13_HST110 Regulation of Calcium and Phosphorus 2017 Flashcards
Homeostasis of Ca2+ and Pi is also regulated by (X) and (Y)
X = intestinal absorption Y = bone exchange
What % of calcium is in bone/teeth?
99%
ECF/ICF calcium plays an important role in what 4 functions?
Nerve impulse transmission
Muscular contraction
Cell signaling
Blood coagulation
What % of ultrafilterable (not protein-bound) calcium is ionized (biologically active form)?
45%
Effect of pH on Ca2+ Distribution
Plasma pH influences the distribution of Ca2+
(X) increases fraction of ionized Ca2+ (decreases bound Ca2+)
(Y) decreases fraction of ionized Ca2+ (increases bound Ca2+)
X = Acidemia (H+ buffered by proteins, displaces Ca2+) Y = Alkalemia (H+ dissociates from proteins, replaced by Ca2+)
What extreme of albumin in the blood increases ionized Ca2+?
Hypoalbuminemia
Intestinal absorption (400 absorbed, 200 mg secreted), Bone exchange, and Renal reabsorption of calcium are all under the control of what?
Hormones
Calcium absorption of Ca2+ occurs in what segments of the small intestine and through what routes?
All 3: duodenum, jejunum, and ileum through 2 routes:
Paracellular (passive)
Driven by concentration gradient
Transcellular (active)
Controlled by calcitriol (1,25(OH)2 vitamin D)
What % of bone is hydroxyapatite and what is its function?
70%, calcium-buffering system, 0.5 g/day exchanged between bone and plasma
Renal Regulation of Ca2+
Ionized Ca2+ and complexed Ca2+ are filtered
(X)% is reabsorbed by tubular system
60-70% PT
20% LOH
10% (Y)
X = 99 Y = DT* (site of regulated Ca2+ excretion)
What is the mechanism of transport for Ca2+ reabsorption in the PT?
Paracellular transport
- Passive diffusion down concentration gradient in second half of PT
- Solvent drag with water
What is the mechanism of transport for Ca2+ reabsorption in the LOH?
Paracellular transport
“Back-leak” of K+ generates positive lumen voltage that drives reabsorption of Ca2+ and other cations
This process can be inhibited by loop diuretics
What are 5 factors that regulate renal Ca2+ handling?
Parathyroid hormone (PTH)
Vitamin D
Plasma [Ca2+]
Metabolic acidosis
Diuretics
Plasma [Ca2+] is sensed by the calcium-sensing receptor (CaSR) on (X) cells
Decrease in serum [Ca2+] (Y) PTH and vice versa
X = chief Y = increases
What are the actions of PTH?
- Increase renal Ca2+ reabsorption
- Increase bone resorption
- increase calcitrol synthesis (kidney), increasing absorbed Ca2+
Net effect of PTH?
Increased serum [Ca2+] back to normal
Where is the receptor that binds PTH located?
Basolateral surface of DT cell
PTH stimulates insertion of (X) channels into apical membrane, increasing the Ca2+ reabsorption. In addition, PTH stimulates 1α-hydroxylase activity in the proximal tubule to increase (Y) (1,25(OH)2-vitamin D) synthesis
X = TRPV5 Y = calcitriol
What are the actions of 1,25(OH)2-Vitamin D?
- Increase intestinal reabsorption of Ca2+
- Increase bone resorption
- Increase renal Ca2+ reabsorption
What is the net effect of 1,25(OH)2-Vitamin D?
Increased serum [Ca2+]
↑ plasma [Ca2+] Effect on Ca2+ regulation
Decreases (X) Ca2+ reabsorption (decreased gradient for paracellular transport)
Decreases Ca2+ reabsorption at loop of Henle by activation of (Y)
Suppresses (Z)
X = PT Y = CaSR Z = PTH
What is the net effect of increased plasma [Ca2+] on excretion of Ca2+?
NET EFFECT: Increases Ca2+ excretion
Hypocalcemia has opposite effect
What % of Phosphorus is in bone/teeth, ECF, and ICF?
85% in bone/teeth
1% in ECF, 14% in ICF
ECF/ICF phosphorus plays important role in what 4 functions?
Energy metabolism
Bone formation
Signal transduction
Phospholipids, nucleic acids
At physiologic pH, ionized Pi exists primarily as (X) and (Y)
Ratio of these forms is determined by equation
At pH 7.4, [X] to [Y] is (Z)
X = HPO42- Y = H2PO4- Z = 4:1
Intestinal absorption of Pi is greatest in (X), little in (Y). This is mediated by the (Z) (Na+-phosphate cotransporter) and regulated by 1,25(OH)2 vitamin D (calcitrol)
X = duodenum and jejunum Y = ileum Z = NaPi-2b
Renal Regulation of Pi:
Ionized and complexed forms ((X)%) are filtered
80% reabsorbed by (Y)
20% is excreted
X = 75 Y = proximal tubule
In the PT, Pi is transported by
(X) transport:
- (Y) symporters
- Type 2a (3Na+/HPO42-)
- Type 2c (2Na+/HPO42-)
- PiT-2 (2Na+/H2PO4-)
-Pi-inorganic anion transporter
X = Transcellular Y = Na+-Pi
In the PT, for Pi transport, (X) of co-transporters determines amount of Pi reabsorbed
X = Abundance
Name 5 methods of regulating urinary Pi excretion
Dietary Pi intake Parathyroid hormone (PTH) Fibroblast growth factor 23 (FGF-23)
ECF volume
Acid-base balance
Increases in dietary phosphorus lead to what effects?
Decreased PT Na+-phosphate cotransport -> Decreased phosphate reabsorption
PTH (X) Pi reabsorption at the proximal tubule
Stimulates endocytic removal of (Y) transporters from the apical surface
Leads to renal phosphate (Z)
X = decreases Y = Na+-Pi Z = excretion
FGF-23 and Pi regulation in the Kidney
Produced in (X) in response to (Y) in serum Pi
Requires (Z) (cofactor) to exert its effects on the kidney
Acts at proximal tubule to:
- Decrease expression and activity of (A) co-transporters
- Decreases expression of 1α-hydroxylase to decrease synthesis of (B)
Promotes renal phosphate (C)
X = osteoclasts Y = increases Z = Klotho A = Na+-Pi B = calcitriol C = excretion
What 3 conditions stimulate production of FGF-23?
⬆ Serum Pi
PTH
Calcitriol
