Chapter 46. The Endocrine Pancreas (Islets of Langerhans)

The islets of Langerhans are microscopic structures 50–250 μm in diameter. They are scattered throughout the pancreas, with a maximum density in the tail. The islets appear to have a great reserve capacity; islet dysfunction is not a major problem even after 90% of the pancreas is removed in a distal pancreatectomy. The islets are not connected to the exocrine duct system; the hormonal products are secreted directly into the bloodstream.

Microscopically, the islets are composed of small uniform cells with round nuclei and scant cytoplasm. Routine microscopy does not permit differentiation of the various types of cells contained within the islets; this requires immunohistochemistry (Table 46-1 and Figure 46-1).

The most important hormone secreted by the pancreas is insulin. The B (beta) cells of the islets are the only source of insulin in the body, and failure of secretion of adequate amounts of insulin results in diabetes mellitus.

Glucagon, secreted by the A (alpha) cells, also plays a role in glucose metabolism. The role of glucagon is a less vital one, and absence of glucagon has not been shown to cause clinical disease. The physiologic functions of pancreatic polypeptide (PP) and vasoactive intestinal polypeptide (VIP) have not been elucidated, and the amount of somatostatin and gastrin normally secreted by the pancreatic islets is thought to be too small to be of any physiologic significance. However, excessive secretion of any of these hormones by pathologic islets causes specific clinical syndromes.

Assessment of islet structure is very difficult because of their small size and scattered distribution in the pancreas. Only large islet cell neoplasms are distinguishable on computerized tomography. The main tests of islet function are serum assays of the various hormones secreted by the islets.

Diabetes mellitus is a chronic disease characterized by relative or absolute deficiency of insulin, resulting in glucose intolerance. It occurs in 4–5 million persons in the United States (approximately 2% of the population).

Normal Insulin Metabolism

Insulin is a polypeptide composed of an A chain, with 21 amino acids, and a B chain, with 30 amino acids (Figure 46-2). It is released from the B cell by a variety of stimuli (Figure 46-2), the most important of which—from a physiologic standpoint—is glucose. Amino acids and drugs of the sulfonylurea group also stimulate insulin release. Insulin is transported in the plasma with the alpha and beta globulins; no specific transport protein has been identified.

Insulin release occurs in three phases: (1) Basal secretion is responsible for the fasting level of insulin in serum; (2) initial rapid secretion after a meal is due to release of stored insulin in the B cells within 10 minutes after eating; and (3) delayed release after meals is due to stimulation of insulin synthesis in response to ingested glucose.

Insulin interacts with target cells that have insulin receptors on their plasma membranes (Figure 46-3). Important target cells are liver, muscle, and fat, although receptors have been demonstrated in many other cells. The number of insulin receptors on individual cells is variable, and the affinity of the receptor to insulin also varies.

The binding of insulin to receptors triggers a chain of events in the cell that mediates the action of the hormone; it is believed that small peptides act as second messengers to activate insulin-dependent enzyme systems.

Metabolic Actions of Insulin

(Figure 46-3)

The major biochemical function of insulin is to regulate the transfer of glucose from the plasma into the cytoplasm of cells.

After a large meal, high insulin levels in the blood induce the tissues to take up and store glucose. Glycogenesis is stimulated in the liver and in skeletal muscle, and lipogenesis increases in adipose tissue. In this state, free glucose represents the major source of immediate energy for muscle cells.

In the fasting state, low levels of insulin result in mobilization of body stores to satisfy energy needs of the body. Glycogenolysis and proteolysis in liver and skeletal muscle provide glucose; lipolysis in adipose tissue produces free fatty acids, which enter the circulation and are metabolized to ketone bodies in the liver. The cells of the body, with the exception of brain cells, utilize fatty acids and ketone bodies for energy in states of low insulin secretion. Brain cells are dependent on a continuous supply of glucose for metabolic needs; in the fasting state, this is supplied mainly by gluconeogenesis from amino acids.

Etiology of Diabetes Mellitus

(Table 46-2)

Diabetes mellitus is caused by a relative or absolute deficiency of insulin. In primary diabetes (95% of cases), there is no underlying disease process that might explain insulin deficiency. Primary diabetes is of two types: I and II (see below and Table 46-3). The remaining 5% of cases of secondary diabetes are due either to pancreatic destruction or to the presence of increased levels of hormones that antagonize the action of insulin.

There is an absolute deficiency of insulin in type I primary diabetes and in those cases of secondary diabetes associated with destruction of the pancreas. In type II primary diabetes—and in the presence of increased levels of antagonistic hormones—the insulin deficiency is relative, and serum insulin levels are usually normal and may even be elevated.

Type I Diabetes Mellitus

Type I diabetes mellitus (insulin-dependent diabetes mellitus [IDDM]) is due to destruction of pancreatic B cells. Plasma insulin levels are very low, and ketoacidosis develops if the patients do not receive exogenous insulin. Rarely, in the early stage of type I diabetes, there may be enough insulin to prevent ketoacidosis, and the patients are not insulin-dependent (this is sometimes known as “type I diabetes in evolution”). The disease affects young patients (juvenile-onset diabetes mellitus), most commonly under 30 years of age, and there is a significant association with human leukocyte antigen (HLA)-B8, -B15, -DR3, and -DR4. The HLA-D locus is closely associated with genes that confer increased susceptibility to type I diabetes. Ninety-five percent of patients with type I diabetes express DR3, DR4, or the heterozygous DR3/DR4 state. Increased susceptibility has also been linked with (1) the absence of aspartic acid in position 57 of the DQβ chain and (2) the presence of the DQw8 allele. The genetic predisposition to type I diabetes is shown by the history of diabetes in about 20% of first-degree relatives—which is not as strong as in type II diabetes.

The cause of B cell destruction in type I diabetes is unknown. A few cases have followed viral infections, most commonly with coxsackievirus B or mumps virus, and several viruses have been shown to cause B cell damage when inoculated into mice. Despite these findings, the role of viruses in the etiology of human diabetes is thought to be that of an inciting factor for autoimmunity.

Autoimmunity is believed to be the major mechanism involved. Islet cell autoantibodies are present in the serum of 90% of newly diagnosed cases. Such antibodies are directed against several cell components, including cytoplasmic and membrane antigens or against insulin itself (IgG and IgE antibodies). Sensitized T lymphocytes with activity against B cells have also been demonstrated in some patients. Microscopic examination of the islets in patients with early type I diabetes shows the presence of a lymphocytic infiltrate in the islet (“insulitis”). One hypothesis is that a mild viral injury of B cells induces an autoimmune reaction against the injured cells. HLA-linked immune response genes may explain the genetic susceptibility; HLA-B8, -B15, -DR3, and -DR4, in addition to their association with diabetes, also occur at increased frequency in Graves' disease, Addison's disease, and pernicious anemia, all of which are characterized by the presence of autoantibodies.

Toxins such as nitrophenylureas (in rat poisons) and cyanide from spoiled food have been implicated in B cell destruction in rare cases.

Type II Diabetes Mellitus

The etiology of type II diabetes (non-insulin-dependent diabetes mellitus [NIDDM]) is even less clearly understood. Two factors have been identified.

Impaired Insulin Release

Basal secretion of insulin is often normal, but the rapid release of insulin that follows a meal is greatly impaired, resulting in failure of normal handling of a carbohydrate load. The delayed phase of insulin secretion is also normal in the early stages but impaired in advanced disease. However, some level of insulin secretion is maintained in most patients, so that the abnormality of glucose metabolism is limited, and ketoacidosis is uncommon. In these patients, insulin secretion can be stimulated by drugs such as sulfonylureas. Exogenous insulin is therefore not essential in treatment. Most patients with type II diabetes first develop disease in adult life (adult-onset diabetes). A subgroup of type II diabetes develops disease at a young age (maturity-onset diabetes of the young [MODY]). These patients have an autosomal dominant single-gene inheritance pattern.

It has been suggested that inheritance of a defective pattern of insulin secretion is responsible for the familial tendency of diabetes. The mechanism of inheritance is highly complex and probably involves multiple genes except in maturity-onset diabetes of the young. The genetic factor is very strong in type II diabetes, with a history of diabetes present in about 50% of first-degree relatives.

Insulin Resistance

A defect in the tissue response to insulin is believed to play a major role. This phenomenon is called insulin resistance and is caused by defective insulin receptors on the target cells.

Insulin resistance occurs in association with obesity and pregnancy. In normal individuals who become obese or pregnant, the B cells secrete increased amounts of insulin to compensate. Patients who have a genetic susceptibility to diabetes cannot compensate because of their inherent defect in insulin secretion. Thus, type II diabetes is frequently precipitated by obesity and pregnancy.

In a few patients with extreme insulin resistance, antibodies against the receptors have been demonstrated in the plasma. These antibodies are mostly of the IgG class and may act in a manner analogous to the action of antiacetylcholine-receptor antibodies in myasthenia gravis. Decreased numbers of insulin receptors, defective binding of insulin to receptors, and abnormalities in the series of cellular events that follow insulin binding have also been postulated as causes of insulin resistance.

Pathology

The pathologic changes in the pancreatic islets in diabetes mellitus are variable from one patient to another and are not specific for diabetes. In type I diabetes, there is frequently a lymphocytic infiltration of the islets in the early phase, followed by a decrease in the total number and size of the islets due to a progressive loss of B cells.

The changes in type II diabetes are often minimal in the early stages. In advanced disease, there may be fibrosis and amyloid deposition in the islets; in diabetes, the amyloid appears to consist in part of precipitated insulin. Similar changes in the islets are sometimes present in elderly nondiabetic patients and are not considered diagnostic for diabetes.

Clinical Features

The classic symptoms of diabetes mellitus result from abnormal glucose metabolism. The lack of insulin activity results in failure of transfer of glucose from the plasma into the cells (“starvation in the midst of plenty”). The body responds as if it were in the fasting state, with stimulation of glycogenolysis, gluconeogenesis, and lipolysis producing ketone bodies (Figure 46-4).

The glucose absorbed during a meal is not metabolized at the normal rate and therefore accumulates in the blood (hyperglycemia) to be excreted in the urine (glycosuria). Glucose in the urine causes osmotic diuresis, leading to increased urine production (polyuria). The fluid loss and hyperglycemia increase the osmolarity of the plasma, stimulating the thirst center (polydipsia). Stimulation of protein breakdown to provide amino acids for gluconeogenesis results in muscle wasting and weight loss. These classic symptoms occur only in patients with severe insulin deficiency, most commonly in type I diabetes.

Many patients with type II diabetes do not have these symptoms and present with one of the complications of diabetes (see below).

Diagnosis

The diagnosis of diabetes mellitus is most certain when there is fasting hyperglycemia. This is defined as a plasma glucose level of > 7.8 mmol/L (> 140 mg/dL) on at least two occasions following an overnight fast.

In mild cases, the patient may have a fasting plasma glucose in the normal range, with the abnormality restricted to deficient handling of a glucose load as assessed in the 75 g oral glucose tolerance test (Figure 46-5). Diabetes is diagnosed in this test if the plasma glucose is > 11.1 mmol/L (> 200 mg/dL) at 2 hours and > 11.1 mmol/L (> 200 mg/dL) on at least one other occasion during the 2-hour test. If the glucose tolerance test is negative, diabetes is excluded. The interpretation of a positive test must take into account the possibility that a false-positive test may result from epinephrine released by the stress associated with the test.

Patients who have a 2-hour plasma glucose concentration between 7.8 and 11.1 mmol/L (140–200 mg/dL) and one other value > 11.1 mmol.L (> 200 mg/dL) during the test are diagnosed as having impaired glucose tolerance. Ten to 25 percent of patients with impaired glucose tolerance will develop overt diabetes.

The presence of glycosuria is of no value in the initial diagnosis of diabetes because there are many causes of glycosuria other than diabetes.

All of the above tests provide information about the patient's glucose metabolism only at the time of the test. Estimation of glycosylated hemoglobin (HbA1c) levels in blood is used as a guide to the degree of control over a long period. The level of HbA1c is dependent on the serum glucose concentration and is increased in uncontrolled diabetes. HbA1c, once formed, remains in the erythrocyte for the 120-day life span of the cell; HbA1c levels thus provide an indication of blood glucose elevation in the previous 2–3 months. Normal HbA1c is around 4% of total hemoglobin.

Acute Complications

Diabetic Ketoacidosis

Ketoacidosis occurs in severe diabetes, where insulin levels are greatly reduced and glucagon levels are increased. It is common in untreated type I diabetes but rare in type II diabetes, where insulin levels, although functionally inadequate, are still sufficient to prevent ketone body formation.

In the absence of insulin, lipolysis is stimulated (Figure 46-4), releasing free fatty acids that are oxidized in the liver cell to form acetylcoenzyme A. The entry of acetyl-CoA into the citric acid cycle is defective in diabetes. As a result, acetyl-CoA is converted in the liver to acetoacetate, β-hydroxybutyrate, and acetone (collectively called ketone bodies). Glucagon excess is an important factor in the pathogenesis of ketoacidosis. While insulin lack mobilizes free fatty acids from adipose tissue, oxidation of fatty acids to ketones in the liver cell is induced by glucagon via its stimulatory effect on the hepatic carnitine-palmitoyltransferase system. Glucagon also stimulates gluconeogenesis, aggravating the hyperglycemia. The ketone bodies enter the blood (ketonemia, ketosis) and represent an important source of energy for skeletal muscle that cannot utilize glucose effectively in diabetes. They also spill over to be excreted in the urine (ketonuria).

Ketone bodies are moderately strong acids and cause a metabolic acidosis with decreased blood pH and low serum bicarbonate. Respiration is stimulated, washing out carbon dioxide and leading to a decrease in Pco2. An acid urine is excreted.

Clinically, patients present with altered consciousness as a result of general failure of energy production and acidosis. Coma occurs in severe cases. Marked volume depletion is usually present. The diagnosis is established by the presence of glycosuria, hyperglycemia, ketonemia, and ketonuria. Treatment requires aggressive fluid replacement, correction of electrolyte imbalance, and insulin therapy.

Hyperosmolar Nonketotic Coma

Patients who develop hyperosmolar coma are usually elderly, with severe uncontrolled diabetes. The disorder results from extremely high serum glucose levels that cause osmotic diuresis and marked fluid depletion, increasing plasma osmolarity. Hyperosmolar coma is treated with aggressive fluid replacement and insulin. It is associated with a high mortality rate.

Hypoglycemic Coma

Hypoglycemic coma is not a direct complication of diabetes but rather a complication of therapy. In treating diabetes it is essential to balance the insulin dose and the dietary intake of carbohydrate (“glucose dose”). A fall in blood glucose may follow overdosage of insulin but is seen more often when the usual daily schedule of insulin injections is given and one or more meals is missed or lost by vomiting (ie, when the “glucose dose” is reduced).

Chronic Complications

(Table 46-4)

Diabetic Microangiopathy (Small Vessel Disease)

Microangiopathy is one of the most characteristic and most important pathologic changes in diabetes. It is characterized by diffuse thickening of the basement membranes of capillaries throughout the body. The kidney (Figure 46-6), retina, skin, and skeletal muscles are commonly involved. A similar change involves other basement membranes in renal tubules, placenta, and peripheral nerves. Basement membrane thickening in capillaries is associated with increased permeability to fluid and protein macromolecules.

The structure of the thick basement membrane in diabetics is abnormal. Increased amounts of collagen and laminin and decreased proteoglycans have been demonstrated. It has been suggested that prolonged elevation of serum glucose increases glycosylation of basement membrane proteins in a manner similar to glycosylation of hemoglobin. This would explain why strict control of diabetes decreases the incidence and severity of microangiopathy. It is widely accepted—although not proved—that tight control of diabetes decreases the risk of microangiopathy.

Large Vessel Disease

Diabetes mellitus is a major risk factor for development of atherosclerotic vascular disease; myocardial infarction and cerebral arterial occlusion (stroke) represent two of the most common causes of death in diabetics. The increased incidence of hyperlipidemia (both hypertriglyceridemia and hypercholesterolemia) in diabetes contributes to the development of atherosclerosis.

Neuropathy and Cataract

Neuropathy and cataract in diabetic patients are believed to result from accumulation of sorbitol within nerve or lens tissue. The enzyme aldose reductase produces sorbitol in these tissues when glucose levels are high, and the accumulated sorbitol, which is osmotically active and nondiffusible, produces cellular swelling or death. It is postulated that nerve and lens tissue (and perhaps small vessels and kidney) may be particularly vulnerable to this effect because glucose can enter these cells even in low-insulin states—unlike other cells of the body, which require normal plasma levels of insulin for entry of glucose. Trials of drugs that inhibit aldose reductase are under way as a possible means of combating some of the chronic effects of diabetes.

Other Complications

Other complications include a general increased susceptibility to infection (Chapter 7: Deficiencies of the Host Response) and impaired wound healing (Chapter 6: Healing & Repair). Chronic foot ulcers are a common and difficult problem.

Clinical Course

The average life expectancy of diabetics is reduced by 9 years for males and 7 years for females when compared with nondiabetics. The reduction is greatest when the onset of disease is at a young age.

Quality of life is seriously affected for all diabetics because of the many disabling complications. In addition, the requirement for strict dietary control and continuous drug treatment for many patients calls for a continuous emotional struggle.

Causes of death in diabetes (in order of frequency) are myocardial infarction, renal failure, cerebrovascular accidents, infections, ketoacidosis, hyperosmolar coma, and hypoglycemia.

Treatment

Type I diabetics require insulin treatment for life. Oral agents that act by stimulating the B cells are not effective in these patients because of their B cell-depleted state. Type II diabetics can be managed with measures to decrease insulin resistance, such as decreasing body weight by diet, and by stimulation of the pancreatic B cells with oral antidiabetic agents such as sulfonylureas. In many type II diabetics, insulin is also necessary for good control. An essential component of treatment is ensuring good control by repeated examinations of blood and urine for glucose. Serum HbA1c levels are useful for checking longer term control.

In animals, transplantation of pancreatic islets has resulted in a cure of experimental diabetes, but this method of treatment is not yet routinely available for treatment of humans.

Excess secretion (Figure 46-7) of any one or several of the hormones of the islets of Langerhans may be caused by islet cell neoplasms or hyperplasia.

Islet Cell Neoplasms

Adenomas derived from the islet cells are relatively common. In 10–15% of cases, multiple adenomas are present. Islet cell carcinomas occur, but less frequently.

Grossly, islet cell neoplasms are firm nodules that typically have a yellowish-brown color. They vary in size from microscopic (microadenomas) to large masses that may weigh several kilograms. They may or may not show encapsulation.

Microscopically, islet cell neoplasms are composed of uniform small cells arranged in nests and trabeculae separated by endothelium-lined vascular spaces. The islet cell origin of a pancreatic neoplasm can be established (1) by the presence of membrane-bound, electron-dense neurosecretory granules in the cytoplasm on electron microscopy; and (2) by positive staining for neuron-specific enolase, chromogranin, or specific hormones by immunoperoxidase techniques.

Differentiation of adenomas from carcinomas of islet cells is difficult by light microscopic examination. Invasion of the capsule and cytologic atypia are common in neoplasms that show benign behavior and cannot be used as evidence of malignant change. Conversely, islet cell carcinomas may be well circumscribed and have little cytologic atypia. Features that favor a diagnosis of carcinoma are extensive invasion of the pancreatic stroma or peripancreatic tissue, venous involvement, and perineural invasion. The only definite evidence of malignancy is the presence of metastatic lesions.

Islet cell adenomas are cured by surgical excision; carcinomas tend to grow slowly but are difficult to control if surgery fails. Even in the presence of metastatic disease, patients may survive several years because of the slow growth rate of islet cell carcinoma.

Most islet cell neoplasms are composed of one cell type; less commonly, multiple cell types are involved. The diagnosis of the cell type is impossible by routine light microscopy and requires (1) electron microscopy, which demonstrates characteristic granules of the different cells; (2) serum assay for the different pancreatic hormones; and (3) demonstration of hormone in the tumor cells by immunoperoxidase techniques. Some islet cell neoplasms do not produce sufficient hormone to be detectable in serum (nonfunctional islet cell neoplasms).

Islet Cell Hyperplasia

Diffuse hyperplasia of the islets is a rare cause of hypersecretion of pancreatic hormones, and there is some doubt about whether it is a real entity. Islet cell hyperplasia is characterized by the presence of islets in the size range 300–700 μm. Islets measuring less than 300 μm are normal; those above 700 μm are microadenomas. Microscopically, hyperplastic islets resemble normal islets; immunohistochemical studies sometimes show a dominance of one cell type.

In adults, the most common situation in which hyperplastic islets are found is in the pancreas adjacent to an islet cell neoplasm. The finding of hyperplastic islets in a surgically removed pancreas should therefore lead to a careful search for an adenoma in the remaining pancreas. Marked islet cell hyperplasia (of insulin-producing B cells) is also seen in fetuses born of diabetic mothers as a fetal response to the high glucose environment.

Clinical Features of Pancreatic Hormone Excess

The clinical features of hypersecretion of the islets depend on which hormone is secreted in excess and to what degree. In most cases, hypersecretion is restricted to one hormone; rarely, two or more hormones are involved.

Hyperinsulinism

The most common clinical syndrome associated with hyperfunctioning islets is hyperinsulinism. Seventy percent of cases are caused by solitary B cell adenomas (insulinomas); 10% by multiple adenomas; 10% by carcinomas; and 10% by islet cell hyperplasia.

Increased insulin secretion is characterized (1) by hypoglycemia, precipitated by fasting or exercise and causing dizziness, confusion, and excessive sweating which, if sustained, are followed by convulsions, coma, and death; (2) by prompt relief of symptoms after glucose administration; and (3) by a plasma glucose level under 40 mg/dL during an attack. The fasting plasma glucose is also decreased to less than half of normal. This symptom complex is known as Whipple's triad.

The diagnosis is established by the finding of an inappropriately high serum insulin level during a period of hypoglycemia.

Glucagon Excess

Glucagon stimulates glycogenolysis and gluconeogenesis, serving to maintain glucose levels between meals. Hyperfunction of A cells is rare and caused by an islet cell neoplasm (glucagonoma), 70% of which are carcinomas and 30% adenomas. Two thirds of patients with carcinomas present with evidence of metastases, commonly in the liver.

Clinically, patients have mild diabetes mellitus due to the insulin antagonistic action of glucagon and a typical erythematous necrotizing migratory skin eruption. Alopecia, increased skin pigmentation, and glossitis are less common manifestations.

The diagnosis is made by finding an elevated serum glucagon level.

Gastrin Excess

Gastrin hypersecretion is second in frequency to hyperinsulinism among this group of diseases. It is usually caused by an islet cell neoplasm composed of G cells (gastrinoma), 70% of which are malignant. In 10% of cases, islet cell hyperplasia is present but no neoplasm is found in the pancreas. In 1% of cases, a microadenoma measuring about 1 mm in diameter is the cause. Gastrinomas rarely occur also in the duodenal and gastric wall.

Secretion of large amounts of gastrin leads to Zollinger-Ellison syndrome, characterized by continuous hypersecretion of gastric acid, causing a low pH of gastric juice. The resting acid secretion is greater than 60% of the maximum acid secretion in response to an injection of histamine or pentagastrin. Unrelenting, recurrent peptic ulcers occur in the stomach, duodenum, esophagus, and jejunum in 90% of patients. Severe diarrhea and hypokalemia—induced by the hyperacidity—are present in 30% of patients. Diarrhea may be associated with malabsorption of fat and steatorrhea due to inactivation of lipase by the low pH in the duodenum. Ten percent of patients with Zollinger-Ellison syndrome present with diarrhea and have no peptic ulcer disease. The gastric mucosal folds are commonly thickened.

The diagnosis of Zollinger-Ellison syndrome is made by demonstrating high serum gastrin levels. It can be distinguished from other causes of elevated serum gastrin by a paradoxic increase in gastrin levels in response to intravenous secretin and calcium injections.

Somatostatin Excess

D cell neoplasms of the pancreas (somatostatinomas) are very rare. Eighty percent are malignant. Clinically, mild diabetes mellitus resulting from impaired release of insulin is the most constant feature; diarrhea and gallstones also occur commonly.

Diagnosis by demonstration of an elevated serum level is difficult because of the short half-life of somatostatin.

Vasoactive Intestinal Polypeptide Excess

Excess secretion of vasoactive intestinal polypeptide (VIP) is rare and caused by a D1 cell neoplasm of the islets (VIPoma). Clinically, the polypeptide stimulates intestinal secretion by an unknown mechanism, causing watery diarrhea with hypokalemia and alkalosis (WDHA syndrome; Verner-Morrison syndrome). The diagnosis is established by demonstrating elevated VIP levels in the serum and in an extract of the tumor.

Pancreatic Polypeptide Excess

Pancreatic polypeptide (PP)-producing neoplasms are extremely rare. They may be present in patients with no clinical symptoms. Some patients have watery diarrhea and hypokalemia; others have peptic ulcer disease.