ENDO - Calcium Homeostasis

Question 1

Why is calcium so important?

Answer 1

Involved in intracellular signalling
- Responsible for exocytosis of neurotransmitter vesicles from pre-synaptic terminals via VGCC
- Important in muscle contraction (including cardiac muscle)

Question 2

Describe regulation of intra- and extracellular calcium.

Answer 2

• Intracellular calcium is tightly buffered by organelles such as the ER and mitochondria
• Extracellular calcium requires hormonal regulation and because calcium enters the cell down its concentration gradient, any significant perturbations could have serious consequences

Question 3

What is the main collective role for calcium and phosphate?

Answer 3

Bone formation
- Come together to form hydroxyapatite crystals - constituting around 70% of bone
- Bone stores calcium

Question 4

Describe the usual levels of calcium in a healthy adult.

Answer 4

2.2 - 2.6 mmol/litre
- 99% bound up in bone in the form of hydroxyapatite. Around 1% of calcium is free-floating for other processes
- Of the free-floating calcium, 45% is uncomplexed and ionised, 45% found bound to albumin and 10% is complexed

Question 5

Describe intake and excretion of serum calcium.

Answer 5

See recording and slide 6

Question 6

Describe where phosphate is involved in the body.

Answer 6

• 80% found in bone
• 7% found in inorganic form in ECF
• 13% found in soft tissue as phospholipids and phosphoproteins

Question 7

Describe intake and excretion of serum phosphate.

Answer 7

See recording and slide 8

Question 8

Describe regulation of serum calcium and phosphate.

Answer 8

By parathyroid gland
- Composed mainly of chief cells - which synthesise and secrete PTH
- Interspersesd amongst cells are oxyphil cells - appear around puberty and increase in number with age - exact function unknown

Full diagram on slide 9

Question 9

How is PTH secretion controlled?

Answer 9

• Chief cells are directly sensitive to [Ca2+]
• High [Ca2+] inhibits PTH secretion - calcium not taken up by bone, not resorbed by kidney and not taken up by the gut.
• Fall in calcium levels - PTH secretion mobilising calcium from stores, increase uptake from gut and DCT. Causing rise in serum levels

Question 10

Describe control of PTH release. PART 1

Answer 10

• Calcium is sensed by the GPCR calcium sensing receptor.
• Upon activation, this receptor activates phospholipase C, which in turn increases inositol 1,4,5 triphosphate leading to the release of intracellular calcium stores.
• This rise in intracellular calcium inhibits the release of stored vesicles of parathyroid hormone.
• Drop in extracellular calcium would in turn reduce the activation of the CaSR, and as such the subsequent intracellular pathways leading to exocytosis of the parathyroid hormone containing vesicles.

Question 11

Describe control of PTH release. PART 2

Answer 11

• PTH also plays an important role in Mg2+ regulation and the CaSR can be activated by serum magnesium (although this is much less studied).
• In patients with low serum magnesium, PTH is secreted in the same way that low calcium stimulates PTH release.
• However, in very depleted Mg2+ states (i.e. premature babies), a paradoxical block of PTH secretion is observed.
• This is because the interaction between the CaSR and its G-proteins is modulated by magnesium, such that very low levels causes aberrant activation of the intracellular pathways leading to inhibition of PTH.
• Hypomagnesiaemia can therefore lead to secondary hypocalcaemia. This type of hypocalcaemia can’t be treated long term with calcium substitution but instead responds to magnesium replenishment

Question 12

Describe control of PTH release. PART 3

Answer 12

• Phosphate also acts to regulate PTH secretion, but stimulates its release by acting as a competitive antagonist at the CaSR
• Sympathetic nervous system also stimulates PTH secretion through actions of beta adrenergic receptors.
• Parasympathetic nervous system does not appear to play a role in PTH secretion

Question 13

Describe calcitonin.

Answer 13

-Thyroid gland contains a sparse population of cells called C-cells
- Secrete a hormone called calcitonin, which in many young animals increases circulating calcium levels.
- However its role in humans is much less remarkable. While calcitonin does have some biological functions in humans, its effects are negligible.
- This is observed in patients who’s thyroid has been removed and who see no significant alteration to their calcium levels.

Question 14

How are the kidneys affected by PTH? PART 1

Answer 14

• Almost all free calcium in the plasma is filtered at the glomerulus.
• Around 95% of calcium reabsorption by the kidneys is independent of PTH.
• PTH-dependent calcium reabsorption occurs in the DCT and CD.
• Cells in these regions express the parathyroid receptor on their basolateral membrane.
• Binding of the hormone increases gene transcription of the TRPV5 channel, a highly selective calcium channel, on the apical membrane, which allows calcium to flow into the cell down its concentration gradient

Question 15

How are the kidneys affected by PTH? PART 2

Answer 15

• PTH-R also increases expression on the basolateral membrane of the sodium-calcium exchanger, which exchanges sodium for calcium.
• Therefore increased calcium levels in the serum
• Phosphate reabsorption occurs through two sodium dependent channels on the apical membrane – NaPi-IIa and NaPi-IIb.
• Activation of the PTH receptor downregulates both of these channels resulting in increased phosphate excretion in urine

Question 16

Describe the effects of PTH on bone. PART 1

Answer 16

• Bone is comprised of two main cell types – osteoblasts, which promote bone formation, and osteoclasts which promote bone resorption.
• Preosteoclasts are precursor cells which have not yet matured into osteoclasts
• PTH acts on bone to increase bone resorption. Osteoblasts express PTH receptors (osteoclasts do not).

Question 17

Describe the effects of PTH on bone. PART 2

Answer 17

• When stimulated, osteoblasts express a protein called receptor activator of nucleuar factor kappa-B ligand (RANK-L), which binds to RANK receptors expressed on pre-osteoclasts.
• Activation of this receptor promotes preosteoclasts to differentiate into osteoclasts, whose activity leads to increased calcium and phosphate release from bone
• Osteoblasts also release interleukin-6, which directly activates existing osteoclasts to increase their resorptive activity

Question 18

Describe vitamin D.

Answer 18

• Vitamin D synthesis begins with the breakdown of 7-dehydrocholesterol in the skin by UV light
• Breakdown forms cholecalciferol (vitamin D3), which can also be found in the diet in foods such as fish and eggs
• Vit D3 is further metabolised to 25-hydroxycholecalfierol in the liver, which in turn travels to the kidneys where it is metabolised into the active form of vitamin D – 1,25-dihydroxycholecalciferol.
• This last step is dependent on PTH levels

Question 19

Describe PTH regulation of Vitamin D. PART 1

Answer 19

• Within renal cells, 25-hydroxycholcalciferol (25-OHD) (which has been made in the liver) diffuses through the plasma membrane of the cell and binds with HSP70.
• PTH binds to the PTH receptor and upregulates the expression of the enzyme 1-alpha-hydroxylase, which is found in the mitochondria.

Question 20

Describe PTH regulation of Vitamin D. PART 2

Answer 20

• HSP70 facilitates 25-OHD entry into the mitochondria where it is hydrolysed by 1-alpha-hydroxylase to the active form 1,25(OH)2D.
• By regulating the expression of 1-alpha-hydroxylase, PTH thus regulates the levels of circulating active Vitamin D
• Mutations in the 1-alpha-hydroxylase gene lead to a form of rickets called Vitamin D-dependent rickets type 1a, which is characterised by normal levels of 25-OHD, but low levels of 1,25 (OH)2D

Question 21

Describe Vitamin D at the intestines. PART 1

Answer 21

• Most calcium absorption from the GI tract occurs through paracellular mechanisms, for which there is little to no hormonal control
• Vitamin D does have control over transcellular calcium absorption
• Transcellular calcium absorption requires TRPV6 channels on the apical surface to facilitate the entry of calcium into the cell

Question 22

Describe Vitamin D at the intestines. PART 2

Answer 22

• In order to maintain very low levels of free intracellular calcium, this calcium binds to the calcium binding protein calbindin, which transports it to the basolateral membrane where it is released into the blood stream via the sodium-calcium exchanger.
• Vitamin D binds to its nuclear receptor, increasing the expression of TRPV6, calbindin and NCX to increase transcellular transport of calcium from the GI tract

Question 23

Describe Vitamin D at the intestines. PART 3

Answer 23

• Vitamin D also increases phosphate absorption in the GI tract by increasing the expression of phosphate transporters on the apical membrane
• Effect of PTH on GI Ca2+ absorption appears to be indirect, through increasing levels of vitamin D

Question 24

How does vitamin D affect the kidneys?

Answer 24

• Acts synergistically with PTH to increase calcium reabsorption in the kidney.
• Although the effects of PTH on calcium reabsorption are greater than that of vitamin D, when put together their effects are greater than the sum of their individual parts
• Vitamin D acts to increase phosphate reabsorption from the kidney, although its effects are less than that of PTH

Question 25

How does vitamin D affect bone? PART 1

Answer 25

• Effects of vitamin D on bone are very complex and are dependent on vitamin D levels
• Both osteoblasts and osteoclasts express vitamin D receptors.
• Activation of these receptors increases the activity of both cell types, essentially resulting in increased bone turnover

Question 26

How does vitamin D affect bone? PART 2

Answer 26

• Also indirect effects due to increased serum calcium as a result of increased GI absorption and renal reabsorption which provide more calcium to the bone for deposition
• However when vitamin D is in excess, it appears to favour bone resorption

Question 27

How does vitamin D affect bone? PART 3

Answer 27

• In states of low vitamin D, weakening of bones occurs.
• In children this results in rickets and in adults osteomalacia – both are essentially the same but the clinical presentation depends on whether the bones are still growing.
• This occurs because PTH levels are still high, favouring bone resorption, but vitamin D levels are low so bone deposition does not occur