Chapter 7. Endocrine Pancreas

The pancreas is a retroperitoneal gland divided into a head, body, and tail that is located near the duodenum. Most of the pancreatic mass is composed of exocrine cells that are clustered in lobules (acini) divided by connective tissue and connected to a duct that drains into the pancreatic duct and into the duodenum. The product of the pancreatic exocrine cells is an alkaline fluid rich with digestive enzymes, which is secreted into the small intestine to aid in the digestive process. Embedded within the acini are richly vascularized, small clusters of endocrine cells called the islets of Langerhans, in which 2 endocrine cell types (β and α) predominate. The β-cells constitute most of the total mass of endocrine cells, and their principal secretory product is insulin. The α-cells account for approximately 20% of the endocrine cells and are responsible for glucagon secretion. A small number of δ-cells secrete somatostatin, and an even smaller number of cells secrete pancreatic polypeptide. The localization of these cell types within the islets has a particular pattern, with the β-cells located centrally, surrounded by α- and δ-cells.

The arterial blood supply to the pancreas is derived from the splenic artery and the superior and inferior pancreaticoduodenal arteries. Although islets represent only 1%–2% of the mass of the pancreas, they receive approximately 10%–15% of the pancreatic blood flow. The rich vascularization by fenestrated capillaries allows ready access to the circulation for the hormones secreted by the islet cells. Venous blood from the pancreas drains into the hepatic portal vein. Therefore, the liver, a principal target organ for the physiologic effects of pancreatic hormones, is exposed to the highest concentrations of pancreatic hormones. Following first-pass hepatic metabolism, the pancreatic endocrine hormones are distributed to the systemic circulation.

Parasympathetic, sympathetic, and sensory nerves richly innervate the pancreatic islets, and the respective neurotransmitters and neuropeptides released from their nerve terminals exert important regulatory effects on pancreatic endocrine hormone release. Acetylcholine, vasoactive intestinal polypeptide, pituitary adenylate cyclase-activating polypeptide, and gastrin-releasing peptide are released from the parasympathetic nerve terminals. Norepinephrine, galanin, and neuropeptide Y are released from sympathetic nerve terminals. Vagal nerve activation stimulates the secretion of insulin, glucagon, somatostatin, and pancreatic polypeptide. Sympathetic nerve stimulation inhibits basal and glucose-stimulated insulin secretion and somatostatin release and stimulates glucagon and pancreatic polypeptide secretion.

Pancreatic Hormones

Insulin

Insulin Synthesis, Release, and Degradation

The process involved in the synthesis and release of insulin, a polypeptide hormone, by the β-cells of the pancreas is similar to that of other peptide hormones, as discussed in Chapter 1 (Figure 1–2). Preproinsulin undergoes cleavage of its signal peptide during insertion into the endoplasmic reticulum, generating proinsulin (Figure 7–1). Proinsulin consists of an amino-terminal β-chain, a carboxy-terminal α-chain, and a connecting peptide; known as the C-peptide, that links the α- and β-chains. Linking of the 2 chains allows proper folding of the molecule and the formation of disulfide bonds between the 2 chains. In the endoplasmic reticulum, proinsulin is processed by specific endopeptidases, which cleave the C-peptide exposing the end of the insulin chain that interacts with the insulin receptor, generating the mature form of insulin. Insulin and the free C-peptide are packaged into secretory granules in the Golgi. These secretory granules accumulate in the cytoplasm in 2 pools; a readily releasable (5%) and a reserve pool of the granules (more than 95%). On stimulation, the β-cell releases insulin in a biphasic pattern; initially from the readily releasable pool followed by the reserve pool of granules. Only a small proportion of the cellular stores of insulin are released even under maximal stimulatory conditions. Insulin circulates in its free form, has a half-life of 3–8 minutes, and is degraded predominantly by the liver, with more than 50% of insulin degraded during its first pass. Additional degradation of insulin occurs in the kidneys as well as at target tissues by insulin proteases following endocytosis of the receptor-bound hormone.

Figure 7–1.

A. Insulin is a peptide hormone synthesized from preproinsulin. Preproinsulin undergoes posttranslational modification in the endoplasmic reticulum (ER) to form proinsulin. The active form of insulin is produced by modification of proinsulin by cleavage of the C-peptide structure linking the alpha and beta chains. Both insulin and the cleaved C-peptide are packaged in secretory granules and are coreleased in response to glucose stimulation. B. Insulin release occurs in a biphasic mode; from readily releasable secretory granules and from granules that must undergo a series of preparatory reactions including mobilization to the plasma membrane. C. In response to a meal, the increase in insulin release results from a higher frequency and amplitude of pulsatile release. Shown are portal insulin concentrations during basal state (left) and after a meal.

Exocytosis of secretory granule content results in the release of equal amounts of insulin and C-peptide into the portal circulation. The importance of C-peptide is that unlike insulin, it is not readily degraded in the liver. Thus, the relatively long half-life of the peptide (35 minutes) allows its release to be used as an index of the secretory capacity of the endocrine pancreas. C-peptide, may have some biologic action as recent evidence indicates that replacement of C-peptide improves renal function and nerve dysfunction in patients with type 1 diabetes. The receptor and signaling mechanisms involved in mediating these responses are still under investigation.

The amino acid sequence of insulin is highly conserved among species. In the past, porcine and bovine insulin were used to treat patients with diabetes. Currently, human recombinant insulin is available and has replaced animal-derived insulin, avoiding problems such as the development of antibodies to nonhuman insulin.

Regulation of Insulin Release

The pancreatic β-cell functions as a neuroendocrine integrator that responds to changes in plasma levels of energy substrates (glucose and amino acids), hormones (insulin, glucagon-like peptide I, somatostatin, and epinephrine), and neurotransmitters (norepinephrine and acetylcholine) by increasing or decreasing insulin release (Figure 7–2). Glucose is the principal stimulus for insulin release from the pancreatic β-cells. In addition, glucose exerts a permissive effect for the other modulators of insulin secretion.

Figure 7–2.

Regulation of insulin release. Glucose is the principal stimulus for insulin release from the pancreatic β-cell. Glucose enters the β-cell cell by a specific glucose transporter protein (GLUT 2) undergoes glycolysis leading to generation of ATP. The increased ATP/ADP ratio leads to inhibition and closure of the ATP-sensitive K+ channels (the target of sulfonylurea drugs), resulting in plasma membrane depolarization and opening of the voltage-dependent Ca2+ channels. The increased Ca2+ influx coupled with mobilization of Ca2+ from intracellular stores leading to the fusion of insulin-containing secretory granules with the plasma membrane and the release of insulin (and C-peptide) into the circulation. Addition factors can also stimulate insulin release from the β-cell, including hormones (glucagon-like peptide 1) and neurotransmitters (acetylcholine). Glucose synergizes with these mediators and enhances the secretory response of the β-cell to these factors. AC, adenylate cyclase; ADP, adenosine diphosphate; ATP, adenosine triphosphate; CCK, cholecystokinin; GLP 1, glucagon-like peptide-1; PLC, phospholipase C. (Modified, with permission, from Fajans SS, Bell GI, Polonsky KS. Mechanisms of disease: molecular mechanisms and clinical pathophysiology of maturity-onset diabetes of the young. N Engl J Med. 2001;345:971. Copyright © Massachusetts Medical Society. All rights reserved.)

The glucose-induced stimulation of insulin release is the result of glucose metabolism by the β-cell (see Figure 7–2). Glucose enters the β-cell through a membrane-bound glucose transporter 2 (GLUT 2) and undergoes immediate phosphorylation by glucokinase in the initial step of glycolysis, leading eventually to the generation of adenosine triphosphate (ATP) by the Krebs cycle. The resulting increase in intracellular ATP to adenosine diphosphate ratio inhibits (closes) the ATP-sensitive K+ channels (KATP) in the β-cell, reducing the efflux of K+. Decreased K+ efflux results in membrane depolarization; activation (opening) of voltage-dependent Ca2+ channels, and increased Ca2+ influx. The increase in intracellular Ca2+ concentrations triggers the exocytosis of insulin secretory granules and the release of insulin into the extracellular space and into the circulation. It is important to note that the regulation of K+ channels by ATP is mediated by the sulfonylurea receptor. This is the basis for the therapeutic use of sulfonylurea drugs in the treatment of diabetes.

The β-cell Ca2+ concentrations can also be elevated by amino acids through their metabolism and ATP generation, or by direct depolarization of the plasma membrane. Other factors (shown in Figure 7–2) that amplify the glucose-induced release of insulin from the β-cell include acetylcholine; cholecystokinin; gastrointestinal peptide, also known as glucose-dependent insulinotropic polypeptide; and glucagon-like peptide 1 (GLP 1). These substances all bind to cell surface receptors and trigger downstream signaling mechanisms controlling insulin release. Acetylcholine and cholecystokinin promote phosphoinositide breakdown, with a consequent mobilization of Ca2+ from intracellular stores, Ca2+ influx across the cell membrane, and activation of protein kinase C. GLP 1 increases levels of cyclic 3′,5′-adenosine monophosphate (cAMP) and activates cAMP-dependent protein kinase A. The generation of cAMP, inositol 1,4,5-trisphosphate, diacylglycerol, and arachidonic acid and the activation of protein kinase C amplify the Ca2+ signal by decreasing Ca2+ uptake by cellular stores and promoting both the phosphorylation and activation of proteins that trigger the exocytosis of insulin. Catecholamines and somatostatin inhibit insulin secretion through G protein–coupled receptor mechanisms, inhibition of adenylate cyclase, and modification of Ca2+ and K+ channel gating.

The short-term regulation of insulin release is mediated through modification of proinsulin mRNA translation. Over longer periods, glucose also increases proinsulin mRNA content by both stimulating proinsulin gene transcription and stabilizing the mRNA. As mentioned above, the release of insulin in response to glucose is biphasic, with an initial rapid release of preformed insulin followed by a more sustained release of newly synthesized insulin. This biphasic response to glucose is a major characteristic of glucose-stimulated insulin secretion. The first phase occurs over a period of minutes, the second over an hour or more. Several hypotheses have been proposed to explain the biphasic nature of insulin secretion; including the involvement of 2 separate pools of insulin granules.

The release of insulin throughout the day is pulsatile and rhythmic in nature (see Figure 7–1). The pulsatile release of insulin is important for achieving maximal physiologic effects. In particular, it appears to be critical in the suppression of liver glucose production and in insulin-mediated glucose disposal by adipose tissue. Insulin release increases after a meal in response to the increases in plasma levels of glucose and amino acids. Secretion is the result of a combination of an increase in the total amount of insulin released in each secretory burst and an increased pulse frequency of a similar magnitude (see Figure 7–1). The synchronized increase in insulin release is thought to be the result of recruitment of β-cells to release insulin. Although it is not clear how the β-cells communicate with each other to synchronize the release of insulin, some of the proposed mechanisms include gap junctions allowing the passage of ions and small molecules; and propagation of membrane depolarization, aiding the synchronization between the cells. In addition, intra-pancreatic neural, hormonal, and substrate factors have been shown to play an important role in the pulsatile pattern of insulin release.

Physiologic Effects of Insulin

Insulin produces a wide variety of effects that range from immediate (within seconds), such as the modulation of ion (K+) and glucose transport into the cell; early (within minutes), such as the regulation of metabolic enzyme activity; moderate (within minutes to hours), such as the modulation of enzyme synthesis; to delayed (within hours to days), such as the effects on growth and cellular differentiation. Overall, the actions of insulin at target organs are anabolic and promote the synthesis of carbohydrate, fat, and protein, and these effects are mediated through binding to the insulin receptor (Table 7–1).

Table 7–1. Insulin Effects on Carbohydrate, Fat, and Protein Metabolism

Table 7–1. Insulin Effects on Carbohydrate, Fat, and Protein Metabolism

Metabolic effects

Insulin stimulates

Insulin inhibits

Carbohydrate metabolism

Glucose transport in adipose tissue and muscle
Rate of glycolysis in muscle and adipose tissue
Glycogen synthesis in adipose tissue, muscle, and liver

Glycogen breakdown in muscle and liver
Rate of glycogenolysis and gluconeogenesis in the liver

Lipid metabolism

Fatty acid and triacylglycerol synthesis in tissues
Uptake of triglycerides from the blood into adipose tissue and muscle
Rate of cholesterol synthesis in the liver

Lipolysis in adipose tissue, lowering the plasma fatty acid level
Fatty acid oxidation in muscle and liver
Ketogenesis

Protein metabolism

Amino acid transport into tissues
Protein synthesis in muscle, adipose tissue, liver, and other tissues

Protein degradation in muscle
Urea formation

Insulin Receptor

The insulin receptor is part of the insulin-receptor family, which includes the insulin-like growth-factor receptor (Figure 7–3). The insulin receptor is a heterotetrameric glycoprotein membrane receptor composed of 2 α- and 2 β-subunits, linked by disulfide bonds. The extracellular α-chain is the site for insulin binding. The intracellular segment of the β-chain has intrinsic tyrosine kinase activity, which on insulin binding, undergoes autophosphorylation on tyrosine residues. The activated receptor phosphorylates tyrosine residues of several proteins known as insulin receptor substrates 1 through 4 (IRS-1–4); facilitating the interaction of the insulin receptor with intracellular substrates. The result is the coupling of insulin receptor activation to signaling pathways, mainly the phosphatidylinositol 3-kinase (PI3K) and the mitogen-activated protein kinase (MAPK) pathways (see Figure 7–3).

Figure 7–3.

Insulin receptor signaling. Insulin binding to the receptor activates the intrinsic kinase activity of the intracellular domain of the receptor. This results in downstream activation of cellular events mediated through the phosphorylation of insulin receptor substrates (IRS). Downstream signaling pathways involved in insulin-mediated effects including the phosphatidylinositol 3-kinase (PI3K) and the mitogen-activated protein kinase (MAPK) cascades. Activation of phosphoinositide-3 kinase is a major pathway in the mediation of insulin-stimulated glucose transport and metabolism. Among the immediate effects of insulin is the active recruitment of glucose transporter 4 (GLUT 4), stored in intracellular vesicles to the cell surface. Exercise can also stimulate glucose transport by pathways that are independent of phosphoinositide-3 kinase and thought to involve 5′-adenosine monophosphate (AMP)-activated kinase.

The PI3K pathway involves phosphorylation of inositol phospholipids and the generation of phosphatidylinositol 3,4,5-trisphosphate and phosphatidylinositol 3,4-bisphosphate. These products, in turn, attract serine kinases to the plasma membrane, including the phosphoinositide-dependent kinase and different isoforms of protein kinase B, which, when activated, catalyze some of the cellular effects of insulin. The PI3K pathway is involved predominantly in mediating the metabolic effects of the hormone, including glucose transport, glycolysis, and glycogen synthesis, and plays a crucial role in the regulation of protein synthesis by insulin. Moreover, this pathway is involved in cell growth and transmits a strong antiapoptotic signal, promoting cell survival. The other main signaling pathway that is activated by insulin binding to its receptor is the MAPK pathway. Although signaling cascades in this pathway do not appear to play a significant role in the metabolic effects of insulin, they are involved in mediating the proliferative and differentiation effects elicited by insulin.

Signal transduction by the insulin receptor is not limited to its activation at the cell surface. The activated ligand-receptor complex is internalized into endosomes. Endocytosis of activated receptors is thought to enhance the insulin receptor tyrosine kinase activity on substrates that are distant from those readily accessible at the plasma membrane. Following acidification of the endosomal lumen, insulin dissociates from its receptor, ending the insulin receptor-mediated phosphorylation events, and promoting the degradation of insulin by proteases such as the acidic insulinase. The insulin receptor can then be recycled into the cell surface, where it becomes available for insulin binding again.

The number of available insulin receptors is modulated by exercise, diet, insulin, and other hormones. Chronic exposure to high insulin levels, obesity, and excess growth hormone all lead to a downregulation of insulin receptors. In contrast, exercise and fasting upregulate the number of receptors, improving insulin responsiveness.

Insulin Effects at Target Organs

Early Effects

Although the expression of insulin receptors is widespread, the specific effects of insulin on skeletal muscle glucose utilization dominate insulin action. Insulin mediates approximately 40% of glucose disposal by the body, the great majority (80%–90%) of which occurs in skeletal muscle. The movement of glucose into the cell is mediated by glucose transporters, with their own unique tissue distribution, summarized in Table 7–2.

Table 7–2. Main Features of Glucose Transporters (GLUTs)

Table 7–2. Main Features of Glucose Transporters (GLUTs)

Transporter

Expression

Function

GLUT 1

Ubiquitous, with particularly high levels in human erythrocytes and in the endothelial cells lining the blood vessels of the brain. Expressed in skeletal muscle and fat.

Glucose uptake by skeletal muscle and fat under basal conditions

GLUT 2

Low-affinity glucose transporter present in pancreatic β-cells, liver, intestine, and kidney

Functions in the glucose sensor system and ensures that glucose uptake by pancreatic β-cells and hepatocytes occurs only when circulating glucose levels are high

GLUT 3

Primarily in neurons

Together, GLUT 1 and GLUT 3 are crucial in allowing glucose to cross the blood-brain barrier and enter neurons

GLUT 4

Predominantly in striated muscle and adipose tissue. In contrast to the other GLUT isoforms, which are primarily localized on the cell membrane, GLUT 4 transporter proteins are sequestered in specialized storage vesicles that remain within the cell’s interior under basal conditions.

The major insulin-responsive transporter

GLUT 5

Spermatozoa and small intestine

Predominantly a fructose transporter

Insulin-stimulated glucose transport is mediated through GLUT 4, most of which is sequestered intracellularly in the absence of insulin or other stimuli such as exercise. Insulin binding to its receptor results in increased GLUT 4 translocation through targeted exocytosis and decreased rate of its endocytosis. This is the underlying mechanism by which insulin stimulates glucose transport into fat and muscle cells.

Intermediate Effects

The intermediate effects of insulin are mediated by modulation of protein phosphorylation of enzymes involved in metabolic processes in muscle, fat, and liver (Figure 7–4). In fat, insulin inhibits lipolysis and ketogenesis by triggering the dephosphorylation of hormone- sensitive lipase and stimulates lipogenesis by activating acetylcoenzyme A (acetyl-CoA) carboxylase. Dephosphorylation of hormone-sensitive lipase inhibits the breakdown of triglycerides to fatty acids and glycerol, the rate-limiting step in the release of free fatty acids mediated by lipolysis. This process thereby reduces the amount of substrate that is available for ketogenesis. Insulin antagonizes catecholamine-induced lipolysis through the phosphorylation and activation of phosphodiesterase, leading to a decrease in intracellular cAMP levels and a concomitant decrease in protein kinase A activity.

Figure 7–4.

Glucagon and insulin effects on hepatic glucose metabolism. Binding of glucagon and insulin to their respective receptors stimulates a cascade of protein phosphorylation steps that activate (or inhibit) key enzymes involved in the regulation of glycogenolysis, gluconeogenesis, and glycolysis. The principal target enzymes for insulin- and glucagon-mediated effects are presented. The overall result is an increase in hepatic glucose output. ADP, adenosine diphosphate; ATP, adenosine triphosphate; G, glucagon; I, insulin; PEP, phosphoenolpyruvate.

In the liver, insulin stimulates the gene expression of enzymes involved in glucose utilization (eg, glucokinase, pyruvate kinase) and lipogenic enzymes and inhibits the gene expression of enzymes involved in glucose production (eg, phosphoenolpyruvate carboxykinase and glucose-6-phosphatase) (see Figure 7–4). Insulin stimulates glycogen synthesis by increasing phosphatase activity, leading to the dephosphorylation of glycogen phosphorylase and glycogen synthase. In addition, insulin-mediated dephosphorylation of inhibitory sites on hepatic acetyl-CoA carboxylase increases the production of malonylcoenzyme A (malonyl-CoA) and simultaneously reduces the rate at which fatty acids can enter hepatic mitochondria for oxidation and ketone body production.

In muscle, insulin stimulates glucose uptake and favors protein synthesis though phosphorylation of a serine/threonine protein kinase known as mammalian target of rapamycin (mTOR). In addition, insulin favors lipid storage in muscle as well as in adipose tissue. As discussed later, insulin deficiency leads to glucose accumulation in blood, a decrease in lipid storage, and protein loss, resulting in negative nitrogen balance and muscle wasting.

Long-Term Effects

Sustained insulin stimulation enhances the synthesis of lipogenic enzymes and the repression of gluconeogenic enzymes. The growth-promoting and mitogenic effects of insulin are long-term responses mediated through the MAPK pathway. Both MAPK and particularly, the chronic activation of extracellular receptor kinase by insulin-receptor binding, lead to excessive cell growth. Although this pathway of insulin action is not as well elucidated as the effects that are mediated through the activation of IRS-PI3K, evidence suggests its involvement in the pathophysiologic consequences of chronic insulin elevations as those that occur in insulin resistant individuals.

Insulin levels are high (reflecting insulin resistance) during the development and early stages of type 2 diabetes. Chronic hyperinsulinemia has been linked to increased risk of cancers including endometrium, postmenopausal breast, colon, and kidney. Conditions that cause elevated insulin levels include high waist circumference, excess visceral fat, high waist-to-hip ratio, high body mass index, sedentary lifestyle, and high energy intake. In addition, the proliferative effects of chronic hyperinsulinemia influence vascular smooth muscle cells, which are responsible for the maintenance of vascular tone. These cells play an important role in the pathogenesis of several diseases, including hypertension, atherosclerosis, cardiovascular disease, and dyslipidemia, all of which are closely associated with insulin resistance and hyperinsulinemia. The molecular basis of insulin’s effect on vascular smooth muscle cell growth and its association with hypertension are currently unclear.

Glucagon

Glucagon Synthesis

Glucagon, is a 29–amino acid polypeptide hormone secreted by the α-cells of the islets of Langerhans, that plays an important role in the regulation of glucose homeostasis by producing antagonistic effects on insulin action. The primary sequence of glucagon is almost perfectly conserved among vertebrates, and it is structurally related to the secretin family of peptide hormones. Glucagon is synthesized as proglucagon and then proteolytically processed to yield glucagon. The prohormone proglucagon is expressed in the pancreas, and also in other tissues, such as enteroendocrine cells in the intestinal tract and in the brain. However, the processing of the prohormone differs among tissues. The 2 main products of proglucagon processing are glucagon in the α-cells of the pancreas and GLP 1 in the intestinal cells. GLP 1 is produced in response to a high concentration of glucose in the intestinal lumen. GLP 1 is known as an incretin, a mediator that amplifies insulin release from the β-cell in response to a glucose load. Glucagon has a short half-life (5–10 minutes) and is degraded mostly in the liver.

Regulation of Glucagon Release

The mechanisms involved in the regulation and stimulus-secretion coupling of glucagon release are not as well understood as those for insulin. Glucagon release is inhibited by hyperglycemia (high blood-glucose levels) and stimulated by hypoglycemia (low blood-glucose levels). A meal rich in carbohydrates suppresses glucagon release and stimulates insulin release from the β-cells through intestinal release of GLP 1. Somatostatin also inhibits glucagon release. High amino acid levels following an amino acid–rich meal stimulate glucagon release. Epinephrine stimulates release of glucagon through a β2-adrenergic mechanism (whereas it suppresses insulin release from β-cells through an α2-adrenergic mechanism). Vagal (parasympathetic) stimulation increases glucagon release.

Physiologic Effects of Glucagon

The principal target tissue for glucagon is the liver. Glucagon’s main physiologic effect is to increase plasma glucose concentrations by stimulating de novo hepatic glucose production through gluconeogenesis and glycogen breakdown; overall, these actions counteract the effects of insulin (Figure 7–5).

Figure 7–5.

Glucagon receptor-mediated cellular effects. Glucagon binds to G protein–coupled receptor (GPCR) on target cells leading to activation of adenylate cyclase, elevation in cAMP and increased protein kinase A activity resulting in phosphorylation of enzymes responsible for control of glucose metabolism. The ultimate result is an increase in hepatic glucose production through increased gluconeogenesis and glycogenolysis. cAMP, cyclic 3′,5′-adenosine monophosphate; GDP, guanosine 5′-diphosphate; G-6-Pase, glucose-6-phosphatase; GTP, guanosine 5′-triphophate; PEPCK, phosphoenolpyruvate carboxykinase; PGC-1, peroxisome proliferator-activated receptor-coactivator-1; PKA, protein kinase A.

Glucagon Receptor

Glucagon mediates its effects by binding to the glucagon Gαs protein–coupled receptor. The glucagon and GLP 1 peptide receptors belong to a family of G protein–coupled receptors that include those for secretin, calcitonin, vasoactive intestinal polypeptide, parathyroid hormone, and growth hormone-releasing factor. The glucagon receptor is expressed in liver, pancreatic β-cells, kidney, adipose tissue, heart, and vascular tissues, as well as in some regions of the brain, stomach, and adrenal glands. Glucagon binding activates adenylate cyclase and results in intracellular accumulation of cAMP, mobilization of intracellular Ca2+, protein kinase A activation, and phosphorylation of effector proteins. The glucagon-receptor complex undergoes endocytosis into intracellular vesicles, where glucagon is degraded. The role of glucagon receptors in tissues other than the liver is still unclear.

Glucagon Effects at Target Organs

Glucagon stimulates hepatic glucose output by stimulating glycogen breakdown and gluconeogenesis and decreasing glycolysis (see Figure 7–5). The key enzymatic steps regulated by glucagon that mediate the stimulation of hepatic glucose output are summarized in Table 7–3. Glucagon has effects on adipose tissue that are relevant primarily during periods of stress or food deprivation, particularly when insulin release is suppressed.

Table 7–3. Effects of Glucagon on Hepatic Glucose Metabolism

Table 7–3. Effects of Glucagon on Hepatic Glucose Metabolism

Effect on target enzyme

Metabolic response

Increased expression of glucose-6-phosphatase

Frees glucose to enter the circulation

Suppression of glucokinase

Decreases glucose entry into the glycolytic cascade

Phosphorylation (activation) of glycogen phosphorylase

Stimulates glycogenolysis

Inhibition of glycogen synthase

Inhibits glycogen synthesis

Stimulation of phosphoenolpyruvate carboxykinase expression

Stimulates gluconeogenesis

Inactivation of phosphofructokinase-2 (PFK-2) and activation of fructose-6-phosphatase. PFK-2 is the kinase activity and fructose-2,6-bisphosphatase (F-2,6-BPase) is the phosphatase activity of the bifunctional regulatory enzyme, phosphofructokinase-2/fructose-2,6-bisphosphatase (PFK-2/F-2,6-BPase).

Inhibits glycolysis
Stimulates gluconeogenesis

Suppression of activity of the pyruvate kinase

Decreases glycolysis

In the adipocyte, glucagon stimulates protein kinase A–mediated phosphorylation (activation) of hormone-sensitive lipase, the enzyme that breaks down triglycerides (stored fat) into diacylglycerol and free fatty acids, releasing them into the circulation. Glycerol released into the circulation can be utilized in the liver for gluconeogenesis or for reesterification. Free fatty acids are used as fuel by most tissues, predominantly skeletal muscle and liver. In the liver, free fatty acids are used for reesterification, or they undergo β-oxidation and conversion into ketone bodies (Figure 7–6). Thus, ketogenesis is regulated by the balance between the effects of glucagon and insulin at their target organs. The importance of this balance is evident during insulin deficiency and glucagon excess, as seen in uncontrolled diabetes (discussed later).

Figure 7–6.

Process of ketogenesis in insulin deficiency. Insulin deficiency and high levels of counterregulatory hormones glucagon, epinephrine, and cortisol combine to increase the activity of hormone sensitive lipase and the release of free fatty acids, and decrease the activity of acetyl CoA carboxylase, thereby impairing the reesterification of FFA promoting fatty acid conversion to ketone bodies. The excess supply of fatty acyl CoA and deficiency in oxaloacetate leads to increased oxidation to ketone bodies with the resulting release of ketone bodies into the blood. CoA, coenzyme A; FFA, free fatty acid; HSL, hormone-sensitive lipase.

Somatostatin

Somatostatin is a 14–amino acid peptide hormone produced by the δ-cells of the pancreas. Its release is stimulated by high-fat, high-carbohydrate, and particularly protein-rich meals, and is inhibited by insulin. Somatostatin has a generalized inhibitory effect on virtually all gastrointestinal and pancreatic exocrine and endocrine functions. The regulation of its release is not well studied because of the difficulty in analyzing the small number of islet cells that produce this hormone. Further, the importance of endogenous paracrine inhibition of insulin and glucagon release is not well established. Because δ-cells are located in the periphery of the β-cells and because blood flows from the center of the islets of Langerhans toward the periphery, pancreatic somatostatin may have a limited contribution toward physiologic control of insulin and glucagon release. However, exogenous administration of somatostatin does suppress the release of both insulin and glucose and is used in the clinical setting for the management of insulin or glucagon producing tumors.

Pancreatic Polypeptide

Pancreatic polypeptide is a 36–amino acid peptide hormone that belongs to a peptide family including neuropeptide Y and peptide YY. It is produced in the endocrine type F cells located in the periphery of pancreatic islets and is released into the circulation after a meal, exercise, and vagal stimulation. The effects of pancreatic polypeptide include inhibition of pancreatic exocrine secretion, gallbladder contraction, modulation of gastric acid secretion, and gastrointestinal motility. Pancreatic polypeptide crosses the blood-brain barrier and has been postulated to play a role in regulating feeding behavior.

Amylin

Amylin, or islet amyloid polypeptide, is a 37–amino acid peptide hormone that belongs to the calcitonin family (calcitonin, calcitonin gene-related peptide, and adrenomedullin). Amylin is synthesized as a small precursor, undergoes posttranslational modification (amidation), is stored in β-granules, and is released along with insulin and C-peptide. Plasma amylin concentrations increase after a meal or glucose infusion. Amylin appears to work with insulin to regulate plasma glucose concentrations in the bloodstream, suppressing the postprandial secretion of glucagon and slowing gastric emptying. In muscle, amylin opposes glycogen synthesis and activates glycogenolysis and glycolysis, thereby increasing lactate production. Circulating amylin is increased in obesity, hypertension, and gestational diabetes; it is low or absent in type 1 diabetes mellitus. Amylin is the main component of pancreatic islet amyloid, found in the vast majority of patients with non-insulin-dependent (type 2) diabetes mellitus, and is thought to contribute to destruction of the pancreatic β-cell. Amylin binds to a variant of the calcitonin G protein–coupled receptor. The modified calcitonin receptor has higher affinity for amylin, an effect mediated by transmembrane proteins known as receptor activity modifying proteins.

Diseases Associated with Pancreatic Hormones

Hormone-Producing Tumors

Excess pancreatic hormone production and release are usually caused by hormone-producing tumors, with insulinoma being the most frequent. Insulinomas produce excessive amounts of insulin, and patients present with episodes of hypoglycemia, confusion, aggressiveness, palpitations, sweating, convulsions, and even unconsciousness. These symptoms are mostly observed before breakfast and following physical exercise. The compensatory or counterregulatory response of the body includes the release of catecholamines, glucagon, cortisol, and growth hormone.

Glucagonomas are unusual tumors that may produce symptoms of diabetes. Excessive glucagon production by the tumor may also result in an overall catabolic effect on fat and muscle, leading to severe weight loss and anorexia. Somatostatinoma is a rare tumor that may cause moderate diabetes.

Diabetes Mellitus

The most common disease resulting from impaired pancreatic hormone release is diabetes mellitus. The 2 forms of diabetes, type 1 and type 2, are characterized by impaired insulin release. Type 1 diabetes, also known as insulin-dependent diabetes mellitus, is the result of β-cell destruction. It accounts for less than 5% of cases, and it occurs more frequently in younger people, hence its other name, juvenile-onset diabetes. Type 1 diabetes is characterized by the development of ketoacidosis in the absence of insulin therapy. Type 2 diabetes results from a loss of normal regulation of insulin secretion and accounts for more than 90% of diabetes cases. It is usually associated with obesity in adults and is characterized by mild hyperglycemia. It rarely leads to ketoacidosis. Type 2 diabetes is often part of “syndrome X” or “insulin-resistance syndrome,” a metabolic syndrome characterized by hypertension, atherosclerosis, and central obesity.

The pathophysiology of the disease involves impaired entry of glucose into the cells and accumulation of glucose in the blood. This process results in increased plasma osmolarity and urinary loss of glucose, accompanied by excess loss of water and sodium (polyuria). The resulting dehydration triggers compensatory mechanisms such as thirst (polydipsia). The inability of the cells to utilize glucose resembles a state of cellular starvation, stimulating hunger (polyphagia) and triggering the activation of compensatory responses to increase the release and availability of fuel substrates through activation of lipolysis and proteolysis. Lack of insulin results in increased circulating levels of free fatty acids and gluconeogenic amino acids. These exceed the liver’s capacity for their metabolic utilization, leading to the buildup of ketone bodies in the blood (diabetic ketoacidosis) and their urinary excretion.

Type 2 Diabetes

Type 2 diabetes is the result of decreased responsiveness of peripheral tissues to insulin action and inadequate responsiveness of the β-cells to glucose, which is ultimately followed by a net reduction in β-cell mass. Patients with type 2 diabetes secrete normal amounts of insulin during fasting, but in response to a glucose load (or a meal), they secrete considerably less insulin (70%) than nondiabetic patients. In addition to a reduction in insulin release, the pattern of insulin release is also altered following a meal, with pulses that are significantly smaller, sluggish, and erratic, particularly after dinner. This abnormality results in significantly higher levels of fasting glucose in these patients.

Regardless of the etiology (eg, abnormalities in glucose transport; abnormal insulin synthesis, processing, storage, or secretion), the earliest physiologic indication of β-cell dysfunction is a delay in the acute insulin response to glucose. The defect in the initial response to a glucose load leads to an excessive rise in plasma glucose, which in turn produces a compensatory and exaggerated second-phase hyperinsulinemic response. This initial period of sustained hyperinsulinemia downregulates the insulin receptors, decreasing the sensitivity of tissues to insulin action and producing a state of insulin resistance. The main pathologic defects in diabetes are excessive hepatic glucose production (reflected in elevated fasting glucose levels), defective β-cell secretory function, and peripheral insulin resistance.

Insulin Resistance

Insulin resistance is the inability of peripheral target tissues to respond properly to normal circulating concentrations of insulin. To maintain euglycemia, the pancreas compensates by secreting increased amounts of insulin. In patients with type 2 diabetes, insulin resistance may precede the onset of the disease by several years. Compensating for insulin resistance by an increase in insulin release is effective only temporarily. As insulin resistance increases, impaired glucose tolerance develops. Ultimately, failure or exhaustion of the pancreatic β-cell results in decreased insulin secretion. The combination of insulin resistance and impaired β-cell function characterizes clinical type 2 diabetes. Exercise has been demonstrated to increase glucose transport in skeletal muscle and to decrease insulin resistance in patients with type 2 diabetes. The increase in glucose transport resulting from exercise is not mediated through the insulin receptor, but involves increased cytosolic Ca2+ and the enzyme adenosine monophosphate (AMP)-activated protein kinase. AMP-activated protein kinase is activated during exercise and has been called a master metabolic switch because it phosphorylates key target proteins that control flux through metabolic pathways.

Clinical Evaluation of Diabetes

The diagnosis of diabetes is based on fasting glucose of at least 126 mg/dL; random glucose levels higher than 200 mg/dL in association with symptoms of diabetes (polyuria, polydipsia, and polyphagia); or persistent elevations in plasma glucose levels following an oral glucose load (greater than 200 mg/dL 2 hours after glucose ingestion). Glycated hemoglobin, resulting from glycosylation of hemoglobin is proportionate to blood glucose concentrations. Because the half-life of erythrocytes is approximately 60 days, the level of glycated hemoglobin reflects the prevailing mean blood glucose concentration over the preceding 6–8 weeks; providing a measure of chronic glycemia. The measurement of glycated hemoglobin is used to monitor glycemic control in patients with known diabetes. Normal values are 5% and targeted values in diabetic patients in treatment is <7%.

Treatment of the Diabetic Patient

The goal of therapy is tight glycemic control, which has been shown to delay the development of microvascular complications associated with diabetes. Because glucose homeostasis relies on regulated balance among hepatic glucose release, dietary glucose absorption, and skeletal muscle and adipose tissue glucose uptake and disposal; all 3 of these components have been targeted for pharmacologic treatment of diabetic patients. Some of the approaches used, in addition to conventional insulin, warrant mention because they affect the physiologic mechanisms of pancreatic hormone release and target organ effects on the control of glucose.

Sulfonylureas

Sulfonylureas increase insulin release by closing K+-ATP channels in the pancreatic β-cell membrane. This action is mediated by binding of the drug to the sulfonylurea receptor subunit of the channel. Because type 1 diabetes is characterized by β-cell destruction, this approach is ineffective in those patients.

Biguanides

Biguanides, such as metformin, reduce hepatic glucose output (primarily through inhibition of gluconeogenesis and, to a lesser extent, glycogenolysis) and increase insulin-stimulated glucose uptake in skeletal muscle and adipocytes. In insulin-sensitive tissues (such as skeletal muscle), metformin facilitates glucose transport by increasing insulin receptor tyrosine kinase activity and enhancing glucose transporter trafficking to the cell membrane.

Alpha-Glucosidase Inhibitors

Alpha-glucosidase inhibitors delay the intestinal absorption of carbohydrates through inhibition of the brush-border enzymes that hydrolyze polysaccharides to glucose.

Thiazolidinediones

Thiazolidinediones reduce insulin resistance in skeletal muscle by activation of the gamma isoform of the peroxisome proliferator-activated receptor in the nucleus, thereby affecting the transcription of several genes involved in glucose and lipid metabolism and energy balance. Among the genes that are affected are those that code for lipoprotein lipase, fatty acid transporter protein, adipocyte fatty acid-binding protein, fatty acetyl-CoA synthase, malic enzyme, glucokinase, and the GLUT4.

Glucagon-Like Peptide

GLP 1 amplifies glucose-induced insulin release. GLP 1 increases insulin biosynthesis and insulin gene expression and has tropic and antiapoptotic effects on the pancreatic β-cells. GLP 1 suppresses glucagon release, hepatic glucose production, gastric emptying, and food intake.

Complications of Diabetes

Complications of diabetes can be categorized as acute and chronic. Acute complications include hypoglycemia; diabetic ketoacidosis; and hyperglycemic, hyperosmolar nonketotic coma. Chronic complications of diabetes involve the vascular system (micro- and macrovascular damage), peripheral nerves, skin, and the lens. End-stage renal disease, autonomic neuropathy, and blindness, are more frequent in type 1 diabetes. Macrovascular disease leading to myocardial infarction and stroke are more likely in type 2 diabetic patients.

Hypoglycemia

A common complication of tight glycemic control is hypoglycemia resulting from excess insulin dose, fasting, or strenuous exercise. This results in an immediate activation of a systemic counterregulatory response involving sympathetic nervous system activation, glucagon release, followed by release of growth hormone and cortisol. Clinical manifestations range from tachycardia, palpitations, sweating, and tremors as glucose levels decrease to approximately 54 mg/dL, to irritability, confusion, blurred vision, tiredness, headache, and difficulty speaking when levels approach 50 mg/dL. Further decreases in glucose can lead to loss of consciousness or seizures.

Diabetic Ketoacidosis

Diabetic ketoacidosis is characterized by hyperglycemia, increased ketone bodies, and metabolic acidosis, resulting directly from decreased insulin availability and simultaneous elevations of the counterregulatory hormones glucagon, catecholamines, cortisol, and growth hormone. Diabetic ketoacidosis can be precipitated by infections, traumatic injury, and discontinuation of, or inadequate, insulin use.

In diabetic ketoacidosis, gluconeogenesis in the liver proceeds unopposed by the physiologic presence of insulin. The excess blood glucose increases osmolarity, which, if severe, can result in diabetic coma (see Figure 7–6). The lack of insulin and the high levels of counterregulatory hormones glucagon, epinephrine, and cortisol combine to increase the activity of hormone-sensitive lipase, increase the release of free fatty acids, and decrease the activity of acetyl-CoA carboxylase, thereby impairing the reesterification of free fatty acids and promoting fatty acid conversion into ketone bodies (see Figure 7–6). In the liver fatty acids undergo β-oxidation to acetyl-CoA. Acetyl-CoA condenses with oxaloacetate to form citrate in the entry step to the Krebs cycle (citric acid cycle or tricarboxylic cycle). However, a low-insulin-to-glucagon ratio favors gluconeogenesis; thus oxaloacetate is preferentially used for gluconeogenesis, decreasing its availability for condensation with acetyl-CoA. As a result, acetyl-CoA is diverted from entering the Krebs cycle and is used preferentially for ketone body formation or ketogenesis, the process by which fatty acids are transformed into acetoacetate and 3-hydroxybutyrate in hepatocyte mitochondria. The steps involved in ketogenesis are β-oxidation of fatty acids to acetyl-CoA, formation of acetoacetyl-CoA, and conversion of acetoacetyl-CoA to 3-hydroxy-3-methylglutaryl-CoA and then to acetoacetate, which is then reduced to 3-β-hydroxybutyrate. The enzymes involved in ketogenesis are summarized in Table 7–4. Acetoacetate can be spontaneously decarboxylated to acetone, a highly fat-soluble compound that is excreted slowly by the lungs and is responsible for the fruity odor of the breath of individuals with diabetic ketoacidosis.

Table 7–4. Three Principal Enzymes Involved in Ketogenesis

Table 7–4. Three Principal Enzymes Involved in Ketogenesis

Enzyme

Tissue

Function

Hormone-sensitive lipase

Adipocytes

Breaks down triglycerides, releasing fatty acids into the circulation

Acetyl-CoA carboxylase

Liver

Catalyzes the conversion of acetyl-CoA to malonyl-CoA, the primary substrate of fatty acid biosynthesis

HMG-CoA synthase

Liver

Involved in the conversion of acetyl-CoA to acetoacetate

CoA, coenzyme A; HMG, 3-hydroxy-3-methylglutaryl.

During diabetic ketoacidosis, high amounts of ketone bodies are released into the blood, and a high ratio (3:1 or higher) of 3-β-hydroxybutyrate to acetoacetate is generated because of the highly reduced state of hepatic mitochondria. These ketone bodies can freely diffuse across cell membranes and serve as an energy source for extrahepatic tissues including the brain, skeletal muscle, and kidneys. Ketone bodies are filtered and reabsorbed in the kidney. At physiologic pH, ketone bodies, with the exception of acetone, dissociate completely. The resulting liberation of H+ from ketone body metabolism exceeds the blood’s buffering capacity, leading to metabolic acidosis with an increased anion gap. If severe, this condition can lead to coma.

Hyperglycemic Hyperosmolar Coma

Hyperglycemic coma is characterized by severe hyperglycemia, hyperosmolality, and dehydration in the absence of significant ketosis. It typically occurs in middle-aged or elderly patients with mild or occult diabetes. Lethargy and confusion develop as serum osmolarity exceeds 300 mOsm/L, and coma can occur if osmolarity exceeds 330 mOsm/L. Underlying medical conditions such as renal insufficiency and congestive heart failure are common, and the presence of either worsens the prognosis. Precipitating events include infections such as pneumonia, cerebrovascular accident, or myocardial infarction, among others.

Key Concepts