Lecture 3 - Effects of PO₄ on CaR/PTH secretion Flashcards
CKD-MBD: what is it and why can it be so serious?
Chronic kidney disease mineral and bone disorder
Renal disease - increase in PTH - Bone demineralisation - vascular calcification - LVH - heart failure
Mechanisms of 2° Hyperparathyroidism with CKD
Hyperphosphataemia causes Pᵢ to bind Ca²⁺ in the blood, reducing free Ca²⁺, meaning there is less negative feedback on pituitary cells, resulting in less decreased PTH release
Reduced calcitriol also results in less PTH negative feedback and therefore no decreased PTH release
PO₄: what is it, what uses are there for it in the body, and how abundant is in the body and earth?
Phosphate
- Intracellular signalling - phosphorylation
- CV system - Cardiac & SM contraction
- Bone mineral (85%)
- ATP
- Biomolecule component (DNA)
6th most abundant element in the body (1%)
but only 12th most abundant earth element
CaSR: what is it, what does it do, and what is it also becoming theorised to be involved in?
Calcium-sensing receptor
Controls PTH secretion (and therefore Ca²⁺ homeostasis) - high Ca²⁺ results in activation, decreasing PTH secretion and decreasing renal Ca²⁺ reabsorption (low Ca²⁺ results in the opposite)
Four Pᵢ binding sites have been described in CaSR - potentially may be a phosphate sensor
Extracellular calcium: why is it so tightly maintained, what is it regulated by, how is homeostasis achieved?
Minor Ca²⁺ deviations cause major problems
The calcium homeostasis system is mainly directed by the parathyroid gland - it senses extracellular calcium by the CaSR and when Ca²⁺ levels are not in the intended range, it targets calciotropic organs to orchestrate an overall response to restore calcium levels back to normal
Secreting PTH:
* PTH secretion increases during hypocalcaemia, raising Ca²⁺ in the blood by increasing renal Ca²⁺ reabsorption
* PTH secretion decreases during hypercalcaemia, lowering Ca²⁺ in the blood by decreasing renal Ca²⁺ reabsorption
What is the relationship between extracellular calcium levels and PTH secretion?
There is a inverse relationship between levels of extracellular calcium and the secretion of PTH that maintains blood levels of Ca²⁺ within a very narrow range
Pᵢ: how is it detected by animals, what cells are thought to be responsible for, and what is the supporting evidence?
It is unclear how mammals sense changes in extracellular phosphate - four Pᵢ binding sites have been described in CaSR, potentially this?
Given that FGF23 and PTH are the 2 major hormones that regulate PO₄ metabolism, bone and PT cells are the most likely candidates for sensing PO₄
- High phosphate causes a decrease in calcium levels, stimulating PTH
- FGF23 is increased to maintain normal PO₄ levels in CKD and it inhibits calcitriol production which in turn produces sPTH (contradictor evidence exists so this is questionable)
- FGF23 inhibits PTH secretion - unclear cause-effect due to contradicting evidence and potential confounding variables
- CKD patients exhibit decreased efficacy to calcimimetics when their serum phosphate levels are high - CaR is inhibited by high phosphate concentrations
CaSR as a phosphate sensor: what is the supporting evidence?
- Four Pᵢ binding sites have been described
- Pᵢ increases murine PTH secretion via the CaSR
- CaSR-induced Ca²⁺ mobilisation in CaSR-HEK cells is sensitive to pathophysiologic changes in [Pᵢ]
- Pᵢ-mediated inhibition of CaSR signalling occurs with other CaSR-positive allosteric modulators but not in the presence of carbachol
- Phosphate acts as a non-competitive CaSR antagonist
- The phosphate effect cannot be explained simply by decreased ionised Ca²⁺ concentration
- Increased serum phosphate (Pᵢ) also increases PTH but by an uncertain mechanism
PO₄: does it increase PTH secretion, what is the difference between acute/chronic effects?
PO₄ stimulates rapid and reversible PTH secretion in freshly isolated human parathyroid cells
Chronic increases in phosphate lead to PTH secretion but short exposition to phosphate is more likely to be a receptor-mediated effect like PO₄ entering the cell
is it mediated by the CaSR?
Where does phosphate act to inhibit CaSR?
CaSR extracellular domain has four PO4 binding sites
Arg-62/Glu-277 salt bridge holds CaSR ECD closed/active
Mutation of CaSRR62 abolishes the phosphate effect
Conclusions of this lesson
- CaSR can sense PO₄ in the parathyroid gland
- PO₄ inhibits the CaSR in a non-competitive manner
- PO₄ elicits rapid & reversible increase in PTH secretion in human and murine parathyroid cells
- PO₄ inhibitory effect is mediated at CaSRR62
- This mechanism could contribute to the 2°HPT of CKD
GoF/LoF CaR mutations: what do they cause and what is the difference between them?
CaSR - activation means high calcium levels, need to reduce resorption, PTH levels decreased and renal Ca²⁺ decreased
GoF mutation means CaR activation irrespective of calcium levels, resulting in continual inhibition of PTH, decreased resorption of Ca²⁺ and eventual hypocalcemia
LoF mutations means CaR is inactive irrespective of calcium levels, resulting in continual secretion of PTH, continual resorption of Ca²⁺ and eventual hypercalcemia
Can calcilytics treat ADH?
High potential - by acting as negative allosteric modulators, the effect of continual CaR activation may be reduced and so are under current investigation as a potential treatment of ADH
