Physiology of calcium regulating hormones

Question 1

At any given time, what percentage of calcium is in the bones

Answer 1

99%

Question 2

What is the division of bound vs free calcium?

Answer 2

50:50 pretty much

Question 3

Normal calcium

Answer 3

8.5-10.5

Question 4

hypocalcemia

Answer 4

less than 8.5

Question 5

signs of hypocalcemia?

Answer 5

nervous system excitement and tetany

Question 6

PTH is secreted by what cells and is what type of chemical

Answer 6

Secreted by Chief Cells. 84 amino acid polypeptide

Question 7

Calcitonin is produced by what cells?

Answer 7

C cells. 32 amino acid polypeptide

Question 8

Chvostek sign

Answer 8

twitching of the facial muscless elicited by tapping on the facial nerve

Question 9

Trousseau sign

Answer 9

carpopedal spasm elicited by inflation of the bp cuff.

THis and Chvostek are signs of hypocalcemia

Question 10

What exactly does decreased extracellular calcium concentration do that leads to twitching?

Answer 10

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.

Question 11

Calcium homeostasis involves the coordinated interaction of 3 organ systems and three hormones

Answer 11

Bone, Kidney , and intestine

PTH, Calcitonin, Vit D

Question 12

If a person ingests 1000mg of calcium in a day, about how much is generally absorbed?

Answer 12

350mg

Question 13

How much is generally secreted back into the intestines to be expelled in the feces

Answer 13

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/

Question 14

Chronic hypocalcemia causes secondary hyperparathyroidism which is characteized by what?

Answer 14

Increased synthesis and storage of PTH and HYPERPLASIA of the parathyroid glands.

Question 15

What 3 organs does PTH interact with

Answer 15

Bone, Kidney, Intestine

Question 16

What specific effect does PTH have on the bone?

Answer 16

Bone Resorption ( this follows a brief period of bone formation since osteoblasts receive direct stimulation from PTH and trigger osteoclast maturatio and activity)

Question 17

What specific effect does PTH have on the kidney

Answer 17

• 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

Question 18

What specific affects does PTH have on the intestine?

Answer 18

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.

Question 19

Major action of calcitonin

Answer 19

inhibit osteoclast bone resorption

Question 20

Major stimulus for calcitonin

Answer 20

Increased serum calcium

Question 21

The majority of renal calcium is reabsorbed from where?

Answer 21

Proximal convoluted tubule

Question 22

Mechanism by which calcitonin lowers plasma calcium and phosphate

Answer 22

• Reduces bone resorption through inhibiting osteoclast activity
• inhibits renal tubular cell reabsorption of calcium which allows it to be excreted in the urine

Question 23

Calcitonin comes from where?

Answer 23

Thyroid gland

Question 24

What is the overall role of Vitamin D?

Answer 24

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.

Question 25

Vitamin D is also called

Answer 25

Cholecalciferol

Question 26

What must occur to cholecalciferol before it is biologically active?

Answer 26

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

Question 27

Whether the renal cells produce 1,25 or 24,25 is decided by what?

Answer 27

Status of Calcium in the body. If we have enough, 24,25 is produced.

Question 28

What enzyme is responsible for the hydroxylation to 1,25

Answer 28

1-alpha hydroxylase

Question 29

What effect does 1,25 have on the intestines?

Answer 29

Induces the synthesis of Calbindin D-28k, a cytosolic protein that can bind four calciums

Question 30

What differs about vitamin D’s affect on the kidney compared to PTH

Answer 30

Cholecalciferol stimulates the reabsorption of both calcium and phosphorus. PTH only reabsorbs calcium

Question 31

Whether calcium and phosphate are laid down in bone or reabsorbed from bone depends on what?

Answer 31

The product of their concentrations rather than their individual concentrations. Product above the solubility product = lay down, below= resorb

Question 32

Physiological importance of phosphate

Answer 32

look at this slide

Question 33

Normal phosphate range

Answer 33

3-4.5

Question 34

85% of total body phosphate is in what

Answer 34

bone….14% intracellular, .1% extracellular

Question 35

What happens to phosphat ein renal failure

Answer 35

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