Regulation of Ca, Pi, and Mg Flashcards
How does PTH affect bone?
increases resorption
How does low and high albumin affect plasma Ca levels?
hypoalbuminemia = increases plasma Ca
hyperalbuminemia = decreases plasma Ca
How does calcitonin work overall?
lowers serum calcium and phosphate levels
How is calcium reabsorbed in the proximal tubule?
primarily paracellularly
some transcellular = diffuses down gradient from lumen –> exits via Ca-ATPase and Na-Ca antiporter
How do chronic acidosis and alkalosis affect Pi excretion?
chronic acidosis: increases Pi excretion
chronic alkalosis: decreases Pi excretion
What is FGF-23 and what does it do?
secreted by bone in response to PTH, calcitriol, and hypoerphosphetemia –> inhibits Na-Pi symporter –> decreases Pi reabsorption in early PT
Where is calcium reabsorbed in the nephron?
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
How does PTH affect the intestines?
Increases Ca/Pi absorption indirectly via Vit D
How does PTH affect the Kidneys?
promotes Ca reabsorption, primarily in the DCT
decreases reabsorption of Pi in PCT
decreases Na/H antiporter action
decreases bicarbonate reabsorption
In general, what does vitamin D/Calcitriol do?
works with PTH to resorb bone (stimulate osteoclasts)
increases Ca and Pi plasma levels
Where is the active form of Vit D made?
made by proximal tubule epithelial cells via 1 alpha-hydroxylase
How does calcitriol affect the intestines?
increases calcium and phosphorus absorption –> increases serum Ca and Pi levels
How is most calcium excreted from the body?
90% excreted through feces
What things affect production/expression of calcitriol?
low Ca, low Pi, and high PTH –> upregulate 1 alpha hydroxylase expression
High calcium –> CaSR –> inhibits 1 alpha hydroxylase
How does insulin affect phosphate?
lowers serum Pi levels by shifting Pi into cells
How does ADH affect Ca reabsorption in the TAL?
increases NaCl reabsorption in TAL –> Ca follows and therefore also is reabsorbed more
What is the % filterability load of calcium?
What does this number mean?
60%
45% is ionized Ca and 15% is in diffusible complexes
What is the other name for vitamin D?
calcitriol
How does calcitriol affect the kidneys?
increases Ca and Pi reabsorption –> increases Serum levels
What does the Calcium sensing receptor do?
monitors calcium levels in IF (in kidney)
If plasma/IF Ca is high –> inhibits NKCC –> Ca can’t be reabsorbed paracellularly
How does calcitonin affect bone?
The Kidneys?
bone: inhibits osteoclast-mediated resorption
kidney: promotes phosphate and excretion
How is Mg reabsorbed in the distal tubule?
10%
electrical potential is primary driver
crosses apical border via TRPM6
basolater border cross = unknown
How do ECF expansion and contraction affect Mg reabsorption?
expansion = decreases
contraction = increases
How does acidemia affect Ca excretion?
Alkalemia?
acidemia = inhibits TRPV5 in DT –> increases Ca excretion
Alkalemia stimulates TRPV5 –> decreases Ca excretion
What does calbindin do?
binds influxing calcium (especially important in the distal tubule) and prevents excessive intracellular levels that might lead to apoptosis
How is Ca reabsorbed in the distal tubule?
DT has a lumen negative TEPD
Ca reabsorption is active = crosses apical membrane via TRPV5 and basolateral via Na-Ca exchanger
How is Pi reabsorbed in the early PT?
via Na-Pi symporter on the apical surface
or via Pi-H antiporter on apical surface
basolateral surface = unknown transporter
How does volume contraction affect Ca reabsorption in the proximal tubule?
increases Ca reabsorption
bc Na and Ca parallel each other
(volume expansion has opposite effect)
What does PTH do, in general?
increases serum Ca and decreases serum Pi
Where is Mg reabsorbed in the nephron?
20% in Proximal tubule
70% thick ascending limb
5% distal tubule
How is Mg reabsorbed in the proximal tubule?
20%
paracellularly following Na and water
How is Ca reabsorbed in the thick ascending limb?
primarily paracellular
reabsorption parallels Na
dependent upon TEPD (lumen positive charge)
What is the relationship btw calcium and hydorgen?
compete for binding sites on plasma albumin
How does PTH affect magnesium reabsorption?
increases reabsorption
Where in the kidney is Phosphate reabsorbed?
80% reabsorbed by PT
How do metabolic acidosis and alkalosis affect Mg reabsorption?
acidosis = decreases reabsorption
alkalosis = increases reabsorption
Is Ca secreted in the kidney?
No!
What is the normal total plasma level of calcium?
5.0 mEq/L
What is the most important hormone that regulates Pi excretion? How does it work?
PTH
inhibits the Na-Pi transporters and Na-H antiporter in apical membrane of PT cells
uses cAMP to do so
How does alkalosis affect free calcium levels?
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
How do thiazide diuretics affect the distal tubule?
inhibit Na reabsorption and stimulate TRPV5 to increase Ca reabsorption
PTH, Vit D, and Calcitriol also stimulate TRPV5
How is Mg reabsorbed in the TAL?
70%
paracellular
depends on function of NKCC, which depends on lumen-positive voltage of TAL
Where is calcium stored in the body?
99% in bone
1% in ICF
0.1% in ECF
How does acidosis affect free calcium levels?
increases free calcium
(H+ has used up all the binding sites on albumin)
How does calcitriol affect bone?
promotes osteoclastic-mediate bone resorption
How do loop diuretics affect Ca reabsorption?
inhibits NKCC –> reduces magnitude of lumen-positive TEPD –> reduces Ca paracellular reabsorption = lowers plasma Ca
How does increased Calcium intake affect Pi?
Increased Ca decreases Pi absorption in GI –> decreases serum Pi
