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What are the three hormones involved in calcium homeostasis?
- Parathyroid hormone (PTH)
- Calcitonin (CT)
- Vitamin D
Where does parathyroid hormone come from? What affect does it have on Ca2+ concentration?
- The parathyroid glands (which are located around the thyroid gland but are embryologically distinct)
- It increases calcium concentration
Where does calcitonin come from? What affect does it have on Ca2+ concentration?
- Calcitonin is secreted by other cells within the thyroid gland called "C" cells (which are also embryologically different from the thyroid gland)
- It decreases calcium concentration
What other electrolyte plays an indirect role in Ca2+ homeostasis?
What is the problem with phosphate?
Phosphate forms an insoluble complex with calcium ions - calcium phosphate. It the phosphate concentration is allowed to increase it removes calcium from solution. The dietary uptake of phosphate cannot be limited - it is continually absorbed from the diet which means it must be continually excreted.
Phosphate excretion is a key renal process regulated by which two hormones?
Parathyroid hormone and calcitonin
What stimulates parathyroid cells to secrete PTH?
Low calcium ion concentration or increased phosphate concentration
What are the four main actions of PTH?
- PTH quickly activates osteocytes and osteoblasts to transfer calcium into the blood.
- PTH slowly stimulates osteoclast cells for bone reabsorption and overall bone remodelling
- PTH acts on the kidney proximal convoluted tubule to decrease reabsorption of phosphate and increase reabsorption of calcium
- PTH increases activation of vitamin D, thereby promoting increased absorption of dietary calcium
What is the main stimulus for calcitonin release?
High plasma calcium concentration
What are the three main actions of CT?
- CT antagonises PTH and reabsorption of calcium from bone
- CT blocks renal calcium absorption and promotes renal phosphate wastage
- Ct blocks vitamin D so inhibits gut calcium absorption
How is vitamin D formed and what role does it play in calcium homeostasis?
- Vitamin D is formed in skin by ultraviolet light.
- It promotes intestinal calcium absorption and calcium deposition in bone
What are the two main disorders of calcium regulation?
- Hypocalcaemia (calcium deficiency)
- Hypercalcaemia (excess of calcium)
What is a common dietary problem that leads to hypercalcaemia in dairy cows?
If the dietary calcium is high leading up to parturition and lactation this will result in high plasma calcium concentrations and will inactivate PTH (and activate CT). At the time of parturition the cow will normally go off her feed. The mechanisms for rapidly raising calcium levels are not there (as she has been relying on dietary calcium which she is no longer getting). This means when her calcium levels drop she will not be able to raise them again, leading to hypercalcaemia. The best preventative measure would be to maintain calcium release from the bone by feeding the cow a low calcium diet.
What condition usually causes hypercalcaemia?
Hyperparathyroidism - excess production of PTH. Lots of PTH will blast the bones liberating lots of calcium, reabsorbing lots of calcium from the kidneys and increasing reabsorption in the gut by stimulating vitamin D.
What is primary hyperparathyroidism caused by?
An overgrowth of the parathyroid gland causing it to produce too much PTH
What is secondary hyperparathyroidism caused by?
Secondary hyperparathyroidism is indirect and can be due to either renal (disease or incompetence of the kidney tubule with failure to both excrete phosphate and reabsorb calcium) or dietary causes (diets with low calcium, excess phosphate or inadequate vitamin D cause excessive PTH production).
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