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Renal > Lecture 8: Ca, Mg, PO4 Excretion > Flashcards

Lecture 8: Ca, Mg, PO4 Excretion Flashcards

1
Q

Explain why the calcium concentration in Bowman’s space is different from its [plasma]

A

About half the calcium is complexed to albumin and other proteins that aren’t filtered by the kidney

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2
Q

For calcium, list the fractional reabsorption in each nephron segment

A

PCT = 60-70%

Thick AL = 20%

DCT = 10%

CD = 5%

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3
Q

Describe calcium regulation in proximal tubule

A

Not tightly regulated!

Passive, paracellular route

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4
Q

What drives paracellular transport of calcium in PT?

A

Electrochemical gradient in lumen (tubular fluid)

Moving down, water and solutes get reabsorbed, while calcium gets more concentrated until it finally gets transported paracellular lay

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5
Q

Describe important transporters in thick AL

A

Apical:
NKCC = brings in Na, K and 2 Cl

ROMK = potassium channel required to keep NKCC going

Basolateral:
CaSR = calcium sensing receptor

CLC-Kb = voltage-gated Cl channel

NaK ATPase = keeps sodium low inside cell

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6
Q

What is the driving force for calcium in the thick AL?

A

Positive charge established by potassium in the lumen

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7
Q

What effect does hypercalcemia have on calcium absorption?

Thick AL

A

Too much calcium at the sensing receptor means ROMK will get inhibited, the positive charge driving force will drop, and calcium absorption via paracellular transport is blocked

More calcium gets excreted in the urine

CaSR can also directly block the NKCC to stop the system as well

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8
Q

What effect does furosemide have on calcium?

A

Induces urine calcium loss to decrease hypercalcemia by blocking NKCC

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9
Q

Distinguish between calcium transport in DCT and thick AL

A

Thick AL = paracellular

Distal convoluted tubule = transcellular

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10
Q

Describe calcium transport in the DCT

A

Transcellular reabsorption only

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11
Q

What does calbindin do in DCT cells?

A

Binds to and traffics calcium within cell so that it is not free/ionized

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12
Q

What is the driving force for calcium transport in DCT cells?

A

NaK ATPase

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13
Q

Describe the steps involved in a DCT cell’s response to hypocalcemia

A

Step 1:
Hypocalcemia induces PTH release from the parathyroid gland. Binding of PTH receptor in DCT occurs

Step 2:
Through GPCR activity, cAMP inc.

Step 3:
cAMP stimulates PKA activity

Step 4:
PKA phosphorylates TRPV5 (calcium channel), increasing its open probability

Step 5:
Cal Indian binds intracellular calcium and transports for removal via Na-Ca Exchanger OR Ca ATPase

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14
Q

What effect do thiazides diuretics have on calcium transport in DCT?

A

Induce greater calcium reabsorption

Can decrease calcium in urine

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15
Q

Describe steps in thiazides activity in DCT on calcium transport

A

Step 1:
Thiazides block NCC. More sodium and water excreted in urine

Step 2:
As a result of thiazides effect in DCT, more Na and H2O are reabsorbed in PT - stimulating more Ca reabsorption in PT also!

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16
Q

Where is calcium actively and primarily regulated?

A

DCT

17
Q

What regulates TRPV5 in DCT, the calcium channel

A

PTH

18
Q

For magnesium, list the fractional reabsorption in each nephron segment

A

PCT = 10-20%

thick AL = 70%

DCT = 10%

19
Q

What is the main storage site of magnesium?

A

Bone

20
Q

What factors decrease Mg reabsorption?

A

Hypermagnesemia
Hypercalcemia

Hypokalemia

Metabolic acidosis

Loop diuretics
Thiazide diuretics

21
Q

What factors increase Mg reabsorption?

A

Hypomagnesemia

Hypocalcemia

22
Q

Describe magnesium reabsorption in PT

A

Likely passive (paracellular)

Greater Na absorption = inc. luminal [Mg] = driving force for paracellular Mg reabsorption

23
Q

How is magnesium transported in the thick AL?

A

Paracellular transport after NKCC/ROMK move potassium to generate positive electric potential driving force in lumen

24
Q

How does furosemide affect Mg reabsorption?

A

Decreases Mg reabsorption by same mechanisms as it decreases Ca reabsorption

(Blocking NKCC = dec. electric potential = less driving force)

25
Q

How does hypercalcemia affect Mg reabsorption in thick AL?

A

Hypercalcemia inhibits Mg reabsorption through CaSR (calcium sensing receptor), which when activated will inhibit ROMK

= less K back leak into lumen = less electrochemical gradient to drive Mg reabsorption

26
Q

How do thiazide diuretics affect Mg transport in DCT?

A

Enhance Mg excretion by suppressing TRPM6 (Mg channel for transcellular transport) expression

27
Q

What is the main site of regulation of phosphate?

A

PT

28
Q

How is phosphate transport different in PT compared to Ca and Mg?

A

Phosphate transport is active (not passive!!) and transcellular

Renal (17 decks)

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