PTH, Calcitonin, Ca and Phosphate, VitD

Question 1

How much calcium is absorbed/ excreted by the intestines?

Answer 1

About 35% is absorbed from the intestines, and 90% is excreted in feces (with the addition of the ones secreted by GI juice)

Question 2

How much calcium is absorbed/ excreted by the kidneys?

Answer 2

About 10% of ingested calcium is excreted via the kidneys. And 99% is reabsorbed by the renal tubules –> controlled by PTH

Question 3

How is phosphate regulated in the kidneys?

Answer 3

If there is low phosphate in the blood, all phosphate will be reabsorbed. If there is more phosphate in the blood, the rate of excretion will be directly proportional to the increase.
PTH can greatly increase excretion

Question 4

What are the 2 types of bone tissues?

Answer 4

1. Compact (cortical)
2. Trabecular (spongy) –> inner, where blood cell production occurs

Question 5

Describe bone calcifcation

Answer 5

1. secretion of collagen and proteoglycans by osteoblast cells
2. uncalcified collagen = osteoid
3. calcium salt precipitates after a few days, forming hydroxyapatite crystals

Question 6

What’s the role of pyrophosphate?

Answer 6

It inhibits hydroxyapatite crystallization and calcification of bone

Question 7

How is the level of pyrophosphate controlled?

Answer 7

regulated by molecules such as tissue-nonspecific alkaline phosphatase (breaks down pyrophosphate)

Question 8

What is bone remodling?

Answer 8

The continuous formation of new bone by osteoblasts and breakdown by osteoclasts

Question 9

What influences bone remodling?

Answer 9

• mechanical stress
• PTH and active Vit D (1,25(OH)2 D3) –> (+) osteoclast
• calcitonin –> (-) osteoclasts

Question 10

What are the 3 steps of active Vit D regualtion?

Answer 10

• U/V can convert 7-dehydrocholesterol to D3 @ the skin
• D3 converted to 25-hydroxycholecalciferol @ the liver
• 25-hydroxycholecalciferol converted to 1,25(OH)2 D3 @ the kidneys, tightly regulated by PTH

Question 11

What are the actions of 1,25(OH)2 D3?

Answer 11

• promotes intestinal absorption of calcium & phosphate
• weak effect to decrease renal excretion of calcium & phosphate
• important in bone resorption and deposition
• PTH bone resorption is diminished without it

Question 12

How does PTH control extracellular calcium level?

Answer 12

1. increased intestinal reabsorption of calcium
2. decrease renal excretion of calcium
3. exchange between bone and ECF

Question 13

Where is PTH formed?

Answer 13

chief cells of the parathyroid gland

Question 14

How does PTH mobilize calcium from bone?

Answer 14

2 phases:
1. fast phase, from existing osteocytes
2. slow phase, proliferation of osteoclasts & increased osteoclast activity

Question 15

How does PTH influence renal calcium and phosphate?

Answer 15

• PTH decreases phosphate reabsorption at the proximal tubule
• PTH increases calcium reabsorption at the ascending loop of Henle and distal tubules

Question 16

How does PTH influence intestinal calcium and phosphate?

Answer 16

PTH increased BOTH calcium and phosphate reabsorption at the intestines due to more 1,25(OH)2 D3 formation @ the kidneys

Question 17

Where is calcitonin produced?

Answer 17

In the parafollicular cells in the interstitial tissue of the thyroid gland

Question 18

What’s the function of calcitonin?

Answer 18

Opposite of PTH, though effects are much weaker

Question 19

How is calcium regulated between ECF and bone?

Answer 19

If there is low ECF calcium:
- readily exchangeable Ca2+ diffuse into ECF
- PTH formation increases –> (+) osteoclasts

Question 20

How is calcium regulated between ECF and GI?

Answer 20

If there is low ECF calcium:
- PTH increases, increases 1,25(OH)2 D3 –> increases calcium binding protein etc –> increased GI absorption

Question 21

How is calcium regulated between ECF and kidneys?

Answer 21

If there is low ECF calcium:
- increased PTH –> increased reabsorption of calcium, decreases excretion
- decreases reabsorption of phosphate, increases excretion

Question 22

What happens is there if a lack of PTH?

Answer 22

• inactive osteoclasts
• low 1,25(OH)2 D3
• decrease in GI calcium absorption
• renal calcium excretion > absorption
• calcium ECF falls below normal
• phosphate level = normal, or elevated

Question 23

What happens with primary hyperparathyroidism?

Answer 23

• excessive PTH production
• excessive osteoclast activity –> bone breakdown most serious
• renal retention of calcium and excretion of phosphate
• increased 1,25(OH)2 D3
• ECF calcium = above normal, phosphate = low

Question 24

What happens with secondary hyperparathyroidism?

Answer 24

ex. Rickets
- increases in PTH to compensate for lack for calcium in the diet or damaged kidneys (ie. 1,25(OH)2 D3 can’t be formed)
- leads to osteomalacia

Question 25

What causes osteoporosis?

Answer 25

• reduced osteoblast activity, so osteoclast > osteoblast
  3 main reasons:
  1. lack of mechanical stress/ physical activity
  2. lack of estrogen/ post menopausal
  3. advanced age, other hormones and GH that contribute to bone growth diminishes greatly

Question 26

Is calcium supplement effective for post menopausal osteoporosis?

Answer 26

nope