Unit 4

• Endocrine Physiology


  The study of the endocrine system and its functions
• Endocrine system


  A collection of glands that coordinate multiple organs/body systems by secreting hormones

  
• Major processes regulated by the endocrine system

  • Reproduction
  • Growth and Development
  • Electrolyte, water, nutrient balance of blood
  • Cellular metabolism and energy balance
  • Immune defenses
• The endocrine system is a principal defender of homeostasis
• Direct communication


  Exchange of ions and molecules between adjacent cells across gap junctions, occurs between two cells of the same type
• Paracrine communication


  Local acting chemicals that affect cells other than those that secrete them
• Autocrine communication


  Messages affect the same cells that secrete them, chemicals involved are autocrines
• Endocrine communication


  Endocrine cells release chemicals (hormones) that are transported in bloodstream and alter metabolic activities of many organs
• Target cells


  Have receptors needed to bind and "read" hormonal messages

  
• Hormones


  Chemical messengers that influence the function of target cells by changing types, quantities or activities of enzymes and structural proteins

  
• Hormone receptors


  Recognise and bind with high affinity to their particular hormones, and initiate a signal to appropriate intracellular effectors

  
• Characteristics of hormone secretion patterns


  • Circadian or diurnal patterns (e.g. Melatonin)
  • Pulsatile and cyclic patterns (e.g. Insulin and glucagon)
  • Patterns that depend on levels of circulating substrates (e.g. calcium, sodium, potassium, or the hormones themselves)

  
• Hormones


  • Operate within feedback systems (positive or negative), to maintain an optimal internal environment
  • Affect cells with appropriate receptors and act on those cells to initiate specific cell functions or activities
  • Steroid hormones are either excreted directly by the kidneys or metabolised by the liver
  • Peptide hormones are catabolised by circulating enzymes and eliminated in the faeces or urine

  
• Classification of hormones


  • Hormone structure
  • Gland of origin
  • Hormone effects

  
• Hormone structure


  • Amino acid derivatives
  • Peptide hormones
  • Lipid derivatives
• Amino acid derivatives


  Small molecules structurally related to amino acids, including thyroid hormones, catecholamines, serotonin and melatonin
• Peptide hormones


  Chains of amino acids, including glycoproteins, short-chain polypeptides, and small proteins
• Lipid derivatives


  Eicosanoids and steroid hormones derived from cholesterol
• Water-soluble hormones (peptides) generally have a short half-life of seconds to minutes because they are catabolised by circulating enzymes
• Lipid-soluble hormones such as cortisol and adrenal androgens are transported bound to a carrier protein and remain in the blood for hours to days
• Hormone receptor


  A protein molecule to which a particular hormone binds strongly, presence or absence of a specific receptor determines hormonal sensitivity of a cell
• Hormone receptor types


  • Catecholamine and peptide hormone receptors (extracellular receptors)
  • Steroid and thyroid hormone receptors (intracellular receptors)
• First messenger


  Hormone that binds to extracellular receptor and promotes release of second messenger in cell

  
• Second messenger


  Intermediary molecule that appears due to hormone-receptor interaction, e.g. cAMP, cGMP, Ca2+

  
• G Proteins and cAMP


  1. Activated G protein activates adenylate cyclase

  2. Adenylate cyclase converts ATP to cyclic AMP

  3. Cyclic AMP functions as a second messenger

  
• Intracellular hormone receptors


  Steroid hormones can alter rate of DNA transcription in nucleus, thyroid hormones bind to receptors within nucleus and on mitochondria

  
• Biological response to hormones


  • Depends on blood levels of the hormone, concentration of target cell receptors, affinity of the receptor to hormone, pH, temperature
• Synergy


  When more than one hormone produces the same effect
• Antagonists


  When hormones work against each other
• Permissive


  When one hormone cannot exert its effect without another hormone being present
• Feedback mechanisms


  Hormones operate within feedback systems, either positive or negative, for homeostasis
• Hormone metabolism and excretion

  • The liver and kidneys are the major organs that remove hormones from the plasma
  • Peptide hormones and catecholamines are rapidly removed from the blood
  • Steroid and thyroid hormones are removed more slowly, in part because they circulate bound to plasma proteins
  • Some hormones are metabolised to more active molecules in their target cells or other organs

  
• Endocrine functions of Hypothalamus

  Releasing hormones, mechanism of hormone action

  
• Anterior pituitary hormones


  • Functions
  • Regulation

  
• Posterior pituitary hormones


  • ADH
  • Oxytocin - functions, regulation

  
• Thyroid hormones


  • Synthesis
  • Functions
  • Fate
  • Regulation
  • Disorders
• Link between nervous and endocrine systems


  Nervous and endocrine systems work together to control the body processes
• Nervous system


  • Short term quick response
  • Send signals through neurotransmitters via nerves
  • Fleeting - go away fast
  • Localized/specific
• Endocrine system


  • Long term slow response
  • Send signals through hormones via bloodstream
  • Longer lasting
  • Body-wide/general
• Major Endocrine Glands


  • Pituitary gland
  • Hypothalamus
  • Pineal Gland
  • Thyroid gland
  • Parathyroid glands
  • Adrenal Glands
  • Pancreas (endocrine portion)
  • Thymus
  • Gonads (ovaries and testes)