Parathyroid Gland and Calcium Homeostasis Flashcards
What is the gross anatomy of the parathyroid glands
- Four glands immediately posterior to thyroid gland.
- Chief cells secrete PTH,
- Oxyphil cells whos function is unknown
What is the function of the parathyroid hormone?
- Change in calcium and phosphate concentrations in the extracellular fluid.
- Release of calcium and phosphate from bones into bloodstream,
- Absorption of calcium and phosphate from ingesta in intestines,
- Conservation of calcium and increase in phosphate excretion by kidneys.
What occurs with excessive and reduced activity of the parathyroid glands?
Excessive activity - rapid release of calcium salts from bones causing hypercalcaemia.
Reduced activity causes hypocalcaemia often with tetany
Explain the synthesis of parathyroid hormones
Ribosomes form a preprohormone which then becomes cleaved to a prohormone and the to a 84 amino acid hormone by the ER and Golgi. The hormone is then packaged in secretory granules in the cytoplasm. Smaller amino acid compounds are also found to have PTH activity. The 84AA hormone is quickly removed by kidneys whilst fragments remain these fragments are thought to cause lots of the PTH activity.
Explain the overall effect that the parathyroid hormone has on extracellular calcium and phosphate ion concentrations
- Calcium slowly rises which is caused by the hormone increasing calcium absorption from bone and decreasing excretion of calcium by kidneys.
- Phosphate quickly falls which is caused by increasing renal phosphate excretion, this effect is big enough to counteract the increased phosphate absorption from bone.
The effects of the parathyroid hormone is mediated via what?
- cAMP second messenger mechanism.
Explain the feedback control of calcium
Calcium concentrations;
- Low plasma calcium causes an increase in secretion rate from the parathyroid gland in the short term however continued reduced concentration of calcium causes an increased secretion due to hypertrophy of glands
What are the common causes of hypertrophy of parathyroid glands?
- Rickets, pregnancy and lactation
What are some common causes of reduced size/decreased output from the parathyroid glands?
- High concentration of calcium in the diet.
- Increased dietary vitamin D,
- Bone reabsorption by factors other than PHT for example, disuse of bones
Explain the two phases in which the parathyroid hormone triggers release of calcium and phosphate from the bone
1) Rapid phase - begins in mins, lasts for hours. Osteocytes liberate calcium and phosphate but no breakdown of bone.
2) Slow phase which lasts for days/weeks - Osteoclasts proliferate and increase reabsorption of bone to liberate calcium and phosphate. This occurs via the osteoclast releasing acid and lysosomes to reabsorb the bone.
Explain the effects of the parathyroid hormone on the kidneys
- Reduces calcium excretion from the kidneys by increasing tubular reabsorption of calcium.
- Increased phosphate excretion from the kidneys via decreased tubular reabsorption.
- Increased reabsorption of magnesium ions and hydrogen ions. Reduced reabsorption of sodium and potassium.
- Stimulates kidney to transform weaker forms of vitamin D into more active forms which increases calcium absorption via small intestine.
What are the effects of the PTH on the small intestines?
Increases calcium and phosphate absorption from the intestines by increasing the formation of 1,25-dihydroxycholecalciferol from vitamin D
Describe features of Vitamin D
- Compound formed in the skin and must be converted into 1,25-dihydroxycholecalciferol (the active form) by the liver and kidneys.
- Control of 1,25(OH)2D3 is via negative feedback from 25-hydroxycholecalciferol and via plasma calcium concentration which controls parathyroid hormone. The PTH is needed for conversion of 25-hydroxycholecalciferol to 1,25 dihydroxycholecalciferol.
- Plasma concs of 1,25-D3 is inversely affected by plasma concs of Ca.
Why is controlled conversion of Vitamin D3 to 25-dihydroxycholecalciferol important?
- Prevention of toxic build ups of 25-dihydroxycholecalciferol,
- Vitamin D storage can last for many months meaning it is hard to become deficient whereas 25-hydroxycholecalciferol lasts for only a few weeks.
What are the actions of Vitamin D?
- 1,25-dihydroxycholecalciferol has effects on intestines, kidneys (Increasing reabsorption by epithelial cells of renal tubules) and bone which causes increase absorption of calcium and phosphate into ECF.
- Forms a complex with another intracellular receptor called retinoid-X receptor. Complex binds to DNA and usually activates transcription
