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Vander's Renal Physiology > 10: Regulation of Calcium, Magnesium, and Phosphate > Flashcards

10: Regulation of Calcium, Magnesium, and Phosphate Flashcards

1
Q

What percentage of total body calcium is stored in the bone?

A

99%

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2
Q

What are the three most important functions of Calcium?

A
  1. constituent of bone - 99% of Ca here
  2. second messanger in cells–> rapid signal transmission
  3. stabilizes the electrical sensitivity of voltage-gated membrane channels
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3
Q

What are the different forms of plasma Calcium?

A
  • free and ionized (40%)
  • protein-bound (40%)
  • complexed (20%)
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4
Q

Name two examples of substances Ca may be complexed with in the plasma.

A
  • phosphate
  • citrate
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5
Q

How does hypocalcemia cause low-calcium tetany?

A

Low levels of Ca –> Na-channels sense more depolarization than actually exists –> spontaneous firing of motor neurons –> inappropriate muscle contractions

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6
Q

How can the acid/base status contribute to or worsen hypocalcemic tetany?

A

Acute alkalosis

hydrogen ions and Ca can both be bound to albumin anionic sites –> if there are less hydrogen ion/protons –> more Ca binds to albumin –> less free ionized Ca available –> hypocalcemic tetany worse

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7
Q

What are the 3 main clinical signs of hypercalcemia?

A
  • CNS depression
  • muscle weakness
  • GI tract immobility
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8
Q

In what form is Ca stored in bones and what are its constituents?

A

hydroxylapatite - complex of: Ca, P, hydroxyl groups

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9
Q

Regarding Calcium absorption in the GI tract: How much of dietary Ca is absorbed, where are the two areas of Ca absorption, how is Ca absorbed there and in what proportion?

A
  • less than half of ingested Ca is usually absorbed
  • main areas of absorption: duodenum and lower small intestines

duodenum:
* active transport - most important when dietary Ca is limited (does not rely on the cc gradient)
* Ca-selective channels on luminal side (TRP familty) –> enters duodenal cells –> binds to mobile cytosolic Ca-binding proteins (calbindins) –> actively excreted on basolateral side via Ca-ATPase and some Na-Ca antiporter

lower small intestines
* paracellular diffusion - following cc gradient
* most absorbed here under normal conditions

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10
Q

What percentage of filtered Ca is reabsorbed and excreted in the kidneys?

A

98% reabsorbed
2% excreted

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11
Q

Regarding calcium reabsorption in the kidneys: What are the areas of reabsorption, what proportion of Ca is reabsorbed in each tubular region, and how is Ca reabsorbed there?

A
  1. proximal tubule
    * 65% of filtered Ca
    * paracellular passive diffusion, following cc gradient, cc gradient achieved by water reabsorption (following the Na reabsorption) causing increased luminal Ca cc compared to interstitium
  2. thick ascending limb of Loop of Henle
    * 20% of filtered Ca
    * paracellular passive diffusion –> caused by lumen-positive potential
  3. distal convoluted tubule and connecting tubule
    * ~15%
    * active and transcellular
    * luminal entrace via calcium specific TPR channels –> transport through cell via calbindins –> exit on basolateral side via Ca-ATPase and Na-Ca antiporter
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12
Q

Where in the kidneys does endocrine control affect Ca absorption?

A
  • distal convoluted tubule and connecting tubule
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13
Q

What is calbindin and how many binding sites does it have?

A

cytosolic calcium-binding protein
4 binding sites for Ca

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14
Q

what percentage of total body calcium is intracellular?

A

0.01%

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15
Q

What cells of the bone lower Ca and P by inserting into bone and what cells increase Ca and P by deliberating it from bone?

A

Osteoclasts –> release Ca and P
Osteoblasts –> build Ca and P into bone

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16
Q

What are the different body compartments of phosphate storage and what are their proportions (%)?

A
  • bone (as hydroxyapatite) 85%
  • intracellular (as phosphorylated proteins and metabolic intermediates) 14%
  • ECF 1%
17
Q

What proportion of the ingested phosphate is typically absorbed?

A

65%

18
Q

Where and how in the GI tract is phosphate absorbed?

A

throughout the small intestines
* transcellular active transport and paracellular diffusion
* active transport = Na-phosphate symporters on apical membrane - basolateral not known (presumed uniporters)

19
Q

What proportion of phosphate in the blood is protein-bound?

A

5-10%

20
Q

What percentage of filtered phosphate is actively reabsorbed under normal conditions?

A

75%

21
Q

Where and how is phosphate reabsorbed in the kidneys?

A
  • proximal tubule
  • active transcellular - Na-phosphate symporters
22
Q

Is phosphorous tubular-maximum or gradient-limited?

A

tubular-maximum limited system

under normal circumstances most transporters are saturated and there is no “spillage” paracellulary

23
Q

What is the active form of vitamin D?

A

1,25-Dihydroxy-Vitamin-D [1,25-(OH)2-D]
syn. Calcitriol

24
Q

What steps does ingested Vitamin D undergo to become active Vitamin D

A

Vitamin D3 = cholecalciferol
Vitamin D2 = ergocalciferol

ingested vitamin D –> travels to the liver –> liver hydroxylates it at the 25 position –> travels to the kidneys –> hydroxylated at the 1 position by proximal tubular cells –> 1,25-Dihydroxy-Vitamin-D

25
Q

How do PTH and FGF23 affect Vitamin D?

A

PTH stimulates vitamin D hydroxylation in proximal tubules

FGF23 inhibits hydroxylation of vitamin D in the proximal tubules

26
Q

How does vitamin affect Ca and P homeostasis?

A

increases transcellular and paracellular (effect on tight functions) Ca absorbtion in intestines –> P to lesser extend

stimulates renal-tubular reabsorption of both calcium and P

27
Q

How does calcitriol affect PTH and FGF-23 levels?

A

–> inhibits PTH synthesis in the parathyroid gland

–> promotes FGF23 production

28
Q

What is the half-life of PTH?

A

less than 10 min –> degradated fast by liver

29
Q

How does ECF Ca cc affect PTH secretion?

A

ECF Ca cc –> binds to Ca receptor couple to G protein-linked signaling cascades –> inhibits PTH secretion

low Ca cc –> removes tonic inhibition –> increased PTH release

30
Q

How does Phosphate affect PTH secretion?

A

High phosphate –> stimulates PTH secretion

31
Q

What are the 5 actions of PTH

A
  1. increases movement of Ca and P from labile pool in bone into ECF
  2. stimulates bone remodeling (net effect on Ca if normal PTH levels, if high –> erosion of hydroxyapatite)
  3. sitmulates hydroxylation of vitamin D in proximal tubules
  4. increaes renal tubular Ca reabsorption in distal convoluted tubules
  5. reduces proximal tubular P reabsorption
32
Q

Where is FGF-23 produced?

A

osteoblasts and ostercytes –> secretin increased in response to elevated P levels

33
Q

here

What are the effects of FGF-23?

A

primarily regulator of P, but secondary effects on Ca (indirect)

  • acts on FGF-23 receptors containing protein called “Klotho”
  • decreases reabsorption of P by causing the internalization of Na-phosphate symporters
  • decreases production of calcitriol
34
Q

Describe the mechanisms behind secondary hyperparathyroidism from chronic kidney disease

A

Decreased GFR –> decreased P excretion –> hyperphosphatemia –> elevated PTH and FGF-23
decreased renal function + high FGF-23 –> decreased active Vitamin D
–> reduced calcium uptake from GI tract –> removal of inhibitory effect on PTH synthesis –> high PTH –> excessive bone resorption

high P –> calcification of vascular smooth muscles

35
Q

What proportion of dietary Mg is absorbed?

A

about 50%

36
Q

How is dietary Mg absorbed?

A

paracellular diffusion mostly through tight-junctions of small intestines + some transcellular uptake

37
Q

What are the fractions of Mg in the blood?

A

60% free
30% albumin-bound
10% complexed

38
Q

Regarding Mg reabsorption in the kidneys: where is it rabsorbed, in what proportion and how?

A
  • 20% parcellular proximal tubule
  • 70% paracellular loop of henle
  • rest distal tubules - transcellular, active

Vander's Renal Physiology (10 decks)

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