Endocrine Transes

• Chemical messenger


  Any substance produced by a cell and plays a physiological role in the control of the activity of another cell
• Hormone


  • Any substance elaborated by one cell to regulate another cell
  • Serve as the body's chemical messengers
• Tropic Hormone


  Hormone which regulates the secretions of other endocrine tissues or organs
• Neurohormone


  Hormone produced by a nerve cell
• Neuropeptide


  Peptidergic neurohormone (i.e. a neurohormone comprised of chains of amino acids)
• Neuromodulator


  Hormone that modulates the response of a neuron to a neurotransmitter or another hormone
• Neuroregulatory


  Generalized term for any neurohormone that acts as a neurotransmitter or neuromodulator
• Pheromones


  Chemical messengers released to the exterior of an animal to stimulate a response in another member of the same species
• Lumones


  Chemical messengers released into the lumen of the gastrointestinal tract
• Chalones


  • Putative cellular mitotic inhibitors
  • Inhibit the proliferation of immature cells
• Growth factors


  • Mitogenic substances
  • Can affect the growth of cells that promote or inhibit mitosis
• The endocrine systems involve secretory cells which lack outflow ducts
• Half-life of a hormone

  Duration of time required to decrease the concentration of a circulating hormone by half
• Oxytocin is a peptide hormone with a half-life of a few hours
• Growth hormone can last several days to several months, depending on the organisms and amount of growth hormone released
• Factors influencing hormone concentrations in the blood

  • Rate of hormone synthesis & secretion from the source gland into the blood
  • Rate of removal of the hormone from the blood
• Ways hormones are cleared from the plasma

  • Metabolic destruction
  • Binding with the tissues
  • Excretion by the liver into the bile
  • Excretion by the kidneys into the urine
• Endocrine mode of hormone delivery

  • Source cell: endocrine gland (usually epithelial)
  • Hormones are released by the endocrine gland and into the bloodstream such that they can be transported to the target tissue
• Neuroendocrine mode of hormone delivery

  • Source cell: nerve cell
  • Neurohormones are released into the bloodstream for transmission
• Neurocrine mode of hormone delivery

  • Source cell: neuron
  • Target cell: neuron
  • Exhibited via chemical synapses
  • Neurotransmitters are the chemical messengers being delivered from the presynaptic cell to the postsynaptic cell
• Paracrine mode of hormone delivery


  The chemical messengers are released into the extracellular environment surrounding the source cell wherein they will be received by the target cells adjacent to the source
• Autocrine mode of hormone delivery

  • Target cell = source cell
  • The cell membrane has a receptor for its own chemical signal
  • A feedback mechanism regulates the source cell
• Synthesis of Eicosanoids

  Arachidonic acid (results from the chemical activity of phospholipase A2 in the phospholipid bilayer) can be acted upon by:

  1. Lipoxygenase (gives rise to Leukotrienes)

  2. Cyclooxygenase (gives rise to Prostaglandins)
• Eicosanoids


  • Local hormones derived from arachidonic acid
  • Unique roles in innate immune responses
• Peptide Hormones


  • Comprised of a chain of amino acids
  • Comprise most autocrine and paracrine regulators
• Steroid Hormones


  Characterized by the presence of a 4-carbon group attached to an R-chain
• Amines


  Characterized by the presence of an amine group (NH2)
• Thyroid Hormones/ Iodothyronine


  • Synthesized in the thyroid gland
  • Red lines and asterisks signify the amount of iodothyronine attached to the hormone

  
• Mechanism of action of amines or peptides (hydrophobic)

  • Can't cross the hydrophobic plasma membrane of target cells
  • Instead bind to receptor proteins on the plasmalemma and are transported (receptor-hormone complex)
• Receptor Signal Transduction


  Recognition: Hormone binds to its receptor on the plasmalemma, causing a conformational change

  2. Transduction: Ligand-receptor complex activates a nucleotide regulatory protein (e.g. G-protein)

  3. Action of second messenger (e.g. cAMP): cAMP diffuses into cytoplasm and binds with a specific cyclic nucleic-dependent kinase
• Activation


  1. Recognition

  2. Transduction

  3. Action of second messenger (ex. cAMP)
• Hormone binding to receptor

  Causes a conformational change in the protein
• Receptor


  May release another chemical signal or bind to another cellular component
• Ligand-receptor complex


  Activates a nucleotide regulatory protein
• 1. protein


  A tertiary complex with three subunits: alpha, beta, and gamma
• 1. protein activation


  1. One protein subunit (like alpha) dissociates and diffuses within the membrane while carrying ATP

  2. This binds to adenylyl cyclase (an enzyme)

  3. The activated regulatory protein interacts with adenylate cyclase → catalyzes cAMP from ATP
• Action of second messenger (ex. cAMP)

  1. Formation of a catalytic subunit: cAMP diffuses into cytoplasm and binds with a specific cyclic nucleic-dependent kinase like kinase-A (regulatory and catalytic components)

  2. cAMP + regulatory subunit interaction releases the catalytic subunit which is now free to function as a kinase

  3. The kinase subunit phosphorylates proteins

  4. Leads to a cascade phenomenon that mediates a particular cellular response

  5. Cellular activity is returned to basal levels
• cAMP


  A second messenger since it's a different molecule from the first ligand that activated the cell, and serves to amplify the message of that ligand
• Steroid and thyroid hormones

  Lipophilic (fat-soluble)
• Mechanism of action for lipophilic hormones

  1. Travel through the bloodstream attached to steroid-binding proteins

  2. Receptors of target cells aren't in the lipid bilayers

  3. Lipid-soluble hormones dissociate from protein carriers and diffuse through the cell membrane

  4. Hormones bind to specific cytosolic receptors and form a hormone-receptor complex

  5. The complex binds to specific regions on genes/DNA (particularly in the nucleus)

  6. mRNA transcription is initiated and gene expression is controlled