Control of Mineral Metabolism Flashcards
roles of calcium
structural (bone), second messenger, exocytosis, neuronal/muscle excitability
hypocalcemia effect on neuronal/muscle excitability
increase excitability
hypercalcemia effect on neuronal/muscle excitability
decreases excitability– due to surface charge screening
why does hypercalcemia result in hypoexcitement
due to Ca in ECF, it binds the negative charges on the cell membrane of neurons/muscles, making the voltage gated channels sense less of a charge difference between inside/outside and less likely to fire
Roles of phosphate
Structure, energy currency, component of DNA/RNA, regulatory role (phosphorylation)
how well is Ca absorbed
not well, about 50% absorbed in gut; also lose about 325 from secretions in intestine, so net intake about 175 mg from diet of 1 g
- excrete about 825 in feces
Ca level set point
between 8 and 10
how is Ca in blood
50% bound to albumin,
about 40% as free Ca,
and 10% as salts (bicarbonate, phosphate)
- free Ca is regulated
how much Ca does kidney filter
about 10 g– retains most of it, excreting about 0.2 g excreted and retains about 9.8 g
main cell types in bone formation/degradation
- Osteoblasts
- complex canals filled with canalicular fluid–contains osteocytes
- osteoclasts– phagocytic cells involved in matrix degradation
does blood or canalicular fluid have higher Ca
blood– this drives Ca into canalicular fluid
How does Ca get into bone
Ca pumped into canalicular fluid via surface osteoblasts and taken up by osteocytes and pumped out
how much Ca is exchanged between blood/canalicular fluid
10 g per day of Ca is rapidly exchanged
NOT Phosphate
Osteoclast role
degrade bone matrix to release Ca and Phosphate; slower than Ca exchange with blood and canalicular fluid
what processes contribute to serum Ca
fast exchange between blood and canalicular fluid and slower osteoclastic process exchanging both Ca and phosphate (net exchange of 280 mg Ca and 210 mg of phosphate every day)
is phosphate or Ca better absorbed
phosphate
how much phosphate taken up per day
1400 mg in diet – 1100 absorbed–200 lost in gut secretions
- net uptake 900 mg, while 500 mg excreted in feces
- 7g filtered through kidneys
- 210 mg exchanged from osteoclast activity
Parathyroid hormone
- peptide hormone
- released by Ca dependent endocytosis by Chief cells of parathyroid
- purpose: regulates Ca levels. If Ca lowers, PTH released to restore Ca back to set point
how is PTH stored
secretory vesicles;
why does decrease in serum Ca increases cytosolic Ca to release PTH
- have Ca sensor on chief cells
- normally bound with Ca– inactive
- removal of Ca triggers receptor, which is a G protein coupled receptor (Gq) – produces IP3 – acts on IP3 receptors in ER to release Ca into cytosol, allowing vesicles of PTH release from vesicles
how does PTH act
on PTH receptors– G coupled protein receptors (Gs or Gq)– increase cAMP or IP3 levels in body
function of PTH
- mobilize sources of Ca to increase serum Ca back to set point
- activate all processes leading to increased Ca in blood
BONE - activates rapid Ca exchange so that equilibrium goes towards Ca release from bone
- the slower osteoclast process activated – equilibrium shifted towards matrix breakdown and Ca going into blood (Phosphate will also be released here)
KIDNEY
-decreased reabsorption of phosphate but retention of Ca (so it doesn’t exceed solubility); also increases synthesis of 1,25 (OH)2 Vit D
GUT - also increases Ca absorption via vitamin D
if too much Ca phosphate
can have calcium-phosphate deposition
-
high PTH
lots of Ca loss from bone – osteoporosis
- also can get kidney stones (lots of Ca retained can exceed solubility even if phosphate filtered out)
