Calcium homeostasis

• Hypocalcaemia = low serum calcium < 2.2 mmol/L
• Normal plasma calcium = 2.2 - 2.6 mmol/L
• Hypercalcaemia = high plasma calcium > 2.6 mmol/L
• PTH causes the rapid release of Ca ions from bone fluid into plasma during acute hypocalcaemia but during chronic hypocalcaemia it causes bone resorption
• During hypercalcemia, parafollicular cells of the thyroid gland sense the high plasma concentration and release their secretory vesicles containing calcitonin. Calcitonin inhibit osteoclastic activity thereby reducing calcium release into the blood
• During hypocalcemia, the parathyroid glands secrete PTH. PTH causes quick uptake of calcium from bone fluid into the blood and bone resorption. PTH also causes the excretion of phosphate ions through urine and retention of calcium ions in blood. In the kidney PTH stimulates the production of 1-alpha hydroxylase which in turn converts prohormone vitamin D to active form vitamin D. Active vitamin D stimulates calcium uptake in the GI tract.
• In the kidney PTH stimulates the production of 1-alpha hydroxylase which in turn converts prohormone vitamin D to active form vitamin D. Active vitamin D stimulates calcium uptake in the GI tract.
• • Vitamin D synthesis begins in skin cells known as keratinocytes. Exposure to UVB radiation causes these cells to convert cholesterol into Vitamin D3
  • Vitamin D3 is carried in the bloodstream to the liver, where it is converted into the prohormone calcidiol.
  • Undergoes chemical modification in kidneys to become active = calcitriol. PTH increases the levels of the enzyme required for this process.
• Principal site of action of vitamin D3 is the intestine, where it increases absorption of calcium into the plasma
• • Vitamin D synthesis begins in skin cells known as keratinocytes. Exposure to UVB radiation causes these cells to convert cholesterol into Vitamin D3
  • Vitamin D3 is carried in the bloodstream to the liver, where it is converted into the prohormone calcidiol.
  • Undergoes chemical modification in kidneys to become active = calcitriol. PTH increases the levels of the enzyme required for this process.
• Principal site of action of vitamin D is the intestine, where it increases absorption of calcium into the plasma
• Causes of hypocalcaemia

  • hypoparathyroidism (complications form thyroidectomy)
  • chronic kidney disease
  • hyperventilation
• Effects of Hypocalcaemia

  • muscle spasms (tetany) - Trousseau's sign
  • cardiac arrhythmia or depression
  • paraesthesia
• Causes of hypercalcaemia

  • hyperparathyroidism
  • multiple myeloma
• Effects of hypercalcaemia

  • stones, bones, groans, moans and psychiatric overtones
• Fish calcitonin (miacalcin) is 10 times more potent than human calcitonin and is prescribed for osteoporosis. The mode of action of this drug is exactly the same as calcitonin.
• Calcitonin's main target is bone, where it can inhibit the activity of osteoclasts
• Disorders in calcium metabolism ususally arise from either too much or too little PTH or a deficiency in vitamin D.
• PTH is essential to life - in its absence, death occurs rapidly due to respiratory arrest (defective neuromuscular junction activity).