1. Animal Hormones

2. Introduction Hormones and Their Actions Hormonal Control of Molting and Development in Insects Vertebrate Endocrine Systems Hormone Actions: The Role of Signal Transduction Pathways

3. 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.

4. Hormones and Their Actions Hormones can be classified into three main groups:  Peptides or proteins. They are water soluble and transported by vesicles out of the cell that made them.  Steroid hormones are lipid-soluble and can diffuse out of the cell that made them but in the blood they must be bound to carrier proteins.  Amine hormones are derivatives of the amino acid tyrosine. Some are water-soluble and some are lipid-soluble.

5. Hormones and Their Actions The receptors for lipid-soluble hormones are inside cells, either in the cytoplasm or in the nucleus. The action of lipid-soluble hormones is mediated by intracellular hormone receptors that usually alter gene expression.

6. Hormones and Their Actions The receptors for water-soluble proteins are large glycoproteins on the cell surface with three domains:  A binding domain projecting outside the plasma membrane  A transmembrane domain that anchors the receptor in the membrane  A cytoplasmic domain that extends into the cytoplasm of the cell The cytoplasmic domain initiates the target cell’s response by activating protein kinases or protein phosphatases.

7. Hormones and Their Actions Some hormones act locally. Autocrine hormones act on the secreting cell itself. Paracrine hormones act on cells near the site of release. Paracrine hormones are released in tiny amounts, or are inactivated rapidly by enzymes, or are taken up efficiently by local cells. They never get into the circulatory system.

8. Figure 42.1 Chemical Signaling Systems

9. Hormones and Their Actions Growth factors, which stimulate growth and differentiation of cells, are a major class of paracrine hormones. Growth factors also act as autocrine hormones: Some of the hormone influences the cell that secreted it, preventing the cell from secreting too much hormone. Neurons may also be considered to be paracrine cells because they use chemicals called neurotransmitters to send messages to another cell.

10. 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.

11. 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.

12. 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. Digestive hormones, for example, are secreted by isolated endocrine cells in the wall of the stomach and small intestine. Some endocrine cells aggregate into secretory organs called endocrine glands. In vertebrates, nine major endocrine glands make up the endocrine system.

13. Figure 42.2 The Endocrine System of Humans

14. Hormonal Control of Molting and Development in Insects Hormonal control is more complex in insects having complete metamorphosis. An example is the silkworm. The egg hatches into a larva that has a high amount of juvenile hormone in its body. As long as the level of juvenile hormone stays high, larvae molt into larvae; when the juvenile hormone level wanes, pupae are formed. No juvenile hormone is found in the pupae, so they molt into adults.

15. Figure 42.4 Complete Metamorphosis Animation

16. Vertebrate Endocrine Systems The posterior pituitary releases two hormones: antidiuretic hormone and oxytocin. They are made by neurons in the hypothalamus, are called neurohormones, 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. This movement of the vesicles is achieved by kinesin proteins, powered by ATP, that “walk” down the microtubules of the axon.

17. Figure 42.5 The Posterior Pituitary Releases Neurohormones Animation

18. Vertebrate Endocrine Systems The anterior pituitary releases four tropic hormones, which control activities of other endocrine glands. They are peptide and protein hormones; each is produced by a different type of pituitary cell. The four tropic hormones are: thyrotropin, adrenocorticotropin, luteinizing hormone, and follicle-stimulating hormone.

19. Figure 42.7 Hormones from the Hypothalamus Control the Anterior Pituitary

20. Table 42.2 Releasing and Release-Inhibiting Neurohormones of the Hypothalamus

21. Figure 42.8 Multiple Feedback Loops Control Hormone Secretion Animation 1, 2

22. Vertebrate Endocrine Systems The adrenal glands are made up of the adrenal medulla and the adrenal cortex. The medulla produces epinephrine and norepinephrine. The medulla develops from the nervous system and remains under its control. The cortex is under hormonal control, mainly by adrenocorticotropin (ACTH) from the anterior pituitary.

23. Figure 42.10 The Adrenal Gland Has an Outer and an Inner Portion

24. Vertebrate Endocrine Systems The adrenal medulla produces epinephrine (adrenaline) in response to stress, initiating fight- or-flight reactions, such as increased heart and breathing rates and elevated blood pressure. It also produces norepinephrine, a neurotransmitter involved in physiological regulation. Epinephrine and norepinephrine are amine hormones. They bind to two types of receptors in target cells:  -adrenergic and  -adrenergic.

25. Vertebrate Endocrine Systems Norepinephrine acts mostly on the alpha type, so drugs called beta blockers, which inactivate only  -adrenergic receptors, can be used to reduce fight-or-flight responses to epinephrine. The beta blockers leave the alpha sites open to norepinephrine and its regulatory functions.

26. 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.

27. Figure 42.11 The Corticosteroid Hormones are Built from Cholesterol

28. Vertebrate Endocrine Systems The main mineralocorticoid, aldosterone, stimulates the kidney to conserve sodium and excrete potassium. The main glucocorticoid, cortisol, mediates the body’s response to stress. The fight-or-flight response ensures that muscles have adequate oxygen and glucose for immediate response.

29. Vertebrate Endocrine Systems Shortly after a frightening stimulus, blood cortisol rises. Cortisol stimulates cells that are not critical to the emergency to decrease their use of glucose. It also blocks the immune system reactions, which temporarily are less critical. Cortisol can therefore be used to reduce inflammation and allergy.

30. Vertebrate Endocrine Systems Cortisol release is controlled by ACTH from the anterior pituitary which, in turn is controlled by adrenocorticotropin-releasing hormone from the hypothalamus. The cortisol response is much slower than the epinephrine response. Turning off the cortisol response is also critical to avoid the consequences of long-term stress. Cortisol has negative feedback effect on brain cells that decreases the release of adrenocorticotropin-releasing hormone.

31. 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.

32. Vertebrate Endocrine Systems Until the seventh week of an embryo’s development, either sex may develop. In mammals, the Y chromosome causes the gonads to start producing androgens in the seven-week-old embryo, and the male reproductive system develops. If androgens are not released, the female reproductive system develops. In birds, the opposite rules apply: male features are produced unless estrogens are present to trigger female development.

33. Figure 42.12 The Development of Human Sex Organs

34. Hormone Actions: The Role of Signal Transduction Pathways Hormones are released in very small amounts, yet they cause large and very specific responses in target organs and tissues. Strength of hormone action results from signal transduction cascades that amplify the original signal. Selective action is keyed to appropriate receptors of cells responding to hormones. Specific receptors can also be linked to different response mechanisms, as is the case with receptors for epinephrine and norepinephrine.

35. Figure 42.14 Some Hormones Can Activate a Variety of Signal Transduction Pathways