42Animal Hormones

42 Introduction

• Chemical messages, or hormones, produce and coordinate anatomical, physiological, and behavioral changes in an animal.

42 Hormones and Their Actions

• Control and regulation require information.

• In multicellular animals, hormones provide chemical signals.

• Hormones are secreted by cells, diffuse into the extracellular fluid, and often are distributed by the circulatory system.

42 Hormones and Their Actions

• Hormone-secreting cells are called endocrine cells.

• Cells receiving the hormonal message are called target cells and must have appropriate receptors.

• The binding of the receptor activates a response.

• The distance over which the signal operates distinguishes hormone groups; some act close to the release site, others at distant body locations.

42 Hormones and Their Actions

• Hormones can be classified into three main groups:

Peptides or proteins: water soluble, transported by vesicles out of the cell that made them.

Steroids: lipid-soluble, diffuse out of the cell that made them but in the blood they must be bound to carrier proteins.

Amine: derivatives of the amino acid tyrosine; water-soluble and lipid-soluble.

42 Hormones and Their Actions

• Some hormones act on distant cells – these get into the circulatory system.

• Some hormones act locally – these never get into the circulatory system.

Autocrine hormones act on the secreting cell itself. Paracrine hormones act on cells near the site of

release. released in tiny amounts are inactivated rapidly by enzymes are taken up efficiently by local cells.

Figure 42.1 Chemical Signaling Systems

42 Hormones and Their Actions

• Most hormones diffuse into the blood, which distributes them throughout the body.

• When the hormone message encounters a cell with the proper receptor, it binds and triggers a response.

• The same hormone can cause different responses in different types of cells.

• An example is epinephrine. The nervous system reacts to an emergency very quickly and stimulates adrenal cells to secrete epinephrine. The result is the fight-or-flight response.

42 Hormones and Their Actions

• The epinephrine acts on different cells in the body:

In the heart, it stimulates faster and stronger heartbeat.

Blood vessels in some areas constrict to send more blood to muscles.

In the liver, glycogen is broken down to glucose to provide quick energy.

In fat tissue, fats are mobilized as another energy source.

42 Hormones and Their Actions

• Endocrine refers to cells or glands that do not have ducts leading to the outside of the body; they secrete their products directly into the extracellular fluid.

• Some endocrine cells are single cells within a tissue.

• Some endocrine cells aggregate into secretory organs called endocrine glands.

• In vertebrates, nine major endocrine glands make up the endocrine system.

Figure 42.2 The Endocrine System of Humans

42 Vertebrate Endocrine Systems

• The pituitary gland of mammals is a link between the nervous system and many endocrine glands and plays a crucial role in the endocrine system.

• The pituitary gland sits in a depression at the bottom of the skull and is attached to the hypothalamus.

• The pituitary is made of two parts: anterior and posterior.

The Pituitary Gland

42 Vertebrate Endocrine Systems

• The posterior pituitary releases two hormones: antidiuretic hormone and oxytocin.

• They are made by neurons in the hypothalamus, and are packaged in vesicles.

• The vesicles are transported down the axons of the neurons that made them and are stored in the posterior pituitary.

- The function of antidiuretic hormone (ADH) is to increase water conservation by the kidney.

- The function of oxytocin is to stimulate uterine muscle contraction for the birth process.

42 Vertebrate Endocrine Systems

• The Anterior Pituitary releases four tropic hormones, which control activities of other endocrine glands.

• The four tropic hormones are: thyrotropin (stimulates the

thyroid gland) adrenocorticotropin luteinizing hormone follicle-stimulating hormone.

Anterior lobe of the human pituitary gland.

42 Vertebrate Endocrine Systems

• Other peptide and protein anterior pituitary hormones influence tissues that are not endocrine glands.

• These include: growth hormone (acts on many tissues to

promote growth) prolactin (stimulates the production and

secretion of milk in female mammals) melanocyte-stimulating hormone endorphins, enkephalins (molecules that act

as neurotransmitters in pain pathways)

42 Vertebrate Endocrine Systems

• The anterior pituitary is controlled by neurohormones from the hypothalamus.

• The hypothalamus obtains data about body conditions and the external environment through both neuronal and hormonal signals.

• Secretions from hypothalamic nerves are transported by blood vessels to the anterior pituitary.

Figure 42.8 Multiple Feedback Loops Control Hormone Secretion

The anterior pituitary cells are also under negative feedback control by the hormones of the glands that they stimulate.

42 Vertebrate Endocrine Systems

• The thyroid gland, located near the trachea, is an example of an endocrine gland that is controlled by negative feedback.

• The thyroid gland produces the hormone thyroxine in specialized structures called follicles.

• Thyroxine has many roles in regulating metabolism.

Thyroid and parathyroid

42 Vertebrate Endocrine Systems

• The four parathyroid glands are embedded in the posterior surface of the thyroid gland.

• It is important in regulating blood calcium levels: Low blood calcium = release of parathyroid hormone

causes specialized cells to dissolve bone and release calcium

promotes calcium resorption by the kidney to prevent loss in the urine

promotes vitamin D (actually a hormone) activation, which stimulates the gut to absorb calcium from food.

High blood calcium = release of calcitonin lower calcium levels in the blood.

42 Vertebrate Endocrine Systems

• Diabetes mellitus is a disease caused by a lack of the protein hormone insulin (Type I) or a lack of insulin receptors on target cells (Type II).

• Insulin binds to receptors on the cell membrane and allows glucose uptake.

• Without insulin or the receptors, glucose accumulates in the blood until it is lost in urine.

42 Vertebrate Endocrine Systems

• High glucose levels in the blood cause water to move from the cells into the blood by osmosis.

• The kidneys then increase urine output to get rid of the fluid excess.

• Cells not taking up glucose use fat and protein for fuel, resulting in the body’s wasting away and tissue and organ damage.

42 Vertebrate Endocrine Systems

• Insulin is produced in the pancreas in cell clusters called islets of Langerhans.

• The remainder of the pancreas acts as an exocrine gland with digestive functions. Human Pancreatic Islet

42 Vertebrate Endocrine Systems

• After a meal, blood glucose levels rise and stimulate the cells to release insulin.

• Insulin stimulates cells to use glucose and to convert it to glycogen and fat.

• When blood glucose levels fall, the pancreas stops releasing insulin, and cells switch to using glycogen and fat for energy.

• If blood glucose falls too low, the cells release glucagon which stimulates the liver to convert glycogen back to glucose.

42 Vertebrate Endocrine Systems

• The adrenal glands are made up of the adrenal medulla and the adrenal cortex.

42 Vertebrate Endocrine Systems

• The medulla produces: epinephrine (response

to stress, initiating fight-or-flight reactions - increased heart, breathing rates, elevated blood pressure)

norepinephrine (a neurotransmitter involved in physiological regulation)

Adrenal medulla

42 Vertebrate Endocrine Systems

• Adrenal cortex cells use cholesterol to produce three classes of steroid hormones called corticosteroids:

Glucocorticoids influence blood glucose concentrations and other aspects of fuel molecule metabolism.

Mineralocorticoids influence extracellular ionic balance.

Sex steroids stimulate sexual development and reproductive activity. These are secreted in only minimal amounts by the adrenal cortex.

Human adrenal gland

42 Vertebrate Endocrine Systems

• The gonads (testes and ovaries) produce steroid hormones synthesized from cholesterol.

• Androgens are male steroids, the dominant one being testosterone.

• Estrogens and progesterone are female steroids, the dominant estrogen being estradiol.

• Sex steroids determine whether a fetus develops into a male or female.

• After birth, sex steroids control maturation of sex organs and secondary sex characteristics such as breasts and facial hair.

42 Vertebrate Endocrine Systems

• Sex steroid production increases rapidly at puberty, or sexual maturation, in humans.

• Control of sex steroids (both male and female) is under the anterior pituitary tropic hormones called luteinizing hormone (LH) and follicle-stimulating hormone (FSH).

• These gonadotropins are controlled by the hypothalamus.

42 Vertebrate Endocrine Systems

• Synthetic androgens (anabolic steroids) can exaggerate body strength and muscle development.

• Negative side effects in females include more masculine body features, such as shrinking the uterus and causing an irregular menstrual cycle.

• In males, the negative side effects include shrinking of the testes, enlarged breasts, and sterility.

• Continued use of anabolic steroids may increase risk of heart disease, some cancers, kidney damage, and personality disorders.