PANCREAS

PANCREAS ANATOMY AND HISTOLOGY

The pancreas is a retroperitoneal organ that lies in an oblique position, where it slopes upward from the duodenum to the hilum of the spleen. The pancreas is 15 to 20 cm long and weighs 75 to 100 g. The four general regions of the pancreas are the head, neck, body, and tail. The head of the pancreas is located in the C-loop of the duodenum, posterior to the transverse mesocolon and anterior to the vena cava, right renal artery, and both renal veins. The uncinate process is the posterior and medial aspects of the head of the pancreas, and it lies behind the portal vein and superior mesenteric vessels. The neck of the pancreas is anterior to the portal vein and first and second lumbar vertebral bodies. The body of the pancreas lies anterior to the aorta at the origin of the superior mesenteric artery. The body and tail of the pancreas lie anterior to the splenic artery and vein. The tail of the pancreas is anterior to the left kidney. The anterior surface of the pancreas is covered by peritoneum. The base of the transverse mesocolon attaches to the inferior margin of the body and tail of the pancreas.

The embryologic origin of the pancreas is the result of fusion of the ventral and dorsal buds. The duct from the smaller ventral bud connects directly to the common bile duct and becomes the duct of Wirsung. The ventral bud becomes the inferior portion of the pancreatic head and uncinate process. The duct from the larger dorsal bud drains directly into the duodenum and becomes the duct of Santorini. The dorsal bud becomes the body and tail of the pancreas. The ducts from each anlage fuse in the pancreatic head so that most of the exocrine pancreas drains through the duct of Wirsung or the main pancreatic duct and then into the common channel formed by the bile duct and pancreatic duct to empty at the ampulla of Vater on the medial aspect of the second portion of the duodenum. The flow of pancreatic and biliary secretions is controlled by the sphincter of Oddi, a group of muscle fibers at the ampulla of Vater.

The blood supply to the pancreas includes multiple branches from the superior mesenteric and celiac arteries. The gastroduodenal artery comes off the common hepatic artery and supplies the head and uncinate process. The body and tail of the pancreas are supplied by multiple branches of the splenic artery. The inferior pancreatic artery arises from the superior mesenteric artery. Three arteries that connect the splenic and inferior pancreatic arteries run perpendicular to the long axis of the pancreas and form an arterial arcade supplying the body and tail of the pancreas. The venous drainage includes an anterior and posterior venous arcade within the head of the pancreas that drains into the portal and mesenteric veins. The venous outflow from the body and tail of the pancreas drain into the splenic vein. The lymphatic drainage of the pancreas includes a profuse network of lymphatic vessels and lymph nodes.

Both the sympathetic and parasympathetic nervous systems innervate the acinar cells (exocrine secretion), islet cells (endocrine secretion), and islet vasculature. In general, the parasympathetic system stimulates endocrine and exocrine secretions, and the sympathetic system inhibits secretions. The neurons that innervate the pancreas also release unique transmitters that include peptides and amines (e.g., somatostatin, galanin, vasoactive intestinal polypeptide, and calcitonin gene–related peptide). A rich supply of afferent sensory nerve fibers is responsible for the intense abdominal pain associated with pancreatic inflammation.

The distribution of pancreatic mass is 85% exocrine, 2% endocrine, 10% extracellular matrix, and 4% blood vessels and ducts. The exocrine cells are clustered in acini (lobules) divided by connective tissue and connected to a duct that drains into the pancreatic duct and into the duodenum. The acinar cells have a high content of endoplasmic reticulum and are apically located eosinophilic zymogen granules. Small clusters of endocrine cells—islets of Langerhans—are embedded within the acini. The three main types of endocrine cells are β-cells (75% of endocrine cell mass) that produce insulin, α-cells (20% of endocrine cell mass) that produce glucagon, and the δ-cells (5% of endocrine cell mass) that secrete somatostatin. Within the islet, the β-cells are in the center and surrounded by the α-cells and δ-cells.

EXOCRINE FUNCTIONS OF THE PANCREAS

Each day the pancreas secretes approximately 1 L of alkaline isosmotic pancreatic juice that originates from the pancreatic acinar cells and pancreatic ducts. The colorless, bicarbonate-rich, and protein-rich pancreatic juice plays key roles in duodenal alkalinization and food digestion. The acinar cells secrete the enzymes required for the digestion of the three main food types: amylase for carbohydrate (starch) digestion, proteases (e.g., trypsin) for protein digestion, and lipases for fat digestion. The acinar cells are pyramidal in shape with the apices facing the lumen of the acinus, where the enzyme-containing zymogen granules fuse with the apical cell membrane for release. Acinar cells, unlike the endocrine cells of the pancreas, are not specialized and produce all three types of pancreatic enzymes from the same cell type.

Amylase is secreted in its active form and hydrolyzes starch and glycogen to the simple sugars of dextrins and maltose; maltose is then metabolized to glucose by intestinal maltase. The proteolytic enzymes are secreted as proenzymes and must be activated in the duodenum. For example, trypsinogen is converted in the duodenum to trypsin by enterokinase. Intrapancreatic conversion of trypsinogen is prevented by a pancreatic secretory trypsin inhibitor, a step that prevents pancreatic autodigestion. Another example of a proteolytic enzyme that is secreted as a proenzyme is chymotrypsinogen, which is activated in the duodenum to chymotrypsin. The actions of trypsin, chymotrypsin, and other proteolytic enzymes (e.g., elastase, carboxypeptidase A and B, intestinal peptidases) cleave bonds between amino acids in peptide chains, yielding smaller peptides that stimulate the intestinal endocrine cells to release cholecystokinin and secretin, which further stimulate the pancreas to release more digestive enzymes and bicarbonate. The amino acids and dipeptides are actively transported into enterocytes.

Pancreatic lipase is secreted in its active form, and it hydrolyzes triglycerides to fatty acids and glycerol. Phospholipase A cleaves the fatty acid off lecithin to form lysolecithin. Phospholipase B cleaves the fatty acid off lysolecithin to form glycerol phosphatidylcholine. Phospholipase A2 is activated by trypsin in the duodenum, where it serves to hydrolyze phospholipids. Hydrolyzed fat is organized in micelles and is transported into the enterocytes.

There are approximately 40 acinar cells per acinus. The acinar cells near the center of the acinus are termed centroacinar cells . Centroacinar cells and pancreatic duct cells secrete electrolytes, bicarbonate, and water into the pancreatic juice. At rest, secretion occurs at a low basal rate (∼2% of maximal). The pancreas' response to a meal occurs in three phases. The cephalic phase—in response to the smell, sight, and taste of food—accounts for 10% of meal-stimulated pancreatic secretion and is mediated by peripherally released acetylcholine. The gastric phase—in response to gastric distension from food—accounts for 10% of meal-stimulated pancreatic secretion. With gastric distension, gastrin is released, and vagal afferents are stimulated to directly mediate pancreatic enzyme secretion and enhance gastric acid secretion and duodenal acidification. The intestinal phase accounts for 80% of meal-stimulated pancreatic secretion. The duodenal hormone secretin is released in response to acid chyme (pH <3.0) and bile passing into the duodenum. Secretin then stimulates increased production of centroacinar cell bicarbonate to buffer the acidic chyme. Cholecystokinin is also released in response to protein and fat in the proximal small intestine, and it enhances the centroacinar cell response to secretin.

NORMAL HISTOLOGY OF PANCREATIC ISLETS

The pancreas is the union of an endocrine gland (pancreatic islets) and an exocrine gland (acinar and ductal cells). Approximately 85% of pancreatic mass is exocrine, 2% endocrine, 10% extracellular matrix, and 3% blood vessels and ducts. The exocrine (acinar) cells are clustered in acini, divided by connective tissue, and connected to a duct that drains into the pancreatic duct and into the duodenum. Small clusters of endocrine cells—islets of Langerhans—are embedded within the acini of the pancreas. The three main types of endocrine cells are β-cells (75% of endocrine cell mass) that produce insulin, α-cells (20% of endocrine cell mass) that produce glucagon, and δ-cells (5% of endocrine cell mass) that secrete somatostatin. The δ ₂ -cells secrete vasoactive intestinal polypeptide. The pancreatic polypeptide–producing (PP) cells secrete pancreatic polypeptide. Within the islet, the β-cells are in the center and surrounded by the α-cells, δ-cells, and PP cells.

The adult pancreas contains about 1 million islets (varying in size from 40–300 μm) that are more densely distributed in the tail of the gland. The entire mass of islets in a single pancreas weighs only approximately 1 g. Each islet contains approximately 3000 cells. The β-cells are polyhedral in shape and are distributed equally in islets across the pancreas. The α-cells are columnar in shape and are located primarily in islets in the body and tail of the pancreas. The δ-cells are smaller than the α- and β-cells and are frequently dendritic. The PP cells are located primarily in islets in the head and uncinate process of the pancreas. The Gomori aldehyde fuchsin and Ponceau techniques stain the insulin-containing granules in β-cells a deep bluish-purple; the α-cells appear pink or red.

Insulin, discovered in 1920 by Banting and Best, is a 56–amino acid peptide with two chains (α and β chains) joined by two disulfide bridges. β-Cell synthesis of insulin is regulated by plasma glucose concentrations, neural signals, and paracrine effects. The enteric hormones gastric inhibitory peptide, glucagon-like peptide-1 (GLP-1), and cholecystokinin also augment insulin secretion. Somatostatin, amylin, and pancreastatin inhibit insulin release. Cholinergic and β-adrenergic sympathetic innervation stimulate insulin release, and α-adrenergic sympathetic innervation inhibits insulin secretion. Insulin acts by inhibiting hepatic glucose production, glycogenolysis, fatty acid breakdown, and ketone formation. Insulin also facilitates glucose transport into cells and stimulates protein synthesis.

Glucagon is a 29–amino acid single-chain peptide hormone that counteracts the effects of insulin by promoting hepatic glycogenolysis and gluconeogenesis. Glucagon release is inhibited by increased levels of plasma glucose and by GLP-1, insulin, and somatostatin. Glucagon secretion is stimulated by the amino acids arginine and alanine. As with insulin, cholinergic and β-adrenergic sympathetic innervation stimulate glucagon release, and α-adrenergic sympathetic innervation inhibits glucagon secretion.

Somatostatin is a peptide that has two bioactive forms—14–amino acid and 28–acid forms. In general, somatostatin inhibits pancreatic endocrine and exocrine secretions.

Pancreatic polypeptide is a 36–amino acid hormone that inhibits bile secretion, gallbladder contraction, and exocrine pancreatic secretion. Pancreatic polypeptide also regulates hepatic insulin receptor expression. Enteral protein and fat stimulate pancreatic polypeptide secretion.

Amylin (also referred to as islet amyloid polypeptide) is a 37–amino acid hormone secreted by β-cells in concert with insulin. Amylin is synergistic with insulin by slowing gastric emptying, inhibiting digestive secretions, and inhibiting glucagon release. The effects of amylin are centrally mediated.

INSULIN SECRETION

Pancreatic β-cell production of insulin is regulated by plasma glucose concentration, neural inputs, and the effects of other hormones by paracrine and endocrine actions. Proinsulin consists of an amino-terminal β-chain, a carboxy-terminal α-chain, and a connecting peptide (C-peptide) in the middle. C-peptide functions by allowing folding of the molecule and the formation of disulfide bonds between the α- and β-chains. C-peptide is cleaved from proinsulin by endopeptidases in the β-cell endoplasmic reticulum (ER) to form insulin. Insulin and C-peptide are packaged into secretory granules in the Golgi apparatus. The secretory granules are released into the portal circulation by exocytosis. Insulin is degraded in the liver, kidney, and target tissues; it has a circulating half-life of 3 to 8 minutes. C-peptide does not act at the insulin receptor and is not degraded by the liver; it has a circulating half-life of 35 minutes. Thus, measurement of serum C-peptide concentration serves as a measure of β-cell secretory capacity. Defects in the synthesis and cleavage of insulin can lead to rare forms of diabetes mellitus (e.g., Wakayama syndrome, proinsulin syndromes).

Insulin is released in a pulsatile and rhythmic background pattern throughout the day and serves to suppress hepatic glucose production and mediates glucose disposal by adipose tissue. Superimposed on the background secretion of insulin is the meal-induced insulin release. There are two phases of caloric intake–induced insulin secretion. In the first phase, prestored insulin is released over 4 to 6 minutes. The second phase is a slower onset and longer sustained release because of the production of new insulin.

The regulators of insulin release include nutrients (e.g., glucose and amino acids), hormones (e.g., glucagon-like peptide 1 [GLP-1], somatostatin, insulin, and epinephrine), and neurotransmitters (e.g., acetylcholine, norepinephrine). The β-cells are exquisitely sensitive to small changes in glucose concentration; maximal stimulation of insulin secretion occurs at plasma glucose concentrations more than 400 mg/dL. Glucose enters the β-cells by a membrane-bound glucose transporter (GLUT 2). Glucose is then phosphorylated by glucokinase as the first step in glycolysis (leading to the generation of acetyl-coenzyme A and adenosine triphosphate (ATP) through the Krebs cycle (see Plate 5-6 ). The rise in intracellular ATP closes (inhibits) the ATP-sensitive potassium (K ⁺ ) channels and reduces the efflux of K ⁺ , which causes membrane depolarization and opening (activation) of the voltage-dependent calcium (Ca ²⁺ ) channels. The resultant Ca ²⁺ influx increases the concentration of intracellular Ca ²⁺ , which triggers the exocytosis of insulin secretory granules into the circulation. The β-cell Ca ²⁺ concentrations can also be increased by the ATP generated from amino acid metabolism.

Insulin release from β-cells can be amplified by cholecystokinin, acetylcholine, gastric inhibitory polypeptide (GIP), glucagon, and GLP-1. Orally administered glucose stimulates a greater insulin response than an equivalent amount of glucose administered intravenously because of the release of enteric hormones (e.g., GLP-1, GIP) that potentiate insulin secretion. This phenomenon is referred to as the incretin effect , a finding that has led to new pharmacotherapeutic options in the treatment of patients with type 2 diabetes mellitus (see Plate 5-20 ). Acetylcholine and cholecystokinin bind to cell surface receptors and activate adenylate cyclase and phospholipase C, which leads to inositol triphosphate (IP ₃ ) breakdown and mobilization of Ca ²⁺ from intracellular stores; activation of protein kinase C also triggers insulin secretion. GLP-1 receptor activation leads to increased cyclic adenosine monophosphate (cAMP) and activation of the cAMP-dependent protein kinase A; the Ca ²⁺ signal is amplified by decreasing Ca ²⁺ uptake by cellular stores and by activation of proteins that trigger exocytosis of insulin. Somatostatin and catecholamines inhibit insulin secretion through G-protein–coupled receptors and inhibition of adenylate cyclase.

Normal insulin secretion is dependent on the maintenance of an adequate number of functional β-cells (referred to as β- cell mass ). The β-cells must be able to sense the key regulators of insulin secretion (e.g., blood glucose concentration). In addition, the rates of proinsulin synthesis and processing must be sufficient to maintain adequate insulin secretion. Defects in any of these steps in insulin secretion can lead to hyperglycemia and diabetes mellitus.

ACTIONS OF INSULIN

Insulin is a 56–amino acid polypeptide that consists of two peptide chains (α and β) that are joined by two disulfide bridges. Insulin is secreted into the portal vein and delivered directly to the liver. Approximately 80% of insulin is cleared by the hepatic cell surface insulin receptors with the first pass through the liver. Insulin acts through the insulin receptor and has anabolic effects at target organs to promote synthesis of carbohydrate, fat, and protein.

The insulin receptor, a member of the growth factor receptor family, is a heterotetrameric glycoprotein membrane receptor that has two α- and two β-subunits that are linked by disulfide bonds. The α-subunits form the extracellular portion where insulin binds. The β-subunits form the transmembrane and intracellular portions of the receptor and contain an intrinsic tyrosine kinase activity. Insulin binding to the receptor triggers autophosphorylation on the intracellular tyrosine residues and leads to phosphorylation of insulin receptor substrates (IRS-1, IRS-2, IRS-3, and IRS-4). The phosphorylation of the IRS proteins activates the phophatidylinositol-3-kinase (PI ₃ kinase) and mitogen-activated protein kinase (MAPK) pathways. The PI ₃ kinase pathway mediates the metabolic (e.g., glucose transport, glycolysis, glycogen synthesis, and protein synthesis) and antiapoptotic effects of insulin. The MAPK pathway has primarily proliferative and differentiation effects. The number of insulin receptors expressed on the cell membrane can be modulated by diet, body type, exercise, insulin, and other hormones. Obesity and high serum insulin concentrations downregulate the number of insulin receptors. Exercise and starvation upregulate the number of insulin receptors.

Glucose oxidation is the major energy source for many tissue types. Cell membranes are impermeable to hydrophilic molecules such as glucose and require a carrier system to transport glucose across the lipid bilayer cell membrane. Glucose transporter 1 (GLUT 1) is present in all tissues and has a high affinity for glucose to mediate a basal glucose uptake in the fasting state. GLUT 2 has a low affinity for glucose and functions primarily at high plasma glucose concentrations (e.g., after a meal). GLUT 3 is a high-affinity glucose transporter for neuronal tissues. GLUT 4 is localized primarily to muscle and adipose tissues.

In muscle, activation of the insulin receptor and the PI ₃ kinase pathway leads to recruitment of the glucose transporter GLUT 4 from the cytosol to the plasma membrane. Increased expression of GLUT 4 leads to active transport of glucose across the myocyte cell membrane. Insulin promotes myocyte glycogen synthesis by increasing the activity of glycogen synthase and inhibiting the activity of glycogen phosphorylase. Insulin also enhances protein synthesis by increasing amino acid transport and by phosphorylation of a serine/threonine protein kinase.

In adipose tissue, insulin inhibits lipolysis by promoting dephosphorylation of hormone-sensitive (intracellular) lipase. The decreased breakdown of adipocyte triglycerides to fatty acids and glycerol leads to decreased substrate for ketogenesis. Insulin also induces the production of the endothelial cell–bound lipoprotein lipase, which hydrolyzes triglycerides from circulating lipoproteins to provide free fatty acids for adipocyte uptake. Insulin stimulates lipogenesis by activating acetyl-coenzyme A carboxylase. Increased glucose transport into adipocytes increases the availability of α-glycerol phosphate that is used in the esterification of free fatty acids into triglycerides. The decreased fatty acid delivery to the liver is a key factor in the net impact of insulin to decrease hepatic gluconeogenesis and ketogenesis.

In the liver, insulin stimulates the synthesis of enzymes that are involved in glucose utilization (e.g., pyruvate kinase, glucokinase) and inhibits the synthesis of enzymes involved in glucose production (e.g., glucose 6-phospatase, phosphoenolpyruvate carboxykinase). Insulin enhances glycogen synthesis by increasing phosphatase activity, causing dephosphorylation of glycogen synthase and glycogen phosphorylase. Insulin also promotes hepatic synthesis of triglycerides, very low-density lipoprotein, and proteins.

GLYCOLYSIS

Glycolysis is the major pathway for glucose metabolism, and it occurs in the cytosol of all cells. Glycolysis breaks down glucose (a 6-carbon molecule) into pyruvate (a 3-carbon molecule). Glycolysis can function either aerobically or anaerobically, depending on the availability of oxygen and the electron transport chain. The ability of glycolysis to provide energy in the form of adenosine triphosphate (ATP) from adenosine diphosphate (ADP) in the absence of oxygen allows tissues to survive anoxia.

Glycolysis occurs when a molecule of glucose 6-phosphate is transformed to pyruvate:

Glucose enters glycolysis by phosphorylation to glucose 6-phosophate, an irreversible reaction catalyzed by hexokinase, and ATP serves as the phosphate donor. Glucose 6-phosphate is converted to fructose-6-phosphate by phosphohexose isomerase. This intermediate is then phosphorylated to yield fructose-1,6-diphosphate. At this stage, the hexose molecule is cleaved by aldolase into two 3-carbon compounds: glyceraldehyde 3-phosphate and dihydroxyacetone phosphate. Dihydroxyacetone phosphate is quickly converted to glyceraldehyde 3-phosphate. The aldehyde group (CHO) of glyceraldehyde 3-phosphate is oxidized by a nicotinamide adenine dinucleotide (NAD)–dependent enzyme, and a phosphate group is attached, yielding 1, 3-bisphosphoglycerate. The energy of this oxidative step now rests in the phosphate bond at position 1. This energy is transferred to a molecule of ADP, forming ATP.

The above reaction yields energy that is not immediately given off as heat but is stored in the form of ATP. Because two molecules of glyceraldehyde 3-phosphate are produced for every molecule of glucose, two molecules of ATP are formed at this step per molecule of glucose undergoing glycolysis. An ensuing transformation of phosphoenolpyruvate to pyruvate (catalyzed by pyruvate kinase) gives rise to another ATP (2 molecules of ATP per molecule of glucose oxidized).

When a tissue possesses the systems for further oxidation of pyruvate, provided oxygen is present, pyruvate is cleaved to acetyl coenzyme A (CoA), and it enters the tricarboxylic acid cycle (see Plate 5-7 ). However, when the oxidative systems are absent (e.g., in erythrocytes that lack mitochondria) or if oxygen is excluded or is present in insufficient amounts (e.g., under anaerobic conditions), pyruvate is reduced to lactic acid by the enzyme lactate dehydrogenase. This system provides for the reoxidation of NADH and thus enables its participation again in oxidizing glyceraldehyde 3-phosphate; otherwise, the latter reaction would stop as soon as all the molecules of NAD were reduced.

The coupling of these two reactions allows the provision of energy by carbohydrates in the absence of oxygen, albeit at the expense of considerable amounts of carbohydrate. Under aerobic conditions, approximately 30 molecules of ATP are generated per molecule of glucose that is oxidized to CO ₂ and H ₂ O, but only two molecules of ATP when oxygen is absent. Glycolysis is regulated by the three enzymes that catalyze nonequilibrium reactions: hexokinase, phosphofructokinase, and pyruvate kinase.

TRICARBOXYLIC ACID CYCLE

The tricarboxylic acid (TCA) cycle, also referred to as the citric acid cycle or the Krebs cycle, is the final common pathway for oxidation of carbohydrate, lipid, and protein. Most of these nutrients are metabolized to acetyl-coenzyme A (acetyl-CoA) or one of the intermediates in the TCA cycle. For example, in protein catabolism, proteins are broken down by proteases into their constituent amino acids. The carbon backbone of these amino acids can become a source of energy by being converted to acetyl-CoA and entering into the TCA cycle. The TCA cycle also provides carbon skeletons for gluconeogenesis and fatty acid synthesis.

The TCA cycle starts with a reaction between the acetyl moiety of acetyl-CoA and the 4-carbon dicarboxylic acid, oxaloacetate, to form a 6-carbon tricarboxylic acid, citrate. In the reactions that follow, two molecules of CO ₂ are released and oxaloacetate is regenerated. This process is aerobic and requires oxygen as the final oxidant of the reduced coenzymes.

From one molecule of glucose, glycolysis (see Plate 5-6 ) provides two molecules of pyruvate. Pyruvate is split to acetyl-CoA and CO ₂ by pyruvate dehydrogenase, a step that generates one molecule of reduced nicotinamide adenine dinucleotide (NADH). Citrate synthase catalyzes the initial reaction between acetyl-CoA and oxaloacetate. Citrate is then isomerized to isocitrate by aconitase. Isocitrate is dehydrogenated by isocitrate dehydrogenase to form oxalosuccinate and then α-ketoglutarate. α-Ketoglutarate then undergoes oxidative decarboxylation to form succinyl-CoA, a step that is catalyzed by a multienzyme complex referred to as the α -ketoglutarate dehydrogenase complex . Succinate thiokinase converts succinyl-CoA to succinate. Succinate is then dehydrogenated to fumarate by succinate dehydrogenase. Fumarase catalyzes the addition of water across the double bond of fumarate to form malate. Malate is converted to oxaloacetate by malate dehydrogenase. Oxaloacetate can then reenter the TCA cycle.

Because of the oxidations catalyzed by the dehydrogenases in the TCA cycle, three molecules of the reduced form of NADH and one molecule of flavin adenine dinucleotide H ₂ (FADH ₂ ) are produced for each molecule of acetyl-CoA catabolized in one turn of the cycle.

In addition, the pyruvate dehydrogenase step provides one molecule of NADH. These reducing equivalents are transferred to the respiratory chain, and reoxidation of each NADH results in approximately 2.5 adenosine triphosphate (ATP) molecules and each FADH ₂ translates to approximately 1.5 ATP molecules. In addition, one ATP equivalent is generated from the phosphorylation step of succinyl-CoA catalyzed by succinate thiokinase. Thus, including the pyruvate dehydrogenase step, approximately 12 ATP molecules are formed per turn of the TCA cycle.

Four of the B vitamins have key roles in the TCA cycle. Riboflavin (vitamin B ₂ ) in the form of FAD is a cofactor for succinate dehydrogenase. Niacin (vitamin B ₃ ) in the form of NAD is the electron acceptor for isocitrate dehydrogenase, α-ketoglutarate dehydrogenase, and malate dehydrogenase. Pantothenic acid (vitamin B ₅ ) is part of CoA. Thiamine (vitamin B ₁ ) serves as the coenzyme for decarboxylation of the α-ketoglutarate dehydrogenase step.

Recent studies have shown a link between intermediates of the TCA cycle and the regulation of hypoxia-inducible factors (HIFs). HIFs have a key role in the regulation of oxygen homeostasis. HIFs are transcription factors that have broad targets, which include apoptosis, angiogenesis, vascular remodeling, glucose use, and iron transport. Dysregulation of HIFs appears central to the development of paragangliomas and pheochromocytomas in individuals with von Hippel–Lindau syndrome, where the VHL tumor suppressor gene encodes a protein that regulates hypoxia-induced proteins (see Plate 8-4 ). In addition, the familial paraganglioma syndromes are associated with mutations in the genes that encode key subunits of succinate dehydrogenase ( SDHB , SDHD , SDHC , SDHA, SDHAF2 ).

GLYCOGEN METABOLISM

Glycogen is a branched polymer of α-D-glucose and is the major depot of carbohydrates in the body, primarily in muscle and liver. Glycogen is the analog of starch, which is a less branched glucose polymer in plants.

CONSEQUENCES OF INSULIN DEPRIVATION

The absence of insulin is incompatible with life. Insulin deprivation can result from surgical removal (pancreatectomy) or autoimmune destruction of β-cells (type 1 diabetes mellitus); both lead to absence or severe curtailment of insulin production and release. In these settings, insulin-sensitive tissues (e.g., muscle, adipose tissue, liver) are deprived of insulin and its actions. Cell membranes are impermeable to hydrophilic molecules such as glucose and require a carrier system (e.g., GLUT 1, 2, 3, 4) to transport glucose across the lipid bilayer cell membrane. Because of decreased insulin-induced activation of the cell membrane glucose transporters, the transit of glucose from the blood into cells is diminished. At the same time, in the absence of insulin, glycogenesis is slowed. The suppressive effect of insulin on glucagon is removed, and glucagon enhances hepatic gluconeogenesis, which is fueled by the increased availability of precursors (e.g., glycerol and alanine) from accelerated fat and muscle breakdown. Thus, in the setting of insulin deprivation, there is impaired glucose utilization in peripheral tissues, increased glycogenolysis, and increased gluconeogenesis.

When the blood glucose concentration increases above 200 mg/dL, the renal tubules begin to exceed their capacity for glucose reabsorption (renal threshold). Excess glucose is lost in the urine (glucosuria) which, because of osmotic forces, takes water and sodium with it. Weight loss, thirst, polyuria, and hunger occur. Patients with indolent uncontrolled diabetes over months can present with wasting and cachexia similar to that seen in those with advanced malignancies.

In insulin-sensitive tissues, metabolic adjustments occur as a consequence of the curtailed glucose supply. Proteins are broken down faster than they can be synthesized; hence, amino acids are liberated from muscle, brought to the liver, and transformed to urea. The nonprotein nitrogen excreted in the urine rises and a negative nitrogen balance results.

Lipolysis is enhanced in the setting of insulin deprivation. There is a net liberation of stored fat as free fatty acids, which are used by many tissues for energy production. Hepatic uptake and metabolism of fatty acids lead to excess production of the ketones acetoacetate and β-hydroxybutyrate, strong organic acids that lead to ketoacidosis (see Plate 5-10 ). Ketones provide an alternate energy source when the utilization of glucose is impaired. The circulating β-hydroxybutyrate and acetoacetate obtain their sodium from NaHCO ₃ , thus leading to a metabolic acidosis. In addition, acetoacetate and β-hydroxybutyrate are excreted readily by the kidney, accompanied by base, and fixed base is lost. The severity of the metabolic acidosis depends on the rate and duration of ketoacid production.

Insulin deprivation also leads to deficits in minerals. A potassium deficit results from urinary losses with the glucose osmotic diuresis and in an effort to maintain electroneutrality as ketoacid anions are excreted. A negative phosphate balance is a result of phosphaturia caused by hyperglycemic-induced osmotic diuresis.

The outcomes of severe insulin deprivation include negative nitrogen balance, weight loss, ketosis, and acidosis. These are the hallmarks of the most severe state of metabolic decompensation characteristic of insulin deprivation in individuals with no endogenous source of insulin (e.g., type 1 diabetes mellitus). Acidosis, when not compensated for, exerts its major effect on brain function. In addition, acidosis affects the contractile responses of the small blood vessels throughout the body that, when coupled with osmotic diuresis-induced volume loss, results in hypotension and vascular collapse. Thus, diabetic coma and death—the fate of all those with type 1 diabetes mellitus before the advent of insulin replacement therapy—are the end result of uncompensated and untreated insulin deprivation.

DIABETIC KETOACIDOSIS

Diabetic ketoacidosis (DKA) is serious complication of diabetes mellitus characterized by the triad of hyperglycemia, anion gap metabolic acidosis, and ketonemia. DKA results from severe insulin deficiency with resultant hyperglycemia, excessive lipolysis, increased fatty acid oxidation, and excess ketone body production. The deficiency of insulin and the excess secretion of glucagon, catecholamines, glucocorticoids, and growth hormone stimulate glycogenolysis and gluconeogenesis while simultaneously impairing glucose disposal. DKA is primarily a complication of type 1 diabetes mellitus because it is usually only seen in the setting of severe insulin deficiency. DKA may be the initial presentation of new-onset type 1 diabetes mellitus.

Most patients with DKA have preceding symptoms of polyuria, polydipsia, and weight loss that result from a partially compensated state. However, with absolute insulin deficiency, metabolic decompensation can intervene rapidly over 24 hours. Typical DKA presenting symptoms include nausea; emesis; abdominal pain; lethargy; and hyperventilation with slow, deep breaths (Kussmaul respirations). On physical examination, most patients with DKA have a low-normal blood pressure, increased heart rate, increased respiratory rate, signs of volume depletion (e.g., decreased skin turgor, low jugular venous pressure, and dry oral mucosa), and breath that smells of acetone (a fruity odor similar to nail polish remover). With profound dehydration, patients may be obtunded or comatose.

The laboratory profile in patients with DKA includes low serum bicarbonate (HCO ₃ ) concentration (<10 mEq/L); increased serum concentrations of ketoacids (acetoacetate, β-hydroxybutyrate); increased anion gap (calculated by subtracting the sum of the serum concentrations of chloride and bicarbonate from that of sodium; reference range, <14 mEq/L; DKA usually >20 mEq/L); increased serum glucose concentration (500–900 mg/dL); and decreased arterial pH (<7.3).

The differential diagnosis of DKA includes other causes of metabolic acidosis (e.g., lactic acidosis, starvation ketosis, alcoholic ketoacidosis, uremic acidosis, and toxin ingestion [e.g., salicylate intoxication]).

TYPE 1 DIABETES MELLITUS

The diagnosis of diabetes mellitus is established when a patient presents with typical symptoms of hyperglycemia (polyuria, polydipsia, weight loss) and has a fasting plasma glucose concentration of 126 mg/dL or higher or a random value of 200 mg/dL or higher, which is confirmed on another occasion. There are three general types of diabetes: type 1, type 2 (see Plate 5-12 ), and gestational (see Plate 5-19 ). Type 1 diabetes mellitus affects less than 10% of all patients diagnosed with diabetes. Type 1 diabetes mellitus is the result of pancreatic β-cell destruction; in more than 95% of the cases, it has an autoimmune basis caused by an apparent selected loss of immune tolerance. If untreated, type 1 diabetes is a fatal catabolic disorder (see Plate 5-10 ). Because of absolute insulin deficiency, all persons with type 1 diabetes require insulin replacement therapy.

Immune-mediated type 1 diabetes is most common in northern Europe, where the approximate annual incidence is 30 per 100,000 persons. The lowest incidence of type 1 diabetes is in China (one per 100,000 persons per year). The peak life stage of onset is in children or young adults. The offspring of a mother with type 1 diabetes have a 3% risk of developing diabetes; the offspring of a father with type 1 diabetes have a 6% risk. Environmental factors (infectious or toxic environmental insult) have a major role in disease development; only 50% of identical twins of type 1 diabetic patients develop diabetes. Individuals with certain human leukocyte antigen (HLA) types are predisposed to type 1 diabetes. HLA class II molecules DQ and DR code for antigens expressed on the surface of B lymphocytes and macrophages. Approximately 95% of individuals with type 1 diabetes have HLA-DR3, HLA-DR4, or both, findings present in 50% of nondiabetic control subjects. Some DQ alleles (e.g., HLA-DQA1*0102, HLA-DQB1*0602) are associated with a decreased risk of diabetes. Non-HLA genes also affect susceptibility to type 1 diabetes. For example, polymorphisms in a lymphocyte-specific tyrosine phosphatase (PTNN22) and in a promoter of the insulin gene are associated with an increased risk of type 1 diabetes.

The immune system mistakenly targets β-cell proteins that share homologies with viral or other foreign peptides, a concept termed molecular mimicry . Most patients with newly diagnosed type 1 diabetes have circulating antibodies (islet cell antibody, antibody to glutamic acid decarboxylase [GAD], antibody to tyrosine phosphatases [insulinoma-associated protein 2], cation efflux zinc transporter, or insulin autoantibody). GAD is an enzyme in pancreatic β-cells that has homology to coxsackievirus B.

The autoimmune destruction of β-cells progresses over months and years, during which time affected individuals are euglycemic and asymptomatic (termed the latent period ). Impaired glucose tolerance usually precedes the onset of overt diabetes. By the time patients come to clinical attention, they have lost more than 90% of their β-cell mass. The progressive hyperglycemia has a toxic effect on the remaining islets with increased rate of apoptosis and impaired insulin secretion. These toxic hyperglycemic effects can be reversed over the short term with exogenous insulin treatment; the pancreas seems to recover for a period of time, termed the honeymoon period . Eventually, the viability of the remaining β-cells is exhausted.

Histopathology studies from the 1960s showed that hydropic changes (vacuolization) were the initial step in islet destruction. This change was actually attributable to infiltration with glycogen as shown with periodic acid–Schiff reagent. There is a selective destruction of β-cells. At the time of clinical presentation, a chronic inflammatory infiltrate of the islets is present (insulitis). The inflammatory infiltrate consists primarily of T lymphocytes (CD8 cells outnumber CD4 cells). Eventually, the islets become hyalinized, a process that partially or completely replaces an islet.

TYPE 2 DIABETES MELLITUS

The diagnosis of diabetes mellitus is established when a patient presents with typical symptoms of hyperglycemia (polyuria, polydipsia, weight loss) and has a fasting plasma glucose concentration of 126 mg/dL or higher or a random value of 200 mg/dL or higher confirmed on another occasion. In asymptomatic individuals, the finding of fasting plasma glucose concentrations higher than 126 mg/dL on more than one occasion is diagnostic of diabetes. Individuals with fasting glucose levels from 100 to 125 mg/dL are considered to have impaired fasting glucose. Individuals with plasma glucose concentrations at or above 140 mg/dL, but not over 200 mg/dL, 2 hours after a 75-g oral glucose load are considered to have impaired glucose tolerance.

There are three general types of diabetes—type 1 (see Plate 5-11 ), type 2, and gestational (see Plate 5-19 ). Type 2 diabetes mellitus accounts for more than 90% of patients diagnosed with diabetes. Unlike type 1 diabetes, in which the individual has an absolute insulin deficiency, individuals with type 2 diabetes have a relative insulin deficiency in part because of a resistance to insulin action. Most patients with type 2 diabetes are obese and are diagnosed after the age of 30 years.

Insulin resistance in patients with type 2 diabetes is related to polygenic factors, abdominal visceral obesity, sedentary lifestyle, and aging. Approximately 40% of patients with type 2 diabetes have a least one parent with the disorder. The concordance of type 2 diabetes in monozygotic twins is 90%. Although many genetic factors are yet to be discovered, several common genetic polymorphisms increase the risk for type 2 diabetes. The basic pathogenesis of type 2 diabetes is inadequate pancreatic β-cell insulin secretory response for the prevailing blood glucose concentration. Sustained hyperglycemia magnifies the underlying insulin resistance and β-cell dysfunction, both of which improve with treatment and improved glycemic control. The impaired insulin secretion in patients with type 2 diabetes is multifactorial but is partly attributable to decreased β-cell mass associated with increased β-cell apoptosis.

Obesity (body mass index [BMI] >30 kg/m ² ) is present in 80% of individuals with type 2 diabetes that are of European, North American, or African descent. Only 30% of individuals with type 2 diabetes of Japanese and Chinese descent are obese. The combination of abdominal obesity, hyperglycemia, hyperinsulinemia, dyslipidemia, and hypertension has been referred to as the metabolic syndrome (see Plate 7-15 ). Abdominal obesity aggravates insulin resistance that results in hyperglycemia leading to further hyperinsulinemia. Type 2 diabetes occurs when the hyperinsulinemia is insufficient to correct the hyperglycemia.

Diffuse damage to more than 70% of the pancreas can cause diabetes. Examples of such insults include pancreatitis, trauma, pancreatic carcinoma, hemochromatosis, and partial pancreatectomy. Excess production of the four insulin counterregulatory hormones can also cause diabetes. For example, diabetes may be the initial presentation of the following endocrine disorders: pheochromocytoma (catecholamines), acromegaly (growth hormone), glucagonoma (glucagon), and Cushing syndrome (glucocorticoids). Patients with thyrotoxicosis or somatostatinomas may also have diabetes. The hyperglycemia in patients with these endocrinopathies typically is cured by effective treatment of the underlying disorder.

Approximately 5% of individuals with type 2 diabetes have a monogenic disorder, maturity-onset diabetes of the young (MODY), resulting in a defect in glucose-induced insulin release. These individuals are usually not obese and are diagnosed with diabetes in late childhood or as young adults. Six types of autosomal dominant MODY have been described. MODY 2 is caused by impaired conversion of glucose to glucose 6-phosphate in the β-cell because of a mutation in the gene encoding the glucokinase enzyme. Glucokinase serves as a glucose sensor in the β-cell. The other forms of MODY are caused by mutations of genes that encode transcription factors that regulate β-cell gene expression. For example, MODY 3 (the most common form of MODY) and MODY 1 are caused by mutations in the gene that encodes hepatocyte nuclear factor 1α (HNF-1α) and HNF-4α, respectively. MODY 4 is caused by mutations in insulin promoter factor-1 (IPF-1), which mediates insulin gene transcription and regulates other β-cell genes (e.g., glucokinase and glucose transporter 2). MODY 5 is caused by mutations in the gene encoding HNF-1β, and MODY 6 is caused by mutations in the gene encoding islet transcription factor neuroD1.