ERS11 Physiology Of Calcium And Phosphate Metabolism

Question 1

Calcium in our body

Answer 1

1. Physiological forms
   - **Bones 99%
   - **Circulation 1%
   —> Free ionised form (50%) - diffusible through capillary —> ***ONLY physiological active form (tightly regulated)
   —> Protein-bound form (40%) (e.g. Albumin) - non-diffusible —> sensitive to pH level —> acidosis ↓ protein binding
   —> Complexed with anion (10%) - diffusible (e.g. PO4, HCO3, citrate) —> inactive
2. Range
   - tightly regulated within narrow range
   - vary with sex, age (higher in younger), physiological status
3. Absorption and excretion
   - adequate intake: Adolescents 1.3g/day, Adult 1g/day
   - diet: 10% absorbed, 90% excreted
   - inefficient absorption without Vit D: Ca absorption via ***paracellular pathway
   - kidney: 99% calcium reabsorbed
   - healthy adult: bone formation = bone resorption
4. Functions
   - build and maintain strong bones + teeth
   - muscle contraction
   - blood coagulation
   - neural transmission
   - IP3-Ca signalling pathway

Regulation of Ca / PO4
- designed to maintain constant level of ***Ca (唔係PO4!!!)
- via 3 hormones:
—> Calcitonin
—> PTH
—> Active Vit D

Question 2

Phosphate in our body

Answer 2

1. Physiological forms
   - **85% bone as Hydroxyapatite (Ca, PO4, OH) / CaPO4
   —> **Hydroxyapatite: stable crystal —> require extended time to digest / form —> long term regulation of serum Ca
   —> ***CaPO4: more simple non-crystalline form, rapid digestion —> used for buffering short term changes in serum Ca, PO4
   - Extra-skeletal tissues (14%): phospholipids, phosphoproteins, nucleic acids
   - Circulation (1%)
   —> Free ionised form (60%) - H2PO4, HPO4
   —> Protein-bound form (10%)
   —> Complexed with cation (30%)
2. Range
   - allow wider range
   - vary with age (higher in infant)
3. Absorption and excretion
   - nearly all ingested is absorbed
   - >90% plasma phosphate freely filtered at glomerulus, >80% actively reabsorbed
   —> via **proximal tubule via Na-PO4 symporter
   —> subject to “transport maximum” but can be regulated by hormone
   —> **control how many reabsorbed = control how many excreted (∵ filtration rate is constant)
4. Functions
   - build and maintain strong bones + teeth
   - growth, maintainance, repair of cells / tissues (protein, DNA, RNA)
   - energy metabolism (ATP synthesis)
   - maintain pH balance (chemical buffer to neutralise acids)

Regulation of Ca / PO4-
- designed to maintain constant level of ***Ca
- via 3 hormones:
—> Calcitonin
—> PTH
—> Active Vit D / Calcitriol

Question 3

Parathyroid gland

Answer 3

• small glands (20-40mg)
• posterior of thyroid gland
• usually 4
• flattened, oval
• tan colour (fat + vasculature) —> easily confused with surrounding fat when performing surgery
• blood supply: Inferior thyroid artery

Cells (arranged in compact mass)

1. Chief cells
   - small
   - numerous, dense
   - produce PTH
2. Oxyphil cells
   - large
   - scattered
   - unknown function

Question 4

PTH: synthesis and secretion

Answer 4

• 84 a.a.
• produced in Chief cells
• ***stored in secretory vesicles
• ***released in response to low plasma (Free, Ionised) Ca
• high plasma Ca —> suppress PTH secretion
• opposite to Calcitonin
• Ca level detected by ultrasensitive ***CaSR (GPCR: Gαq) on Chief cells

Low Ca
—> CaSR more relaxed, less activation
—> removes inhibitory signal
—> ***↓ PTH degradation
—> ↑ PTH release

High Ca
—> Activation of CaSR
—> Gαq activation
—> Phospholipase C activation
—> Phospholipase A2 activation
—> Arachidonic acid cascade
—> ***PTH degradation
—> ↓ PTH release

High PO4
—> ↓ Phospholipase A2 activity
—> removes inhibitory signal (↓ PTH degradation)
—> ↑ PTH release
—> ∴ chronic failure / renal disease patients have ↑ PTH

Vit D
—> ↓ stability of PTH mRNA
—> ↓ PTH release

簡單而言:
Low Ca, High PO4: ↑ PTH
High Ca, Calcitriol: ↓ PTH

Question 5

Physiological effects of PTH

Answer 5

Bind to PTH receptor (GPCR) in Bone + Kidney
—> activation of Gαq + Gαs (***depend on PTH conc)
—> activation of PLC + Adenyl cyclase
—> IP3-Ca release + ↑ cAMP

Bone (PTH receptor on ***Osteoblast):
1. High [PTH]
—> activate Gαs
—> ↑ cAMP/PKA pathway
—> RANKL production + inhibition of Osteoprotegerin expression (inactivate RANKL)
—> RANKL allowed to bind to RANK on Osteoclast precursors
—> Osteoclastogenesis (proliferation + maturation)
—> Osteoclasts resorb bone by pumping out HCl
—> Dissolve bone mineral
—> ↑ serum Ca

2. Low [PTH]
   —> activates Gαq
   —> PKC activation + IP3-Ca pathway
   —> MAPK activation + intracellular Ca release
   —> Osteoblast proliferation (accompany (High [Calcitonin]))
   —> helping bone formation
   —> ↓ serum Ca

Kidney (PTH receptor on Proximal + Distal tubule):

Proximal tubule:
1. ↑ expression of enzymes (1α-Hydroxylase) (from CYP27B1 gene)
—> ***Calcitriol formation (1,25-dihydroxycholecalciferol from 25 hydroxycholecalciferol)

2. Stimulates both PLC/PKC + cAMP/PKA pathway
   —> PKC: NPT2a mRNA degradation + PKA: NHERF-1 phosphorylation (dissociate from NPT2a —> NPT2a internalisation)
   —> NPT2a mRNA + protein degradation
   —> ↓ expression of Na/PO4 cotransporter (NPT2a)
   —> ↓ renal absorption of PO4
   —> ↑ renal excretion of PO4 (vs Calcitriol: ↑ intestinal PO4 absorption)
   —> further liberate additional ionised Ca (∵ PO4 bind to Ca)

Distal tubule:
3. Stimulate cAMP/PKA pathway
—> ↑ expression of Ca transport protein: TRPV5 (apical) + NCX1 (basolateral, pump 2 Ca out / 3 Na in)
—> ↑ Ca reabsorption
(Ca reabsorption in distal tubule: regulated by both PTH + Calcitriol —> reinforcement effect)

Question 6

Physiological effects of Calcitriol

Answer 6

Calcitriol bind to Vit D receptors (steroid receptors acting as transcription factor)

1. Intestinal cells
   - ↑ expression of apical membrane Ca channel TRPV6
   - ↑ expression of intracellular Ca binding protein Calbindin-D9k (trafficking of Ca)
   - ↑ expression of basolateral Ca-ATPase (PMCA1b)
   —> Allow ***transcellular absorption of Ca
   —> ↑ Intestinal Ca absorption

• ↑ expression of Na/PO4 cotransporter (NPT2b) on intestinal brush border membrane
  —> ↑ Intestinal PO4 absorption

2. Osteoblasts (Negative Ca balance)
   - ↑ RANKL expression
   —> ↑ Osteoclast formation + action
   —> ↑ serum Ca

• ↑ expression of 2 potent mineralisation inhibitors (in extracellular region)
  —> Pyrophosphate (PPi) + Osteopontin (OPN) (Glycosylated phosphoprotein)
  —> Pyrophosphate / Osteopontin bind Ca
  —> Stop PO4 to crystallise with Ca to form Hydroxyapatite
  —> Inhibit bone matrix mineralisation

3. Distal tubules
   - ↑ expression of basolateral Ca-ATPase (PMCA1b)
   - ↑ expression of intracellular Ca binding protein Calbindin-D28k
   —> ↑ Ca reabsorption
   (Ca reabsorption in distal tubule: regulated by both PTH + Calcitriol —> reinforcement effect)

Question 7

Normal / Negative Ca balance, In vitro / In vivo Vit D

Answer 7

Normal / Positive Ca balance (Normal Ca level —> Normal Calcitriol level):

• Act on intestinal cells —> ↑ serum Ca
• inhibit PTH —> ↑ bone mineral density

Negative Ca balance (Low Ca level —> ↑↑ Calcitriol):
- Osteoblast (i.e. Osteoclastic) effect become dominant —> Bone resorption

Exogenous / In vitro Vit D (e.g. Drugs):
- ↑ bone mineral density under ***normal / positive Ca balance

Endogenous / In vivo Vit D:
- stimulate osteoclastic bone resorption

Question 8

***Summary of PTH and Calcitriol

Answer 8

Intestine
PTH: N/A

Calcitriol:

• ↑ Ca absorption (via ↑ TRPV6, PMCA1b, Calbindin-D9k)
• ↑ Pi absorption (via ↑ NPT2b)

Kidney
PTH:
- ↑ Ca reabsorption (via ↑ TRPV5, NCX in distal tubules)
- ↓ Pi reabsorption (via ↓ NPT2a in proximal tubules)
- Stimulate Calcitriol production (via 1α-hydroxylase)

Calcitriol:
- ↑ Ca reabsorption (via ↑ PMCA1b, Calbindin-D28k in distal tubules)

Bone:
PTH:
- ↑ Ca + Pi resorption by stimulating osteoclastogenesis (via ↑ RANKL, ↓ Osteoprotegerin)

Calcitriol:
Negative Ca balance
- ↑ Ca + Pi resorption by stimulating osteoclastogenesis (via ↑ RANKL)
- Inhibit bone matrix mineralisation (via ↑ Pyrophosphate, Osteopontin)

• **Normal / Positive Ca balance
• ↑ bone mass by reducing osteoclastogenesis (via ↓ RANKL, ↑ Osteoprotegerin)
• ↓ PTH synthesis and secretion (via retinoid X receptor RXR, Vit D receptor form heterodimer with RXR —> bind to Vit D response element —> block PTH transcription)

Net effect:
PTH: ↑ Ca, ***↓ Pi
Calcitriol: ↑ Ca, ↑ Pi

Question 9

Disorders related to PTH

Answer 9

1. Hyperparathyroidism
   - ↑↑ PTH

Primary hyperparathyroidism (自動放多D PTH)
- gland disorder
- tumour in one of parathyroid gland (e.g. Parathyroid adenoma)
—> 結果: ***Hypercalcaemia (↑↑ PTH —> bone demineralisation, ↑ intestinal Ca absorption (via ↑ Calcitriol), ↓ renal Ca excretion)

Secondary hyperparathyroidism (被刺激放多D PTH)
- disorder outside gland that leads to ↑↑ PTH
- Causes (**Hypocalcaemia):
1. Calcium deficiency
2. Vit D / Fat deficiency in diet
3. Lack of exposure of skin to sunlight
4. **Chronic kidney failure —> insufficient production of Calcitriol + low PO4 excretion
5. Calcitonin age-related decrease
—> 結果: ***Hypocalcaemia (or Normal Ca) —> ALL 4 glands becomes enlarged and overproduce PTH

Tertiary hyperparathyroidism (自動放多D PTH)
- **long-term secondary hyperparathyroidism
- parathyroid gland loses regulation
- **automatically secrete excess PTH (even after renal transplantation)
—> 結果: ***Hypercalcaemia

2. Hypoparathyroidism
   - parathyroid glands produce too little PTH

• Causes:
  —> Accidental injury to parathyroid glands during head+neck surgery / radiation therapy to neck for thyroid cancer
  —> Autoimmune destruction
  —> Congenital

Question 10

Hypercalcaemia caused by Hyperparathyroidism

Answer 10

1. Painful bones
   - weakening of bones (**osteoporosis, **bone pain, osteitis fibrosa)
2. Renal stones
   - kidney problems (**polyuria, **renal stones)
3. Abdominal groans
   - digestive problems (***constipation, indigestion, abdominal pain)
4. Psychic moans
   - nervous system problems (fatigue, ***depression, memory loss, worsening concentration)
5. ***Muscle weakness (Negative bathmotropic effect: Ca inhibit Na channel), slow heart rate)
6. CVS problems
   - **Hypertension
   - **Arrhythmia

Question 11

Symptoms of Hypocalcaemia

Answer 11

mnemonic CATS:

• Convulsion
• Arrhythmia
• Tetany (***Chvostek’s sign —> hyperexcitability of face when tap on CN7)
• Spasm (***Trousseau’s sign —> use of BP cuff —> lack of blood flow to distal arm exacerbate hypoparathyroidism —> hyperexcitability)

Question 12

Disorders related to Calcitriol

Answer 12

Vit D toxicity (Hypervitaminosis D)
- excess intake
- extrarenal elevation of 1α-hydroxylase activity e.g. granulomatous disease (Sarcoidosis)
—> Hypercalcaemia —> ↓ PTH

Vit D deficiency (Hypovitaminosis D)
- inadequate intake
- inadequate sunlight exposure
- mutation in CYP27B1 gene (↓ 1α-hydroxylase activity)
- problems in digestive tract (e.g. Crohn’s)
—> Hypocalcaemia —> ↑ PTH

Question 13

***Summary of blood test

Answer 13

Hyperparathyroidism:
↑ PTH
—> ↑ Ca, ↓ Pi, ↑ Calcitriol

Hypoparathyroidism:
↓ PTH
—> ↓ Ca, ↑ Pi, ↓ Calcitriol

Vit D toxicity:
↑ 25(OH)D, ↑ Calcitriol (may be normal due to compensation from ↓ PTH)
—> ↑ Ca, ↑ Pi, ↓ PTH

Vit D deficiency:
↓ 25(OH)D, ↓ Calcitriol (may be normal due to compensation from ↑ PTH)
—> ↓ Ca, ↓ Pi, ↑ PTH

記住:

• ↑ PTH —> ↑ Calcitriol —> ↓ PTH
• ↑ PTH —> ↓ Pi
• ↑ Calcitriol —> ↑ Pi
• 兩個都↑ Ca