Principles of Endocrine Control

• Glands are epithelial tissue derivatives specialised for secreting
• Secretion is the biochemical release from a particular type of cell upon stimulation
• Endocrine - secretions enter bloodstream - ductless gland
• Exocrine - gland with ducts that channels its secretions
• What sort of gland is the pancreas?

  Both endocrine and exocrine
• Autocrine - signal released by cell binds to receptor on cell itself
• Paracrine - chemical messenger released by cell binds to receptor on nearby cell
• Cancer cells release growth peptides which stimulate themselves to grow and divide. What kind of cell to cell signalling is this?

  Autocrine
• Prostaglandins produced by cells close to the uterus stimulate uterine muscle cells to contract during menstrual cramps. What kind of cell to cell signalling is this?

  Paracrine
• Secretions from immune cells e.g. interleukins are usually autocrine or paracrine.
• Hormones are chemical messengers released from endocrine glands that enter the systemic circulation and travel to distant sites or organs.
• The production of testosterone in the testes is stimulated by FSH and LH from the pituitary gland, which travel to the testes in the systemic circulation. What kind of cell to cell signalling is this?

  Endocrine
• Neuroendocrine - hormones produced by glands in the central nervous system, for example the hypothalamus and pituitary.
• Neurotransmitters take messages from one neurone to another - they are not hormones.
• Hormones - chemical messengers secreted into the blood by endocrine glands in response to an appropriate signal, and exerting their effects on target cells that have receptors that bind with the hormone.
• Hormones can be classified based on solubility

  • hydrophilic (water soluble)
  • lipophilic (lipid soluble)
• Hormones can be classified based on their structure:

  • peptide - chain of specific amino acids
  • amines - derivatives of amino acids, tryptophan or tyrosine
  • steroids - cholesterol derived lipids
• Peptide hormones are produced by normal protein synthesis machinery. They travel in blood in solution.
• Peptide hormones cannot enter cells through the lipid bilayer cell membrane, so instead they bind to cell surface receptor and trigger event on inside surface of membrane via second messenger system.
• Peptide hormones e.g. insulin are fast acting.
• Steroids are produced by modification of cholesterol molecules by enzymes. They are not soluble in water so they travel in blood bound to plasma proteins.
• Steroid hormones can enter cells easily. They bind to an intracellular receptor, which acts on DNA to alter cell function.
• Steroid hormones e.g. testosterone are slow acting due to transcription delays.
• Contraceptives can be taken orally or given as a patch because they can be absorbed across membranes (e.g. GI tract) without breaking down.
• Insulin cannot be taken as an oral tablet as it will be broken down by enzymes in the gut into amino acids.
• Peptide hormones bind to outer cell surface receptors and induce conformational change. This stimulates the conversion of ATP to cyclic AMP inside the cell, which induces further events leading to cell response.
• Prolactin and growth hormone use JAK/STAT pathway via receptor/enzyme complex.
• Insulin uses tyrosine kinase pathway via receptor-enzyme complex.
• Adrenaline, noradrenaline, angiotensin II, ADH, GnRH and TRH use Ca2+ second messengers via G-protein coupled receptors
• Atrial natriuretic hormone and nitric oxide use cyclic GMP second messengers via G-protein coupled receptors
• Adrenaline, noradrenaline, glucagon, FSH, LH, TSH, calcitonin, parathyroid hormone and ADH use cyclic AMP second messengers via G-protein coupled receptors.
• Steroid hormone crosses lipid bilayer, binds to receptor and acts as transcription factor to stimulate production of mRNA which gets converted into protein.
• Regulatory role of endocrine system

  • metabolism, water and electrolyte balance
  • stress response
  • growth and development
  • reproduction
  • RBC production
  • coordination of circulation
  • coordination of digestion
• Thyroid function:

  • control of metabolic rate
  • increases heart rate
  • needed for normal growth and development
• Thyroid hormones

  • triiodothyronine (T3)
  • thyroxine (T4)
• Parathyroid function:

  • calcium metabolism
  • raises free plasma Ca2+
  • mobilises bone store
  • reduces urine loss
  • increases gut absorption
• Kidney endocrine function:

  • releases renin if circulating volume or BP is low - sodium is retained and volume restored
  • releases erythropoietin when hypoxic - increases red blood cell formation in bone marrow
• Kidney hormones:

  • renin
  • erythropoietin
• Adrenal cortex function:

  • metabolic response to stress
  • body sodium levels
• Adrenal cortex hormones:

  • cortisol (glucocorticoid)
  • aldosterone (mineralocorticoid)
  • DHEA