Download VI. The vertebrate endocrine system coordinates homeostasis and

CHAPTER
41
CHEMICAL SIGNALS
IN ANIMALS
OUTLINE
I.
A variety of chemical signals coordinate body functions: an overview
A.
B.
C.
II.
Hormone binding to specific receptors triggers signaling mechanisms at the cellular level
A.
B.
III.
V.
The hypothalamus and pituitary integrate many functions of the vertebrate endocrine
system
Structural and Functional Relationships of the Hypothalamus and the Pituitary Gland
Posterior Pituitary Hormones
Anterior Pituitary Hormones
Hypothalamic Hormones
The vertebrate endocrine system coordinates homeostasis and regulates growth,
development, and reproduction
A.
B.
C.
D.
E.
F.
VII.
Cyclic AMP
Signal Amplification and Specificity
Cytoplasmic Ca2+ and Signal Transduction
Invertebrate control systems often integrate endocrine and nervous system functions
A.
B.
C.
D.
VI.
Steroid Hormones and Gene Expression
Peptide Hormones and Signal Transduction
Many chemical signals are relayed and amplified by second messengers and protein kinases
A.
B.
C.
IV.
Hormones
Pheromones
Local Regulators
The Thyroid Gland
The Parathyroid Glands
The Pancreas
The Adrenal Glands
The Gonads
Other Endocrine Organs
The endocrine system and the nervous system are structurally, chemically, and functionally
related
Chemical Signals in Animals
753
OBJECTIVES
After reading this chapter and attending lecture, the student should be able to:
1. Compare the response times of the two major systems of internal communication: the nervous system
and the endocrine system.
2. On the basis of structure and function, distinguish among types of chemical messengers.
3. Distinguish between endocrine and exocrine glands.
4. Describe the relationships among endocrine system components: hormones, endocrine glands, target
cells and target cell receptors.
5. List the three general classes of hormones and give examples of each.
6. Explain how pheromone function differs from hormone function.
7. Provide indirect evidence that humans may communicate with pheromones.
8. State which of the two classes of hormones is lipid soluble and explain how this property affects
hormone function.
9. Describe the mechanism of steroid hormone action and explain the location and role of steroid
hormone receptors.
10. Using epinephrine as an example, explain how nonsteroid hormones act via second messengers,
including the roles of adenylate cyclase, cyclic AMP, G proteins and cAMP-dependent protein
kinase.
11. Explain how adenylate cyclase activity is controlled.
12. Using epinephrine as an example, explain how an enzyme cascade amplifies response to a hormone.
13. Explain how to account for specificity in target cell response to hormonal signals.
14. Outline the sequence of events which occurs as inositol triphosphate mediates between a hormonal
signal and an increase in cytoplasmic calcium ions.
15. Compare and contrast the two general modes of hormone action.
16. Describe hormonal regulation of insect development including the roles of ecdysone, brain hormone
and juvenile hormone.
17. Describe the location of the hypothalamus and explain how its hormone-releasing cells differ from
both endocrine gland secretory cells and other neurons.
18. Describe the location of the pituitary and explain the functions of the posterior and anterior lobes.
19. List the posterior pituitary hormones and describe their effects on target organs.
20. Using antidiuretic hormone as an example, explain how a hormone contributes to homeostasis and
how negative feedback can control hormone levels.
21. Define tropic hormone and describe the functions of tropic hormones produced by the anterior
pituitary.
22. Explain how the anterior pituitary is controlled.
23. List hormones of the thyroid gland and explain their role in development and metabolism.
24. Diagram the negative feedback loop which regulates the secretion of thyroid hormones.
25. State the location of the parathyroid glands and describe hormonal control of calcium homeostasis.
26. Distinguish between α and β cells in the pancreas and explain how their antagonistic hormones
(insulin and glucagon) regulate carbohydrate metabolism.
27. List hormones of the adrenal medulla, describe their function and explain how their secretion is
controlled.
28. List hormones of the adrenal cortex, describe their function and explain how their secretion is
controlled.
29. Describe both the short term and long term endocrine responses to stress.
30. Identify male and female gonads and list the three categories of gonadal steroids.
31. Define gonadotropin and explain how estrogen and androgen synthesis is controlled.
754
Chemical Signals in Animals
32. Describe the location of the pineal and thymus glands, list their hormone products and describe their
functions.
33. Explain how the endocrine and nervous systems are structurally, chemically and functionally related.
KEY TERMS
hormone
steroid
tyrosine
peptide hormone
signal molecules
target cells
endocrine gland
exocrine gland
pheromone
neurotransmitter
histamine
interleukin
nerve growth
factors (NGF)
epidermal growth
factors (EGF)
prostaglandins (PGs)
second messengers
cytoplasmic
intermediates
cyclic AMP (cAMP)
inositol triphosphate
melanocyte-stimulating
hormone (MSH)
Earl W. Sutherland
G protein
adenylate cyclase
phosphodiesterase
hormone-receptor
complex
enzyme cascade
protein kinase
inositol triphosphate
calmodulin
ecdysone
juvenile hormone (JH)
hypothalamus
neurosecretory cells
releasing factors
posterior pituitary
(neurohypophysis)
antidiuretic
hormone (ADH)
oxytocin
anterior pituitary
(adenohypophysis)
growth hormone (GH)
insulinlike growth
factors (IGFs)
prolactin (PRL)
follicle-stimulating
hormone (FSH)
luteinizing
hormone (LH)
thyroid-stimulating
hormone (TSH)
pro-opiomelonocortin
adrenocorticotropin
(ACTH)
melanocyte-stimulating
hormone (MSH)
endorphins
hypothalamic
releasing factors
median eminence
thyroid gland
triiodothyrone
thyrodoxine
hyperthyroidism
hypothyroidism
calcitonin
parathyroid
hormone (PTH)
insulin
β islet cells
proinsulin
α islet cells
glucagon
adrenal medulla
epinephrine
norepinephrine
catecholamines
acetylcholine
adrenal cortex
corticosteroids
glucocorticoids
cortisol
mineralocorticoids
aldosterone
estrogen
androgen
testosterone
progestins
gonadotropins
gonadotropinreleasing
hormone (GnRH)
gastrin
secretin
erythropoietin
melatonin
thymosin
Chemical Signals in Animals
755
LECTURE NOTES
The activities of the various specialized parts of an animal are coordinated by the two major systems of
internal communication: the nervous system and the endocrine system.
• The nervous system is involved with high-speed messages.
• The endocrine system is slower and involves the production, release, and movement of chemical
messages.
I.
A variety of chemical signals coordinate body functions: an overview
All animals exhibit some coordination by chemical signals:
• Hormones produced by the endocrine system convey information between organs of the
body.
• Pheromones are chemical signals used to communicate between different individuals.
• Other messengers, such as neurotransmitters, act between cells on a localized scale.
A.
Hormones
The endocrine system has 3 key components: hormones, endocrine glands, and target cells
with their molecular receptors for hormones.
Hormone = Secreted by endocrine or neurosecretory cells, a chemical signal that travels in
body fluids to target cells where it elicits a specific response.
Neurosecretory cell = Neuron that receives signals from other nerve cells and responds by
releasing hormones into body fluids or into a storage organ from which they are later
released.
Target cell = A cell equipped to respond to a given hormone.
Endocrine glands = Ductless glands that secrete hormones into the body fluids for
distribution through the body.
• Composed of the specialized cells which secrete hormones.
• May comprise only a portion of organs that produce other materials (e.g. pancreas).
Exocrine glands = Glands that produce a variety of substances (e.g. sweat, mucus, digestive
enzymes) and convey their products by means of ducts.
• Many organs perform both exocrine and endocrine functions.
• Exocrine glands are not a part of the endocrine system.
756
Chemical Signals in Animals
Hormones are grouped into two general classes according to similarities in chemical
structure: steroid hormones and hormones derived from amino acids.
• Steroid hormones are lipid molecules fashioned from cholesterol.
⇒ Includes the sex hormones.
• Hormones derived from amino acids may be small or large. Includes
⇒ Amine hormones (modified versions of single amino acids).
⇒ Peptide hormones (short chains of amino acids).
⇒ Protein hormones.
⇒ Glycoproteins.
A hormone’s target cell possesses a hormone receptor to which the hormone molecule must
bind.
• The hormone receptor is a protein within the cell or on the plasma membrane of the
target cell.
• The hormone and receptor are shape specific and must bind together to elicit a
response in the cell to the hormone.
The actions of antagonistic hormones are important in maintaining homeostasis.
• Feedback mechanisms are usually involved in adjusting hormone actions.
B.
Pheromones
Pheromones = Chemical signals that function between animals of the same species.
• Classified according to function (e.g. mate attractant, territorial marker, alarm
substance).
• Are small, volatile, easily dispersed molecules that are active in minute amounts.
• Have an important role in social behavior.
C.
Local Regulators
Local regulators are chemical messengers that affect target cells adjacent to or near their
point of secretion. Includes synaptic signaling and paracrine signaling.
• Synaptic signaling is direct and involves the release of a neuron’s transmitter into a
synapse.
• Paracrine signaling is less direct but very localized.
⇒ Regulatory substances are released into interstitial fluids but affect only nearby
target cells (i.e. histamine and interleukins).
Local regulators may be gases produced by cells, growth factors, or prostaglandins.
1. Gases produced by cells. Many types of cells produce gases which can act as
neurotransmitters and paracrine signals.
• Nitric oxide (NO) released by endothelial cells of blood vessels makes the
adjacent smooth muscle cells relax, dilating the vessel.
Chemical Signals in Animals
757
• Nitric oxide released by white blood cells kills certain cancer cells and bacteria
in the body fluids.
758
Chemical Signals in Animals
2. Growth Factors
Growth factors = Peptides and proteins that regulate the behavior of cells in growing
and developing tissues.
• Must be present in the extracellular environment for certain cell types to grow
and develop normally.
• Studied mainly in cultures of mammalian cells but also regulate development
within the animal body.
• Binding of the growth factor to a specific receptor protein on the outer
membrane surface triggers the target cell's response – a change in gene
expression in the target-cell nucleus.
3. Prostaglandins
Prostaglandins (PGs) = Modified fatty acids released into interstitial fluid that function
as local regulators.
• Often derived from lipids of the plasma membrane.
• Very subtle differences in their molecular structure profoundly affect how these
signals affect target cells (e.g. antagonistic actions of PGE and PGF).
• PGs secreted by the placenta help induce labor during childbirth by causing
chemical changes in the nearby uterine muscles.
• Other PGs help defend the body by inducing fever and inflammation.
II.
Hormone binding to specific receptors triggers signaling mechanisms at the cellular
level
Hormones can act in very low concentration and can affect different target cells in an animal or
different species of animals very differently.
• A hormone-induced change in a target organ cell results from one of two possible signal
transduction pathways.
• Signal transduction pathways include a series of steps between hormone-receptor binding
and target-cell response.
A.
Steroid Hormones and Gene Expression
Steroid hormones enter the target cell by passing through the target cell membrane and
binding to a receptor protein in the cytoplasm. (See Campbell, Figure 41.3)
• The hormone-receptor complex then has the proper configuration to bind to specific
regulatory sites located along the genome.
• Binding of the hormone-receptor complex to regulatory sites may induce or suppress
expression of specific genes.
• When expression is stimulated, mRNA molecules are produced, exit to the cytoplasm,
and direct the synthesis of new proteins.
• Two cell types can respond differently to a single hormone since the hormone-receptor
complex probably binds with regulatory sites controlling different genes in the two
kinds of cells.
Chemical Signals in Animals
Steroid hormones primarily affect the synthesis of proteins.
759
760
Chemical Signals in Animals
B.
Peptide Hormones and Signal Transduction
Peptide hormones and most hormones derived from amino acids are unable to pass through
the target cell plasma membrane, they thus require a different mechanism of action.
• These hormones attach to specific receptor proteins embedded in the target cell’s
plasma membrane.
• The receptors are components of a signal-transduction pathway that converts the
extracellular signal of the hormone to intracellular signals that alter the target-cell’s
behavior.
Peptide hormones usually affect the activity of enzymes and other proteins already present in
the cell.
III.
Many chemical signals are relayed and amplified by second messengers and protein
kinases
Earl W. Sutherland and his colleagues discovered the signal-transduction pathway used by
nonsteroid hormones.
• They discovered that a second messenger transmits the signal from the plasma membrane to
the metabolic machinery in the cytoplasm.
A.
Cyclic AMP
Cyclic AMP (cyclic adenosine monophosphate) was discovered as the second messenger in
Sutherland’s studies of how epinephrine stimulates glycogen hydrolysis in liver and muscle
cells. (See Campbell, Figure 41.4)
• Binding of a hormone with its receptor activates a G protein, which binds with GTP
(guanosine triphosphate), an energy carrying molecule.
• Activated G protein binds to adenylyl cyclase, an effector enzyme embedded in the
plasma membrane.
• Adenylyl cyclase catalyzes the conversion of ATP to cyclic AMP (cAMP).
• cAMP relays the signal from the membrane to the metabolic machinery of the
cytoplasm.
• A second type of G protein inhibits adenylyl cyclase, when activated by an inhibitory
hormone-receptor complex. This reduces cAMP concentration in the cytoplasm.
• Regulation of adenylyl cyclase by the opposing effects of two G proteins enables a cell
to respond to slight changes in the proportions of antagonistic hormones.
Chemical Signals in Animals
B.
761
Signal Amplification and Specificity
Many metabolic responses to hormones involve an enzyme cascade, where each step
activates an enzyme that in turn activates the next enzyme in the series (Campbell, Figure
41.6). For example, a cell’s response to epinephrine:
• cAMP activates a cAMP dependent protein kinase (an enzyme that catalyzes the
transfer of a phosphate group from ATP to proteins = phosphorylation), which in turn
can activate phosphorylase kinase.
• Phosphorylase kinase adds a phosphate group to glycogen phosphorylase which
actually hydrolyzes glycogen.
• The cascade thus amplifies response to the hormone since the number of activated
products increases with each step.
Although a single basic mechanism is used to mediate responses of diverse cells to many
different hormones, response is still specific. Specificity arises since:
• Particular cells can only interact with certain hormones — those for which they have
receptors.
• cAMP-dependent protein kinases differ in structure and function from tissue to tissue.
• Post-protein kinase activation steps in the cascade differ between cell types; thus,
individual cell types produce a unique set of proteins in response to cAMP-dependent
protein kinase.
In addition to working as a second messenger, cAMP has been found to inhibit or stimulate
key enzyme components of the signal-transduction pathway of many growth factors.
C.
Cytoplasmic Ca2+ and Signal Transduction
Many chemical messengers in animals (neurotransmitters, growth factors, some hormones)
stimulate responses in their target cells by increasing the calcium ion concentration of the
cytoplasm.
• The second messenger in this pathway is inositol triphosphate.
Inositol triphosphate (IP3) acts as follows: (See Campbell, Figure 41.7)
• The hormone (first messenger) binds to its receptor, activating a G protein (different
from those that function in the cAMP system) that in turn stimulates an enzyme in the
plasma membrane called phospholipase C.
• Phospholipase C cleaves a membrane phospholipid into IP3 and diacylglycerol; both
may function as second messengers.
• Diacylglycerol stimulates another membrane enzyme, protein kinase C, which triggers
various target cell responses by phosphorylating specific proteins.
• IP3 released from the plasma membrane stimulates movement of Ca2+ from the ER into
the cytoplasm.
• The Ca2+ regulates cellular protein activity either by itself or by first binding to
calmodulin which binds to other proteins.
• The signal is thus transmitted to various enzymes and other proteins by the increased
concentration of Ca2+ in the cytoplasm.
• The target cell then produces a specific response to the original hormonal signal.
762
IV.
Chemical Signals in Animals
Invertebrate control systems often integrate endocrine and nervous system functions
Invertebrates possess a diversity of hormones which function in homeostasis, reproduction,
development and behavior.
In many cases, the hormone may stimulate one activity while inhibiting another.
• In Hydra, one hormone stimulates growth and budding while inhibiting sexual reproduction.
• In the sea slug, a peptide hormone secreted by specialized neurons stimulates egg laying
while inhibiting feeding and locomotion.
All arthropods have extensive endocrine systems that regulate growth and reproduction, water
balance, pigment movement in the integument and eyes, and metabolism.
Hormones may act together such as in the case of insect development and molting.
• Ecdysone is secreted by a pair of prothoracic glands; it triggers molting and favors
development of adult characteristics and metamorphosis.
⇒ Ecdysone stimulates transcription of specific genes, the same mechanism of action as
vertebrate steroid hormones.
• Brain hormone promotes development by stimulating production of ecdysone.
• Brain hormone and ecdysone are balanced by juvenile hormone (JH), which actively
promotes retention of larval characteristics; JH is secreted by the corpora allata.
• When JH levels decrease, ecdysone-induced molting produces a pupa; in the pupa, adult
anatomy replaces larval anatomy during metamorphosis.
V.
The hypothalamus and pituitary integrate many functions of the vertebrate
endocrine system
Vertebrates possess a number of tissues and organs that secrete hormones. (See Campbell, Figure
41.9 and Table 41.1)
• Some hormones affect most tissues of the body (e.g. sex hormones).
• Some hormones affect only one or a few tissues.
• Some affect other endocrine glands; these are called tropic hormones.
A.
Structural and Functional Relationships of the Hypothalamus and the Pituitary Gland
The hypothalamus is a region of the lower brain that receives information from nerves
throughout the body and brain and initiates endocrine signals appropriate to the
environmental conditions.
• Contains two sets of neurosecretory cells whose secretions are stored in or regulate
activity of the pituitary gland.
Chemical Signals in Animals
763
The pituitary gland is an extension of the brain located at the base of the hypothalamus. It
consists of two lobes and has numerous endocrine functions. (See Campbell, Figure 41.10)
1. The neurohypophysis, or posterior pituitary, stores and secretes two peptide hormones
which are made by the hypothalamus: oxytocin and antidiuretic hormone.
• These hormones act directly on muscles of the uterus and on the kidneys.
2. The adenohypophysis, or anterior pituitary, is composed of non-nervous tissues and
synthesizes its own hormones.
• Controlled by two kinds of hormones secreted by neurosecretory cells in the
hypothalamus: releasing hormones and inhibiting hormones.
• Releasing hormones stimulate the anterior pituitary to secrete its hormones.
• Inhibiting hormones stop the anterior pituitary from secreting its hormones.
B.
Posterior Pituitary Hormones
The hypothalamic peptide hormones oxytocin and antidiuretic hormone are stored in and
released from the posterior pituitary.
• They are synthesized in the hypothalamus and secreted into the posterior pituitary.
Oxytocin induces uterine muscle contraction and causes the mammary glands to eject milk
during nursing.
Antidiuretic hormone (ADH) is part of the feedback mechanism that helps regulate blood
osmolarity.
• It acts on the kidneys to increase water retention which results in a decrease in urine
volume.
• Osmoreceptors (specialized nerve cells) in the hypothalamus monitor blood
osmolarity.
• When plasma osmolarity increases, the osmoreceptors shrink slightly (lose water by
osmosis) and transmit nerve impulses to certain hypothalamic neurosecretory cells.
• These neurosecretory cells respond by releasing ADH into the general circulation from
their tips in the posterior pituitary.
• The target cells for ADH are cells lining the collecting ducts of nephrons in the
kidneys.
• ADH binds to receptors on the target cells and activates a signal-transduction pathway
that increases the water permeability of the collecting duct.
• Water retention is increased as water exits the collecting ducts and enters nearby
capillaries.
• The osmoreceptors also stimulate a thirst drive.
⇒ Drinking water reduces blood osmolarity to the set point.
• As more dilute blood (lower osmolarity) arrives at the brain, the hypothalamus
responds by reducing ADH secretion and lowering thirst sensations.
⇒ This prevents over compensation by stopping hormone secretion and quenching
thirst.
764
Chemical Signals in Animals
Note that this negative feedback scheme includes a hormonal action and a behavioral
response (drinking).
Chemical Signals in Animals
C.
765
Anterior Pituitary Hormones
The adenohypophysis produces many different hormones and is regulated by releasing
factors from the hypothalamus.
• Four are tropic hormones that stimulate other endocrine glands to synthesize and
release their hormones.
Growth hormone (GH) is a protein hormone which affects a wide variety of tissues.
• It promotes growth of some tissues directly and promotes growth of others indirectly
by stimulating the production of growth factors by other tissues.
• For example, GH stimulates the liver to secrete insulin-like growth factors (IGFs)
which stimulate bone and cartilage growth.
Prolactin (PRL) is a protein hormone similar in structure to GH although their physiological
roles are very different.
• PRL produces a diversity of effects in different vertebrates:
⇒ It stimulates mammary gland development and milk synthesis in mammals.
⇒ Regulates fat metabolism and reproduction in birds.
⇒ Delays metamorphosis and may function as a larval growth hormone in
amphibians.
⇒ Regulates salt and water balance in freshwater fish.
Follicle-stimulating hormone (FSH) is a tropic hormone which affects the gonads (=
gonadotropin).
• In males, it is necessary for spermatogenesis.
• In females, it stimulates ovarian follicle growth.
Luteinizing hormone (LH) is another gonadotropin.
• It stimulates ovulation and corpus luteum formation in females and spermatogenesis in
males.
Thyroid-stimulating hormone is a tropic hormone which stimulates the thyroid gland to
produce and secrete its own hormones.
The remaining hormones from the anterior pituitary are formed by the cleaving of a single
large protein, pro-opiomelanocortin, into short fragments.
• At least three of these fragments become active peptide hormones:
⇒ Adrenocorticotropin (ACTH) stimulates the adrenal cortex to produce and secrete
its steroid hormones.
⇒ Melanocyte-stimulating hormone (MSH) regulates the activity of pigmentcontaining skin cells in some vertebrates.
⇒ Endorphins inhibit pain perception.
766
Chemical Signals in Animals
D.
Hypothalamic Hormones
Regulation of the anterior pituitary by the hypothalamus involves releasing hormones
produced by neurosecretory cells in the hypothalamus into capillaries.
• The releasing hormones are secreted into capillaries that drain into short portal vessels
that subdivide into a second capillary bed within the anterior pituitary.
• These hormones then travel through the second capillary network within the anterior
pituitary, where they stimulate or inhibit release of specific hormones by pituitary
cells.
• Every hormone produced by the anterior pituitary is controlled by at least one
releasing hormone; some have both a releasing and an inhibiting hormone.
VI.
The vertebrate endocrine system coordinates homeostasis and regulates growth,
development, and reproduction
There are several endocrine glands that play major roles in the control of homeostasis, growth,
development, and reproductive functions of vertebrates.
A.
The Thyroid Gland
The thyroid gland consists of two lobes located on the ventral surface of the trachea in
mammals, and on the two sides of the pharynx in other vertebrates.
• Produces two hormones, T3 (triiodothyronine) and T4 (thyroxine or tetraiodothyronine)
derived from the amino acid tyrosine.
⇒ These differ in structure by only one iodine atom.
• Both have the same effects on their target although T3 is usually more reactive than T4
in mammals.
The thyroid gland plays a major role in vertebrate development and maturation.
• Thyroid hormones control metamorphosis in amphibians.
• Normal function of bone-forming cells and the branching of nerve cells during
embryonic brain development of nonhuman animals also requires the presence of the
thyroid hormones.
The thyroid gland is critical for maintaining homeostasis in mammals.
• Help maintain normal blood pressure, heart rate, muscle tone, digestion, and
reproductive functions.
• T3 and T4 tend to increase the rate of oxygen consumption and cellular metabolism.
• Serious metabolic disorders can result from a deficiency or excess of thyroid
hormones.
⇒ Hyperthyroidism, excessive secretion of thyroid hormones, causes high body
temperature, sweating, weight loss, irritability and high blood pressure.
⇒ Hypothyroidism, low secretion of thyroid hormones, can cause cretinism in infants
and weight gain, lethargy and cold-intolerance in adults.
Chemical Signals in Animals
767
⇒ Goiter (enlarged thyroid) is caused by a dietary iodine deficiency and thyroid
hormones cannot be synthesized.
768
Chemical Signals in Animals
Thyroid hormone secretion is regulated by the hypothalamus and pituitary through a negative
feedback system.
• The hypothalamus secretes TRH (TSH-releasing hormone) which stimulates TSH
(thyroid-stimulating hormone) secretion by the anterior pituitary.
• When TSH binds to receptors in the thyroid, cAMP is generated and triggers release of
T3 and T4.
• High levels of T3, T4, and TSH inhibits TRH secretion.
The thyroid gland in mammals also produces and secretes calcitonin which is a peptide
hormone that lowers blood calcium levels.
B.
The Parathyroid Glands
Four parathyroid glands are embedded in the surface of the thyroid and function in the
homeostasis of calcium ions.
• They secrete PTH (parathyroid hormone) which raises blood Ca2+ levels (antagonistic
to calcitonin) and needs vitamin D to function.
• PTH stimulates Ca2+ reabsorption in the kidney and induces osteoclasts to decompose
bone and release Ca2+ into the blood.
PTH and calcitonin (which have opposite affects) work in an antagonistic manner and their
balance in the body maintains the proper blood calcium levels.
C.
The Pancreas
The pancreas is composed primarily of exocrine tissue which produces digestive enzymes;
however, scattered among the exocrine tissue are clusters of endocrine cells called the islets
of Langerhans.
• Each islet is composed of alpha (α) cells, which secrete the peptide hormone
glucagon, and beta (β) cells which secrete the hormone insulin.
Glucagon and insulin work together in an antagonistic manner to regulate the concentration
of glucose in the blood. (See Campbell, Figure 41.13)
• Blood glucose levels must remain near 90 mg/mL in humans for proper body function.
• At glucose levels above the set point, insulin is secreted and lowers blood glucose
concentration by stimulating body cells to take up glucose from the blood.
⇒ It also slows glycogen breakdown in the liver and inhibits the conversion of amino
acids and fatty acids to sugar.
• When blood glucose levels drop below the set point, glucagon is secreted and increases
blood glucose concentrations by stimulating the liver to increase the hydrolysis of
glycogen, convert amino acids and fatty acids to glucose, and slowly release glucose
into the blood.
Chemical Signals in Animals
769
Glucose homeostasis is critical due to its function as the major fuel for cellular respiration
and a key source of carbon for the synthesis of other organic compounds.
Serious conditions can result when glucose homeostasis is unbalanced. Diabetes mellitus is
caused by a deficiency of insulin or a loss of response to insulin in target tissues. Diabetes
occurs in two forms:
• Type I diabetes mellitus is an autoimmune disorder that appears in juveniles and is
insulin dependent.
⇒ Results from an immune system attack on the cells of the pancreas.
⇒ Usually occurs suddenly during childhood and destroys the ability of the pancreas
to produce insulin.
⇒ Treated by insulin injections several times each day.
• Type II diabetes mellitus occurs most frequently in adults over 40.
⇒ May be due to an insulin deficiency but more commonly results from reduced
responsiveness in target cells because of changes in insulin receptors.
⇒ Is non-insulin-dependent and can be treated with exercise and dietary controls.
Both types of diabetes mellitus will result in high blood sugar concentrations if untreated.
• Kidneys excrete glucose, resulting in higher concentrations in the urine.
• More water is excreted due to the high concentration of glucose (results in the
symptoms of copious urine production accompanied by thirst).
• Fat must serve as the major fuel source for cellular respiration since glucose does not
enter the cells.
• In severe cases, acidic metabolites formed during fat metabolism may lower the blood
pH to a critical level.
D.
The Adrenal Glands
Adrenal glands are located adjacent to kidneys.
• In mammals, each gland has an outer adrenal cortex and inner adrenal medulla which
are composed of different cell types, have different functions, and are of different
embryonic origin.
• Different arrangements of these same tissues are found in other vertebrates.
The adrenal medulla synthesizes and secretes catecholamines (epinephrine and
norepinephrine).
• Catecholamines are secreted in times of stress when nerve cells excited by stressful
stimuli release the neurotransmitter acetylcholine in the medulla. Acetylcholine
combines with cell receptors, stimulating release of epinephrine.
⇒ Norepinephrine is released independently of epinephrine.
• Epinephrine and norepinephrine released into the blood results in rapid and dramatic
effects on several targets:
⇒ Glucose is mobilized in skeletal muscle and liver cells.
⇒ Fatty acid release from fat cells is stimulated (may serve as extra energy sources).
770
Chemical Signals in Animals
⇒ The rate and stroke volume of the heartbeat is increased.
⇒ Blood is shunted away from the skin, gut, and kidneys to the heart, brain, and
skeletal muscles by stimulation of smooth muscle contraction in some blood
vessels and relaxation in others.
⇒ Oxygen delivery to the body cells is increased by dilation of bronchioles in the
lungs.
The adrenal cortex synthesizes and secretes corticosteroids. In humans the two primary
types are: glucocorticoids (cortisol) and mineralocorticoids (aldosterone).
• Stressful stimuli cause the hypothalamus to secrete a releasing hormone that stimulates
release of ACTH from the anterior pituitary.
⇒ ACTH stimulates release of corticosteroids from the adrenal cortex.
• Glucocorticoids promote glucose synthesis from noncarbohydrate substances such as
proteins.
⇒ Skeletal muscle proteins are broken down and the carbon skeletons transported to
the liver and kidneys.
⇒ The liver and kidneys convert the carbon to glucose which is released into the
blood to increase the fuel supply.
⇒ Also have immunosuppressive effects and are used to treat inflammation.
• Mineralocorticoids affect salt and water balance.
⇒ Aldosterone stimulates kidney cells to reabsorb sodium ions and water from the
filtrate.
⇒ This raises blood volume and blood pressure.
• Aldosterone from the RAAS (renin-angiotensin-aldosterone system), ADH, and ANF
(atrial natriuretic factor) from the heart form a regulatory complex which influences
the kidneys ability to maintain the blood’s ion and water concentrations.
⇒ The RAAS regulates aldosterone secretion in response to changes in plasma ion
concentrations.
⇒ Severe stress also stimulates aldosterone secretion by causing the hypothalamus to
secrete releasing hormones that increase ACTH secretion from the anterior
pituitary that increases aldosterone secretion by the adrenal cortex.
Current evidence indicates glucocorticoids and mineralocorticoids are important to
maintaining body homeostasis during extended periods of stress.
A third group of corticosteroids are sex hormones.
• Include androgens (male hormones) similar to testosterone and small amounts of
estrogens and progesterone (female hormones).
• The roles of adrenal sex hormones are not well understood.
Chemical Signals in Animals
E.
771
The Gonads
The testes of males and ovaries of females produce steroid hormones that affect growth and
development as well as regulate reproductive cycles and behaviors.
The gonads of both males and females produce all three categories of gonadal steroids
(androgens, estrogens, and progestins) although the proportions differ.
Androgens generally stimulate the development and maintenance of the male reproductive
system.
• Produced in greater quantities in males than females.
• Primary androgen is testosterone.
• Androgens produced during early embryonic development determine whether the fetus
will be male or female.
• High androgen concentrations at puberty stimulate development of male secondary sex
characteristics.
Estrogens perform the same functions in females as androgens do in males.
• Estradiol is the primary estrogen produced.
• Maintain the female system and stimulate development of female secondary sex
characteristics.
Progestins are primarily involved with preparing and maintaining the uterus for reproduction
in mammals.
• Include progesterone.
The gonadotropins from the anterior pituitary (FSH and LH) control the synthesis of both
androgens and estrogens.
• FSH and LH are in turn controlled by gonadotropin-releasing hormone (GnRH) from
the hypothalamus.
F.
Other Endocrine Organs
Many organs with primarily nonendocrine functions also secrete hormones.
• The digestive tract secretes at least eight hormones (e.g. gastrin, secretin).
• Kidneys secrete erythropoietin which stimulates red blood cell production.
• Two other important endocrine glands are the pineal gland and the thymus.
The pineal gland is a small mass of tissue near the center of the mammalian brain although
it is closer to the surface in some other vertebrates.
• The pineal contains light sensitive cells or has nervous connections with the eyes.
• It secretes melatonin, which is a modified amino acid.
• Melatonin regulates functions related to light and to seasons marked by changes in day
length, such as biological rhythms associated with reproduction.
• Melatonin is secreted only at night; larger amounts are thus secreted during the winter
than the summer.
772
Chemical Signals in Animals
⇒ The role of melatonin in regulating biological rhythms is not yet understood.
Chemical Signals in Animals
773
The thymus is located under the breast-bone in humans.
• It is large in children and reduced in adults.
• It secretes several messengers including thymosin which stimulates T lymphocyte
development and differentiation after they leave the thymus.
VII.
The endocrine system and the nervous system are structurally, chemically, and
functionally related
The endocrine system and nervous system interact and are often inseparable.
between the two systems may be: structural, chemical, or functional.
Relationships
Structural relationships result from the fact that many endocrine glands are either made of nerve
tissue or evolved from nervous tissue.
• The hypothalamus and posterior pituitary are nervous tissues that secrete hormones into the
blood.
• The adrenal medulla evolved from the same cells that produce certain ganglia of the nervous
system.
Chemical relationships center around those vertebrate hormones that are used as signals by both
systems.
• Norepinephrine functions as both an adrenal hormone and as a neurotransmitter.
Functional relationships relate to the coordination of physiological processes by both nervous and
hormonal components arranged in series.
• Each system can affect the output of the other.
REFERENCES
Campbell, N. Biology. 4th ed. Menlo Park, California: Benjamin/Cummings, 1996.
Marieb, E.N. Human Anatomy and Physiology. 3rd ed. Redwood City, California:
Benjamin/Cummings, 1995.