Parathyroid Hormone Metabolism

Question 1

What types of cells comprise the parathyroid glands?

Answer 1

• Oxyphil:

- Cheif cells: secrete PTH

Question 2

Where did the parathyroid glands emerge from during embryological development? Where might the parathyroid glands be located?

Answer 2

• derived from 3rd and 4th branchial pouches that migrate
• typically found on the back of the thyroid gland, but extra parathyroid glands or aberrant locations are not uncommon especially along the path of embryologic migration.

Question 3

Function of PTH

Answer 3

• controls Ca2+ (increase) and phosphate homeostasis in the body
• conversion of Vit D in the kidney to Calcitriol

Question 4

How do parathyroid cells know when to secrete PTH?

How does the kidney respond to Ca2+?

Answer 4

• parathryoid cells have calcium-sensing receptors (CaR)

- kidney possesses CaR receptors that regulates ca2+ handling. Hypercalcemia=excretion of Ca2+ in urine.

Question 5

PTH Homeostasis
-what happens in times of low ca2+

-what happens in times of high ca2+?

Answer 5

• decreased plasma ca2+….increased PTH……bone release of ca2+ and phosphate, Kidney increasing calcitriol formation(intestinal reabsorption), phosphate excretion/calcium reabsorption
• the opposite of the above answer.

Question 6

PTH-Related Protein (PTHrP)

• what is this?
• when is this secreted?
• how does it work?

Answer 6

• a hormone that acts like PTH
• secreted by nonmetastatic solid tumor and some patients with non-hodgkin lymphoma.
• works like PTH in that it increases bone re-absorption and distal tubule reabsoprtion BUT does not increase intestinal reabsorption.

Question 7

Calcitonin

• what is this?
• How does it work?

Answer 7

• peptide hormone secreted by thyroid gland (parafollicular cells)
• stimulated in high ca2+ levels, decreases plasma levels of ca2+ (decreases absorptive activities and formation of new osteoclasts)

Question 8

Where is calcium in the body?

Answer 8

1% in extracellular fluid/flows in and out of cells
–50% in the ECF is bound to albumin/50% ionized state

-99% is in the bone

Question 9

Where is phosphate in the body?

Answer 9

• 85% of phosphate is in the bone
• 14% in the cells
• less than 1% in ECF
• -phospholipids, buffers for acid-base changes

Question 10

What is calciums role in the body?

Answer 10

• normal bone density
• clotting cascade
• muscle function
• transmission of nerve signals
• intracellular signaling

Question 11

Whats important to remember about ca and phosphate homeostasis?

Answer 11

they are indirectly related. When ca2+ goes up P04- goes down.

Question 12

Vitamin D
-sources
-

Answer 12

• Photoisomerization (sun)
• Intestinal absorption from diet (fortified milk)
• liver stores some, excess is in adipose tissue

Question 13

How is Vitamin D synthesized?

Answer 13

UV light and diet/supplements make Vit D3, which is stored in the liver, and converted to 25-hydroxy Vit D. The kidney metabolizes it to Calcitriol. (which is the most active form of Vit D.)

Question 14

Where is calcitonin secreted from? calcitriol? PTH?

Answer 14

• calcitonin=thyroid
• calcitriol=kidney
• PTH= parathyroid

Question 15

Function of:

• osteoblasts
• osteoclasts

Answer 15

• Blasts: deposit bone on outer surfaces or in bone cavities
• Clasts: break down bone or (absorb)
• in adults deposition and absoprtion are equal.