Lecture 8

• Three main types of hormones

  • Peptide/Protein
  • Steroid
  • Amine

  
• Hydrophilic hormones


  Water soluble, can dissolve in plasma, not lipid soluble (lipophobic), cannot cross plasma membranes, examples: peptide hormones, protein hormones and catecholamines
• Hydrophobic hormones


  Not water soluble, do not dissolve in plasma, lipid soluble (lipophilic), readily cross plasma membrane, examples: steroid and thyroid hormones

  
• Peptide/Protein hormone synthesis, packaging and release

  1. Signal sequence + hormone (1+ copies) + cleaved peptide fragments

  2. Post-translational processing produces biologically active peptide

  3. Single preprohormone can contain: several copies of the same hormone, one than one type of hormone

  4. Active peptides released depends on specific proteolytic processing enzymes

  
• Insulin


  Formation of disulfide bonds

  
• Steroid hormones


  Synthesized only from cholesterol, made on demand, not stored in vesicles, released from cell by simple diffusion, water insoluble (bound to carriers in blood), long half life, diffuse into target cells or taken up by endocytosis of steroid hormone carrier proteins, cytoplasm or nucleus receptors (but can also act on plasma membrane receptors)

  
• Amine hormones


  Synthesized only from tryptophan or tyrosine, tryptophan derivative: melatonin (behaves like peptides or steroids), tyrosine derivatives: catecholamines (behave like peptides), thyroid hormones (behave like steroids)

  
• Melatonin


  Darkness hormone, secreted at night (sleep), made in pineal gland (also gi tract, leukocytes, other brain regions), diverse effects: transmits information (light-dark cycles to govern the biological clock), immune modulation, anti-oxidant
• Synthesis of catecholamines


  Synthesized in adrenal medulla (mainly in cytosol), stored in vesicles prior to release, released via exocytosis, lipophobic, water soluble, bind to membrane receptors (adrenaline)

  
• Control of hormone release

  Endocrine cells directly sense stimuli, then secrete the hormone

  
• Stimuli triggering hormone release


  Act through intracellular pathways to: change the membrane potential, increase free cytosolic Ca2+, change enzymatic activity, increase the transport of hormone substrates into the cell, alter transcription of genes coding for hormones or for enzymes needed for hormone synthesis, promote survival and in some cases growth of the endocrine cell

  
• Hormones released from the hypothalamus and anterior pituitary regulate the release of several hormones

  
• Hormone interactions


  Most cells sensitive to more than one hormone and exhibit interactive effects, synergistic effects: multiple hormones act together for greater effect, permissive effects: one hormone enhances the target organ's response to a second later hormone, antagonistic effects: one hormone opposes the action of another