41Animal Hormones

41 Animal Hormones

• 41.1 What Are Hormones and How Do They Work?

• 41.2 How Do the Nervous and Endocrine Systems Interact?

• 41.3 What Are the Major Mammalian Endocrine Glands and Hormones?

• 41.4 How Do We Study Mechanisms of Hormone Action?

41.1 What Are Hormones and How Do They Work?

Hormones are

chemical signals secreted by cells of the endocrine system.

Endocrine cells:

cells that secrete hormones

Target cells:

cells that have receptors for the hormones

41.1 What Are Hormones and How Do They Work?

Circulating hormones diffuse into the blood and can activate target cells far from the site of release.

Paracrine hormones: affect only target cells near the site of release.

Autocrine hormones: affect the cells that released the hormones.

Figure 41.1 Chemical Signaling Systems (Part 1)

Figure 41.1 Chemical Signaling Systems (Part 2)

41.1 What Are Hormones and How Do They Work?

Some endocrine cells are single cells (e.g., in the digestive tract.)

Endocrine glands: aggregations of secretory cells. Hormones are secreted to the extracellular space.

Exocrine glands: ducts carry products to the outside of the body.

41.1 What Are Hormones and How Do They Work?

Chemical communication arose early in evolution.

Plants, sponges, protists all use chemical signals.

In arthropods, hormones control molting and metamorphosis.

41.1 What Are Hormones and How Do They Work?

Arthropods: the rigid exoskeleton is shed during molts to allow growth.

Growth stages between molts are called instars.

Figure 41.2 Diffusible Substance Triggers Molting (Part 1)

Figure 41.2 Diffusible Substance Triggers Molting (Part 2)

41.1 What Are Hormones and How Do They Work?

Two hormones regulate molting:

PTTH (prothoracicotropic hormone) from cells in the brain. Stimulates the prothoracic gland to secrete ecdysone.

Ecdysone diffuses to target tissues and stimulates molting.

41.1 What Are Hormones and How Do They Work?

The nervous system and hormonal system are linked.

[Nervous system (brain) controls the endocrine gland (prothoracic gland), which produces the ecdysone that orchestrates the physiological response.]

41.1 What Are Hormones and How Do They Work?

Juvenile hormone: also released from the brain—prevents maturation to adult form.

Control of development by juvenile hormone important in insects with complete metamorphosis.

Figure 41.3 Complete Metamorphosis

41.1 What Are Hormones and How Do They Work?

Three types of hormones:

• Peptides or polypeptides: water-soluble, transported in blood but not across membranes.

• Steroid hormones: lipid-soluble; must be bound to carrier proteins to be carried in blood.

• Amine hormones: derivatives of tyrosine

41.1 What Are Hormones and How Do They Work?

Hormone receptors:

• Lipid soluble hormones: receptors are inside the cell

• Water-soluble hormones cannot readily pass cell membrane—receptors are on the outside

41.1 What Are Hormones and How Do They Work?

Receptors are glycoproteins with three domains:

• Binding domain: projects outside plasma membrane

• Transmembrane domain

• Cytoplasmic domain: extends into cytoplasm—initiates target cell response

41.1 What Are Hormones and How Do They Work?

One hormone can trigger different responses in different types of cells.

Example: epinephrine (amine), fight-or-flight response

Figure 41.4 Epinephrine Stimulates “Fight or Flight” Responses

Figure 41.5 The Endocrine System of Humans (Part 1)

Figure 41.5 The Endocrine System of Humans (Part 2)

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41.2 How Do the Nervous and Endocrine Systems Interact?

The pituitary gland is attached to the hypothalamus of the brain.

Posterior pituitary secretes neurohormones (synthesized by neurons in the hypothalmus): oxytocin and ADH.

Figure 41.6 The Posterior Pituitary Releases Neurohormones

41.2 How Do the Nervous and Endocrine Systems Interact?

The anterior pituitary secretes:

• Tropic hormones: control other endocrine glands

• Growth hormone: promotes growth

• Prolactin: breast development and milk production

• Enkephalins and endorphins: natural opiates

Figure 41.7 The Anterior Pituitary Produces Many Hormones

41.2 How Do the Nervous and Endocrine Systems Interact?

Hormones from the hypothalamus control the anterior pituitary.

The hypothalamus produces releasing hormones—carried to the anterior pituitary by portal blood vessels.

Negative feedback loops control hormone secretion from the anterior pituitary.

Figure 41.8 Multiple Feedback Loops Control Hormone Secretion

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41.3 What Are the Major Mammalian Endocrine Glands and Hormones?

The thyroid gland produces thyroxine (T4)

and triiodothyronine (T3)

41.3 What Are the Major Mammalian Endocrine Glands and Hormones?

Thyroxine regulates cell metabolism.

Anterior pituitary secretes thyrotropin (TSH), which activates the follicles to produce thyroxine.

Figure 41.9 The Thyroid Gland Consists of Many Follicles (A)

Thyroxine is produced by the follicles.

Figure 41.9 The Thyroid Gland Consists of Many Follicles (B)

41.3 What Are the Major Mammalian Endocrine Glands and Hormones?

Goiter is an enlarged thyroid gland. It can result from either hyperthyroidism (thyroxine excess) or hypothyroidism (thyroxine deficiency).

Epithelial cells produce excess thyroglobulin and the follicles enlarge.

Figure 41.9 The Thyroid Gland Consists of Many Follicles (C)

41.3 What Are the Major Mammalian Endocrine Glands and Hormones?

The thyroid gland also produces calcitonin.

Regulation of calcium levels in the blood controlled by calcitonin, parathyroid hormone, and vitamin D.

Calcitonin inhibits osteoclasts: more Ca2+ is put into bone by osteoblasts; levels of Ca2+ in blood decrease.

Figure 41.10 Hormonal Regulation of Calcium (Part 1)

Figure 41.10 Hormonal Regulation of Calcium (Part 2)

41.3 What Are the Major Mammalian Endocrine Glands and Hormones?

The parathyroid glands secrete parathyroid hormone (PTH).

PTH raises blood calcium levels:

• Stimulates osteoclasts

• Stimulates kidneys to reabsorb calcium

• Activates vitamin D, which stimulates digestive tract to absorb calcium.

41.3 What Are the Major Mammalian Endocrine Glands and Hormones?

Vitamin D (calciferol) is produced by skin cells from cholesterol, by UV light.

In cells it combines with a receptor to form a transcription factor—synthesis of calcium pumps, channels, and calcium-binding proteins enhance uptake of calcium in the digestive tract.

41.3 What Are the Major Mammalian Endocrine Glands and Hormones?

PTH also acts on kidneys to stimulate removal of phosphates from the blood.

Precipitation of calcium phosphate salts result in kidney stones and hardening of the arteries.

41.3 What Are the Major Mammalian Endocrine Glands and Hormones?

Insulin binds to receptors on target cells and allows uptake of glucose.

Lack of insulin: Type I diabetes

Lack of insulin receptors on target cells: Type II diabetes

41.3 What Are the Major Mammalian Endocrine Glands and Hormones?

Islets of Langerhans: clusters of endocrine cells in the pancreas.

cells produce insulin

cells produce glucagon: stimulates liver to convert glycogen back to glucose

cells produce somatostatin: inhibits release of both (also released by the hypothalamus)

41.3 What Are the Major Mammalian Endocrine Glands and Hormones?

Adrenal gland: two glands

Adrenal medulla: epinephrine and norepinephrine

On target cells: -adrenergic and -adrenergic receptors.

-blockers block -adrenergic receptors.

Figure 41.11 The Adrenal Gland Has an Outer and an Inner Portion

41.3 What Are the Major Mammalian Endocrine Glands and Hormones?

Adrenal cortex produces corticosteroids from cholesterol:

• Glucocorticoids: cortisol

• Mineralocorticoids: aldosterone

• Sex steroids

Figure 41.12 The Corticosteroid Hormones are Built from Cholesterol

41.3 What Are the Major Mammalian Endocrine Glands and Hormones?

Aldosterone stimulates kidneys to retain sodium and excrete potassium.

Cortisol: mediates reaction to stress; cells not critical for action decrease use of blood glucose; blocks immune system reactions.

41.3 What Are the Major Mammalian Endocrine Glands and Hormones?

Cortisol release is controlled by corticotropin from the anterior pituitary, in turn controlled by corticotropin-releasing hormone from the hypothalamus.

Stress response is turned off by negative feedback of cortisol to brain.

41.3 What Are the Major Mammalian Endocrine Glands and Hormones?

Gonads produce sex steroids:

• Androgens: testosterone

• Estrogens (estradiol) and progesterone

In development, sex hormones determine whether fetus will become male or female.

Figure 41.13 The Development of Human Sex Organs (Part 1)

Figure 41.13 The Development of Human Sex Organs (Part 2)

Figure 41.13 The Development of Human Sex Organs (Part 3)

41.3 What Are the Major Mammalian Endocrine Glands and Hormones?

At puberty, production of sex hormones increases.

Controlled by gonadotropins from the anterior pituitary:

Luteinizing hormone (LH)

Follicle-stimulating hormone (FSH)

41.3 What Are the Major Mammalian Endocrine Glands and Hormones?

The pineal gland produces melatonin from tryptophan.

Melatonin is released in the dark; light inhibits release.

Involved in photoperiodicity: seasonal changes in light trigger physiological changes.

Figure 41.14 The Release of Melatonin Regulates Seasonal Changes

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41.4 How Do We Study Mechanisms of Hormone Action?

To study hormone action:

• Identify and measure the hormone

• Identify the receptors

• Determine the signal transduction pathways in different tissues.

41.4 How Do We Study Mechanisms of Hormone Action?

Hormones occur in extremely small concentrations.

Immunoassay techniques are used to measure concentration in the blood.

Half-life: time required for one half of the hormone molecules to be depleted.

Figure 41.15 An Immunoassay Measures Hormone Concentration (Part 1)

Figure 41.15 An Immunoassay Measures Hormone Concentration (Part 2)

Figure 41.16 Dose–Response Curves Quantify Response to a Hormone

Immunoassay is also used to determine dose–response curves.

41.4 How Do We Study Mechanisms of Hormone Action?

One hormone may bind to different receptors.

Drugs can be created to block specific responses.

Receptors can be identified by affinity chromatography.

Receptors can also be identified by genomic analyses.

41.4 How Do We Study Mechanisms of Hormone Action?

Abundance of hormone receptors can be regulated by negative feedback.

Downregulation: continuous high levels of hormone decreases number of receptors.

Upregulation: when hormone secretion is suppressed, receptors increase.

41.4 How Do We Study Mechanisms of Hormone Action?

Type II diabetes mellitus results from downregulation of insulin receptors.

Possibly due to overstimulation of pancreatic release of insulin by excessive carbohydrate intake.

41.4 How Do We Study Mechanisms of Hormone Action?

Beta blockers can result in upregulation—if beta receptors are blocked over time, more receptors are produced.

41.4 How Do We Study Mechanisms of Hormone Action?

Receptors can be linked to different signal transduction pathways.

Example: epinephrine and norepinephrine connect with different pathways—can have different effects even in the same cell.

Figure 41.17 Some Hormones Can Activate a Variety of Signal Transduction Pathways (Part 1)

Figure 41.17 Some Hormones Can Activate a Variety of Signal Transduction Pathways (Part 2)

41.4 How Do We Study Mechanisms of Hormone Action?

Signal transduction pathways can be “cascades,” in which each step amplifies the response.

One hormone molecule binding to a receptor might result in millions of molecules of final product.

Example: response of liver cells to epinephrine.