Regulation of Ca, Pi, and Mg

Question 1

How does PTH affect bone?

Answer 1

increases resorption

Question 2

How does low and high albumin affect plasma Ca levels?

Answer 2

hypoalbuminemia = increases plasma Ca

hyperalbuminemia = decreases plasma Ca

Question 3

How does calcitonin work overall?

Answer 3

lowers serum calcium and phosphate levels

Question 4

How is calcium reabsorbed in the proximal tubule?

Answer 4

primarily paracellularly

some transcellular = diffuses down gradient from lumen –> exits via Ca-ATPase and Na-Ca antiporter

Question 5

How do chronic acidosis and alkalosis affect Pi excretion?

Answer 5

chronic acidosis: increases Pi excretion

chronic alkalosis: decreases Pi excretion

Question 6

What is FGF-23 and what does it do?

Answer 6

secreted by bone in response to PTH, calcitriol, and hypoerphosphetemia –> inhibits Na-Pi symporter –> decreases Pi reabsorption in early PT

Question 7

Where is calcium reabsorbed in the nephron?

Answer 7

PT: 65-70% reabsorbed; mostly passive and follows Na and H2O

Thick ascending limb: lumen positive voltage drives Ca reabsorption via paracellular path

Distal tubule: reabsorbs 8%, but major site of regulation via Vit D

Question 8

How does PTH affect the intestines?

Answer 8

Increases Ca/Pi absorption indirectly via Vit D

Question 9

How does PTH affect the Kidneys?

Answer 9

promotes Ca reabsorption, primarily in the DCT

decreases reabsorption of Pi in PCT

decreases Na/H antiporter action

decreases bicarbonate reabsorption

Question 10

In general, what does vitamin D/Calcitriol do?

Answer 10

works with PTH to resorb bone (stimulate osteoclasts)

increases Ca and Pi plasma levels

Question 11

Where is the active form of Vit D made?

Answer 11

made by proximal tubule epithelial cells via 1 alpha-hydroxylase

Question 12

How does calcitriol affect the intestines?

Answer 12

increases calcium and phosphorus absorption –> increases serum Ca and Pi levels

Question 13

How is most calcium excreted from the body?

Answer 13

90% excreted through feces

Question 14

What things affect production/expression of calcitriol?

Answer 14

low Ca, low Pi, and high PTH –> upregulate 1 alpha hydroxylase expression

High calcium –> CaSR –> inhibits 1 alpha hydroxylase

Question 15

How does insulin affect phosphate?

Answer 15

lowers serum Pi levels by shifting Pi into cells

Question 16

How does ADH affect Ca reabsorption in the TAL?

Answer 16

increases NaCl reabsorption in TAL –> Ca follows and therefore also is reabsorbed more

Question 17

What is the % filterability load of calcium?

What does this number mean?

Answer 17

60%

45% is ionized Ca and 15% is in diffusible complexes

Question 18

What is the other name for vitamin D?

Answer 18

calcitriol

Question 19

How does calcitriol affect the kidneys?

Answer 19

increases Ca and Pi reabsorption –> increases Serum levels

Question 20

What does the Calcium sensing receptor do?

Answer 20

monitors calcium levels in IF (in kidney)

If plasma/IF Ca is high –> inhibits NKCC –> Ca can’t be reabsorbed paracellularly

Question 21

How does calcitonin affect bone?

The Kidneys?

Answer 21

bone: inhibits osteoclast-mediated resorption
kidney: promotes phosphate and excretion

Question 22

How is Mg reabsorbed in the distal tubule?

Answer 22

10%

electrical potential is primary driver

crosses apical border via TRPM6

basolater border cross = unknown

Question 23

How do ECF expansion and contraction affect Mg reabsorption?

Answer 23

expansion = decreases

contraction = increases

Question 24

How does acidemia affect Ca excretion?

Alkalemia?

Answer 24

acidemia = inhibits TRPV5 in DT –> increases Ca excretion

Alkalemia stimulates TRPV5 –> decreases Ca excretion

Question 25

What does calbindin do?

Answer 25

binds influxing calcium (especially important in the distal tubule) and prevents excessive intracellular levels that might lead to apoptosis

Question 26

How is Ca reabsorbed in the distal tubule?

Answer 26

DT has a **lumen negative** TEPD Ca reabsorption is active = crosses apical membrane via TRPV5 and basolateral via Na-Ca exchanger

Question 27

How is Pi reabsorbed in the early PT?

Answer 27

via Na-Pi symporter on the apical surface or via Pi-H antiporter on apical surface basolateral surface = unknown transporter

Question 28

How does volume contraction affect Ca reabsorption in the proximal tubule?

Answer 28

increases Ca reabsorption bc Na and Ca parallel each other (volume expansion has opposite effect)

Question 29

What does PTH do, in general?

Answer 29

increases serum Ca and decreases serum Pi

Question 30

Where is Mg reabsorbed in the nephron?

Answer 30

20% in Proximal tubule 70% thick ascending limb 5% distal tubule

Question 31

How is Mg reabsorbed in the proximal tubule?

Answer 31

20% paracellularly following Na and water

Question 32

How is Ca reabsorbed in the thick ascending limb?

Answer 32

primarily paracellular reabsorption parallels Na dependent upon TEPD (lumen positive charge)

Question 33

What is the relationship btw calcium and hydorgen?

Answer 33

compete for binding sites on plasma albumin

Question 34

How does PTH affect magnesium reabsorption?

Answer 34

increases reabsorption

Question 35

Where in the kidney is Phosphate reabsorbed?

Answer 35

80% reabsorbed by PT

Question 36

How do metabolic acidosis and alkalosis affect Mg reabsorption?

Answer 36

acidosis = decreases reabsorption alkalosis = increases reabsorption

Question 37

Is Ca secreted in the kidney?

Answer 37

No!

Question 38

What is the normal total plasma level of calcium?

Answer 38

5.0 mEq/L

Question 39

What is the most important hormone that regulates Pi excretion? How does it work?

Answer 39

PTH inhibits the Na-Pi transporters and Na-H antiporter in apical membrane of PT cells uses cAMP to do so

Question 40

How does alkalosis affect free calcium levels?

Answer 40

decreases free calcium (bc fewer H+ to bind albumin, so Ca binds to it) predisposes to hypocalcemic tetany acute alkalosis can induce symptoms mimicking hypocalcemia

Question 41

How do thiazide diuretics affect the distal tubule?

Answer 41

inhibit Na reabsorption and stimulate TRPV5 to increase Ca reabsorption PTH, Vit D, and Calcitriol also stimulate TRPV5

Question 42

How is Mg reabsorbed in the TAL?

Answer 42

70% paracellular depends on function of NKCC, which depends on lumen-positive voltage of TAL

Question 43

Where is calcium stored in the body?

Answer 43

99% in bone 1% in ICF 0.1% in ECF

Question 44

How does acidosis affect free calcium levels?

Answer 44

increases free calcium (H+ has used up all the binding sites on albumin)

Question 45

How does calcitriol affect bone?

Answer 45

promotes osteoclastic-mediate bone resorption

Question 46

How do loop diuretics affect Ca reabsorption?

Answer 46

inhibits NKCC --\> reduces magnitude of lumen-positive TEPD --\> reduces Ca paracellular reabsorption = lowers plasma Ca

Question 47

How does increased Calcium intake affect Pi?

Answer 47

Increased Ca decreases Pi absorption in GI --\> decreases serum Pi