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Hormones
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Classical definition of the endocrine system:
Endocrine cells within endocrine glands release substances (hormones), which
are conveyed by the blood stream and act on distant cells
Current awareness is
that hormones may be
produced in many
tissues and have both
local and distant effects

Non-classical producers:
 Kidney
 Heart muscle
 Endothelium
 Platelets
 Adipocytes
 White blood cells
Hormone: Chemical messenger, synthesised by specialised cells, secreted into the
blood in small amount which acts on a specific receptor in target organs to regulate
cellular function
In contrast with:
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Neurotransmitter: Chemical messenger which is secreted by neurons into a
synapse between the neuron and its effector
Pheromone: Chemical messenger which is secreted into the environment and
alters the physiology or behaviour of other individuals of the same species

The endocrine system and hormones: Interaction with target cell
Hormones act on target cells by binding to specific receptors located either on the cell
surface or within the cell
 High affinity: Hormones are effective in low concentration
 Synergistic: The effect of two hormones is greater than the effect of one
 Permissive: The presence of one hormone is necessary for another to have effect
 Antagonistic: Two hormones oppose each other’s effects
 Competitive: Two hormones, similar in structure, compete for the same receptor

Classes of hormone and their properties
Classes of hormone:
There are 3 classes of hormone based on their chemical structure
1
...
g
...
) Peptides (e
...
growth hormone) – Synthesised form amino acids
3
...
g
...
) Steroid Hormones
 Small hydrophobic (lipophilic) molecules synthesised primarily from cholesterol
 Released immediately following synthesis
 Circulate in bound form (they bind to proteins when moving around the body)
 Act on intracellular receptors which then bind to DNA (hormone response
elements) and regulate gene transcription
 Have slow long lasting effects
Action on receptors: Steroid hormones

Steroid hormone synthesis, storage and release: Adrenal glands
Cortex:
Comprises 80-90% of adrenal
weight and has 3 distinct zones
1
...
) Zona fasiculta (middle)
–
Large lipid containg
cells
(75% cortical volume) –
Produces cortisol and
androgens
3
...
6%
Cortisol (Hydrocortisone):
 Major glucocorticoid – Can alter glucose levels
...
Plasma
(cortisol) has around 100X more than that of aldosterone
Mineralocorticoid effects of cortisol would be greater if not for the action of
11beta – hydroxysteroid dehydrogenase in aldosterone responsive cells

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Cushings syndrome:
Chronic glucocorticoid (cortisol) excess
Commonly iatrogenic (chronic glucocorticoid therapy)
Spontaneous Cushing’s syndrome result of pituitary or adrenal abnormality or ectopic
ACTH secretion
Cushing’s disease is specifically Cushing’s syndrome resulting from a pituitary tumour
Clinical feature:
 Weight gain – Generally only in the central and not the extremities
 Moon face – Accumulation of fat in the face
 Atrophy of epidermis
 Hypertension
 Psychological disturbances
 Muscle weakness
 Osteoporosis
 Thirst and polyuria
Steroid hormone synthesis, storage and release: Gonads
The gonads:
 The gonads have a dual function in that they produce germ cells (gametogenesis)
and secrete sex hormones
 Androgens are masculinising
 Oestrogens are feminising
Androgens are secreted from the adrenal cortex in both sexes and some are converted
to oestrogen in fat and other tissues
Testes:
 Secrete large amounts of androgens (testosterone) and small amounts of
oestrogen
Ovaries
 Secrete large amounts of oestrogen and small amounts of androgens
 Secrete progesterone; prepares uterus for pregnancy
 Secrete relaxin during pregnancy; loosens ligaments in pubic symphysis and
softens the cervix to facilitate delivery

Classes of hormone and their properties
Classes of hormone:
There are 3 classes of hormone based on chemical structure
1
...
g
...
) Peptides (growth hormone) – Synthesised from amino acids
3
...
They are
longer than they need to be and
so some amino acids need to be
removed by enzymes to
become activated
...

They act on receptors instead
which will be associated with a
G protein that causes a secondary messenger cascade
 The N end of the receptor is usually outside the cell
 The C terminus is often associated with the G protein
 The binding of the hormone changes the shape of the receptor releasing the G
protein
 G proteins can be inhibitory or stimulating
 Adding a phosphate group to a protein will change its function
Amino Acids
 Amino acid hormones include thyroid hormones and epinephrine
 Stored for instant release
 Different modes of action
...
) Hormone arrives at the receptor and binds
2
...
) This then binds to the
“effector” (adenyle cyclase)
4
...
) cAMP activate protein kinase
A
6
...


Action on receptors & IP3: peptide & amino acid hormones
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Hormone binds to receptor
Activate G protein is released
The G protein will activate Phospholipase C
Phosphlipase C breaks down phospholipids in the membrane
Phosphatydyl inositol bisphosphate (PIP2 ) is broken down, releasing IP3
IP3 will bind to a ligand gated channel on the sarcoplasmic reticulum releasing
calcium ions
Dicyglycerol, which is left behind will bind to protein kinase C which will
phosphorylate

Structure of the hypothalamus and pituitary gland
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The hypothalamus and pituitary are the major coordinators of the endocrine
system
...
Neurohypophysis
The anterior pituitary contains true endocrine cells
...

-Major site of interaction between the nervous system and endocrine system
-Exerts control over several endocrine glands and a number of physiological
activities
Hypothalamus:
-Region on the brain that plays a key role in homeostasis
Pituitary:
-Aka the hypophysis consists of two lobes

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Hypothalamic hormones:
Release hypothalamic hypophysiotropic hormones into the portal circulation and acts
on the anterior pituitary

Posterior pituitary hormones:
Hormones produces in the magnocellular neurones of the hypothalamus and stored in
the posterior pituitary prior to release

Anterior pituitary hormones
Go out to act on other endocrine tissues to release hormones
Release hormones into systemic circulation

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