Learning Objectives
By the end of this section, you will be able to:
- Describe catabolic and anabolic hormones and function in the body
- Discuss the role of ATP in metabolism
- Describe the hormones that regulate anabolic and catabolic reactions
| Catabolic Hormones (Table 24.1) | |
|---|---|
| Hormone | Function |
| Cortisol | Released from the adrenal gland in response to stress; its main role is to increase blood glucose levels by gluconeogenesis (breaking down fats and proteins) |
| Glucagon | Released from alpha cells in the pancreas either when starving or when the body needs to generate additional energy; it stimulates the breakdown of glycogen (glycogenolysis) and the production of glucose (gluconeogenesis) in the liver to increase blood glucose levels; its effect is the opposite of insulin; glucagon and insulin are a part of a negative-feedback system that stabilizes blood glucose levels |
| Adrenaline/epinephrine | Released in response to the activation of the sympathetic nervous system; increases heart rate and heart contractility, constricts blood vessels, is a bronchodilator that opens (dilates) the bronchi of the lungs to increase air volume in the lungs, and stimulates gluconeogenesis |
| Anabolic Hormones (Table 24.2) | |
|---|---|
| Hormone | Function |
| Growth hormone (GH) | Synthesized and released from the pituitary gland; stimulates the growth of cells, tissues, and bones |
| Insulin-like growth factor (IGF) | Stimulates the growth of muscle and bone while also inhibiting cell death (apoptosis) |
| Insulin | Produced by the beta cells of the pancreas; plays an essential role in carbohydrate and fat metabolism, controls blood glucose levels, and promotes the uptake of glucose into body cells; causes cells in muscle, adipose tissue, and liver to take up glucose from the blood and store it in the liver and muscle as glycogen (glycogen synthesis); its effect is the opposite of glucagon; glucagon and insulin are a part of a negative-feedback system that stabilizes blood glucose levels |
| Testosterone | Produced by the testes in males and the ovaries in females; stimulates an increase in muscle mass and strength as well as the growth and strengthening of bone |
| Estrogen | Produced primarily by the ovaries, it is also produced by the liver and adrenal glands; its anabolic functions include increasing metabolism and fat deposition |
Disorders of the…Metabolic Processes: Cushing Syndrome and Addison’s Disease
As might be expected for a fundamental physiological process like metabolism, errors or malfunctions in metabolic processing lead to a pathophysiology or—if uncorrected—a disease state. Metabolic diseases are most commonly the result of malfunctioning proteins or enzymes that are critical to one or more metabolic pathways. Protein or enzyme malfunction can be the consequence of a genetic alteration or mutation. However, normally functioning proteins and enzymes can also have deleterious effects if their availability is not appropriately matched with metabolic need. For example, excessive production of the hormone cortisol (see Table 24.1) gives rise to Cushing syndrome. Clinically, Cushing syndrome is characterized by rapid weight gain, especially in the trunk and face region, depression, and anxiety. It is worth mentioning that tumors of the pituitary that produce adrenocorticotropic hormone (ACTH), which subsequently stimulates the adrenal cortex to release excessive cortisol, produce similar effects. This indirect mechanism of cortisol overproduction is referred to as Cushing disease.
Patients with Cushing syndrome can exhibit high blood glucose levels and are at an increased risk of becoming obese. They also show slow growth, accumulation of fat between the shoulders, weak muscles, bone pain (because cortisol causes proteins to be broken down to make glucose via gluconeogenesis), and fatigue. Other symptoms include excessive sweating (hyperhidrosis), capillary dilation, and thinning of the skin, which can lead to easy bruising. The treatments for Cushing syndrome are all focused on reducing excessive cortisol levels. Depending on the cause of the excess, treatment may be as simple as discontinuing the use of cortisol ointments. In cases of tumors, surgery is often used to remove the offending tumor. Where surgery is inappropriate, radiation therapy can be used to reduce the size of a tumor or ablate portions of the adrenal cortex. Finally, medications are available that can help to regulate the amounts of cortisol.
Insufficient cortisol production is equally problematic. Adrenal insufficiency, or Addison’s disease, is characterized by the reduced production of cortisol from the adrenal gland. It can result from malfunction of the adrenal glands—they do not produce enough cortisol—or it can be a consequence of decreased ACTH availability from the pituitary. Patients with Addison’s disease may have low blood pressure, paleness, extreme weakness, fatigue, slow or sluggish movements, lightheadedness, and salt cravings due to the loss of sodium and high blood potassium levels (hyperkalemia). Victims also may suffer from loss of appetite, chronic diarrhea, vomiting, mouth lesions, and patchy skin color. Diagnosis typically involves blood tests and imaging tests of the adrenal and pituitary glands. Treatment involves cortisol replacement therapy, which usually must be continued for life.
Oxidation-Reduction Reactions
The chemical reactions underlying metabolism involve the transfer of electrons from one compound to another by processes catalyzed by enzymes. As the series of reactions progresses, energy accumulates that is used to form ATP, the high-energy molecule that the body uses for fuel.
Chapter Review
Metabolism is the sum of all catabolic (break down) and anabolic (synthesis) reactions in the body. The metabolic rate measures the amount of energy used to maintain life. An organism must ingest a sufficient amount of food to maintain its metabolic rate if the organism is to stay alive for very long.
Catabolic reactions break down larger molecules, such as carbohydrates, lipids, and proteins from ingested food, into their constituent smaller parts. They also include the breakdown of ATP, which releases the energy needed for metabolic processes in all cells throughout the body.
Anabolic reactions, or biosynthetic reactions, synthesize larger molecules from smaller parts, using ATP as the energy source for these reactions. Anabolic reactions build bone, muscle mass, and new proteins, fats, and nucleic acids. Errors in metabolism alter the processing of carbohydrates, lipids, proteins, and nucleic acids, and can result in a number of disease states.
Critical Thinking Questions
1. Describe how metabolism can be altered.
2. Describe how Addison’s disease can be treated.
Glossary
- anabolic hormones
- hormones that stimulate the synthesis of new, larger molecules
- anabolic reactions
- reactions that build smaller molecules into larger molecules
- biosynthesis reactions
- reactions that create new molecules, also called anabolic reactions
- catabolic hormones
- hormones that stimulate the breakdown of larger molecules
- catabolic reactions
- reactions that break down larger molecules into their constituent parts
- metabolism
- sum of all catabolic and anabolic reactions that take place in the body
- oxidation
- loss of an electron
- oxidation-reduction reaction
- (also, redox reaction) pair of reactions in which an electron is passed from one molecule to another, oxidizing one and reducing the other
- reduction
- gaining of an electron
Solutions
Answers for Critical Thinking Questions
- An increase or decrease in lean muscle mass will result in an increase or decrease in metabolism.
- Addison’s disease is characterized by low cortisol levels. One way to treat the disease is by giving cortisol to the patient.
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