Control of Mineral Metabolism

Question 1

Ca++ physiological roles

Answer 1

-structural role (constituent of the mineral matrix of bone) - biochemical role.

Question 2

Normal plasma Ca++ levels

Answer 2

8-10 mg/dl (~2.5 mM)

Question 3

Normal Phosphate Levels

Answer 3

between 3 and 4 mg/dl (~1.1 mM)

Question 4

major compartments of Ca++

Answer 4

1. Bone-99% in form of hydroxyapatite.
2. Intracellular- (10 g) maintained by intracellular mobile calcium buffers, compartmentalization into ER calcium stores, by an ATP linked calcium pump and a Na/Ca antiporter.
3. Extracellular fluid- (blood and interstitial). (2.5 mM

Question 5

T of F: majority of Ca++ filtered by kidney is reabsorbed

Answer 5

True. About 98% of this is reabsorbed.

Question 6

forms of Ca++ i ECF

Answer 6

• 1/2 is free, and filterable by the kidney,
• 10% as salts (bicarbonate and phosphate) which can also be filtered by the kidney and
• remainder bound to albumin.

Question 7

Functions of phosphate

Answer 7

• structural role, (mineral matrix of bone).
• intracellular buffer. It is
• required for phosphorylation reactions,

Question 8

T of F: there is a fast exchange of up to 20 g/day between the ECF and labile bone, mediated by osteocytes.

Answer 8

true

Question 9

How does PTH increase plasma Ca++

Answer 9

• BONE: Rapid effect-increased efflux of labile bone calcium, not accompanied by phosphate Slow effect-increased bone remodeling, releases both calcium and phosphate (seen mainly in pathological conditions)
• KIDNEY: Inc Ca++ reabsorption in, DT Dec phosphate reabsorption, Inc synthesis of 1,25 (OH)2 Vitamin D
• GI TRACT: Vitamin D, enhances Ca++ absorption-(requires 1 day)

Question 10

Net effect of PTH

Answer 10

increase serum calcium and decrease serum phosphate levels.

Question 11

PTH secretion is stimulated by

Answer 11

• Dec free ionized Ca++ in the plasma and

- inhibited by a rise.

Question 12

Release of Calcitonin

Answer 12

• elevated calcium

- certain GI hormones (gastrin, cholecystokinin, secretin and glucagon)

Question 13

Fxn of calcitonin

Answer 13

It acts on bone to decrease efflux of labile bone calcium

Question 14

Summerize Vit D synthesis

Answer 14

• in skin 7-dehydrocholesterol is acted on by sunlight to produce Vitamin D (biologically inert).
• In the liver, one hydroxyl group is added to form 25-OH Vitamin D.
• In the kidney, a second hydroxyl is added in a reaction catalyzed by 1hydroxylase yielding 1,25 (OH)2 Vitamin D, the most active f

Question 15

1,25 (OH)2 Vitamin D blood transporter

Answer 15

mostly transported in the blood bound to transcalciferin

Question 16

Action of Vit D

Answer 16

• GI tract: inc Ca++ and phosphate absorbtion (transciption of a calcium binding protein that appears in the lumen of the intestine
• mobilizes bone in a way similar to PTH, possibly simply by sensitizing the bone to PTH action.

Question 17

Enzyme responsible for active form of Vit D

Answer 17

1-hydroxylase

Question 18

Regulation of Vit D release

Answer 18

• 1,25 (OH)2 Vitamin D acts in a negative feedback loop inhibiting 1-hydroxylase
• PTH increases vit D

Question 19

24-hydroxylase

Answer 19

• In Kidney\

- Inactivates vit D (forms 24,25 (OH)2 Vitamin D)

Question 20

Regulation of Vit D by PTH & phosphate

Answer 20

• Increased PTH induces 1-hydroxylase & inhibits 24hydroxylase
• Decreased phosphate induces 1 hydroxylase and inhibits 24-hydroxylase

Question 21

SHORT TERM REGULATION OF BLOOD CALCIUM

Answer 21

• PTH: mobilize calcium into the plasma when levels begin to become low.mobilize calcium movement from the bone compartment into the blood. If this is continued for a prolonged time, calcium balance is affected negatively.
• Calcitonin may be useful in increasing the rate of storage of an acute calcium load.

Question 22

LONG TERM REGULATION OF CALCIUM BALANCE

Answer 22

Vitamin D: regulating the intestinal absorption of these minerals