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Chapter 44
CHEMICAL SIGNALS IN ANIMALS
The endocrine system consists of a collection of glands, cells and tissues that secrete
hormones.
Its function is to regulate many aspects of metabolism, growth and reproduction.
Endocrine glands produce hormones and secrete them to the surrounding tissues and
eventually into the capillaries.
Hormones are chemical messengers that are responsible for regulating body processes.
Hormones are transported by the blood to the target tissues.
Endocrinology is the study of endocrine gland function and hormonal effect on target tissues.
Exocrine glands release their secretions into ducts.
Some neurons secrete hormones (neurohormones) and are known are neuroendocrine cells.
Some cells release hormones that act on nearby cells. This is called paracrine regulation.
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Interleukins that regulate immune responses.
Prostaglandins are modified fatty acids that have a wide range of activities.
Lungs, liver, digestive tract and reproductive organs release prostaglandins.
Affect cells in their immediate vicinity.
Mimic cyclic AMP and interact with other hormones that regulate many metabolic
activities.
In autocrine regulation, the hormone influences the very cell that produces it.
Some animals produce pheromones for communication.
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Do not regulate metabolism.
Produced by exocrine glands.
They do not have a regulatory function in metabolism and are not considered hormones.
Most hormones are regulated by a negative feedback mechanism.
HORMONE TYPES
1. Steroid hormones are synthesized from cholesterol, e.g. cortisol, progesterone,
testosterone.
2. Amino acid derivatives called amines, e.g. thyroid, melatonin.
3. Peptide hormones are short chains of amino acids, e.g. secretin, ADH, oxytocin.
4. Fatty acid derivatives, e.g. prostaglandins
VERTEBRATE HORMONES
Endocrine hormones regulate growth, development, fluid balance, metabolism and
reproduction.
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Hyposecretion: secretion lower than normal.
Hypersecretion: secretion higher than normal.
Homeostasis depends on the normal concentrations of hormones.
Hypothalamus
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Part of the brain.
Links the endocrine system with the nervous system.
Most endocrine activity is controlled directly or indirectly by the hypothalamus.
Produces growth releasing and growth inhibiting hormones.
Anterior lobe of the pituitary is the target tissue.
Stimulates and inhibits secretion.
Pituitary
At the base of the brain.
1. Posterior lobe of the pituitary
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Produces oxytosin.
Causes the uterus to contract during birth.
Causes the mammary glands to eject milk.
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Produces antidiuretic hormone (ADH).
Causes the collecting ducts of the kidneys to reabsorb water.
Secretes growth-hormone-releasing hormone or GHRH and growth-hormone-inhibiting
hormone or GHIH also called somatostatin.
2. Anterior lobe of the pituitary.
Tropic hormones stimulate other endocrine glands.
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Growth hormone (GH) stimulates linear body growth and tissue and organ growth
by promoting protein synthesis.
 GH stimulates the liver to produce peptides called somatomedins including
insulin-like growth factor.
 Secretion of GH is regulated by growth-hormone releasing hormone or
GHRH and growth-hormone-inhibiting hormone or GHIH also called
somatostatin. Both are released by the hypothalamus.
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Prolactin stimulates the mammary glands to produce milk.
Thyroid-stimulating hormone (TSH) causes the thyroid to secrete hormones.
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Adrenocorticotropic hormone (ACTH) stimulates the secretion of hormones by
adrenal cortex.
Gonadotropic hormones (follicle-stimulating hormone, FSH, luteinizing hormone,
LH) stimulate gonad functions.
Thyroid gland
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Thyroxine (T4) and triiodothyromine (T3) contain iodine.
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T4 is converted to T3 in many cases.
Stimulate general growth and development, and the metabolic rate in most tissues.
T3 induce or suppress the synthesis of enzymes.
Hypothyroidism in childhood leads to cretinism, retarded mental and physical development.
Hypersecretion causes a fast use of nutrients, hunger, nervousness, irritability and
emotional instability.
 These hormones require iodine. Lack of iodine causes goiter due to an over production of
THS by the anterior pituitary.
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Produces calcitonin, which works antagonistically to the parathyroid hormone.
 Lowers the blood calcium level by inhibiting calcium release form bones.
Parathyroid glands
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These glands are embedded in the connective tissue surrounding the thyroid.
Secrete the parathyroid hormone (PTH) that regulates calcium level in the blood
and tissue fluid.
Stimulate calcium release from bones and calcium reabsorption from the kidney
tubules.
Islets of the pancreas or of Langerhans.
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About 1 million little clusters of cells scattered throughout the pancreas.
Alpha cells secrete glucagon, which increases the concentration of glucose in the
blood.
Beta cells secrete insulin, which lowers the concentration of glucose in the blood.
Insulin stimulates cells to take up glucose, inhibits the release of glucose from the
liver, stimulates the deposit of fat in the adipose tissue, and inhibits the use of amino
acids.
Glucose concentration regulates the secretion of glucagon and insulin.
Diabetes mellitus is an endocrine disorder.
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Type II diabetics produce enough insulin but the receptors on target cells cannot bind
to it.
Type I diabetics do not produce enough insulin. The immune system or a virus
destroys beta cells.
Metabolic disturbances of diabetes.
1. Hyperglycemia with a concentration of glucose of above 200 mg/100 ml. The
normal concentration is 90mg/100ml.
2. Increased fat mobilization increasing the blood lipid level up to five times the normal
amount.
3. Increase protein use.
4. In hypoglycemia the glucose concentration is too low.
Adrenal medulla
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Helps the body cope with stress, increases the heart rate, blood pressure, metabolic
rate, reroutes blood, mobilize fats and increase glucose level in the blood.
Secretes epinephrine (adrenalin) and norepinephrine.
Sympathetic nervous system can increase their production.
Adrenal cortex
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Secretes three hormone in significant amounts, but more than 30 steroids
have been isolated.
Mineralocorticoids (aldosterone) maintain sodium and potassium balance by
increasing sodium reabsorption and potassium excretion in the kidney
tubules.
Glucocorticoids (cortisol) help the body adapt to stress, raise glucose level
and mobilize fats.
DHEA (dehydroepiandrosterone) is converted in the tissues to testosterone
Testes
Testes produce testosterone. This hormone regulates and develops secondary sexual
characteristics in males.
Ovaries
Ovaries produce estradiol. Estradiol regulates and maintains secondary sexual characteristics
in females.
Thymus produces thymosin that stimulates the T cells after they leave the thymus and makes
them immunologically active.
Kidneys release renin that regulates blood pressure.
Pineal body in the brain releases melatonin, which influences biological rhythms, sleep and
the onset of sexual maturity.
The heart releases atrial natriurectic factor (ANF) that promotes sodium excretion and lowers
the blood pressure.
HORMONE ACTIVITY
Hormones coordinate the activities of cells in response to three general situations:
1. Environmental changes.
2. Growth, developments and reproduction
3. Homeostasis.
Acidic food in the duodenum causes the release of secreting cholecystokinin.
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Secretin causes the pancreas to release an alkaline solution and the gall bladder to release
bile.
Cholecystokinin activates the secretion of the pancreatic enzymes into the duodenum.
Action potentials from the sympathetic nervous system cause the adrenal medulla to release
epinephrine.
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Epinephrine leads to heightened state of alertness and rapid utilization of energy:
Increase of pulse rate, blood pressure and oxygen consumption by the brain.
Changes in blood distribution: away from digestive system and skin, and redirected to
muscles, heart and brain.
Increase feelings of anxiety and excitement.
During periods of long stress, the adrenal cortex secretes glucocorticoids.
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Glucocorticoids increase glycogen formation in the liver and increase glucose level in the
blood.
Release of amino acids and lipids from muscle and fat cells respectively, which promotes
their use in ATP production.
They reserve glucose for use by the brain during stress.
Hormones direct developmental processes.
1. Early embryonic development. Primary sex determination, the development of testes or
ovaries, is genetically controlled. Afterwards, the gonads will produce male or female
hormones.
2. Juvenile-to adult transition. Hormones control the development of secondary sexual
characteristics like the development of facial hair in boy and the enlargement of breast in
girls at the onset of puberty.
3. Seasonal or cyclic sexual activity. Most long-lived animals reproduce seasonally.
Environmental cues like increase in day length or seasonal rains trigger the release of sex
hormones. Humans do not reproduce seasonally but sex hormones regulate the menstrual
cycle.
Hormones are involved in homeostasis.
E.g. Antidiuretic hormone, ADH, is released from the pituitary gland when there is a loss of
water. ADH increases the permeability to water of the distal tubes of the nephron. As a result,
water is reabsorbed from the urine and conserved.
E. g. Aldosterone is released from the adrenal cortex when Na+ concentration is low, and
increases the absorption of ions in the distal tubule of the nephron.
Regulation of hormone production.
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The pituitary gland is directly connected to the brain region called the hypothalamus.
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The pituitary has two regions: the anterior and the posterior lobes. Each secretes different
hormones to different signal from the hypothalamus.
Example of the regulation of hormone production.
The hypothalamus releases corticotropin releasing factors called CRF.
CRF triggers the release of the adrenocorticotropic hormone, ACTH, by the anterior pituitary.
In response to ACTH in the blood, the adrenal cortex releases glucocorticoids.
ACTH also inhibits the production of CRF through a feedback mechanism.
The hypothalamic-pituitary axis
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Neurosecretory cells of the hypothalamus extend directly into the posterior pituitary and
secrete ADH and oxytosin.
The hypothalamus and the anterior pituitary communicate indirectly via blood vessels.
Hormones produced in the hypothalamus travel to the anterior pituitary in the blood.
The anterior pituitary releases its hormones in response to the hypothalamic hormones.
How do hormones act on target cells.
Target cells have receptors that combine with a specific hormone. They are responsible for the
specificity of the hormone. The receptors may be in or out of the cell.
Steroid hormones are lipid soluble and pass through the cell membrane and bind to receptors
inside the cell.
Amino acid derivatives and polypeptides are not lipid soluble and bind to the receptors located
in the membrane of target cells.
In both cases the response to a hormone is tissue specific because only certain cells express
certain receptors.
Receptors are continuously synthesized and degraded by receptor up-regulation or receptor
down-regulation.
In many cases, the receptors act as intermediaries between the hormone and the cellular
compound.
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Signal transducers.
Hormone converts an extracellular signal to an intracellular signal that affects the
function of the cell.
The hormone relays information to a second messenger or intracellular signal.
Most steroid hormones act as inducers of gene expression.
Steroid hormones bind to receptor proteins inside the cell, and hormone-receptor complexes
trigger the transcription of certain genes by binding to regulatory DNA segments called
hormone-response elements.
Signal transduction occurs when a chemical message at the cell surface elicits a response
inside the cell.
Epinephrine is a peptide hormone and binds to receptors on the surface of the plasma
membrane. when this occurs, a cascade of events inside the cell takes place that results in the
production of glucose from glycogen.
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Epinephrine binds to a receptor on cell membrane.
The hormone-receptor complex in turn activates the G protein, which in turns stimulates
adenyl cyclase to catalyze the formation of cAMP inside the cell.
The molecule cAMP is called a secondary messenger.
cAMP binds to protein kinase, A which in turns phosphorylates aand enzyme initiating a
chain reaction of chemical events resulting in the production of glucose from glycogen.