Hormones in Animals

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Hormones in Animals

• Endocrinology
• D R Davies
• School of Biological Sciences
• Purves Life the Science of Biology Chapters 41
  (Animal Hormones) and 15 (Cell Signalling)

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Lecture 13 Learning Outcomes

• At the end of this lecture you should be able to
  
• Describe the general properties of hormones, with
  examples
• Know the location of the major endocrine glands
  and the types of hormone secreted
• Outline the steps in the study of hormones
• Explain the different types of hormone receptor
  and describe in outline the transduction mechanism

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Communication between cells in multicellular
organisms

• For a multicellular organism to survive it must
  be able to respond to changes in the external and
  internal environment - individual cells must be
  able to communicate with one another
  communication between cells occurs via 4 distinct
  mechanisms
• Cell-to-cell communication via gap junctions in
  the plasma membrane
• Paracrine control via locally acting chemical
  signals
• Electrical signals via the nervous system
• By chemical signals (hormones) released into the
  bloodstream

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Definition of hormone

• A specific chemical substance formed in one organ
  (endocrine gland) and transported in the
  bloodstream to another organ (Target Organ)where
  it affects the metabolism of that organ.
• First coined in 1902 by Bayliss and Starling who
  were the first to show the existence of the
  hormone - secretin.

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Definition of hormone (2)

• Hormones are information transferring molecules
  which move from one cell to another for the
  benefit of the organism as a whole Huxley)

Endocrine Cell
Target Cell
H
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Physiological effects of hormones are
proportional to hormone concentration

• Hormones are only effective over a narrow
  concentration range
• The EC50 value is the hormone concentration
  required to produce 50 of the maximal response

100
50
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Hormones

• do not initiate reactions but rather they effect
  the rate of pre-existing metabolic functions in a
  positive or negative fashion
• some hormones have specific effects on a single
  cell type, others a more general effect
• hormones are effective at minute concentrations -
  range 10-12 to 10-8 M
• hormones have a very short half-life in
  circulation ( ranging from minutes to hours)

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Hormone concentration

• Hormones are inactivated or degraded to an
  inactive form (H)at a constant rate
• It follows that the level of hormone in
  circulation is dependent on the rate of secretion

Endocrine Cell
Target Cell
H
H
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Physiological effects of hormones are timed
responses

• The effects of hormones occur in a regulated and
  timed manner
• Hormone levels increase in response to a
  physiological signal which results in an
  increased secretion of hormone

• hormone levels decrease when secretion ceases

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Hypothalamus releasing peptide hormone acting on
the anterior pituitary GHRH, CRH, TRH GnRH
Anterior Pituitary Growth Hormone (GH),
Corticotrophin (ACTH), Thyroid Stimulating
hormone (TSH), Luteinizing hormone (LH), Follicle
stimulating hormone (FSH) Posterior Pituitary
oxytocin, vasopressin (ADH) Pancreatic Islets of
Langerhans insulin and glucagon Adrenal cortex
aldosterone and cortisol Adrenal Medulla
Adrenaline and Noradrenaline Thyroid Gland
thyroxine and Tri-iodothyronine Testis
testosterone (androgen) Ovary and placenta
oestradiol (oestrogens), progesterone
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Different types of hormones
Glycoprotein hormones e.g TSH, LH and FSH Small
peptide hormones oxytocin, vaspressin (ADH)
GnRH Larger peptide hormones insulin,
ACTH Catecholamine hormones adrenaline and
noradrenaline (a.k.a. epinephrine and
norepinephrine in the USA) Thyroid hormone
thyroxine and triiodothyronine Steroid hormones
cortisol, progesterone, testosterone,
(o)estradiol Fatty Acid -based hormones
prostaglandins
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Peptide Hormones from the Posterior Pitutary
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Insulin Structure
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Steroid Hormones

• Lipid hormones
• Cross plasma membranes readily
• Interact with intracellular receptors
• Regulate gene transcription

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Adrenaline (epinephrine)
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Methods of studying hormones and their effects

• Removal of the source of hormones
• If you remove the endocrine gland there is a
  measurable physiological consequence
• remove pancreas - causes diabetes mellitus (high
  blood glucose)
• destroy b-cells in pancreas by injecting
  streptozotocin - also results in diabetes
• castration - results in loss of secondary sexual
  characteristics

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Methods of studying hormones and their effects

• Make extracts of the removed tissue and
  administer the the hormone deficient animal
• there is a measurable reversal physiological
  consequence
• pancreatic extracts lower blood glucose) in
  streptozotocin - induced in diabetes
• administration of testosterone restores
  secondary sexual characteristics
• Proof that the endocrine gland secretes a
  chemical agent with hormonal effects

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Methods of studying hormones and their effects

• Identify and characterise the hormone
• Make a synthetic version of the hormone and show
  that it has similar effects
• Identify the target tissue (s)
• What are the metabolic and physiological effects
  of the hormone?
• Make radiolabelled hormone (e.g. I125-insulin,
  3H-oestradiol
• Hormone specfically retained in target tissue (by
  a high affinity receptor)
• Examine properties of hormone receptor
• Purify and characterise receptor
• characterise the metabolic effects of hormone
  within the target tissue

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Hormones which bind to plasma membrane receptors

• Hormones bind to specific receptors on the
  outside of the plasma membrane of the cell and
  exert rapid and specific effects on metabolism
  and longer term effects gene transcription

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G-protein linked receptors e.g. adrenaline and
glucagon receptors
Activated!
Second Messenger
Effects
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Activation of Insulin Receptor
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Effects of Hormones

• In all cases activation of the receptor can lead
  to a cascade of related and consequential
  molecular events inside the cell.
• Events including generation of second messengers,
  changes in ion fluxes, activation or inhibition
  of protein kinases, activation or inhibition of
  transcription factors
• eventually lead to the regulation of the activity
  of key metabolic enzymes or other cellular
  function or changes in the level of transcription
  of genes coding for key proteins

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Intracellular Signalling Cascades
Signal Transfer
Signal Transformed and Relayed
Signal Amplified
Signal Diverges
Modulated Effect
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Signalling via the insulin receptor
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Mode of Action of Steroid and Thyroid Hormones

• These hormones are produced continuously
• They are not stored in the endocrine gland
• The hormones cross cell membranes readily
• Bind to receptors in the cytoplasm or nuclei of
  target cells
• These steroid-receptor complexes act as
  transcription factors regulating specific gene
  expression
• The effects of steroid and thyroid hormones
  usually take several hours to take effect

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Activation of Protein Kinases