May 9, 2016 - Calcium Flashcards
Parathyroid Hormone
The “ultimate control freak”
Extremely tight control of calcium.
Synthesized and secreted by chief cells in the parathyroid gland.
Calcium-Sensing Receptor
The calcium sensing receptor senses calcium on the surface of the chief cells in the parathyroid.
When they detect high calcium, they rapidly decrease PTH.
Influences on PTH Production
High calcium decreases PTH (the trump card)
High PO4 increases PTH
High Vitamin D decreases PTH
Effects of PTH on Organs
- Releases calcium from bone
- Conserve calcium in kidneys
- Absorb calcium and phosphate in gut
PTH and Vitamin D
PTH stimulates the renal enzyme (VDR) that converts inactive Vitamin D to active Vitamin D.
Activate vitamin D induces duodenal calbindin - which can double or more the net GI absorption of dietary calcium.
Vitamin D Pathway
Pro Vitamin D3 gets sunlight exposure and is converted into…
Pre Vitamin D3 (you can also get this in diet)
This will go through the liver with no rate-limiting step to make 25-OH Vitamin D or Calcidiol (this is what you measure when looking at Vitamin D)
Calcidiol is converted with the help of 1-A-Hydroxylase in the kidney (stimulated by PTH) to Calcitriol or 1,25-OH2 Vitamin D, which is the active form.
1-Alpha-Hydroxylase Activity
Increased by PTH
Increased hy low serum phosphate
Decreased by high calcium
Decreased by high calcitriol
Osteoclasts
The ruffled border secretes acid which increases the solubility of calcium phosphate in the bone and causes demineralization. One of the lysosomal hydrolases is Cathepsin K (cathepsin K inhibitors are in clinical trials).
RANK is needed for bone resorption.
Resorption and New Bone
Resorption is needed to signal the osteocyte to produce new bone.
It is all about the ratio of RANKL : OPG
With a high ratio, bone will be resorbed, and with a low ratio, it will be deposited (to a point).
