Animal Hormones

Animal Hormones

30

Chapter 30 Animal Hormones

Key Concepts

• 30.1 Hormones Are Chemical Messengers

• 30.2 Hormones Act by Binding to Receptors

• 30.3 The Pituitary Gland Links the Nervous and Endocrine Systems

• 30.4 Hormones Regulate Mammalian Physiological Systems

Chapter 30 Opening Question

How do the transformative effects of testosterone exemplify the way many hormones work?

Concept 30.1 Hormones Are Chemical Messengers

Endocrine secretion—cells secrete substances into the extracellular fluid

Exocrine secretion—cells secrete substances into a duct or a body cavity that communicates to the external world


Endocrine cells—cells that secrete endocrine signals

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

Endocrine glands—secretory organs composed of aggregations of endocrine cells


Endocrine signaling molecules are paracrine signals, autocrine signals, or hormones.

Hormones are “long-distance” endocrine signals that are released into the bloodstream and circulate throughout the body.


Target cells—cells that have receptors for the chemical signals

The same hormone can have a variety of different target cells, all distant from the site of release.


Hormones are in three chemical groups:

• Peptide and protein hormones—water-soluble, transported in blood with receptors on exterior of target cells

• Steroid hormones—synthesized from cholesterol; lipid-soluble; bound to carrier proteins in blood; receptors inside target cells


• Amine hormones—synthesized from single amino acids; may be lipid-soluble or water-soluble, depending on the charge of the amino acid

Figure 30.1 Three Classes of Hormones (Part 1)




Chemical communication was critical for evolution of multicellular organisms.

Plants, sponges, and protists all use chemical signals.

Signaling molecules are highly conserved, but their functions differ.


In arthropods, hormones control molting and metamorphosis

The rigid exoskeleton is shed during molts to allow growth.

Growth stages between molts are called instars.

Figure 30.2 A Diffusible Substance Triggers Molting (Part 1)

Figure 30.2 A Diffusible Substance Triggers Molting (Part 2)


Two hormones regulate molting:

PTTH (prothoracicotropic hormone), from cells in the brain, is stored in the corpora cardiaca

PTTH stimulates the prothoracic gland to secrete ecdysone.

Ecdysone diffuses to target tissues and stimulates molting.


A third hormone, juvenile hormone, is also released from the brain—prevents maturation to adult form.

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

Figure 30.3 Hormonal Control of Metamorphosis

Concept 30.2 Hormones Act by Binding to Receptors

Hormone receptors can be membrane-bound with three domains:

• Binding domain—projects outside plasma membrane

• Transmembrane domain—anchors receptor

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


Hormone receptors can also be intracellular:

• Lipid soluble hormones—receptors are inside the cell, usually in the cytoplasm

When hormone binds, the hormone–receptor complex moves into the nucleus.


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

Epinephrine and norepinephrine are secreted by adrenal glands in the fight-or-flight response.

These hormones bind to adrenergic receptors.

Figure 30.4 The Fight-or-Flight Response


Two categories: -adrenergic and -adrenergic receptors

Stimulation of one receptor can cause diverse effects, depending on its location.

Example: -adrenergic stimulation causes sweating in skin and shutdown of digestive enzymes and decreased blood flow in gut.

Figure 30.5 Adrenergic Receptors (Part 1)

Figure 30.5 Adrenergic Receptors (Part 2)


Abundance of hormone receptors can be regulated by negative feedback.

Downregulation—continuous high level of hormone decreases number of receptors.

Upregulation—when hormone secretion is suppressed, receptors increase.

Figure 30.6 The Human Endocrine System

Concept 30.3 The Pituitary Gland Links the Nervous and Endocrine Systems

The nervous system communicates via molecules—neurotransmitters.

The endocrine system communicates via molecules released into the blood.

The systems are complementary—nervous system is rapid and specific, endocrine system is broader and longer-term.


The nervous and endocrine systems also interact.

Nervous system controls activity of many endocrine glands.

Some neurons secrete hormones directly—neurohormones.

Endocrine system can also influence the nervous system—steroids promote sexual behavior.


The pituitary gland connects the nervous and endocrine systems.

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

Two parts—the anterior pituitary and posterior pituitary

Figure 30.7 The Posterior Pituitary (Part 1)


The hypothalamus secretes two neurohormones into the posterior pituitary: antidiuretic hormone (vasopressin) and oxytocin.

Antidiuretic hormone (ADH) serves to increase the water retained by the kidneys when necessary.

Oxytocin stimulates contractions, milk flow, promotes bonding—the “cuddle chemical”

Figure 30.7 The Posterior Pituitary (Part 2)


The anterior pituitary secretes four tropic hormones that control other endocrine glands:

• Thyroid-stimulating hormone (TSH)

• Luteinizing hormone (LH)

• Follicle-stimulating hormone (FSH)

• Adrenocorticotropin hormone (ACTH)

Figure 30.8 The Anterior Pituitary


The anterior pituitary also secretes other peptide hormones including prolactin and growth hormone.

Growth hormone (GH) stimulates cells to take up amino acids.

GH stimulates the liver to produce somatomedins or insulin-like growth factors (IGFs).

Overproduction of GH causes gigantism; underproduction causes pituitary dwarfism.


Neurohormones from the hypothalamus control subsequent hormone production in the anterior pituitary.

The hypothalamus sends secretions to the anterior pituitary via the portal blood vessels.


Hypothalamic neurohormones are released in minute quantities measurable by immunoassay.

The first releasing hormone to be purified was thyrotropin-releasing hormone (TRH).

TRH causes anterior pituitary cells to release thyroid-stimulating hormone (TSH).

TSH causes the thyroid gland to release thyroxine.


Negative feedback loops control hormone secretion from the anterior pituitary.

Corticotropin is released by pituitary—adrenal produces cortisol in response.

Circulating cortisol in bloodstream reaches pituitary and inhibits production.

Hypothalamus slows release of corticotropin-releasing hormone.

Figure 30.9 Multiple Feedback Loops Control Hormone Secretion

Concept 30.4 Hormones Regulate Mammalian Physiological Systems

The thyroid gland contains two cell types that produce two different hormones, thyroxine and calcitonin.

In or near the thyroid gland are the parathyroid glands, which produce parathyroid hormone.

Thyroxine (T4) is synthesized from the amino acid tyrosine and iodine.

T3 is a similar hormone that is more active.


In birds and mammals, thyroxine raises metabolic rate.

Thyroxine regulates cell metabolism by acting as a transcription factor for many genes and is crucial during development.

Hypothalamus releases thyrotropin-releasing hormone (TRH), which causes anterior pituitary to secrete thyroid-stimulating hormone (TSH).

TSH causes the thyroid to produce thyroxine.


Goiter is an enlarged thyroid gland.

Hyperthyroidism (thyroxine excess) is often caused by an autoimmune disease.

Antibody-binding activates TSH receptors on follicle cells and increases thyroxine.

Thyroid remains stimulated and grows bigger.


Hypothyroidism (thyroxine deficiency) is the result of low circulating thyroxine.

The most common cause is iodine deficiency—thyroid cannot produce thyroxine.

TSH levels remain high and stimulate the thyroid to grow bigger.

Figure 30.10 Goiter


Blood calcium concentration is controlled by calcitonin, calcitriol (from vitamin D), and parathyroid hormone (PTH).

Mechanisms for changing calcium levels:

• Deposition or absorption by bone

• Excretion or retention by kidneys

• Absorption of calcium from digestive tract


Calcitonin, released by thyroid, lowers blood calcium (Ca2+) by regulating bone turnover.

Osteoclasts break down bone, increasing blood Ca2+.

Ca2+ is deposited into bone by osteoblasts; levels of Ca2+ in blood decrease.

Calcitonin decreases osteoclast activity and favors adding calcium to bones.

Figure 30.11 Hormonal Regulation of Calcium


Vitamin D (calciferol) is synthesized from cholesterol in skin cells by UV light.

Once synthesized it is converted to calcitriol, a hormone that stimulates calcium absorption from food.

If light is insufficient, vitamin D must be obtained from diet or supplements.


The parathyroid glands secrete parathyroid hormone (PTH).

PTH raises blood calcium levels:

• Stimulates osteoclasts and osteoblasts

• Stimulates kidneys to reabsorb calcium

• Activates synthesis of calcitriol from vitamin D


Each of the two adrenal glands is a gland within a gland.

The core, or adrenal medulla, produces epinephrine and norepinephrine.

Release of these neurohormones is under control of the nervous system and is very rapid in the stress response.

Figure 30.12 The Adrenal Is a Gland within a Gland


The outer adrenal cortex produces two types of corticosteroid hormones:

• Mineralocorticoids influence salt and water balance

Aldosterone, the main mineralocorticoid, stimulates kidneys to conserve sodium and excrete potassium.


• Glucocorticoids influence blood glucose concentration

Cortisol, the main glucocorticoid in humans and mammals, mediates metabolic stress response.

After a stressful stimulus, blood cortisol rises.

Cells not critical for action decrease their use of blood glucose—immune system reactions are also blocked.


Gonads produce sex steroids.

Androgens—male steroids, testosterone

Estrogens and progesterone—female steroids

Both sexes use both types, in varying levels.

In embryos, sex hormones determine sex of fetus; at puberty, they stimulate maturation and secondary sex characteristics.


Sex hormones exert their effects by the seventh week of human development.

If a Y chromosome is present, gonads begin producing testosterone and MIS (Müllerian-inhibiting substance)—these produce male reproductive organs and inhibit female reproductive structures.

Without androgens, female reproductive structures develop.

Figure 30.13 Sex Steroids Direct the Development of Human Sex Organs (Part 1)

Figure 30.13 Sex Steroids Direct the Development of Human Sex Organs (Part 2)


At puberty, production of sex hormones increases.

Controlled by tropic hormones called gonadotropins from the anterior pituitary:

• Luteinizing hormone (LH)

• Follicle-stimulating hormone (FSH)


Gonadotropins are controlled by hypothalamic gonadotropin-releasing hormone (GnrH)—its release increases at puberty.

Increase in gonadotropins leads to increase in sex steroids and development of secondary sex characteristics.

Answer to Opening Question

Hormones and their receptor complexes can have varying effects depending on the type of target cells.

The receptors are essential because without them the circulating hormones are unable to have the desired effect.

Hormones may also be modified during development, with different effects at different stages.

Figure 30.14 Real People, Real Lives

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Animal Hormones