Calcium homeostasis

• Calcium Homeostasis


  The regulation of calcium levels in the body
• Phosphate Homeostasis


  The regulation of phosphate levels in the body
• Principal regulators of Calcium Homeostasis

  • Parathyroid hormone (PTH)
  • Vitamin D
• Other hormones influencing Calcium Homeostasis

  • Calcitonin
  • Prolactin
  • Growth Hormone
  • Insulin
  • Thyroid Hormone
  • Glucocorticoids
  • Sex Hormones
• Other factors influencing Calcium Homeostasis

  • Calcium
  • Phosphate
  • Na
  • F
  • Diuretics
  • Plicamycin
  • Diphosphates
• Plicamycin


  An antineoplastic antibiotic that can also be used to treat hypercalcemia
• Parathyroid hormone (PTH)


  Essential to life; synthesized by chief cells in the parathyroid glands
• Low blood calcium


  Triggers PTH release
• PTH actions are aimed at raising calcium levels back to their normal physiological concentration
• Restoration of blood calcium levels

  Inhibits PTH production
• Low plasma calcium


  Triggers PTH secretion
• Sites responding to PTH to increase calcium levels

  • Kidneys
  • Bones
  • Intestines
• An excess of PTH causes hypercalcaemia
• PTH deficiency results in hypocalcaemia
• Other causes of hypocalcaemia can cause a rise in PTH
• Excess PTH always results in decalcified, weak bones
• Actions of PTH on the kidneys

  1. Increase calcium ion reabsorption

  2. Increase phosphate ion excretion

  3. Stimulation of 1α-hydroxylase
• Actions of PTH on the bones

  1. Inhibition of osteoblast collagen synthesis

  2. Stimulation of osteoclast bone erosion

  3. Decreased collagenase synthesis

  4. Increased hydrogen ion release
• Actions of PTH on the intestines

  1. Direct action on calcium absorption

  2. Indirect action through activation of vitamin D
• Vitamin D


  Absorbed by the small intestine or synthesized from cholesterol in the skin
• Vitamin D synthesis requires ultraviolet (UV) light
• Vitamin D


  Raises blood calcium and phosphate levels mainly through its actions on the intestines
• Cholecalciferol


  Inactive steroid form of vitamin D (vitamin D3)
• Vitamin D is stored in adipose tissue
• Activation of Vitamin D

  Two reactions in liver and kidney
• Vitamin D can be inactivated by 24-hydroxylase in the kidney
• Vitamin D transport


  Transported in blood by specific plasma protein or within chylomicrons
• Vitamin D action


  Acts via specific intracellular receptors found in the same locations as PTH receptors
• Actions of activated Vitamin D on the kidney

  1. Increase calcium reabsorption

  2. Increase phosphate reabsorption

  3. Inhibition of 1α-hydroxylase activity
• Activated Vitamin D increases both calcium and phosphate levels
• PTH increases calcium but decreases phosphate
• Actions of Vitamin D on the bones

  Stimulates osteoblast activity to increase bone mass and calcification
• Disorders of vitamin D absorption or activation can cause weak bones
• Rickets in children or osteomalacia in adults can result from vitamin D disorders
• Actions of Vitamin D on the intestines

  Stimulates active calcium and phosphate absorption
• The exact mechanism of vitamin D action on intestines is unclear
• Vitamin D increases the synthesis of calcium-binding proteins in intestinal cells
• Calcitonin


  Secreted by parafollicular cells in the thyroid gland; lowers calcium levels
• Calcitonin is not essential to life
• Calcitonin fine-tunes blood calcium levels