The Endocrine System

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The Endocrine System

• General aspects
• Major endocrine organs
• Chemistry of hormones
• Target cell specificity
• Mechanisms of hormone action
• Control of hormone release
• The Pituitary and the Thryroid gland
• Other endocrine glands

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Midterm Results
Average 37.2
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Major Endocrine Organs

• Endocrine - to secrete within
• ductless glands that release hormones into the
  surrounding tissue fluid
• - rich vascular and lymphatic drainage

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Chemistry of Hormones

• Amino-acid Based Hormones
• Hydrophilic, so they are unable to cross the
  cell membrane
• - Interact with receptors found in the plasma
  membrane

Amino acid derivatives thyroxin, epinephrine,
norepinephrine (catecholamines) Peptides
glucagon Proteinsgrowth hormone, insulin
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Chemistry of Hormones
Steroids derived from cholesterol - hydrophobic,
enters plasma membrane easily
Adrenal cortex hormones mineralocorticoids,
glucocorticoids Gonadal steroids estrogen,
progesterone, testosterone Eicosanoids Derived
from arachidonic acid (membrane lipid)
leukotrienes and prostaglandins
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Target Cell Specificity
Receptors - specialized proteins associated
with target cells that bind to specific
hormones - dynamic structure (can be up- or
down-regulated) Level of activation of the
target cell depends on - concentration of
hormone - of receptors expressed in target
cell - affinity of hormone and receptor for each
other
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Mechanisms of Hormone Action

• Effect of hormone depends on the type of
  target cell hormones may
• Affect membrane permeability
• Stimulate protein synthesis
• Modulate enzyme activity
• Induces secretion of particular compounds
• Stimulate mitosis (cells to divide)
• Hormones act like signals
• 2nd Messenger Systems
• Direct Gene Activation

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2nd Messenger Systems
Used by protein peptide hormones i.e.
lipophobic cannot diffuse into cell
Signalling effect of hormone is through
intracellular 2nd messengers (inside the
cell) e.g. cyclic AMP (cAMP), (cGMP, IP3 )
2nd messengers activated by hormone binding to
plasma membrane receptors Great amplification
effect one 1st messenger ? many 2nd messengers
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2nd Messenger Systems cAMP
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2nd Messenger Systems IP3
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Direct Gene Activation
Thryroxin!
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Control of Hormone Release
1) Humoral Activated by content of blood 2)
Neural Direct stimulation by the nervous
system 3) Hormonal In response to hormones from
other endocrine glands Provides a mechanism for
positive and negative feedback loops
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Control of Hormone Release
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inhibitory hormones
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Pituitary Gland (Hypophysis)

• Anterior Pituitary (adenohypophysis)
• produces hormones in response to hormones
  produced in the hypothalamus
• Posterior Pituitary (neurohypophysis)
• secretes hormones produced in the hypothalamus

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Anterior Pituitary (adenohypophysis)

• Growth Hormone - increased growth in bones and
  muscles

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Anterior Pituitary (adenohypophysis)

• Hormones secreted by the anterior pituitary
• Growth Hormone (GH) increased growth in bones
  and muscles
• Thyroid-Stimulating Hormone (TSH) activates
  thyroid gland
• Adrenocorticotropic Hormone (ACTH) - causes
  adrenal cortex to release corticosteroids
• Gonadotropins (FSH, LH) regulate gonads, both
  endocrine and reproductive functions
• Prolactin- (PRL) Stimulates gonads and milk
  production

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Posterior Pituitary (Neurohypophysis)

• Hormones secreted by the posterior pituitary
• Oxytocin
• uterine contraction, milk letdown, orgasm,
  bonding
• Vasopressin (antidiuretic hormone, ADH)
• water balance, vasoconstriction
• reproductive behaviors, aggression

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The Thyroid Gland

• Thyroxine (T4) primary hormone secreted from
  thyroid
• Triiodothyronine (T3) more active form of the
  hormone
• -both hormones are combinations of thyroglobulin
  and iodine
• Calcitonin
• -produced in the parafollicular cells
• -lowers blood calcium levels by inhibiting
  osteoclast activity

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Biosynthesis of Thyroid Hormones
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Effects of TH
- The Thyroid Hormone affects all of the body
except the adult brain, spleen, testes, uterus,
and thyroid gland. - Primary effect is to
stimulate enzymes concerned with glucose
oxidation increasing basal metabolic rate.
Increase expression of sodium/potassium ATPase
pumps
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Effects of TH
- Increases adrenergic receptors in blood
vessels, helping to maintain blood pressure - TH
is also involved in controlling tissue growth and
development
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Other endocrine organs Parathyroid glands
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Other endocrine organs Adrenal Cortex

• Mineralcorticoids - aldosterone
• Water / ion balance
• Glucocorticoids - cortisol
• Stress Response
• Gonadocorticoids - primarily androgens
• May help with onset of puberty

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Other endocrine organs Pancreas

• Glucagon - increase glucose in blood
• glycogen to glucose
• gluconeogenesis
• glucose into blood from liver
• Insulin - decreases glucose in blood
• enhances glucose uptake in fat and muscle
• inhibits breakdown of glycogen glucose to fat

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Other endocrine organs Pancreas
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Other important endocrine organs

• Gonads ovaries ?estrogen, progesterone
• testes ? testosterone
• Pineal gland melatonin ? biological clock
• Thymus thymopoietins, thymosins ? T-cell
  development

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Other lesser sources of hormone signals

• Heart atrial natriuretic peptide (ANP), reduces
  blood volume, pressure and sodium by signaling
  the kidneys to increase urine production and by
  inhibiting aldosterone release
• Gastrointestinal tract enteroendocrine cells
• Placenta influence course of pregnancy
• Kidneys erythropoietin ? RBC production
• Skin cholecalciferol ? vitamin D3
• Adipose tissue leptin ? control of appetite

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The endocrine organs work together Control of
feeding behavior and satiety (see pg 983)
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Endocrine dysfunctions Adenomas and
adenocarcinomas (see p147) Diabetes
mellitus polyuria, polydipsia,
polyphagia Hyperinsulism and Hypoglycemia Gigantis
m, acromegaly and pituitary dwarfism Myxedema,
endemic goiter, cretinism and Graves
disease Cushings disease and Addinsons
disease Xenoestrogens