Public flashcard deck · 33 cards

11 · PTH Actions on Bone, Kidney & Gut (and the Calcitriol/FGF-23 Axis)

Topic 17 · Calcium, Phosphate & Biomineralization · High-yield · ~60% likely · drill after reading book section 11.

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1. Front

   What change in plasma Ca²⁺ triggers PTH secretion?
   Back

   A FALL in plasma Ca²⁺ (low calcium) triggers PTH release.
2. Front

   By what mechanism does LOW plasma Ca²⁺ cause PTH to be released?
   Back

   Low Ca²⁺ leaves the parathyroid CaSR (calcium-sensing receptor) inactivated → this disinhibition releases PTH .
3. Front

   When plasma Ca²⁺ is HIGH, what happens to the parathyroid CaSR and to PTH?
   Back

   High Ca²⁺ activates the CaSR → PTH secretion is suppressed (the opposite of the low-Ca state).
4. Front

   Right or wrong? "When plasma Ca²⁺ falls, the parathyroid CaSR is activated."
   Back

   ✗ Wrong. Low Ca²⁺ INACTIVATES the CaSR ; that disinhibition is exactly what releases PTH. The CaSR is only activated by HIGH Ca²⁺.
5. Front

   Name the three organs through which PTH raises serum Ca²⁺.
   Back

   Bone , kidney , and gut (gut indirectly, via calcitriol).
6. Front

   What cell type does PTH ultimately activate in bone to liberate Ca²⁺?
   Back

   Osteoclasts → bone resorption releases Ca²⁺ and phosphate.
7. Front

   PTH receptors are on osteoBLASTs, not osteoclasts — so how does PTH activate osteoclasts?
   Back

   Indirectly: PTH acts on osteoblasts to express RANKL , which then activates osteoclasts .
8. Front

   Besides Ca²⁺, what else does PTH-driven bone resorption release?
   Back

   Phosphate (resorption liberates both Ca²⁺ and phosphate from mineral).
9. Front

   What does PTH do to renal Ca²⁺ reabsorption, and in which tubular segment?
   Back

   ↑ STIMULATES Ca²⁺ reabsorption in the distal tubule (conserves calcium).
10. Front

    What does PTH do to renal phosphate reabsorption, and in which segment?
    Back

    ↓ DECREASES phosphate reabsorption in the proximal tubule = phosphaturic (phosphate dumped in urine).
11. Front

    PTH lowers serum phosphate by down-regulating which proximal-tubule transporter?
    Back

    The NaPi-IIa (Na⁺/phosphate IIa) cotransporters — down-regulated → less phosphate reabsorbed.
12. Front

    Right or wrong? "PTH increases renal phosphate reabsorption."
    Back

    ✗ Wrong. PTH is phosphaturic — it DECREASES proximal phosphate reabsorption. This is the classic "NOT an action of PTH" answer.
13. Front

    Right or wrong? "PTH inhibits / decreases renal Ca²⁺ reabsorption."
    Back

    ✗ Wrong (reversed). PTH STIMULATES distal-tubule Ca²⁺ reabsorption to conserve calcium. Decreasing it would lower serum Ca — the opposite of PTH's goal.
14. Front

    Which renal enzyme does PTH induce, and what does it make?
    Back

    PTH induces renal 1-α-hydroxylase , which makes 1,25(OH)₂D (calcitriol) .
15. Front

    What is the active, hormonal form of vitamin D produced by 1-α-hydroxylase?
    Back

    1,25(OH)₂D = calcitriol (1,25-dihydroxycholecalciferol).
16. Front

    Through what intermediate does PTH increase intestinal Ca²⁺ absorption (PTH's gut effect is indirect)?
    Back

    Via calcitriol [1,25(OH)₂D] made by renal 1-α-hydroxylase — PTH does not act on the gut directly.
17. Front

    Which transport protein does calcitriol INDUCE to facilitate intestinal Ca²⁺ absorption?
    Back

    Calbindin — calcitriol induces it to facilitate Ca²⁺ transport across the enterocyte.
18. Front

    Cloze: Calcitriol induces ______, which facilitates intestinal Ca²⁺ transport.
    Back

    calbindin
19. Front

    What is the NET effect of PTH on serum calcium and serum phosphate?
    Back

    Serum Ca²⁺ ↑ , serum phosphate ↓ .
20. Front

    Why does serum phosphate fall even though PTH-driven bone resorption RELEASES phosphate?
    Back

    The renal phosphaturic effect dumps phosphate in urine, which outweighs the phosphate liberated from bone → net serum phosphate ↓ .
21. Front

    Which hormone is the physiological ANTAGONIST of PTH that lowers serum Ca²⁺?
    Back

    Calcitonin — it LOWERS serum Ca²⁺.
22. Front

    Which cells secrete calcitonin?
    Back

    Thyroid C cells (parafollicular cells) .
23. Front

    By what mechanism does calcitonin lower serum Ca²⁺?
    Back

    It inhibits osteoclasts → less bone resorption → serum Ca²⁺ ↓ .
24. Front

    Right or wrong? "Calcitonin raises serum Ca²⁺."
    Back

    ✗ Wrong. Calcitonin (thyroid C cells) LOWERS Ca²⁺ by inhibiting osteoclasts — it is the antagonist of PTH.
25. Front

    What stimulus triggers FGF-23 release?
    Back

    HIGH phosphate (hyperphosphatemia).
26. Front

    Which co-receptor does FGF-23 require to act?
    Back

    Klotho (Klotho co-receptor).
27. Front

    What are FGF-23's two main effects on phosphate and on calcitriol?
    Back

    It is phosphaturic (dumps phosphate) AND it suppresses 1-α-hydroxylase → ↓ calcitriol .
28. Front

    Right or wrong? "FGF-23 responds to LOW phosphate."
    Back

    ✗ Wrong. FGF-23 responds to HIGH phosphate , then acts phosphaturically and suppresses calcitriol.
29. Front

    Discriminate: which agent suppresses calcitriol synthesis — PTH or FGF-23?
    Back

    FGF-23 SUPPRESSES it. PTH does the opposite — it stimulates 1-α-hydroxylase to MAKE calcitriol.
30. Front

    Right or wrong? "PTH inhibits 1,25-DHCC (calcitriol) synthesis."
    Back

    ✗ Wrong. PTH STIMULATES 1-α-hydroxylase to MAKE calcitriol. The suppressors of calcitriol are FGF-23 (and calcitonin).
31. Front

    When plasma Ca²⁺ FALLS, which one of these does NOT occur: (a) PTH secretion, (b) osteoclast activation, (c) CaSR inactivation, (d) decreased renal Ca²⁺ reabsorption?
    Back

    (d) decreased renal Ca²⁺ reabsorption does NOT occur — PTH actually ↑ increases renal Ca²⁺ reabsorption. (a), (b), (c) all genuinely occur.
32. Front

    Which compartment/segment handles PTH's Ca²⁺ reabsorption vs its phosphate effect? (distal vs proximal)
    Back

    Ca²⁺ reabsorption ↑ in the DISTAL tubule ; phosphate reabsorption ↓ in the PROXIMAL tubule .
33. Front

    Capstone (predicted exam Q): A patient's plasma Ca²⁺ falls and PTH is secreted. Which is NOT an action of PTH? A. Activation of osteoclasts, increasing bone resorption B. Increased reabsorption of Ca²⁺ in the renal tubule C. Increased reabsorption of phosphate in the renal tubule D. Stimulation of renal 1-α-hydroxylase to form calcitriol E. Increased intestinal Ca²⁺ absorption (via calcitriol/calbindin)
    Back

    ✓ Answer C. PTH is phosphaturic — it DECREASES renal phosphate reabsorption (down-regulating proximal NaPi-IIa cotransporters). All other options are real PTH actions, giving the net Ca up , phosphate down .