Endocrinology

• Nervous System

  Network of neurons that transmit signals along dedicated pathways
• Endocrine System

  The nervous and endocrine systems often overlap in function! Hormones are chemical signaling molecules that travel through the blood and reach every part of the body but only target cells have receptors that allow them to respond.
• Homeostasis


  Organisms use homeostasis to maintain a "steady state" or internal balance regardless of external environment
• Examples of homeostasis in humans

  • Body temperature
  • Blood pH
  • Glucose concentration
• Mechanisms of Homeostasis


  1. Fluctuations above or below a set point serve as a stimulus

  2. Detected by a sensor

  3. Trigger a response

  4. Response returns the variable to the set point
• Target cell recognition

  Stages of the second messenger model:

  The hormone itself is the first messenger.

  It binds to specific receptors on the cell-surface membrane of target cells.

  Forms a hormone-receptor complex.

  Activates an enzyme within the cell.

  Production of a chemical that acts as a second messenger.
• Diabetes Mellitus

  A group of disorders characterised by abnormalities of metabolism resulting from deficiency of or resistance to the action of insulin. Characterised by elevated levels of glucose in blood.
• Different forms of diabetes mellitus

  • Type 1 diabetic patient before and after insulin therapy
  • Type 2 diabetes mellitus
• Type 2 diabetes mellitus

  Combination of:

  insulin secretory defect and

  peripheral insulin resistance
• Insulin secretory defect


  May be the major factor in the genetic transmission of diabetes
• Hormone actions


  • Affect growth, development, metabolic activity and function of tissues
  • May be stimulatory or inhibitory
  • May act on several tissues or just one specific target tissue – major difference between endocrine and nervous system
  • Responsive tissues must have specific receptors for that hormone
• The hypothalamus secretes hormones that regulate the release of hormones from the anterior pituitary, such as GnRH (gonadotropin-releasing hormone), TRH (thyrotropin-releasing hormone), CRH (corticotrophin-releasing hormone), and GHRH (growth hormone-releasing hormone).
• The posterior pituitary gland is also known as the neurohypophysis.
• The anterior pituitary gland is also known as the adenohypophysis.
• BI0BF1/BI0BIO Foundation Biology An Introduction to Endocrinology Dr Andrew Bicknell [e-mail]
• Suggested Reading

  • Essential Endocrinology and Diabetes (Holt and Hanley)
  • Earlier version called Essential Endocrinology (Brook and Marshall)
• Learning outcomes for these lectures

  • Understand the term Endocrinology and why animals and humans need an Endocrine system
  • Define the term hormone and explain the different types
  • Understand the concepts of hormone regulation and hormone action
  • Be able to give an explanation of the pituitary gland and give an example of a system that the pituitary regulates
  • Be able to give a basic explanation of the regulation of glucose levels in man
• ENDOCRINE SYSTEM

  • BASIC CONCEPTS
  • ENDOCRINE SYSTEMS
• Why we need an endocrine system

  • Required for effective cell-cell communication in large, complex multicellular organisms (10^14 cells in human body!!)
  • Monitor and coordinate internal environment and make appropriate adaptive changes (homeostasis!)
  • Regulate growth, development, reproduction, senescence
  • Enable you to respond and adapt to changes in external environment (eg: food intake, water availability, temperature change, "stressors" etc)
• Hormones


  Chemical signaling molecules that travel through the blood and reach every part of the body but only target cells have receptors that allow them to respond
• Homeostasis

  Organisms use homeostasis to maintain a "steady state" or internal balance regardless of external environment
• Mechanisms of Homeostasis


  1. Fluctuations above or below a set point serve as a stimulus; these are detected by a sensor and trigger a response

  2. The response returns the variable to the set point
• Hormone


  A chemical messenger produced and secreted by a specialized endocrine gland that is transported in the bloodstream to a distant target organ/cell where it elicits a physiological response
• Classes of Hormones


  • Proteins / Peptides
  • Cholesterol Derivatives (Steroids, Vitamin D)
  • Modified Amino Acids (Adrenaline, Thyroid hormones)
• Non lipid-soluble hormones


  Cannot diffuse through the phospholipid bilayer, their action upon target cells must be carried out indirectly by binding to an extra-cellular receptor and triggering an intracellular signalling cascade "second messenger system"
• Lipid-soluble hormones


  Diffuse through the phospholipid bilayer, they bind to an intracellular receptor that directly activates target molecules (normally changes in gene expression)
• Hormone actions


  • Affect growth, development, metabolic activity and function of tissues
  • May be stimulatory or inhibitory
  • May act on several tissues or just one specific target tissue - major difference between endocrine and nervous system
  • Responsive tissues must have specific receptors for that hormone
• Factors influencing circulating hormone levels

  • Rate of secretion by endocrine gland
  • Rate of metabolism by target tissue, blood, liver, kidney
  • Serum binding proteins (transport in blood - solubility issue etc..)
• Regulation of Hormone Secretion

  • By physiological changes (e.g. blood glucose regulates insulin & glucagon release from pancreas, 'stress' regulates adrenaline release from adrenal medulla)
  • By endogenous rhythms (e.g. ultradian cycles in minutes, circadian daily cycles, infradian monthly cycles)
  • By Feedback Mechanisms (mostly negative "closed-loop" feedback to maintain homeostasis, some positive feedback)
• Main Types of Endocrine Dysfunction

  • Hyposecretion
  • Hypersecretion
  • Ectopic hormone secretion (hormone made outside normal endocrine gland e.g. Tumour)
• Causes of Hyposecretion


  • Genetic
  • Dietary
  • Immunological (auto-immune disease)
  • Cancer
  • Iatrogenic (doctor-caused)
  • Idiopathic
• Treatments for Hyposecretion


  • Hormone Replacement (Steroids - straightforward, absorbed in GI tract, high success)
  • Protein Hormones (difficult, must be injected, species specific)
• Causes of Hypersecretion


  • Functional tumour
  • Ectopic hormone-secreting tumour
  • Immunological
  • Substance abuse
• Treatments for Hypersecretion


  • Surgical removal of the gland that is hyper-secreting (but will then need replacement therapy!)
  • Irradiation of Gland - reduce function (but will then need replacement therapy!)
  • Drugs that block the actions of the hormone
• The endocrine system and nervous system are the two major control systems of the vertebrate body
• The pituitary gland is a major interface between the brain (nervous system) and the periphery, and plays a major role in coordinating the entire endocrine system
• What does the Hypothalamus/Pituitary regulate?

  • Growth
  • Metabolism
  • Response to stress
  • Reproduction
  • Lactation
  • Water and salt balance
  • Birth
  • Feeding behaviour
• Hormones of the Adrenal Gland

  • Secreted by the adrenal medulla: Adrenaline (epinephrine) - secretion in response to stress and exercise, increases heart rate
  • Secreted by adrenal cortex: Cortisol (Glucocorticoid) - essential for adaptation to stress, mobilizes glucose, anti-inflammatory actions
  • Aldosterone (Mineralcorticoid) - maintains balance of Na/K in blood
• Regulation of Thyroid Hormone Release

  1. Thyrotropin Releasing Hormone (TRH) from hypothalamus

  2. Thyroid Stimulating Hormone (TSH) from pituitary
• Effects of Thyroid Hormone

  • Increases basal metabolic rate (BMR), O2 consumption, CO2 production, heat production
  • Increases heart rate and force of contraction
  • Increases activity of nervous system
  • Enhances the sensitivity to other hormones
  • Essential for growth, maturation, embryo development, CNS development, linear growth