Physiology of calcium regulating hormones Flashcards
At any given time, what percentage of calcium is in the bones
99%
What is the division of bound vs free calcium?
50:50 pretty much
Normal calcium
8.5-10.5
hypocalcemia
less than 8.5
signs of hypocalcemia?
nervous system excitement and tetany
PTH is secreted by what cells and is what type of chemical
Secreted by Chief Cells. 84 amino acid polypeptide
Calcitonin is produced by what cells?
C cells. 32 amino acid polypeptide
Chvostek sign
twitching of the facial muscless elicited by tapping on the facial nerve
Trousseau sign
carpopedal spasm elicited by inflation of the bp cuff.
THis and Chvostek are signs of hypocalcemia
What exactly does decreased extracellular calcium concentration do that leads to twitching?
Decreased extracellular calcium leads to increased excitability of muscle cells because it lowers the threshold potential. That is, less inward current is required to depolarize the threshold and fire action potentials.
Calcium homeostasis involves the coordinated interaction of 3 organ systems and three hormones
Bone, Kidney , and intestine
PTH, Calcitonin, Vit D
If a person ingests 1000mg of calcium in a day, about how much is generally absorbed?
350mg
How much is generally secreted back into the intestines to be expelled in the feces
150mg (this combined with the 650 that was left over from the 1000 originally ingested means that 800mg in total is expelled from the intestines.
This means there is a net absorption of 200 mg per day through the diet. In order to stay in balance, the kidneys must then excrete 200. This process ensures that bone is constantly being destroyed and remodelled/
Chronic hypocalcemia causes secondary hyperparathyroidism which is characteized by what?
Increased synthesis and storage of PTH and HYPERPLASIA of the parathyroid glands.
What 3 organs does PTH interact with
Bone, Kidney, Intestine
What specific effect does PTH have on the bone?
Bone Resorption ( this follows a brief period of bone formation since osteoblasts receive direct stimulation from PTH and trigger osteoclast maturatio and activity)
What specific effect does PTH have on the kidney
- Less Phosphate reabsorption (this is due to inhibition of the Na-P cotransporter in the proximal convoluted tubule), causes phosphaturia
- Increased Calcium reabsorption
- Increased urinary cAMP
What specific affects does PTH have on the intestine?
Increases Calcium reabsorption (this is indirect,it activates the renal 1Alpha hydroxylase, the enzyme that converts the inactive form of vitamin D to the active form and increases calcium rebsorption in teh small intestine.
Major action of calcitonin
inhibit osteoclast bone resorption
Major stimulus for calcitonin
Increased serum calcium
The majority of renal calcium is reabsorbed from where?
Proximal convoluted tubule
Mechanism by which calcitonin lowers plasma calcium and phosphate
- Reduces bone resorption through inhibiting osteoclast activity
- inhibits renal tubular cell reabsorption of calcium which allows it to be excreted in the urine
Calcitonin comes from where?
Thyroid gland
What is the overall role of Vitamin D?
Vitamin D exists to promote mineralization of new bone and its actions are coordinated to increase BOTH Ca2+ and phosphate concentrations in plasma so that these elements may be deposited in new bone material.
Vitamin D is also called
Cholecalciferol
What must occur to cholecalciferol before it is biologically active?
Must be successively hydroxylated
1) Hydroxylated in the liver to 25- hydroxycholecalciferol (also inactive)
2) Hydroxylated in the kidney to 1, 25 DIhydroxycholecalciferol….may be hydroxylated to 24,25 dihydroxycholecalciferol (this is inactive0
Whether the renal cells produce 1,25 or 24,25 is decided by what?
Status of Calcium in the body. If we have enough, 24,25 is produced.
What enzyme is responsible for the hydroxylation to 1,25
1-alpha hydroxylase
What effect does 1,25 have on the intestines?
Induces the synthesis of Calbindin D-28k, a cytosolic protein that can bind four calciums
What differs about vitamin D’s affect on the kidney compared to PTH
Cholecalciferol stimulates the reabsorption of both calcium and phosphorus. PTH only reabsorbs calcium
Whether calcium and phosphate are laid down in bone or reabsorbed from bone depends on what?
The product of their concentrations rather than their individual concentrations. Product above the solubility product = lay down, below= resorb
Physiological importance of phosphate
look at this slide
Normal phosphate range
3-4.5
85% of total body phosphate is in what
bone….14% intracellular, .1% extracellular
What happens to phosphat ein renal failure
In renal failure, dietary phosphate restriction cannot keep up with the loss of phosphate excretion so you have positive phosphate balance. Bone is resorbed more rapidly than it is formed as well. You get vascular calcification
