Endocrine control mechanisms

• Describe the difference between endocrine, Paracrine and autocrine messengers

  • Endocrine: hormone is secreted into the circulatory system and targets a distant cell
  • Paracrine: hormone targets a nearby or adjacent cells
  • Autocrine: hormone targets the cell that it is produced by

  (Neurocrine: hormone released from the axon terminal and binds to the target cell)
• Describe the anterior-pituitary hormones

  FLATPiG

  • Follicle Stimulating Hormone (FSH)
  • Luteinising Hormone (LH)
  • Adrenocorticotropic hormone (ACTH)
  • Thyroid Stimulating Hormone (TSH)
  • Prolactin (PRL)
  • Growth Hormone (GH)

  PRL and GH are non-tropic hormones, which means they exert direct effects on tissues. The remaining hormones are tropic, which means that they exert their effects on primary endocrine glands that secrete hormones.
• Describe FSH and LH

  These are both synthesised, stored and secreted from gonadotropes within the anterior pituitary gland. FSH stimulates ovaries to produce oestrogen and testes to produce testosterone. LH has a role in ovulation and growth of the corpus luteum. LH stimulates androgen secretion by interstitial cells in the testes.
• Describe ACTH

  These are synthesised, stored and secreted from corticotropes in the anterior pituitary gland. ACTH stimulates the adrenal gland cortex to produce corticosteroids such as aldosterone and cortisol.
• Describe TSH

  This is synthesised, stored and secreted from the thyrotropes within the posterior pituitary gland. TSH stimulates the thyroid gland to produce thyroid hormones such as T3 and T4.
• Describe HPT axis in thyroid hormone secretion

  1. Low temperature stimulates the hypothalamus to release thyrotropin releasing hormone (TRH) via the long portal vessels.
  2. Then the anterior pituitary gland releases thyroid stimulating hormone (TSH).
  3. TSH can act tropically by stimulating the thyroid gland to release thyroid hormones; T3 and T4, or TSH can act non-tropically by directly stimulating tissues in the body.
  4. This leads to increased metabolism, which increase heat production.
• Describe the feedback loops in thyroid hormone secretion

  T3 acts on the anterior pituitary gland to inhibit TSH release. T3 also inhibits the hypothalamus from producing TRH, which is a longer feedback loop
• Describe the production of CRH and ACTH in the HPT-axis

  1. The hypothalamus secretes CRH, which stimulates the anterior pituitary corticotropes to release ACTH.
  2. This stimulates the adrenal gland to release cortisol.
• Describe the three feedback loops for CRH and ACTH

  Short feedback loop consists of cortisol inhibiting the release of ACTH from the pituitary gland. The long feedback loop consits of cortisol inhibiting the release of CRH from the hypothalamus. Finally, the shortest feedback loop consists of CRH extorting an autocrine effect by inhibiting its own secretion by the hypothalamus.
• Describe prolactin

  This is synthesised, stored and secreted by lactotropes in the anterior pituitary gland. Prolactin exerts effects on mammary tissue for lactation.
• Describe growth hormone

  This is synthesised, stored and secreted by somatotropes in the anterior pituitary gland. GH stimulates growth, cell reproduction and regeneration of specific cells.
• Describe the impact of dysregulation growth hormone

  • Hypersecretion of growth hormone causes gigantism in children. This is occurs in teens years before the bones fuse, and can therefore still grow.
  • Acromegaly: this occurs in adults and is characterised by enlargement of the head, hands, feet, etc. and leads to excessive sweating and visual field loss due to pressure on the optic nerve. Can be treated using somatostatin.
• Describe the causes of growth hormone deficiency

  This can be caused by insufficient hormone production as a result of a hypothalamic or pituitary tumour, mutation in growth hormone genes, or head injury. Furthermore, deficiency may be due to growth hormone resistance, which is due to mutations in growth hormone binding proteins or growth hormone receptors.
• Describe the function of the posterior pituitary hormones

  Magnocellular neurones in the hypothalamus connect with the posterior pituitary gland and synthesise and secrete oxytocin and vasopressin. These hormones diffuse into the capillary network of the posterior lobe.
• Describe the functions of vasopressin

  Vasopressin binds to receptors on cells in the collecting ducts of the kidney. This leads to synthesis and insertion of aquaporins into the kidney tubules, which promotes reabsorption of water back into the circulation. In the absence of vasopressin, the collecting ducts are virtually impermeable to water, which therefore flows out as urine.
• Descrice the different types of Diabetes insipidus

  1. Hypothalamic: this is due a deficiency in vasopressin secretion, which is due to head trauma, infections of tumours of the hypothalamus. This can be treated using exogenous vasopressin.
  2. Nephrogenic: this is when the kidney is insensitive to vasopressin, which is caused by renal disease, or mutations in the vasopressin receptor gene or the aquaporin-2 gene. This can be treated by increasing water consumption.