glucokinase activity.
• Glucokinase converts glucose to glucose-6-phosphate, maintaining a low intracellular free glucose concentration. This favors continued movement of glucose into liver cells.

Stimulation of Glycolysis

• Insulin increases glycolysis by activating key enzymes such as phosphofructokinase and pyruvate kinase. These pathways convert glucose to pyruvate for energy production.
• Insulin also enhances pyruvate dehydrogenase activity, promoting oxidative metabolism of pyruvate.

Promotion of Glycogen Storage

• Insulin strongly stimulates glycogen synthesis in the liver.
• It activates glycogen synthase, increasing conversion of glucose into hepatic glycogen stores. This helps buffer postprandial rises in blood glucose.

Inhibition of Glycogen Breakdown

• Insulin suppresses hepatic glycogenolysis.
• It inhibits glycogen phosphorylase and reduces glucose release from glycogen stores. This decreases hepatic glucose output.

Inhibition of Gluconeogenesis

• Insulin inhibits formation of glucose from non-carbohydrate precursors. It suppresses gluconeogenic enzymes such as pyruvate carboxylase, phosphoenolpyruvate carboxykinase, and fructose-1,6-bisphosphatase.
• It also reduces hepatic uptake of amino acids used for glucose synthesis.

Overall Effect

• By increasing glucose uptake and storage while reducing glucose production, insulin maintains normal blood glucose homeostasis.

• Deficiency or resistance to insulin leads to fasting and postprandial hyperglycemia.

In Adipose Tissue

• In adipose tissue, insulin increases glucose entry by stimulating GLUT4 translocation and hexokinase activity.
• Glucose is converted to glycerol-3-phosphate, which helps esterify fatty acids into triglycerides.
• Insulin therefore promotes fat storage.

In Skeletal Muscle

• In skeletal muscle, insulin enhances glucose uptake through GLUT4 and hexokinase activation.
• Glucose is oxidized for energy by stimulating pyruvate dehydrogenase.
• Insulin also increases muscle glycogen synthesis and storage.

On Fat Metabolism

• Insulin strongly promotes fat storage and suppresses breakdown of stored fat. It lowers plasma free fatty acids and reduces formation of ketone bodies.
• Insulin is the principal anti-ketogenic hormone because it inhibits excessive ketogenesis.
• Major effects occur in adipose tissue and the liver.

In Adipose Tissue

Inhibition of Lipolysis

• Insulin inhibits hormone-sensitive lipase, the enzyme that hydrolyzes triglycerides into free fatty acids and glycerol.

• Reduced lipolysis decreases release of free fatty acids into blood.
• Lower plasma free fatty acids reduce hepatic ketone body production.
• Insulin also promotes peripheral utilization of ketone bodies.

Promotion of Fat Storage

• Dietary triglycerides circulate in lipoproteins and must be hydrolyzed before entering adipocytes.
• Insulin stimulates lipoprotein lipase on capillary endothelium. This enzyme releases fatty acids from circulating triglycerides.
• Fatty acids then enter adipose cells and are re-esterified into triglycerides.
• Insulin increases formation of glycerol-3-phosphate from glucose, providing the backbone for triglyceride synthesis.
• Net effect is increased storage of fat in adipose tissue.

In Liver

Anti-Ketogenic Effect

• Insulin stimulates acetyl-CoA carboxylase, increasing formation of malonyl-CoA.
• Malonyl-CoA inhibits carnitine palmitoyltransferase-1, which transports long-chain fatty acids into mitochondria.
• Reduced mitochondrial fatty acid entry decreases beta-oxidation and ketone body formation.

Promotion of Fatty Acid Synthesis

• Insulin activates acetyl-CoA carboxylase, the rate-limiting enzyme of fatty acid synthesis. It also increases expression and activity of fatty acid synthase.
• Insulin stimulates the pentose phosphate pathway by enhancing glucose-6-phosphate dehydrogenase activity. This pathway provides NADPH, required for fatty acid synthesis.
• Excess carbohydrate can therefore be converted into fatty acids and triglycerides in the liver.

Cholesterol Synthesis

• Insulin promotes hepatic cholesterol synthesis by stimulating HMG-CoA reductase, the rate-limiting enzyme in this pathway.

Overall Metabolic Significance

• Insulin shifts metabolism from fuel mobilization to energy storage.
• Deficiency of insulin increases lipolysis, raises free fatty acids, and may cause diabetic ketoacidosis.

On Protein Metabolism

• Insulin is an important anabolic hormone that promotes protein conservation and synthesis. It increases amino acid uptake into skeletal muscle and liver cells. This lowers plasma amino acid concentration and reduces substrate availability for gluconeogenesis.
• Insulin stimulates ribosomal protein synthesis through enhanced gene transcription and translation.
• It supports cellular growth and tissue repair.
• Insulin reduces breakdown of ribonucleic acid and helps preserve cellular protein-forming capacity.
• It inhibits proteolysis by decreasing lysosomal protein degradation.
• Reduced muscle protein breakdown decreases release and oxidation of amino acids.

On Plasma K+ Concentration

• Insulin rapidly shifts potassium from extracellular fluid into cells. This occurs mainly by stimulating membrane sodium-potassium adenosine triphosphatase activity.
• As a result, plasma potassium concentration decreases.
• Insulin is therefore an important physiological regulator of potassium balance.
• Combined insulin and glucose therapy is used to temporarily lower potassium in hyperkalemia.
• Patients receiving insulin treatment may develop hypokalemia, especially if potassium stores are already depleted.

Other Actions of Insulin

• Insulin increases intracellular uptake of phosphate and magnesium.
• It enhances renal tubular reabsorption of sodium, potassium, and phosphate.
• Insulin can reduce food intake partly through hypothalamic effects, including suppression of neuropeptide Y signaling.
• Chronic excess insulin often increases body weight and adipose tissue mass.
• Rising leptin levels during weight gain contribute to satiety regulation.

• Insulin stimulates synthesis of macromolecules in cartilage and bone, supporting normal growth.
• It promotes gene transcription for growth factors such as insulin-like growth factor I and insulin-like growth factor II.
• Insulin also enhances protein synthesis and tissue repair.
• Deficiency of insulin during childhood may impair linear growth, maturation, and normal development.
• Insulin increases glucose uptake and storage, decreases lipolysis, promotes lipogenesis, and stimulates protein synthesis.

Applied Physiology

Insulin Deficiency (Diabetes Mellitus)

• Deficiency of insulin leads to diabetes mellitus, a common endocrine and metabolic disorder.
• Diabetes prevalence is increasing worldwide, including rapidly rising rates in India.
• Both impaired glucose tolerance and overt diabetes are major public health concerns.
• Important contributing factors include sedentary lifestyle, excess calorie intake, ultra-processed foods, obesity, stress, and environmental influences.
• Early detection, weight control, regular exercise, and healthy diet help reduce disease burden.

Table 60.3: Etiological classification of diabetes.

Diabetes Category	Main Causes / Features
Type 1 Diabetes Mellitus	Autoimmune beta-cell destruction or idiopathic loss of insulin secretion.
Type 2 Diabetes Mellitus	Insulin resistance with progressive pancreatic beta-cell dysfunction.
Other Specific Types	Monogenic diabetes (MODY), defects in insulin action, pancreatic disorders (pancreatitis, pancreatectomy, cystic fibrosis, hemochromatosis, neoplasms), endocrine excess states, drug-induced diabetes, viral infections, genetic syndromes.
Gestational Diabetes Mellitus	Glucose intolerance first recognized during pregnancy.

Causes of DM

• Diabetes mellitus commonly results from destruction of pancreatic beta cells or reduced responsiveness of tissues to insulin.
• Beta-cell destruction causes inadequate insulin secretion.
• Reduced cellular response causes insulin resistance.
• In research animals, diabetes can be induced by alloxan or streptozotocin, which damage beta cells.

• In untreated diabetes, blood glucose remains elevated (hyperglycemia).
• Despite high plasma glucose, many tissues cannot utilize glucose efficiently.This occurs because insulin is absent, insufficient, or ineffective.
• Cells therefore experience relative energy deprivation.
• Hence, diabetes is described as starvation in the midst of plenty.
• Some endogenous substances, such as proinsulin and insulin-like growth factors, have weak insulin-like effects.Their metabolic activity is much lower than true insulin and cannot compensate adequately.Therefore, significant insulin deficiency leads to diabetes.
• Type 1 diabetes mellitus is usually caused by autoimmune beta-cell destruction and severe insulin deficiency.Most patients require lifelong insulin therapy.
• Type 2 diabetes mellitus is characterized by insulin resistance with gradual beta-cell failure.It may initially be managed with lifestyle modification and oral medicines.Many patients later develop insulin deficiency and may need insulin treatment.
• Older terms such as insulin-dependent and non-insulin-dependent diabetes are now outdated.
• Current classification is based on cause and pathophysiology.

Major Types of DM

• Diabetes mellitus is mainly classified into type 1 and type 2 diabetes mellitus.
• Both disorders cause chronic hyperglycemia but differ in pathogenesis and treatment approach.

Type-1 Diabetes Mellitus

• Type 1 diabetes mellitus is characterized by marked insulin deficiency.It usually results from autoimmune destruction of pancreatic beta cells.
• Many patients have antibodies against islet or beta-cell antigens.
• Genetic susceptibility exists, but concordance in identical twins is well below 100%, indicating environmental influence.
• The disorder commonly begins in childhood or adolescence, but it may occur at any age.
• Diabetic ketoacidosis is more common in this type because insulin deficiency is severe.
• Lifelong insulin replacement therapy is usually required.
• Rare monogenic forms may result from mutations affecting insulin synthesis or secretion.

Type-2 Diabetes Mellitus

• Type 2 diabetes mellitus is characterized by insulin resistance with progressive beta-cell dysfunction.
• Early in the disease, insulin secretion may be normal or increased.
• Genetic predisposition is strong, with high concordance among identical twins. It commonly develops in adulthood, but increasing obesity has led to earlier onset.
• Overweight status, central obesity, physical inactivity, excess calorie intake, and environmental factors increase risk.
• Chronic psychological stress and poor sleep may worsen metabolic control.
• Acute severe presentations may include hyperosmolar hyperglycemic state.
• Management usually begins with lifestyle modification and oral antidiabetic drugs.
• Some patients later require insulin therapy as beta-cell function declines.

Mechanism of insulin resistance:

• Insulin resistance is a reduced biological response of tissues to normal or elevated insulin levels. It is central to the pathophysiology of type 2 diabetes mellitus.
• The exact mechanism is multifactorial and involves genetic, metabolic, and environmental influences.

Major Contributing Factors

Obesity

• Obesity, especially visceral adiposity, strongly correlates with insulin resistance.

• Excess adipose tissue reduces glucose uptake in muscle and other tissues. It is commonly associated with hyperinsulinemia, dyslipidemia, hypertension, and accelerated atherosclerosis.
• Weight reduction often improves insulin sensitivity.

Adipokines and Inflammation

• Adipose tissue secretes signaling molecules called adipokines.
• Important adipokines include leptin, adiponectin, resistin, and tumor necrosis factor alpha.
• Lower adiponectin levels are linked with reduced insulin sensitivity.
• Increased resistin and inflammatory cytokines may worsen insulin resistance.
• Chronic low-grade inflammation in obesity contributes to impaired insulin signaling.

Post-Receptor Signaling Defects

• Insulin resistance may result from abnormalities in intracellular signaling after receptor activation.
• Impaired phosphorylation pathways reduce downstream metabolic responses.
• Defects in insulin receptor substrate pathways are commonly implicated.

Reduced Glucose Transport

• Decreased expression or translocation of GLUT4 in skeletal muscle and adipose tissue lowers glucose uptake.
• This contributes significantly to postprandial hyperglycemia.

Free Fatty Acids

• Elevated circulating free fatty acids interfere with insulin signaling and glucose utilization.
• They also increase hepatic glucose production.

Clinical Importance

• Insulin resistance predisposes to type 2 diabetes mellitus, fatty liver disease, and cardiovascular disease.

Secondary Diabetes Mellitus

• Secondary diabetes mellitus develops as a consequence of another disease or treatment.
• Pancreatic disorders such as chronic pancreatitis, pancreatic surgery, or loss of pancreatic tissue can cause diabetes.
• Endocrine disorders may also produce hyperglycemia.
• Cushing syndrome increases glucose through excess cortisol.
• Acromegaly increases glucose because of excess growth hormone.
• This group represents a smaller proportion of total diabetes cases.

Maturity Onset Diabetes of the Young (MODY)

• MODY is a monogenic form of diabetes caused by inherited defects in beta-cell function. It usually presents in children, adolescents, or young adults.
• Several genetic subtypes are recognized.
• HNF1A-related MODY is common and often responds well to sulfonylureas.
• Glucokinase-related MODY usually causes mild stable fasting hyperglycemia and may need only dietary management.
• Accurate diagnosis is important because treatment differs from type 1 and type 2 diabetes mellitus.

Gestational DM

• Gestational diabetes mellitus is glucose intolerance first recognized during pregnancy. It increases risks of fetal macrosomia, birth complications, and future maternal diabetes.
• Many women return to normal glucose levels after delivery, but long-term risk of type 2 diabetes remains increased.

Features of DM

• Typical symptoms include polyuria, polydipsia, polyphagia, fatigue, and unexplained weight loss.
• Laboratory findings include hyperglycemia and, in some cases, glycosuria.
• Untreated severe disease may progress to ketosis, acidosis, dehydration, or coma.

Table: 60.4: Major differences between Type 1 and Type 2 diabetes mellitus.

Feature	Type 1 Diabetes Mellitus	Type 2 Diabetes Mellitus
Usual onset	Commonly childhood or young adulthood	Usually adulthood, but now seen earlier
Body habitus	Often lean or normal weight	Commonly overweight or obese
Relative frequency	About 5–10% of cases	About 90–95% of cases
Genetic tendency	Moderate familial association	Strong familial association
Onset pattern	Rapid	Gradual
Ketoacidosis risk	Higher	Lower, but possible
Pancreatic beta cells	Autoimmune destruction	Progressive dysfunction with insulin resistance
Severe acute state	Diabetic ketoacidosis	Hyperosmolar hyperglycemic state

Polyphagia

• Polyphagia means excessive hunger or increased food intake.
• In diabetes, cells cannot utilize glucose effectively because insulin is deficient or ineffective.
• Despite high blood glucose, the brain perceives relative cellular energy deficiency.
• Reduced satiety signaling from the hypothalamus may increase appetite. This leads to persistent hunger and increased eating.

Polyuria

• Polyuria means increased urine output.
• When blood glucose rises above the renal threshold (about 180 milligrams per deciliter, variable), glucose appears in urine.
• Glucose in tubular fluid causes osmotic diuresis by retaining water in the urine.
• Excess urinary water loss increases urine volume and frequency.

Polydipsia

• Polydipsia means excessive thirst.
• Large water losses from polyuria reduce extracellular fluid volume.
• Dehydration stimulates the hypothalamic thirst center.
• Patients therefore drink excessive amounts of water.

Weight Loss

• Weight loss may occur despite increased appetite.
• Glucose cannot be efficiently used for energy in insulin deficiency.
• The body shifts to breakdown of fat and protein stores for fuel.
• Loss of calories in urine also contributes.
• Children may develop growth impairment if diabetes is prolonged and uncontrolled.

Hyperglycemia and Glycosuria

• Hyperglycemia occurs because insulin is the major hormone that lowers blood glucose.
• Reduced insulin action increases hepatic glucose output and decreases peripheral glucose uptake.
• Once renal threshold is exceeded, glycosuria develops.

Ketosis

• Insulin normally suppresses lipolysis and ketone formation.
• In severe insulin deficiency, free fatty acids are released from adipose tissue.
• The liver converts them to ketone bodies such as acetoacetate and beta-hydroxybutyrate.
• Excess ketones cause ketosis, especially in type 1 diabetes mellitus.

Acidosis

• Accumulation of acidic ketone bodies produces metabolic acidosis.
• Buffer systems become overwhelmed in severe disease.
• Patients may develop deep rapid breathing (Kussmaul respiration) as compensation.
• Urinary loss of water and electrolytes causes dehydration, hypovolemia, and hypotension.
• Diabetic ketoacidosis is a medical emergency requiring urgent fluids, electrolytes, and insulin.

Coma

• Severe diabetes may lead to coma.
• Diabetic ketoacidosis can impair consciousness through acidosis and dehydration.
• Hyperosmolar hyperglycemic state causes profound hyperglycemia and plasma hyperosmolality.
• Brain cell dehydration may produce confusion, stupor, or coma without significant acidosis.

Diagnosis

• Diagnosis is based on demonstrating persistent hyperglycemia using standardized blood glucose tests.
• Glycosuria may occur, but urine glucose alone is not sufficient for diagnosis.

Fasting Blood Glucose

• Fasting plasma glucose is commonly used after at least 8 hours of fasting.
• A value of 126 milligrams per deciliter or higher on two separate occasions supports diagnosis of diabetes mellitus.
• Values between 100 and 125 milligrams per deciliter suggest impaired fasting glucose (prediabetes).

Oral Glucose Tolerance Test

• The oral glucose tolerance test evaluates the body’s response to a glucose load. It is useful when diagnosis is uncertain or during pregnancy screening.
• Persistent elevation of blood glucose after glucose intake suggests diabetes.
• Abnormal results may also occur in some other medical conditions.

Glycated Hemoglobin

• Hemoglobin A1c reflects average blood glucose over the previous 2–3 months.
• Normal values are usually below 5.7%.
• Values of 5.7–6.4% indicate prediabetes.
• A value of 6.5% or higher supports diagnosis of diabetes when appropriate standards are met.

Treatment

• Management includes lifestyle modification, medical nutrition therapy, regular exercise, and weight control.
• Drug therapy may include oral antihyperglycemic agents, non-insulin injectables, or insulin.
• Stress reduction and long-term follow-up improve glycemic control and reduce complications.

Table 60.5: Categories of glucose tolerance based on the levels of fasting and post-prandial blood glucose (mg%), and HbA1c.

Category	Fasting Glucose	Post-Meal Glucose	HbA1c
Normal	60–99 mg/dL	<140 mg/dL	4.0–5.6%
Prediabetes	100–125 mg/dL	140–199 mg/dL	5.7–6.4%
Diabetes Mellitus	≥126 mg/dL	≥200 mg/dL	≥6.5%

Insulin Therapy

• Insulin replacement is the essential treatment for type 1 diabetes mellitus. It is also used in some patients with type 2 diabetes mellitus when oral therapy is insufficient.
• Intravenous regular insulin acts rapidly and is commonly used in emergencies such as diabetic ketoacidosis.
• Subcutaneous insulin is used for routine long-term management.
• Commercial insulin preparations are broadly classified as:
  • Rapid-acting insulin
  • Intermediate-acting insulin
  • Long-acting insulin

• Modern therapy mainly uses recombinant human insulin and insulin analogs produced by biotechnology.
• Animal-derived insulins are now rarely used.
• Dose selection depends on blood glucose pattern, meals, activity, and comorbidities.

Oral Hypoglycemic Agents

• These agents are mainly used for type 2 diabetes mellitus.

Sulfonylureas

• Examples include glipizide and glyburide.

• They close ATP-sensitive potassium channels in pancreatic beta cells. This increases calcium entry and stimulates insulin secretion.

Biguanides

• Metformin is the most widely used drug in this class. It decreases hepatic gluconeogenesis and lowers hepatic glucose output.
• It also improves insulin sensitivity.

Thiazolidinediones

• These drugs improve peripheral insulin sensitivity.
• They activate peroxisome proliferator-activated receptor gamma, influencing genes involved in glucose and lipid metabolism.
• Older agents such as troglitazone are no longer routinely used because of toxicity.

Clinical Principles

• Lifestyle modification remains fundamental with all drug therapies.
• Treatment is individualized according to glycemic control, body weight, renal function, and cardiovascular risk.
• Regular monitoring helps prevent hypoglycemia and chronic complications.

Change in Lifestyle

• Long-term management of diabetes mellitus requires lifestyle measures along with medicines.
• Calorie intake should match age, body weight, and activity level.
• Excess energy intake should be avoided to prevent weight gain.
• A balanced diet should include controlled carbohydrates, limited saturated fat, adequate protein, vitamins, and dietary fiber.
• Regular physical activity such as brisk walking, resistance exercise, and stretching improves insulin sensitivity and glycemic control.
• Exercise also helps weight management and cardiovascular health.
• Stress reduction, adequate sleep, and mental well-being support glucose control.
• Practices such as yoga may improve fitness, flexibility, and stress management when used with standard care.

Complications

• Chronic uncontrolled diabetes damages both small and large blood vessels.

Microvascular Complications

• Diabetic retinopathy causes retinal hemorrhage, exudates, edema, and abnormal new vessel formation, which may impair vision.
• Diabetic nephropathy damages renal microvasculature and may progress to chronic kidney disease.
• Diabetic neuropathy affects peripheral and autonomic nerves, causing pain, numbness, postural hypotension, gastroparesis, or bladder dysfunction.

Macrovascular Complications

• Accelerated atherosclerosis increases risk of myocardial infarction, stroke, and peripheral arterial disease.
• Dyslipidemia, hypertension, and smoking further increase risk.

Infection and Foot Problems

• Hyperglycemia impairs immune function and wound healing.
• Neuropathy and poor circulation predispose to ulcers, infection, and diabetic foot.
• Severe untreated cases may lead to gangrene or amputation.

Prevention

• Good glycemic control, blood pressure control, lipid management, foot care, and regular screening reduce complications.

Insulin Excess

• Insulin excess may occur in pancreatic insulin-secreting tumors (insulinoma) or after excessive insulin therapy.
• Excess insulin causes hypoglycemia.
• Chronic recurrent hypoglycemia may produce poor coordination, confusion, behavioral change, and slurred speech.
• Symptoms can be mistaken for alcohol intoxication.
• Episodes may occur in the early morning after overnight fasting.
• Acute hypoglycemia commonly causes sweating, palpitations, tremor, anxiety, hunger, and altered consciousness.
• Severe untreated hypoglycemia may cause seizures or coma.

Other Pancreatic Hormones

Glucagon

• Glucagon generally exerts metabolic effects opposite to insulin. It increases blood glucose and promotes ketone body formation during fasting.
• Therefore, it is considered a hyperglycemic and ketogenic hormone.

Source and Structure

• Glucagon is secreted mainly by pancreatic alpha cells, which form about 20–25% of islet cells.
• Related glucagon-derived peptides are also produced in intestinal endocrine cells and certain neurons.
• Glucagon is a polypeptide hormone containing 29 amino acids. Its approximate molecular weight is 3,485 daltons.

Synthesis

• Glucagon is synthesized from a precursor called preproglucagon. This precursor is expressed in pancreatic alpha cells, intestinal L cells, and parts of the brain.
• Tissue-specific processing of preproglucagon generates different biologically active peptides.

In Pancreatic Alpha Cells

• Preproglucagon is processed mainly to glucagon and related peptide fragments.

In Intestinal L Cells

• Processing produces glucagon-like peptide-1, glucagon-like peptide-2, oxyntomodulin, glicentin, and other fragments.

Regulation

• Elevated glucose and insulin generally suppress glucagon gene expression and secretion.

Functions of Related Peptides

• Glucagon-like peptide-1 enhances glucose-dependent insulin secretion, slows gastric emptying, and improves glucose control.
• Glucagon-like peptide-2 supports intestinal growth and nutrient absorption rather than directly reducing food intake.
• Oxyntomodulin may reduce appetite and decrease gastric acid secretion.
• Glicentin has gastrointestinal regulatory actions.
• Some preproglucagon-derived peptides also function as neurotransmitters in the brain.

Secretion

• Glucagon secretion increases with amino acids, especially gluconeogenic amino acids, cortisol, cholecystokinin, gastrin, stress, exercise, infection, beta-adrenergic agonists, and theophylline.
• Secretion decreases with glucose, somatostatin, insulin, secretin, ketone bodies, free fatty acids, and alpha-adrenergic agonists.

Metabolism and Mechanism of Action

• Glucagon circulates freely in blood and is mainly degraded by the liver.
• Its plasma half-life is about 5–10 minutes.
• It acts primarily through cyclic adenosine monophosphate and may also activate inositol trisphosphate signaling pathways.

Physiological Actions

• Glucagon generally opposes many metabolic actions of insulin.
• It promotes mobilization of stored fuels during fasting.
• Its principal targets are the liver and, to a lesser extent, adipose tissue.
• Major effects include increased hepatic glucose output and enhanced ketone production.

On Carbohydrate Metabolism

Stimulation of Glycogenolysis

• Glucagon rapidly stimulates glycogenolysis in the liver. It activates glycogen phosphorylase through cyclic adenosine monophosphate and protein kinase A signaling.
• Hepatic glycogen is broken down to glucose-6-phosphate and then converted to free glucose.
• Released glucose enters the bloodstream.
• Glucagon has little direct glycogenolytic effect on skeletal muscle because muscle lacks significant glucagon receptors.

Stimulation of Gluconeogenesis

• Glucagon increases gluconeogenesis in the liver. It enhances enzymes such as phosphoenolpyruvate carboxykinase.
• Amino acids and other substrates are used to form new glucose. This action is important during prolonged fasting.

Inhibition of Glycogen Synthesis

• Glucagon inhibits glycogenesis by suppressing glycogen synthase activity.
• Therefore, glucose is not readily stored as glycogen during fasting states.

On Fat Metabolism

Lipolysis

• Glucagon can promote lipolysis, mainly in the liver and under specific physiological states. It increases availability of free fatty acids for oxidation.
• In humans, adipose lipolysis is influenced more strongly by catecholamines than by glucagon.

Ketogenesis

• Glucagon strongly promotes ketogenesis in the liver.
• It lowers malonyl-coenzyme A, facilitating fatty acid entry into mitochondria.
• Beta-oxidation then generates ketone bodies.

On Calorigenesis

Glucagon may modestly increase thermogenesis and energy expenditure through hepatic metabolic activity.

On Heart

• At pharmacological doses, glucagon increases heart rate and myocardial contractility by raising cyclic adenosine monophosphate.
• It may be used selectively in certain beta-blocker overdose situations rather than routine heart disease treatment.

On Other Hormones

• Glucagon can stimulate secretion of insulin, growth hormone, and pancreatic polypeptide under some conditions.

Insulin-Glucagon Ratio

• Insulin promotes glucose utilization, glycogen formation, fat storage, and suppression of ketogenesis.
• Glucagon promotes glucose release, gluconeogenesis, lipolysis, and ketone production.
• Because these hormones have opposing actions, their balance is metabolically important.
• Therefore, the insulin-glucagon ratio often reflects metabolic state better than either hormone alone.

Typical Changes in the Ratio

• After a balanced meal, the ratio is relatively high and commonly around 3.
• After overnight fasting, it falls toward 1.
• During prolonged fasting, it may decline further to about 0.4.
• After glucose administration, the ratio can rise markedly, sometimes approaching 30.

Physiological Significance:

Fed State

• A high ratio favors energy storage.
• Glycogen synthesis, protein synthesis, and fat deposition are promoted.
• Hepatic glucose production is suppressed.

Fasting or Starvation

• A low ratio favors energy mobilization.
• Glycogen breakdown, gluconeogenesis, lipolysis, and ketogenesis increase.
• These changes help maintain nutrient supply to vital organs.

Neonatal Period

• Shortly after birth, a relatively low ratio supports adaptation to interruption of maternal nutrient supply.
• It helps maintain endogenous fuel production until feeding is established.

Diabetes Mellitus

• In insulin deficiency, glucagon may remain inappropriately elevated.
• The resulting low ratio worsens hyperglycemia and ketone body formation.
• Correcting this imbalance improves metabolic control.

Applied Physiology

• Glucagon excess may occur in pancreatic alpha-cell tumors called glucagonomas. It can cause hyperglycemia, weight loss, ketosis, and diabetes-like metabolic abnormalities.
• Relative glucagon excess also worsens insulin-deficient diabetes.
• True glucagon deficiency is rare.

Somatostatin

Source and Structure

• Somatostatin is secreted by pancreatic delta cells.
• It is also produced in the hypothalamus and gastrointestinal tract.
• It is a peptide hormone with two principal forms:
  • 14 amino acid form, mainly in the pancreas
  • 28 amino acid form, mainly in the gastrointestinal tract

Synthesis and Secretion

• Somatostatin is synthesized as preprosomatostatin, then converted to prosomatostatin and active hormone.
• Secretion is stimulated by glucose, amino acids, free fatty acids, gastrointestinal hormones, glucagon, and autonomic neural input.
• Both vagal activity and some sympathetic beta-adrenergic signals may enhance release.

Functions

• Somatostatin strongly inhibits secretion of insulin and glucagon by paracrine and neuroendocrine mechanisms.
• It slows absorption and assimilation of nutrients from the intestine.
• It inhibits gastric, duodenal, and gallbladder motility.
• It reduces secretion of gastric acid, pepsin, pancreatic enzymes, and intestinal fluids.
• It suppresses release of hormones such as gastrin and secretin.
• It decreases intestinal absorption of glucose and lipids.
• It helps regulate gastric emptying, preventing rapid delivery of chyme into the intestine.

Clinical Significance

Therapeutic Use

• Somatostatin analogs are used to treat certain forms of diarrhea by reducing intestinal secretion and motility.
• They are also used in conditions such as acromegaly, neuroendocrine tumors, and variceal bleeding.

Somatostatinoma

• Somatostatinoma is a rare tumor of delta cells that causes excess somatostatin secretion.
• Patients may develop steatorrhea or malabsorption, weight loss, and mild to moderate hyperglycemia due to reduced insulin release.
• Reduced gastric secretion and delayed emptying may cause dyspepsia.
• Inhibition of gallbladder contraction can predispose to gallstones.

Pancreatic Polypeptide

Source and Structure

• Pancreatic polypeptide is secreted by pancreatic F cells (PP cells). It is a peptide hormone composed of 36 amino acids.
• The molecule has a characteristic carboxyl-terminal tyrosine-amide residue. It belongs to the same peptide family as neuropeptide Y and peptide YY.

Synthesis and Secretion

• It is synthesized as a peptide precursor and processed to the active hormone.
• Secretion rises mainly after food intake, especially protein-rich meals.
• Gastrointestinal hormones and vagal stimulation enhance release during digestion.
• Secretion may also increase during fasting, exercise, and hypoglycemia.
• Hyperglycemia and somatostatin inhibit release.
• Because secretion depends partly on cholinergic pathways, atropine reduces pancreatic polypeptide secretion.

Functions

• Pancreatic polypeptide inhibits exocrine pancreatic secretion, including digestive enzyme output.
• It may reduce uptake of precursor amino acids by pancreatic acinar cells.
• It slows gastrointestinal transit and may modestly delay nutrient absorption.
• It also appears to participate in regulation of appetite and satiety.

Clinical Significance

• Elevated plasma pancreatic polypeptide levels may occur in neuroendocrine tumors of the pancreas and can serve as a supportive biochemical marker.
• Absent or blunted rise after hypoglycemia may suggest autonomic, especially cholinergic, dysfunction affecting pancreatic islets.
• Altered levels may also be seen in diabetes mellitus and chronic pancreatic disease.