Chapter 18: The Endocrine System

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Chapter 18 The Endocrine System
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Endocrine System

• Regulates long-term processes
• growth
• development
• Reproduction
• Uses chemical messengers to relay information and
  instructions between cells

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What are the modes of intercellular communication
used by the endocrine and nervous systems?
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Direct Communication

• Exchange of ions and molecules between adjacent
  cells across gap junctions
• Occurs between 2 cells of same type
• Highly specialized and relatively rare

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Paracrine Communication

• Chemical signals transfer information from cell
  to cell within single tissue
• paracrine factors may enter blood stream and
  function as hormones
• Most common form of intercellular communication

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Endocrine Communication

• Endocrine cells
• release chemicals (hormones) into bloodstream
• Alters metabolic activities of many tissues and
  organs simultaneously
• Target Cells
• possess receptors needed to bind and read
  hormonal messages

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• Hormones
• Stimulate synthesis of enzymes or structural
  proteins
• Increase or decrease rate of synthesis
• Turn existing enzyme or membrane channel on or
  off

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Synaptic Communication

• Releases neurotransmitter at a synapse that is
  very close to target cells
• Ideal for crisis management

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What are the cellular components of the endocrine
system?
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• Includes all endocrine cells and tissues that
  produce hormones or paracrine factors
• Endocrine Cells
• Glandular secretory cells that release their
  secretions into extracellular fluid
• Exocrine Cells
• Secrete their products onto epithelial surfaces

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Endocrine System
Figure 181
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What are the major structural classes of hormones?
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Hormones

• Can be divided into 3 groups
• amino acid derivatives
• peptide hormones
• lipid derivatives

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Amino Acid Derivatives

• Small molecules structurally related to amino
  acids
• Synthesized from the amino acids tyrosine and
  tryptophan

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• Tyrosine Derivatives
• Thyroid hormones
• Catacholamines epinephrine (E), dopamine
  norepinephrine (NE)
• Tryptophan Derivative
• Melatonin produced by pineal gland

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Peptide Hormones

• Chains of amino acids
• Synthesized as prohormones
• inactive molecules converted to active hormones
  before or after secretion
• ADH, prolactin, oxytocin, growth hormone

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Lipid Derivatives

• Eicosanoids
• important paracrine factors
• Leukotrienes, Prostaglandins
• Steroid hormones
• derived from cholesterol
• androgens (estrogens, and progestins)
  corticosteroids, calcitriol

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What are the general mechanisms of hormonal
action?
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• Receptors are protein molecules to which a
  particular molecule binds strongly
• Different tissues have different combinations of
  receptors
• Presence or absence of specific receptor
  determines hormonal sensitivity

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• Hormones bind to receptors in cell membrane
• Cannot have direct effect on activities inside
  target cell

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• Hormones indirectly
• alter cellular operations by altering protein
  composition and activity
• stimulate protein synthesis
• activate proteins
• modulate current levels of protein synthesis

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• First messenger
• leads to second messenger (cAMP, cGMP, Ca)
• A single hormone promotes release of more than 1
  type of second messenger

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• Down-regulation
• Presence of a hormone triggers decrease in number
  of hormone receptors
• Up-regulation
• Absence of a hormone triggers increase in number
  of hormone receptors

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G Protein

• Enzyme complex coupled to membrane receptor
• Involved in link between first messenger and
  second messenger
• Binds GTP
• Activated when hormone binds to receptor at
  membrane surface

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• Activated G protein
• activates enzyme adenylate cyclase
• Adenylate cyclase
• converts ATP to cyclic-AMP
• Cyclic-AMP (second messenger)
• activates kinase
• Activated kinases affect target cell
• depends on nature of proteins affected

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• Activated G proteins trigger
• opening of calcium ion channels in membrane
• release of calcium ions from intracellular stores

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• Activated G protein stimulates phosphodiesterase
  (PDE) activity
• Inhibits adenylate cyclase activity
• Levels of cAMP decline
• cAMP breakdown accelerates cAMP synthesis is
  prevented

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G Proteins and Hormone Activity
Figure 183
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Steroid Hormones

• Cross cell membrane
• Bind to receptors in cytoplasm or nucleus,
  activating or inactivating specific genes

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Steroid Hormones
Figure 184a
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Steroid Hormones

• Alter rate of DNA transcription in nucleus
• change patterns of protein synthesis
• Directly affect metabolic activity and structure
  of target cell

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How are endocrine organs controlled?
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Hypothalamus

• Integrates activities of nervous and endocrine
  systems in 3 ways
• Secretes regulatory hormones
• special hormones control endocrine cells in
  pituitary gland

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• Acts as an endocrine organ
• manufactures hormones released into the
  bloodstream via the posterior pituitary
• Contains autonomic centers
• exert direct neural control over endocrine cells
  of adrenal medullae

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Hypothalamus
Figure 185
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Where is the pituitary gland located, and what
is its relationship to the hypothalamus?
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Pituitary Gland

• Hangs inferior to hypothalamus
• connected by infundibulum
• Releases 9 important peptide hormones
• Hormones bind to membrane receptors
• use cAMP as second messenger

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Pituitary Gland
Figure 186
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What are the hormones produced by the anterior
lobe, and what are the functions of those
hormones?
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Anterior Pituitary

• Thyroid-Stimulating Hormone (TSH)
• Also called thyrotropin
• Triggers release of thyroid hormones
• hypothalamic regulatory hormone
• Thyrotropin Releasing Hormone (TRH)

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• Adrenocorticotropic Hormone (ACTH)
• Also called corticotropin
• Stimulates release of steroid hormones by adrenal
  cortex
• hypothalamic regulatory hormone
• Corticotropin Releasing Hormone (CRH)

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• Prolactin (PRL)
• Also called mammotropin
• Stimulates development of mammary glands and milk
  production
• hypothalamic regulatory hormone
• prolactin-inhibiting hormone (PIH)

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• Growth Hormone (GH)
• Also called somatotropin
• Stimulates cell growth and replication
• hypothalamic regulation
• growth hormonereleasing hormone (GHRH)
• growth hormoneinhibiting hormone (GHIH)

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• Melanocyte-Stimulating Hormone (MSH)
• Also called melanotropin
• Stimulates melanocytes to produce melanin
• Inhibited by dopamine

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• Gonadotropins
• Regulate activities of gonads (testes, ovaries)
• hypothalamic regulation
• gonadotropin-releasing hormone (GnRH)
• GnRH production inhibited by estrogens,
  progestins, and androgens

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• Follicle-Stimulating Hormone (FSH)
• Also called follitropin
• Stimulates follicle development and estrogen
  secretion in females
• Stimulates sustentacular cells in males
• promotes physical maturation of sperm
• Production inhibited by inhibin released by
  testes and ovaries

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• Luteinizing Hormone (LH)
• Also called lutropin
• Causes ovulation and progestin production in
  females
• Causes androgen production in males

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What hormones are secreted by the posterior lobe,
and what are their functions?
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• Antidiuretic Hormone
• Decreases amount of water lost at kidneys
• Elevates blood pressure
• Release inhibited by alcohol

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• Oxytocin
• Stimulates contractile cells in mammary glands
• Stimulates smooth muscles in uterus

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Summary The Hormones of the Pituitary Gland
Figure 189