Insulin, Glucagon, and Diabetes Mellitus

The pancreas, in addition to its digestive functions, secretes two major hormones, insulin and glucagon , that are crucial for normal regulation of glucose, lipid, and protein metabolism. Although the pancreas secretes other hormones, such as amylin, somatostatin, and pancreatic polypeptide, their functions are not as well established. The main purpose of this chapter is to discuss the physiological roles of insulin and glucagon and the pathophysiology of diseases, especially diabetes mellitus , caused by abnormal secretion or activity of these hormones.

Physiological Anatomy of the Pancreas

The pancreas is composed of two major types of tissues, as shown in Figure 79-1 : (1) the acini, which secrete digestive juices into the duodenum, and (2) the islets of Langerhans, which secrete insulin and glucagon directly into the blood. The digestive secretions of the pancreas are discussed in Chapter 65 .

Figure 79-1., Physiological anatomy of an islet of Langerhans in the pancreas.

The human pancreas has 1 to 2 million islets of Langerhans. Each islet is only about 0.3 millimeter in diameter and is organized around small capillaries, into which its cells secrete their hormones. The islets contain three major types of cells— alpha, beta, and delta cells—that are distinguished from one another by their morphological and staining characteristics.

The beta cells, constituting about 60% of all the cells of the islets, lie mainly in the middle of each islet and secrete insulin and amylin, a hormone that is often secreted in parallel with insulin, although its function is not well understood. The alpha cells, about 25% of the total, secrete glucagon, and the delta cells, about 10% of the total, secrete somatostatin. In addition, at least one other type of cell, the PP cell , is present in small numbers in the islets and secretes a hormone called pancreatic polypeptide.

The close interrelations among these cell types in the islets of Langerhans allow cell-to-cell communication and direct control of secretion of some of the hormones by the other hormones. For example, insulin inhibits glucagon secretion, amylin inhibits insulin secretion, and somatostatin inhibits the secretion of both insulin and glucagon.

Insulin and its Metabolic Effects

Insulin was first isolated from the pancreas in 1922 by Banting and Best, and almost overnight rescued patients with severe cases of diabetes mellitus from a rapid decline in health and early death. Historically, insulin has been associated with “blood sugar,” and true enough, insulin has profound effects on carbohydrate metabolism. However, abnormalities of fat metabolism that cause conditions such as acidosis and arteriosclerosis are also important causes of morbidity and death in patients with diabetes mellitus. Patients with prolonged, untreated diabetes have diminished ability to synthesize proteins which leads to wasting of the tissues and many cellular functional disorders. Therefore, it is clear that insulin affects fat and protein metabolism almost as much as it affects carbohydrate metabolism.

Insulin is a Hormone Associated With Energy Abundance

As we discuss insulin in the next few pages, it will become apparent that insulin secretion is associated with energy abundance. That is, when a person’s diet includes a great abundance of foods that provide energy, especially excess amounts of carbohydrates, insulin secretion increases. In turn, the insulin plays an important role in storing the excess energy. In the case of excess carbohydrates, it causes them to be stored as glycogen, mainly in the liver and muscles. Furthermore, all the excess carbohydrates that cannot be stored as glycogen are converted under the stimulus of insulin into fats and stored in adipose tissue. In the case of proteins, insulin has a direct effect in promoting amino acid uptake by cells and conversion of these amino acids into protein. In addition, it inhibits breakdown of proteins that are already in the cells.

Insulin Chemistry and Synthesis

Human insulin, which has a molecular weight of 5808, is composed of two amino acid chains, shown in Figure 79-2 , that are connected to each other by disulfide linkages. When the two amino acid chains are split apart, insulin’s functional activity is lost.

Figure 79-2., A schematic of the human proinsulin molecule, which is cleaved in the Golgi apparatus of the pancreatic beta cells to form connecting peptide (C peptide), and insulin, which is composed of the A and B chains connected by disulfide bonds. The C peptide and insulin are packaged in granules and secreted in equimolar amounts, along with a small amount of proinsulin.

Insulin is synthesized in beta cells by the usual cell machinery for protein synthesis, as explained in Chapter 3 , beginning with translation of the insulin RNA by ribosomes attached to the endoplasmic reticulum to form preproinsulin . This initial preproinsulin has a molecular weight of about 11,500, but it is then cleaved in the endoplasmic reticulum to form a proinsulin with a molecular weight of about 9000 and consisting of three chains of peptides, A, B, and C. Most of the proinsulin is further cleaved in the Golgi apparatus to form insulin, which is composed of the A and B chains connected by disulfide linkages, and the C chain peptide, called connecting peptide (C peptide). The insulin and C peptide are packaged in secretory granules and secreted in equimolar amounts. About 5% to 10% of the final secreted product is still in the form of proinsulin.

The proinsulin and C peptide have virtually no insulin activity. However, C peptide binds to a membrane structure, most likely a G protein–coupled membrane receptor, and elicits activation of at least two enzyme systems, sodium-potassium adenosine triphosphatase and endothelial nitric oxide synthase. Although both of these enzymes have multiple physiological functions, the importance of C peptide in regulating these enzymes is still uncertain.

C peptide levels can be measured by radioimmunoassay in insulin-treated diabetic patients to determine how much of their own natural insulin they are still producing. Patients with type 1 diabetes who are unable to produce insulin will usually also have greatly decreased levels of C peptide.

When insulin is secreted into the blood, it circulates almost entirely in an unbound form. Because it has a plasma half-life that averages only about 6 minutes, it is mainly cleared from the circulation within 10 to 15 minutes. Except for the portion of the insulin that combines with receptors in the target cells, the insulin is degraded by the enzyme insulinase mainly in the liver, to a lesser extent in the kidneys and muscles, and slightly in most other tissues. This rapid removal from the plasma is important because, at times, it is as important to rapidly turn off the control functions of insulin as it is to turn them on.

Activation of Target Cell Receptors by Insulin and the Resulting Cellular Effects

To initiate its effects on target cells, insulin must first bind with and activate a membrane receptor protein that has a molecular weight of about 300,000 ( Figure 79-3 ).

Figure 79-3., A schematic of the insulin receptor. Insulin binds to the α subunit of its receptor, which causes autophosphorylation of the β-subunit receptor, which in turn induces tyrosine kinase activity. The receptor tyrosine kinase activity begins a cascade of cell phosphorylation that increases or decreases the activity of enzymes, including insulin receptor substrates, that mediate the effects on glucose, fat, and protein metabolism. For example, glucose transporters are moved to the cell membrane to assist glucose entry into the cell.

The insulin receptor is a combination of four subunits held together by disulfide linkages: two alpha subunits that lie entirely outside the cell membrane and two beta subunits that penetrate through the membrane, protruding into the cell cytoplasm. Insulin binds with the alpha subunits on the outside of the cell, but because of the linkages with the beta subunits, portions of the beta subunits protruding into the cell become autophosphorylated. Thus, the insulin receptor is an example of an enzyme-linked receptor, discussed in Chapter 75 . Autophosphorylation of the beta subunits of the receptor activates a local tyrosine kinase, which in turn causes phosphorylation of multiple other intracellular enzymes, including a group called insulin-receptor substrates (IRS) . Different types of IRS (e.g., IRS-1, IRS-2, and IRS-3) are expressed in different tissues. The net effect is to activate some of these enzymes while inactivating others. In this way, insulin directs the intracellular metabolic machinery to produce the desired effects on carbohydrate, fat, and protein metabolism. The following are the main end effects of insulin stimulation:

1.
Within seconds after insulin binds with its membrane receptors, the membranes of about 80% of the body’s cells markedly increase their uptake of glucose. This action is especially true of muscle cells and adipose cells, but it is not true of most neurons in the brain. The increased glucose transported into the cells is immediately phosphorylated and becomes a substrate for all the usual carbohydrate metabolic functions. The increased glucose transport is believed to result from translocation of multiple intracellular vesicles to the cell membranes; these vesicles carry multiple molecules of glucose transport proteins, which bind with the cell membrane and facilitate glucose uptake into the cells. When insulin is no longer available, these vesicles separate from the cell membrane within about 3 to 5 minutes and move back to the cell interior to be used again and again, as needed.
2.
The cell membrane becomes more permeable to many of the amino acids, potassium ions, and phosphate ions, causing increased transport of these substances into the cell.
3.
Slower effects occur during the next 10 to 15 minutes to change the activity levels of many more intracellular metabolic enzymes. These effects result mainly from the changed states of phosphorylation of the enzymes.
4.
Much slower effects continue to occur for hours and even several days. These result from changed rates of translation of messenger RNAs at the ribosomes to form new proteins and still slower effects from changed rates of transcription of DNA in the cell nucleus. In this way, insulin remolds much of the cellular enzymatic machinery to achieve some of its metabolic effects.

Effect of Insulin on Carbohydrate Metabolism

Immediately after a high-carbohydrate meal is consumed, glucose that is absorbed into the blood causes rapid secretion of insulin, which is discussed in detail later in the chapter. The insulin in turn causes rapid uptake, storage, and use of glucose by almost all tissues of the body but especially by the muscles, adipose tissue, and liver.

Insulin Promotes Muscle Glucose Uptake and Metabolism

During much of the day, muscle tissue depends not on glucose but on fatty acids for its energy. The principal reason for this dependence on fatty acids is that the normal resting muscle membrane is only slightly permeable to glucose, except when the muscle fiber is stimulated by insulin; between meals, the amount of insulin that is secreted is too small to promote significant amounts of glucose entry into the muscle cells.

However, under two conditions the muscles do use large amounts of glucose. One of these is during moderate or heavy exercise. This usage of glucose does not require large amounts of insulin because muscle contraction increases translocation of glucose transporter 4 (GLUT 4) from intracellular storage depots to the cell membrane, which, in turn, facilitates diffusion of glucose into the cell.

The second condition for usage of large amounts of glucose by muscles is during the few hours after a meal. At this time the blood glucose concentration is high and the pancreas is secreting large quantities of insulin. The extra insulin causes rapid transport of glucose into the muscle cells, which causes the muscle cell to use glucose preferentially over fatty acids during this period, as will be discussed later.

Storage of Glycogen in Muscle

If the muscles are not exercised after a meal and yet glucose is transported into the muscle cells in abundance, instead of being used for energy, most of the glucose is stored in the form of muscle glycogen, up to a limit of 2% to 3% concentration. The glycogen can be used by the muscle later for energy. Glycogen is especially useful for short periods of extreme energy usage by the muscles and even to provide spurts of anaerobic energy for a few minutes at a time via glycolytic breakdown of the glycogen to lactic acid, which can occur even in the absence of oxygen.

Quantitative Effect of Insulin to Facilitate Glucose Transport Through the Muscle Cell Membrane

The quantitative effect of insulin to facilitate glucose transport through the muscle cell membrane is demonstrated by the experimental results shown in Figure 79-4 . The lower curve labeled “control” shows the concentration of free glucose measured inside the cell, demonstrating that the glucose concentration remained almost zero despite increased extracellular glucose concentration up to as high as 750 mg/100 ml. In contrast, the curve labeled “insulin” demonstrates that the intracellular glucose concentration rose to as high as 400 mg/100 ml when insulin was added. Thus, it is clear that insulin can increase the rate of transport of glucose into the resting muscle cell by at least 15-fold.

Figure 79-4., The effect of insulin in enhancing the concentration of glucose inside muscle cells. Note that in the absence of insulin (control), the intracellular glucose concentration remains near zero, despite high extracellular glucose concentrations.

Insulin Promotes Liver Uptake, Storage, and Use of Glucose

One of the most important effects of insulin is to cause most of the glucose absorbed after a meal to be rapidly stored in the liver in the form of glycogen. Then, between meals, when food is not available and the blood glucose concentration begins to fall, insulin secretion decreases rapidly and the liver glycogen is split back into glucose, which is released back into the blood to keep the glucose concentration from falling too low.

The mechanism by which insulin causes glucose uptake and storage in the liver includes several almost simultaneous steps:

1.
Insulin inactivates liver phosphorylase, the principal enzyme that causes liver glycogen to split into glucose. This inactivation prevents breakdown of the glycogen that has been stored in liver cells.
2.
Insulin enhances uptake of glucose from the blood by the liver cells by increasing the activity of the enzyme glucokinase, which is one of the enzymes that causes the initial phosphorylation of glucose after it diffuses into the liver cells. Once phosphorylated, the glucose is temporarily trapped inside the liver cells because phosphorylated glucose cannot diffuse back through the cell membrane.
3.
Insulin increases the activities of enzymes that promote glycogen synthesis, including especially glycogen synthase. This is responsible for polymerization of the monosaccharide units to form glycogen molecules.

The net effect of all these actions is to increase the amount of glycogen in the liver. The glycogen can increase to a total of about 5% to 6% of the liver mass, which is equivalent to almost 100 grams of stored glycogen in the entire liver.

Glucose Is Released From the Liver Between Meals

When the blood glucose level begins to fall to a low level between meals, several events transpire that cause the liver to release glucose back into the circulating blood:

1.
The decreasing blood glucose causes the pancreas to decrease its insulin secretion.
2.
The lack of insulin then reverses all the effects listed earlier for glycogen storage, essentially stopping further synthesis of glycogen in the liver and preventing further uptake of glucose by the liver from the blood.
3.
The lack of insulin (along with increased glucagon, which is discussed later) activates the enzyme phosphorylase, which causes the splitting of glycogen into glucose phosphate.
4.
The enzyme glucose phosphatase, which had been inhibited by insulin, now becomes activated by the lack of insulin and causes the phosphate radical to split away from the glucose, allowing the free glucose to diffuse back into the blood.

Thus, the liver removes glucose from the blood when it is present in excess after a meal and returns it to the blood when the blood glucose concentration falls between meals. Ordinarily, about 60% of the glucose in the meal is stored in this way in the liver and then returned later.

Insulin Promotes Conversion of Excess Glucose Into Fatty Acids and Inhibits Gluconeogenesis in the Liver

When the quantity of glucose entering the liver cells is more than can be stored as glycogen or can be used for local hepatocyte metabolism, insulin promotes the conversion of all this excess glucose into fatty acids. These fatty acids are subsequently packaged as triglycerides in very low density lipoproteins, which are transported in the blood to adipose tissue, and deposited as fat.

Insulin also inhibits gluconeogenesis mainly by decreasing the quantities and activities of the liver enzymes required for gluconeogenesis. However, part of the effect is caused by an action of insulin that decreases release of amino acids from muscle and other extrahepatic tissues and in turn the availability of these necessary precursors required for gluconeogenesis. This phenomenon is discussed further in relation to the effect of insulin on protein metabolism.

Lack of Effect of Insulin on Glucose Uptake and Usage by the Brain

The brain is quite different from most other tissues of the body in that insulin has little effect on uptake or use of glucose. Instead, most of the brain cells are permeable to glucose and can use glucose without the intermediation of insulin .

The brain cells are also quite different from most other cells of the body in that they normally use only glucose for energy and can use other energy substrates, such as fats, only with difficulty. Therefore, it is essential that the blood glucose level always be maintained above a critical level, which is one of the most important functions of the blood glucose control system. When the blood glucose level falls too low, into the range of 20 to 50 mg/100 ml, symptoms of hypoglycemic shock develop, characterized by progressive nervous irritability that leads to fainting, seizures, and even coma.

Effect of Insulin on Carbohydrate Metabolism in Other Cells

Insulin increases glucose transport into and glucose usage by most other cells of the body (with the exception of most brain cells, as noted) in the same way that it affects glucose transport and usage in muscle cells. The transport of glucose into adipose cells mainly provides substrate for the glycerol portion of the fat molecule. Therefore, in this indirect way, insulin promotes deposition of fat in these cells.

Effect of Insulin on Fat Metabolism

Although not quite as visible as the acute effects of insulin on carbohydrate metabolism, the effects of insulin on fat metabolism are, in the long run, equally important. Especially dramatic is the long-term effect of insulin deficiency in causing extreme atherosclerosis, often leading to heart attacks, cerebral strokes, and other vascular accidents. First, however, let us discuss the acute effects of insulin on fat metabolism.

Insulin Promotes Fat Synthesis and Storage

Insulin has several effects that lead to fat storage in adipose tissue. First, insulin increases glucose utilization by most of the body’s tissues, which automatically decreases fat utilization, thus functioning as a fat sparer. However, insulin also promotes fatty acid synthesis, especially when more carbohydrates are ingested than can be used for immediate energy, thus providing the substrate for fat synthesis. Almost all this synthesis occurs in the liver cells, and the fatty acids are then transported from the liver by way of the blood lipoproteins to the adipose cells to be stored. The following factors lead to increased fatty acid synthesis in the liver:

1.
Insulin increases glucose transport into the liver cells. After the liver glycogen concentration reaches 5% to 6%, further glycogen synthesis is inhibited. All the additional glucose entering the liver cells then becomes available to form fat. The glucose is first split to pyruvate in the glycolytic pathway, and the pyruvate subsequently is converted to acetyl coenzyme A (acetyl-CoA), the substrate from which fatty acids are synthesized.
2.
An excess of citrate and isocitrate ions is formed by the citric acid cycle when excess amounts of glucose are used for energy. These ions then have a direct effect to activate acetyl-CoA carboxylase, the enzyme required to carboxylate acetyl-CoA to form malonyl-CoA , the first stage of fatty acid synthesis.
3.
Most of the fatty acids are then synthesized within the liver and used to form triglycerides , the usual form of storage fat. They are released from the liver cells to the blood in the lipoproteins. Insulin activates lipoprotein lipase in the capillary walls of the adipose tissue, which splits the triglycerides again into fatty acids, a requirement for them to be absorbed into adipose cells, where they are again converted to triglycerides and stored.

Role of Insulin in Storage of Fat in the Adipose Cells

Insulin has two other essential effects that are required for fat storage in adipose cells:

1.
Insulin inhibits the action of hormone-sensitive lipase. Lipase is the enzyme that causes hydrolysis of triglycerides already stored in fat cells. Therefore, release of fatty acids from adipose tissue into the circulating blood is inhibited.
2.
Insulin promotes glucose transport through cell membranes into fat cells in the same way that it promotes glucose transport into muscle cells. Some of this glucose is then used to synthesize minute amounts of fatty acids, but more important, it also forms large quantities of α-glycerol phosphate. This substance supplies the glycerol that combines with fatty acids to form triglycerides, the storage form of fat in adipose cells. Therefore, when insulin is not available, even storage of the large amounts of fatty acids transported from the liver in lipoproteins is almost blocked.

Insulin Deficiency Increases Use of Fat for Energy

All aspects of fat breakdown and its use for providing energy are greatly enhanced in the absence of insulin. This enhancement occurs even normally between meals when secretion of insulin is minimal, but it becomes extreme in persons with diabetes mellitus when secretion of insulin is almost zero.

Insulin Deficiency Causes Lipolysis of Storage Fat and Release of Free Fatty Acids

In the absence of insulin, all the effects of insulin noted earlier that cause storage of fat are reversed. The most important effect is that the enzyme hormone-sensitive lipase in the fat cells becomes strongly activated. This activation causes hydrolysis of stored triglycerides, releasing large quantities of fatty acids and glycerol into the circulating blood. Consequently, plasma concentration of free fatty acids begins to rise within minutes. These free fatty acids then become the main energy substrate used by essentially all tissues of the body except the brain.

Figure 79-5 shows the effect of a lack of insulin on the plasma concentrations of free fatty acids, glucose, and acetoacetic acid. Note that almost immediately after removal of the pancreas, the free fatty acid concentration in the plasma begins to rise, more rapidly even than the concentration of glucose.

Figure 79-5., The effect of removing the pancreas on the approximate concentrations of blood glucose, plasma free fatty acids, and acetoacetic acid.

Insulin Deficiency Increases Plasma Cholesterol and Phospholipid Concentrations

The excess of fatty acids in the plasma associated with insulin deficiency also promotes liver conversion of some of the fatty acids into phospholipids and cholesterol, two of the major products of fat metabolism. These two substances, along with excess triglycerides formed at the same time in the liver, are then discharged into the blood in the lipoproteins. Occasionally the plasma lipoproteins increase as much as threefold in the absence of insulin, giving a total concentration of plasma lipids of several percent rather than the normal 0.6%. This high lipid concentration—especially the high concentration of cholesterol—promotes development of atherosclerosis in people with severe diabetes.

Excess Usage of Fats During Insulin Deficiency Causes Ketosis and Acidosis

Insulin deficiency also causes excessive amounts of acetoacetic acid to be formed in liver cells. In the absence of insulin but in the presence of excess fatty acids in the liver cells, the carnitine transport mechanism for transporting fatty acids into the mitochondria becomes increasingly activated. In the mitochondria, beta oxidation of the fatty acids then proceeds rapidly, releasing extreme amounts of acetyl-CoA. A large part of this excess acetyl-CoA is then condensed to form acetoacetic acid, which is then released into the circulating blood. Most of this acetoacetic acid passes to the peripheral cells, where it is again converted into acetyl-CoA and used for energy in the usual manner.

At the same time, the absence of insulin also depresses utilization of acetoacetic acid in peripheral tissues. Thus, so much acetoacetic acid is released from the liver that it cannot all be metabolized by the tissues. As shown in Figure 79-5 , the concentration of acetoacetic acid rises during the days after cessation of insulin secretion, sometimes reaching concentrations of 10 mEq/L or more, which is a severe state of body fluid acidosis.

As explained in Chapter 69 , some of the acetoacetic acid is also converted into β-hydroxybutyric acid and acetone . These two substances, along with the acetoacetic acid, are called ketone bodies, and their presence in large quantities in the body fluids is called ketosis . We will see later that in severe diabetes, the acetoacetic acid and the β-hydroxybutyric acid can cause severe acidosis and coma, which may lead to death.

Effect of Insulin on Protein Metabolism and Growth

Insulin Promotes Protein Synthesis and Storage

Proteins, carbohydrates, and fats are stored in the tissues during the few hours after a meal when excess quantities of nutrients are available in the circulating blood; insulin is required for this storage to occur. The manner in which insulin causes protein storage is not as well understood as the mechanisms for both glucose and fat storage. Here are some of the facts:

1.
Insulin stimulates transport of many of the amino acids into the cells. Among the amino acids most strongly transported are valine, leucine, isoleucine, tyrosine, and phenylalanine. Thus, insulin shares with growth hormone the capability of increasing uptake of amino acids into cells. However, the amino acids affected are not necessarily the same ones.
2.
Insulin increases translation of messenger RNA, thus forming new proteins. Insulin “turns on” the ribosomal machinery and, in the absence of insulin, the ribosomes stop working, almost as if insulin operates by an “on-off” mechanism.
3.
Over a longer period, insulin also increases the rate of transcription of selected DNA genetic sequences in the cell nuclei, thus forming increased quantities of RNA and still more protein synthesis—especially promoting a vast array of enzymes for storage of carbohydrates, fats, and proteins.
4.
Insulin inhibits catabolism of proteins, thus decreasing the rate of amino acid release from the cells, especially from muscle cells. Presumably this results from the ability of insulin to diminish the normal degradation of proteins by cellular lysosomes.
5.
In the liver, insulin depresses the rate of gluconeogenesis by decreasing activity of the enzymes that promote gluconeogenesis. Because the substrates used most for synthesis of glucose by gluconeogenesis are plasma amino acids, this suppression of gluconeogenesis conserves amino acids in the protein stores of the body.

In summary, insulin promotes formation of protein and prevents degradation of proteins.

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